ASTM E910-18

Standard Test Method for Application and Analysis of Helium Accumulation Fluence Monitors for Reactor Vessel Surveillance

SIGNIFICANCE AND USE
5.1 The HAFM test method is one of several available passive neutron dosimetry techniques (see, for example, Test Methods E854 and E1005). This test method can be used in combination with other dosimetry methods, or, if sufficient data are available from different HAFM sensor materials, as an alternative dosimetry test method. The HAFM method yields a direct measurement of total helium production in an irradiated sample. Absolute neutron fluence can then be inferred from this, assuming the appropriate spectrum integrated total helium production cross section. Alternatively, a calibration of the composite neutron detection efficiency for the HAFM method may be obtained by exposure in a benchmark neutron field where the fluence and spectrum averaged cross section are both known (see Guide E2005).  
5.2 HAFMs have the advantage of producing an end product, helium, which is stable, making the HAFM method very attractive for both short-term and long-term fluence measurements without requiring time-dependent corrections for decay. HAFMs are therefore ideal passive, time-integrating fluence monitors. Additionally, the burnout of the daughter product, helium, is negligible.  
5.2.1 Many of the HAFM materials can be irradiated in the form of unencapsulated wire segments (see 1.1.2). These segments can easily be fabricated by cutting from a standard inventoried material lot. The advantage is that encapsulation, with its associated costs, is not necessary. In several cases, unencapsulated wires such as Fe, Ni, Al/Co, and Cu, which are already included in the standard radiometric (RM) dosimetry sets (Table 1) can be used for both radiometric and helium accumulation dosimetry. After radiometric counting, the samples are later vaporized for helium measurement. (A) Evaluated  235U fission neutron spectrum averaged helium production cross section and energy range in which 90 % of the reactions occur. All values are obtained from ENDF/B-V Gas Production Dosimetry File data. Bracke...
SCOPE
1.1 This test method describes the concept and use of helium accumulation for neutron fluence dosimetry for reactor vessel surveillance. Although this test method is directed toward applications in vessel surveillance, the concepts and techniques are equally applicable to the general field of neutron dosimetry. The various applications of this test method for reactor vessel surveillance are as follows:  
1.1.1 Helium accumulation fluence monitor (HAFM) capsules,  
1.1.2 Unencapsulated, or cadmium or gadolinium covered, radiometric monitors (RM) and HAFM wires for helium analysis,  
1.1.3 Charpy test block samples for helium accumulation, and  
1.1.4 Reactor vessel (RV) wall samples for helium accumulation.  
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Jan-2018
• ICS: 17.240 - Radiation measurements
• Technical Committee: E10 - Nuclear Technology and Applications
• Drafting Committee: E10.05 - Nuclear Radiation Metrology

Relations

• Replaces: ASTM E910-07(2013) - Standard Test Method for Application and Analysis of Helium Accumulation Fluence Monitors for Reactor Vessel Surveillance, E706 (IIIC)
• Effective Date: 01-Feb-2018
• Refers: ASTM C859-24 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM E1018-20 - Standard Guide for Application of ASTM Evaluated Cross Section Data File
• Effective Date: 01-Mar-2020
• Refers: ASTM E854-19 - Standard Test Method for Application and Analysis of Solid State Track Recorder (SSTR) Monitors for Reactor Surveillance
• Effective Date: 01-Nov-2019
• Refers: ASTM E844-18 - Standard Guide for Sensor Set Design and Irradiation for Reactor Surveillance
• Effective Date: 01-Jun-2018
• Refers: ASTM E170-17 - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 01-Jun-2017
• Refers: ASTM E170-16a - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 01-Oct-2016
• Refers: ASTM E170-16 - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 15-Feb-2016
• Refers: ASTM E170-15a - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 01-Sep-2015
• Refers: ASTM E1005-15 - Standard Test Method for Application and Analysis of Radiometric Monitors for Reactor Vessel Surveillance
• Effective Date: 01-Jul-2015
• Refers: ASTM E261-15 - Standard Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
• Effective Date: 01-Jun-2015
• Refers: ASTM E170-15 - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 15-Mar-2015
• Refers: ASTM E900-15 - Standard Guide for Predicting Radiation-Induced Transition Temperature Shift in Reactor Vessel Materials
• Effective Date: 01-Feb-2015
• Refers: ASTM E900-15e1 - Standard Guide for Predicting Radiation-Induced Transition Temperature Shift in Reactor Vessel Materials
• Effective Date: 01-Feb-2015
• Refers: ASTM E170-14a - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 15-Oct-2014

Overview

ASTM E910-18 is the internationally recognized standard test method for the application and analysis of Helium Accumulation Fluence Monitors (HAFM) in reactor vessel surveillance. Developed by ASTM, this standard is a cornerstone in passive neutron dosimetry, providing direct, stable measurements of neutron fluence through the detection of helium production in irradiated materials. Designed specifically for the nuclear industry-particularly for reactor vessel surveillance-ASTM E910-18 offers robust methodologies applicable to both power reactor monitoring and general neutron dosimetry.

This test method outlines the principles of helium accumulation, suitable materials, required instrumentation, and analysis protocols for deriving accurate neutron fluence data. The stability of helium as a measured end-product makes HAFMs ideal for both short-term and long-term monitoring, without the need for decay corrections.

Key Topics

• Passive Neutron Dosimetry: HAFM is one of several passive methods (such as radiometric monitors and solid-state track recorders) for measuring neutron fluence.
• Helium Production Measurement: The method yields direct measurements of helium yield, which is proportional to incident neutron exposure.
• Sensor Materials & Forms:
  • HAFM capsules
  • Unencapsulated wire segments (Fe, Ni, Al, Cu, etc.)
  • Charpy test blocks
  • Reactor vessel wall samples
• Data Analysis:
  • High-sensitivity gas mass spectrometry for helium quantification
  • Correction factors for sample composition, isotope purity, and neutron environment
• Calibration: Calibration in benchmark fields with known neutron spectra for increased accuracy.
• Material Selection:
  • Coverage of a wide range of neutron energies through composite sensor sets
  • Consideration for self-shielding, chemical stability, and encapsulation needs.

Applications

ASTM E910-18 is primarily applied in the nuclear power industry for:

• Reactor Vessel Surveillance: Monitoring neutron fluence to ensure the structural integrity and safety of reactor pressure vessels over operational life.
• Material Irradiation Studies: Evaluating test block and vessel wall materials for neutron-induced changes.
• Benchmark Dosimetry: Reference measurements during reactor shield and flux mapping programs.
• Research Reactors & General Dosimetry: Applicable to any scenario where precise, time-integrated neutron fluence data are required.

Practical Advantages:

• High Stability: Helium, as a noble gas, is not subject to radioactive decay, allowing for flexible timing in post-irradiation analysis.
• Passive Monitoring: HAFMs accumulate the neutron exposure history, offering a permanent record.
• Dual-use: Certain materials from standard radiometric sets can serve both radiometric and helium dosimetry, optimizing sample usage.

Related Standards

A number of other ASTM standards complement or are referenced by ASTM E910-18, including:

• ASTM E854: Application and Analysis of Solid State Track Recorder Monitors for Reactor Surveillance
• ASTM E1005: Application and Analysis of Radiometric Monitors for Reactor Vessel Surveillance
• ASTM E706: Master Matrix for Light-Water Reactor Pressure Vessel Surveillance Standards
• ASTM E2005: Guide for Benchmark Testing of Reactor Dosimetry in Standard and Reference Neutron Fields
• ASTM E261: Practice for Determining Neutron Fluence by Radioactivation Techniques
• ASTM E844: Guide for Sensor Set Design and Irradiation for Reactor Surveillance

Conclusion

By defining reliable procedures for the use and analysis of Helium Accumulation Fluence Monitors, ASTM E910-18 plays a critical role in the accurate monitoring of reactor vessel neutron exposure-a central concern for nuclear safety, reactor lifespan, and regulatory compliance. Its direct, stable, and robust methodology makes it an essential standard for anyone engaged in reactor dosimetry or neutron flux assessment.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E910 − 18
Standard Test Method for
Application and Analysis of Helium Accumulation Fluence
Monitors for Reactor Vessel Surveillance
This standard is issued under the fixed designation E910; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E170Terminology Relating to Radiation Measurements and
Dosimetry
1.1 This test method describes the concept and use of
E261Practice for Determining Neutron Fluence, Fluence
helium accumulation for neutron fluence dosimetry for reactor
Rate, and Spectra by Radioactivation Techniques
vessel surveillance. Although this test method is directed
E482Guide for Application of Neutron Transport Methods
toward applications in vessel surveillance, the concepts and
for Reactor Vessel Surveillance
techniquesareequallyapplicabletothegeneralfieldofneutron
E706MasterMatrixforLight-WaterReactorPressureVessel
dosimetry. The various applications of this test method for
Surveillance Standards
reactor vessel surveillance are as follows:
E844Guide for Sensor Set Design and Irradiation for
1.1.1 Helium accumulation fluence monitor (HAFM)
Reactor Surveillance
capsules,
E853PracticeforAnalysisandInterpretationofLight-Water
1.1.2 Unencapsulated, or cadmium or gadolinium covered,
Reactor Surveillance Results
radiometric monitors (RM) and HAFM wires for helium
E854Test Method for Application and Analysis of Solid
analysis,
State Track Recorder (SSTR) Monitors for Reactor Sur-
1.1.3 Charpy test block samples for helium accumulation,
veillance
and
E900Guide for Predicting Radiation-Induced Transition
1.1.4 Reactor vessel (RV) wall samples for helium accumu-
Temperature Shift in Reactor Vessel Materials
lation.
E944Guide for Application of Neutron Spectrum Adjust-
1.2 This standard does not purport to address all of the
ment Methods in Reactor Surveillance
safety concerns, if any, associated with its use. It is the
E1005Test Method for Application and Analysis of Radio-
responsibility of the user of this standard to establish appro-
metric Monitors for Reactor Vessel Surveillance
priate safety, health, and environmental practices and deter-
E1018Guide for Application of ASTM Evaluated Cross
mine the applicability of regulatory limitations prior to use.
Section Data File
1.3 This international standard was developed in accor-
E2005Guide for Benchmark Testing of Reactor Dosimetry
dance with internationally recognized principles on standard-
in Standard and Reference Neutron Fields
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3. Terminology
mendations issued by the World Trade Organization Technical
3.1 Definitions—For definition of terms used in this test
Barriers to Trade (TBT) Committee.
method, refer to Terminologies C859 and E170. For terms not
defined therein, reference may be made to other published
2. Referenced Documents
glossaries.
2.1 ASTM Standards:
C859Terminology Relating to Nuclear Materials
4. Summary of the HAFM Test Method
4.1 Helium accumulation fluence monitors (HAFMs) are
passive neutron dosimeters that have a measured reaction
ThistestmethodisunderthejurisdictionofASTMCommitteeE10onNuclear
product that is helium. The monitors are placed in the reactor
Technology and Applicationsand is the direct responsibility of Subcommittee
locations of interest, and the helium generated through (n,α)
E10.05 on Nuclear Radiation Metrology.
Current edition approved Feb. 1, 2018. Published March 2018. Originally
reactions accumulates and is retained in the HAFM (or HAFM
approved in 1982. Last previous edition approved in 2013 as E910–07 (2013).
capsule) until the time of removal, perhaps many years later.
DOI: 10.1520/E0910-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on See Dictionary of Scientific Terms, 3rd Edition, Sybil P. Parker, Ed., McGraw
theASTM website. Hill, Inc.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E910 − 18
Theheliumisthenmeasuredverypreciselybyhigh-sensitivity cussed more fully in Section 13. Generally, they total less than
gas mass spectrometry (1, 2). The neutron fluence is then 5%ofthemeasuredheliumconcentration.Sincetheindividual
directly obtained by dividing the measured helium concentra- corrections are usually known to within 50%, the total error
tion by the spectrum-averaged cross section. Competing he- from these corrections amounts to ≤2%. Sources of uncer-
lium producing reactions, such as (γ,α) do not, except for tainty also lie in the HAFM material mass, isotopic
Be(γ,α), affect the HAFM results. The range of helium composition, and mass spectrometric helium analysis. As
concentrations that can be accurately measured in irradiated indicated in Section 13, however, these uncertainties generally
−14 −1
HAFMs extends from 10 to 10 atom fraction. This range contribute less than 1% of the total uncertainty for routine
permits the HAFMs to be tested in low fluence environments analyses.
yet to work equally well for high fluence situations.
4.5 Applying the above corrections to the measured HAFM
4.2 Typically, HAFMs are either individual small solid
helium concentration, the total incident neutron fluence (over
samples, such as wire segments (3) or miniature encapsulated
the energy range of sensitivity of the HAFM) can be obtained
samplesofsmallcrystalsofpowder (4),asshowninFig.1.As
directly from a knowledge of the spectrum-integrated total
with radiometric dosimetry, different materials are used to
helium production cross section for the particular irradiation
provide different energy sensitivity ranges. Encapsulation is
environment.Atthepresenttime,theuncertaintyinthederived
necessary for those HAFM materials and reactor environment
neutron fluence is mainly due to uncertainty in the spectrum-
combinations where sample melting, sample contamination, or
integrated cross section of the HAFM sensor material rather
loss of generated helium could possibly occur. Additionally,
than the combined uncertainties in the helium determination
encapsulation generally facilitates the handling and identifica-
process. This situation is expected to improve as the cross
tion of the HAFM both prior to and following irradiation. The
sections are more accurately measured, integrally tested in
contentsofHAFMcapsulestypicallyrangefrom0.1to10mg.
benchmark facilities (6), and reevaluated.
4.3 Followingirradiation,encapsulatedHAFMsarecleaned
5. Significance and Use
and identified in preparation for helium analysis. Helium
analysis is then accomplished by vaporizing both the capsule
5.1 The HAFM test method is one of several available
anditscontentsandanalyzingtheheliumintheresultinggases
passive neutron dosimetry techniques (see, for example, Test
inahighsensitivitymassspectrometersystem (5).Theamount
Methods E854 and E1005). This test method can be used in
4 4 3
of He is determined by measuring the He-to- He isotopic
combinationwithotherdosimetrymethods,or,ifsufficientdata
ratio in the sample gases subsequent to the addition of an
are available from different HAFM sensor materials, as an
accurately calibrated amount of He “spike.” Unencapsulated
alternativedosimetrytestmethod.TheHAFMmethodyieldsa
HAFMs,forexample,pureelementwires,areusuallyetchedto
direct measurement of total helium production in an irradiated
remove a predetermined layer of outer material before helium
sample. Absolute neutron fluence can then be inferred from
analysis (3). This eliminates corrections for both cross con-
this,assumingtheappropriatespectrumintegratedtotalhelium
tamination between samples and α-recoil into or out of the
production cross section. Alternatively, a calibration of the
sample during the irradiation.
composite neutron detection efficiency for the HAFM method
4.4 The He concentration in the HAFM, in general terms, may be obtained by exposure in a benchmark neutron field
wherethefluenceandspectrumaveragedcrosssectionareboth
is proportional to the incident neutron fluence. Consideration
must, however, be made for such factors as HAFM material known (see Guide E2005).
burnup, neutron self-shielding and flux depression, α-recoil,
5.2 HAFMs have the advantage of producing an end
and neutron gradients. Corrections for these effects are dis-
product, helium, which is stable, making the HAFM method
very attractive for both short-term and long-term fluence
measurements without requiring time-dependent corrections
The boldface numbers in parentheses refer to the list of references appended to
fordecay.HAFMsarethereforeidealpassive,time-integrating
this test method.
FIG. 1 Helium Accumulation Fluence Monitor Capsule
E910 − 18
fluence monitors. Additionally, the burnout of the daughter nation of the boron content (7). Boron level down to less than
product, helium, is negligible. 1 wt. ppm can be obtained in this manner.
5.2.1 Many of the HAFM materials can be irradiated in the
5.5 By careful selection of the appropriate HAFM sensor
form of unencapsulated wire segments (see 1.1.2). These
material and its mass, helium concentrations ranging from
−14
segments can easily be fabricated by cutting from a standard
−1
;10 to10 atomfractioncanbegeneratedandmeasured.In
inventoried material lot. The advantage is that encapsulation, 12 27
termsoffluence,thisrepresentsarangeofroughly10 to10
with its associated costs, is not necessary. In several cases,
n/cm . Fluence (>1 MeV) values that may be encountered
unencapsulatedwiressuchasFe,Ni,Al/Co,andCu,whichare
during routine surveillance testing are expected to range from
already included in the standard radiometric (RM) dosimetry
14 20 2
;3×10 to2×10 n/cm , which is well within the range of
sets (Table 1) can be used for both radiometric and helium
the HAFM technique.
accumulation dosimetry. After radiometric counting, the
5.6 The analysis of HAFMs requires an absolute determi-
samples are later vaporized for helium measurement.
nation of the helium content. The analysis system specified in
5.3 The HAFM method is complementary to RM and solid
this test method incorporates a specialized mass spectrometer
state track recorder (SSTR) foils, and has been used as an
in conjunction with an accurately calibrated helium spiking
integral part of the multiple foil method. The HAFM method
system. Helium determination is by isotope dilution with
followsessentiallythesameprincipleastheRMfoiltechnique,
subsequentisotoperatiomeasurement.Thefactthatthehelium
which has been used successfully for accurate neutron dosim-
is stable makes the monitors permanent with the helium
etry. Various HAFM sensor materials exist which have signifi-
analysis able to be conducted at a later time, often without the
cantly different neutron energy sensitivities from each other.
inconvenience in handling caused by induced radioactivity.
10 6
HAFMs containing B and Li have been used routinely in
Such systems for analysis exist, and additional analysis facili-
LMFBR applications in conjunction with RM foils. The
ties could be reproduced, should that be required. In this
resulting data are entirely compatible with existing adjustment
respect, therefore, the analytical requirements are similar to
methods for radiometric foil neutron dosimetry (refer to Guide
other ASTM test methods.
E944 ).
5.4 An application for the HAFM method lies in the direct
6. Apparatus
analysis of pressure vessel wall scrapings or Charpy block
6.1 High-Sensitivity Gas Mass Spectrometer System, ca-
surveillance samples. Measurements of the helium production
pable of vaporizing both unencapsulated and encapsulated
in these materials can provide in situ integral information on
HAFM materials and analyzing the resulting total helium
the neutron fluence spectrum. This application can provide
content is required. A description of a suitable system is
dosimetry information at critical positions where conventional
contained in Ref (5).
dosimeter placement is difficult if not impossible. Analyses
must first be conducted to determine the boron, lithium, and 6.2 Analytical Microbalance for Accurate Weighing of
other component concentrations, and their homogeneities, so HAFM Samples, minimum specifications: 200-mg capacity
that their possible contributions to the total helium production with an absolute accuracy of 60.5 µg. Working standard
can be determined. Boron (and lithium) can be determined by
masses must be traceable to appropriate national or interna-
converting a fraction of the boron to helium with a known tional mass standards.Additionally, a general purpose balance
thermal neutron exposure. Measurements of the helium in the with a capacity of at least 200 g and an accuracy of 0.1 mg is
material before and after the exposure will enable a determi- required for weighing larger specimens.
TABLE 1 Neutron Characteristics of Candidate HAFM Materials for Reactor Vessel Surveillance
Fission Neutron Spectrum
Principal Helium Producing Thermal Neutron Cross
HAFM Sensor Material
90 % Response
A
Reaction Section, (b)
Cross Section, (mb)
A
Range, (MeV)
Li Li(n,α)T 942 457 0.167–5.66
9 6 6
Be Be(n,α) He ;ra Li . 284 2.5–7.3
10 7
B B(n,α) Li 3838 494 0.066–5.25
14 11
N N(n,α) B . 86.2 1.7–5.7
19 16
F F(n,α) N . 27.6 3.7–9.7
B 27 24
Al Al(n,α) Na . 0.903 6.47–11.9
32 29
S S(n,α) Si . . .
35 32
Cl Cl(n,α) P . ;13 (Cl) 2.6–8.3
B 47 44
Ti Ti(n,α) Ca . 0.634 (Ti) 6.5–12.8
B 56 53
Fe Fe(n,α) Cr . 0.395 (Fe) 5.2–11.9
B 58 55
Ni Ni(n,α) Fe . 5.58 (Ni) 3.9–10.1
B 63 60
Cu Cu(n,α) Co . 0.330 4.74–11.1
316-SS
Helium Production Largely
56 58
from Fe and Ni
PV Steel
J
Charpy Block
A 235
Evaluated U fission neutron spectrum averaged helium production cross section and energy range in which 90 % of the reactions occur. All values are obtained from
ENDF/B-V Gas Production Dosimetry File data. Bracketed terms indicate cross section is for naturally occurring element.
B
Often included in dosimetry sets as a radiometric monitor, either as a pure element foil or wire or, in the case of aluminum, as an allaying material for other elements.
E910 − 18
6.3 Laminar flow (optional) clean benches, for use in the HAFMs or combination of HAFMs, RM, and SSTR multiple
preparation of HAFM samples and capsules. foils must be chosen to cover the entire neutron energy range
(refer to Guide E844). The majority of potential HAFM
6.4 Stereo microscope, with 7 to 30 magnification, a
materials fall into the threshold reaction category. That is,
;0.1-mm graticule, and an optical illuminator.
below the threshold energy (usually in the 1–10 MeV range),
6.5 Electron beam welder, with moveable platform stage,
these materials produce essentially no helium from neutron
for sealing HAFM capsules, minimum specifications: variable
reactions. Above this energy, however, the (n, total helium)
beam power to 0 to 1 kW, variable beam size capable of
cross section generally rises fairly rapidly to a plateau from
focusing down to a diameter of 0.5 mm. Controls must also be
where it continues to rise relatively slowly. Generally, the
available to permit automatic control of beam duration and
higher the threshold energy, the lower the total cross section.
onset and offset beam power slopes.
The threshold reaction HAFM isotopes presently identified as
9 14
6.6 High temperature vacuum furnace for out-gassing beingmostsuitableforreactorvesselsurveillanceare Be, N,
19 27 32 35 56 58 63
F, Al, S, Cl, Fe, Ni and Cu (see Table 1).
HAFM materials, capsules, and mass spectrometer system
furnace components. Minimum specifications: 1000°C at a
7.2.1.1 The two stable isotopes that have significant non-
−5
6 10
maximum pressure of 10 Torr.
threshold helium production cross sections are Li and B.
The cross sections of these two isotopes, which are large and
6.7 Micro-sandblaster/cleaner,forcleaningmassspectrom-
well known, vary inversely with the neutron velocity below
eter vacuum furnace parts.
about 0.1 MeV. Above 0.1 MeV, the cross section behavior
6.8 X-ray machine, for quality assurance test of HAFM
becomes more irregular, with the Li exhibiting a significant
capsules. Minimum specifications; 300 kV, 10 mA, 4-mm spot
resonance near 0.24 MeV.
sizewithcontrolofsourcedistanceto1.0mandexposuretime
7.2.1.2 Other stable isotopes exist which have nonthreshold
to 5 min.
helium production cross sections, but all are much less than 1
−24 2 59
6.9 General Laboratory Supplies:
barn (10 cm ). Of the radioactive isotopes, Ni, which has
6.9.1 Ultrasonic Cleaner—100 to 200 W,
a ;12barnthermalneutron(n,α)crosssection,istheonlyone
6.9.2 Heat Lamp—250 W, and
important for HAFM neutron dosimetry through the two-stage
58 59 56
6.9.3 Optical Pyrometer—700 to 2000°C.
reaction Ni(n,γ)· Ni(n,α) Fe.Also included in Table 1 are
additional potential HAFM materials which are already in-
6.10 Radioactive Material Handling:
cluded in the standard specified RM foil and metallurgical sets
6.10.1 Lead shielding,
(refer to Matrix E706) and thus may serve a double purpose
6.10.2 Portable radioactive (β-γ) counters (0.01 mrem/h to
(see 11.1).These materials include the natural elementsTi, Fe,
100 rem/h), and
Cu, and Ni; stainless steel dosimetry capsule material, RV
6.10.3 Radioactive waste disposal capability.
steel; and Charpy block metallurgical specimens. Relevant
6.11 Reagents and Materials:
characteristicsofthevariousHAFMisotopesandmaterialsare
6.11.1 Hydrochloric Acid (HCl), (37%),
listed in Table 1.Aluminum is also often included in RM sets
6.11.2 Hydrofluoric Acid (HF), (48%),
in the form of alloys of Co and Au.
6.11.3 Nitric Acid (HNO ), (70%),
7.2.2 Activation Cross Sections—Also to be considered in
6.11.4 Sulfuric Acid (H SO ), (96%),
2 4
the selection of HAFM materials is their relative activation
6.11.5 Acetone [(CH ) CO]—Reagent grade (>99.7%),
3 2
crosssectionsintypicalreactorvesselneutronfields.Although
6.11.6 Alcohol (C H OH)—Pure (200 proof),
2 5
activation reactions in general do not interfere with helium
6.11.7 Chloroform (CHCl )—Reagent grade (>99.2%),
production(exceptionsarecasesoftwo-stagereactionsaswith
6.11.8 Distilled and Deionized Water, and
Ni, and cases where daughter products have contributing
6.11.9 Detergent Cleaning Solution (Alconox or equiva-
9 6 6
(n,α) reactions such as Be(n,α) He → Li), the resulting
lent).
radioactive decay contributes to post-irradiation handling and
analysis difficulties and, to this extent, should be minimized.
7. HAFM Materials
7.2.3 Neutron Self-Shielding—High cross section isotopes,
7.1 General Requirements—The general requirements con-
6 10
such as Li and B, exhibit significant neutron self-shielding
cerning the characteristics of HAFM materials fall into two
and surface flux depression in thermal and epithermal neutron
broad categories: (1) nuclear properties, and (2) chemical
environments. In order to apply these isotopes to reactor
properties. These two categories are discussed separately
surveillance dosimetry, dilution of these materials by alloying
below.
is required to reduce their effective isotopic concentrations.
7.2 Nuclear Properties:
Suitable alloying materials for boron and lithium at the 0.1 to
7.2.1 Helium Production Cross Section—Consideration
0.5 weight percent level are vanadium, niobium, and alumi-
must be made for the energy range or energy sensitivity of the
num. Additional details on self-shielding are given in Section
(n, total helium) cross section of the potential HAFM sensor
13.
material. For any given neutron environment, the set of
7.2.4 Neutron Screening at Low Energies—An alternate
technique,oronethatcanbeusedinconjunctionwithalloying
to reduce neutron self-shielding, is to protect the boron and
Alconox is a registered trademark ofAlconox Inc., 215 ParkAve. South, New
York, NY 10003. lithiumfromlow-energyneutronsbycoveringwithappropriate
E910 − 18
materials. Cadmium or gadolinium provides a low-energy containinglowweightcontentsofboronandlithium(discussed
neutroncutoffof ;0.5eV.Aconsiderablyhighercutoffenergy earlier in 7.2.3) can also be determined in this way.
can be achieved by shielding with boron carbide (B C). For 1
keV neutrons, ;4.5 cm of B C provides ;90% attentuation.
9. Manufacture of HAFMs
Because of the neutron perturbation effects of B C, however,
9.1 HAFM Capsules:
this latter technique would be useful only at ex-vessel surveil-
9.1.1 Fabrication and X-ray Qualification—As discussed
lance locations.
previously, encapsulation of HAFM sensor material is neces-
7.3 Sensor Chemical Properties—Various considerations
sary in those cases where contamination, loss of sensor
must be made concerning the chemical properties of the
material, or loss of internally generated helium could occur.A
HAFM sensor materials. Many of the HAFM isotopes, such as
typical HAFM capsule is shown in Fig. 1. These capsules
6 7 14
Li, Li, N, etc., are conveniently usable only in compound
generallyare6.4-mmlong,withoutsidediametersof0.9or1.3
6 7
form.Examplesofsuitablecompoundsare LiF, LiF,TiN,and
mm and inside diameters ranging from 0.5 to 1 mm.To ensure
ZrN.Inthechoiceofthemostusefulcompound,consideration
nolossofinternallygeneratedhelium,capsulewallsmusthave
must be given to such factors as: (1) helium production and
a minimum thickness of 0.17 mm. This is most easily verified
activation cross sections of the host element (F, Ti, and Zr in
by X-ray inspection of each empty capsule from two perpen-
theaboveexamples),(2)homogeneityandstoichiometryofthe
dicularangles.Tominimizetimeandcost,thecapsulesmaybe
compound,(3)residualimpuritiessuchasboronorlithium,(4)
X-rayed in groups of approximately 100. Various X-ray con-
stability and resistance to decomposition at higher
ditionshavebeeninvestigated,andfromthesetests,ithasbeen
temperatures, (5) alloying potential with the encapsulating
determined that optimum capsule definition is obtained by
material,and(6)meltingandvaporizationtemperatures,which
enclosing the capsules in stainless steel hypodermic tubing
are important when it comes to releasing the helium for mass
during the X-ray procedure. The stainless steel serves both as
spectrometric analysis.
a convenient holder and aligning material, and it has the effect
7.4 HAFM Material Encapsulation—Encapsulation is nec-
of lowering the X-ray exposure to the film at the capsule edge.
essary for those HAFM sensor materials and irradiation con-
In this manner, a “sharp’’ material density edge for the X-rays
ditions for which there is a potential for either contamination,
isachieved,resultinginawell-definedcapsuleedge.Following
loss of generated helium from α-recoil or diffusion, or loss of
theX-rayprocedure,eithertheX-raynegativesorenlargement
sensor material itself. This includes those HAFM compounds
prints can be visually scanned using a calibrated magnifier to
whichareintheformoffinepowdersorcrystals,orwhichmay
locate capsules whose central holes are not concentric and
melt at the temperatures anticipated in the irradiation environ-
whose minimum wall thicknesses may fall outside the allow-
ment. The encapsulating material must be chosen so as to
able limits. The X-ray negatives or prints should be kept on
completelycontaintheHAFMsensoranditsgeneratedhelium,
permanent file, with some means of identification for later
whileatthesametimehavingrelativelylowheliumproduction
tracing individual capsules back to the X-ray records.
and activation cross sections. The former is of importance for
9.1.1.1 In addition to the capsule X-ray number, each
total helium production since the entire HAFM sensor plus
HAFM capsule should have an alphanumeric identification
capsuleislateranalyzedforhelium.Thelatterisofimportance
code stamped on the solid base, and as well may have one or
in minimizing induced radioactivity in the HAFM capsule.
two identifying grooves around the circumference. In this
Further requirements are that the encapsulating material must
manner, individual capsules or groups of capsules can be
be reasonably durable to withstand handling before and after
identified remotely during post-irradiation hot cell recovery.
irradiation and that the material be both machinable and
9.1.2 HAFM Material Mass—Encapsulated HAFM sensor
weldable to facilitate HAFM capsule fabrication. Generally,
materials can range in mass from single crystals (for example,
when it has been determined that the HAFM sensor material
10 6
Bor LiF) weighing less than 0.1 mg to fine crystalline
has itself the required helium retention, strength, and chemical
powders weighing up to 10 mg. In each case, the total HAFM
inertness, the HAFM is used in the form of a “bare’’ wire
material mass should be determined using a microbalance and
segment without being encapsulated (3).
preferablyadoublesubstitutionweighingscheme,inwhichthe
samples are compared with the working standard masses.
8. HAFM Material Processing
Periodic calibration of the working standards must be made
8.1 HAFM sensor and encapsulating materials must be
relativetoappropriatenationalorinternationalmassstandards.
analyzed for possible residual helium by pre-irradiation analy-
Total mass accuracy, using this technique, is generally better
sis of the various lot materials. In this regard, precautions
than 60.3 µg. For single crystals, the mass is best determined
should be taken to ensure that no helium has been used (as an
priortoloading.Forthefinercrystallinepowders,however,the
inert gas) during any stage of material fabrication.
most reliable and accurate method of determining the mass is
by weighing the HAFM capsule before and after loading.
8.2 HAFM and encapsulating materials must also be ana-
lyzed for thermal neutron helium producing impurities (for 9.1.3 Capsule Welding—Because of the need to exclude air,
6 10
with its natural helium content, from the HAFM interior, weld
example, Li and B at sub-ppm levels). As discussed
previously, this is most effectively done by helium analysis of closure of the capsule top is best accomplished by electron
beam under vacuum. This form of welding has the additional
a sample of each lot of material following a thermal neutron
irradiation. The concentration and homogeneity of alloys advantage of precise control of weld power and heating zone.
E910 − 18
TIG welding, an alternate technique, would involve closure the case of helium, when sequential pumping is employed. A
under an inert gas atmosphere which could complicate later rotarypumpandthenaturbomolecularpumpfirstremovemost
helium analysis. of the helium very rapidly. As soon as the lower limit is
9.1.3.1 After HAFM material loading and prior to capsule reached, an ion pump is used to reduce the vacuum to a lower
welding, thin spacer disks should be placed above the sensor level. Finally, another ion pump is used only to maintain the
−9
materialtoreflecttheheatfromtheweldzone(seeFig.1).This mass spectrometer in the 10 Torr range between analyses.
is followed by partially closing the capsule top to facilitate the
10.2.3 Furnaces—Several methods have been successfully
;
weld process. This can be accomplished either by insertion of
used to vaporize HAFM materials. For small samples ~, 2
a solid plug or by squeezing the top portion of the capsule
mg)withmeltingtemperatureslessthan ;1800°C,thesamples
together. Some gaps should be left in the capsule top to allow
can be readily vaporized in small resistance-heated 0.25-mm
for complete evacuation (or inert gas backfilling) prior to final
diameter tungsten wire coil baskets (2). Larger samples (>2
closure. To further reduce HAFM sensor material heating
mg), including HAFM capsules or samples with melting
during welding, the lower portion of each capsule should be in
temperatures above 1800°C, can be vaporized in larger
firmcontactwithasuitableheatsink,“chillblock.”Thelength
resistance-heated cylindrical graphite crucibles (4.8-mm OD,
of the weld zone should be limited to the top ;1mmof
20-mmlong) (2).Priortoloading,thetungstencoilbasketsand
capsule.
graphite crucibles should be degassed in vacuum by heating to
9.1.4 Final Capsule Weighing—As an additional aid in pre-
;1750°C for about 2 min. Vacuum furnaces have been
and post-irradiation identification, the final welded capsules
constructed that contain up to ten individual tungsten coils or
should be weighed to an accuracy of at least 610 µg.
graphite crucibles. The design of the vacuum furnaces must
Therefore, if part of the alphanumeric identification base code
allow vaporization of samples with masses ranging from about
becomes unreadable, capsule identification would still be
0.5 to 200 mg (the heavier masses are associated with
likely. Additionally, this additional weighing step reveals any
encapsulated HAFMs). During analysis, the current through
possibleHAFMmaterialmasslossduringtheweldingprocess.
the baskets or crucibles is steadily increased until decomposi-
In this respect, capsule weighings before and after loading
tion of the tungsten or graphite occurs. In this manner,
should include the actual spacer disks and weld cap (if
vaporization of the enclosed sample and total helium release is
applicable) to be used (see 9.1.3).
assured. For maximum sensitivity for very low level samples,
the heating can be stopped prior to tungsten or graphite
10. HAFM Analysis
decomposition provided it can be ascertained that all HAFM
sensor material has been vaporized. This reduced heating
10.1 Outline of Test Method—Determination of the helium
generallyreducestheamountofhelium“background”released
content in HAFM materials is made by vaporizing the materi-
by the furnace itself.
als under vacuum. Immediately before the sample is vaporized
4 3
and the He is released, a precisely-known amount of He is
10.2.3.1 A third furnace type has been used to vaporize
added ( He “spike”).After mixing of the two isotopes, the gas
largermetallicsampleswithmeltingpointsupto ;1200°C (8).
passes over getters that remove unwanted gases, then passes
Thisfurnaceusesagraphitecruciblewhichisresistanceheated
into the mass spectrometer volume, which is isolated from its
and then maintained at a constant temperature of ;2000°C.
vacuum pump for “static mode” operation. The measurement
Samples are dropped individually by remote means into the
4 3
ofthe He/ HeratioandaknowledgeofthemassoftheHAFM
heated crucible and vaporized. The fact that the furnace
material then produces the helium concentration. A recom-
temperature remains essentially constant during the analysis
mended helium analysis system has been described previously
procedure reduces the uncertainty in the furnace “blank’’ – the
(5). Precautions must be taken to account for He that might
amount of helium attributable to the furnace itself. This
already be present in the HAFM (see 10.3.1).
reduced uncertainty has the effect of lowering the effective
detection limit of the mass spectrometer system. Using this
10.2 Apparatus:
technique, samples with masses up to ;1 g can be analyzed,
10.2.1 Mass Spectrometer—Magnetic sector mass spec-
witharesultingheliumanalysisuncertaintyof ;1×10 atoms.
trometer with all-metal tube and an interior volume of about 1
In copper, this is equivalent to a helium concentration of
L. The instrument should have an electron impact ion source,
−14
;10 atomfraction.
electronmultiplier,andanelectrometerwithcurrentmeasuring
−13 −14
10.2.4 Getters—Asystemofgettersshouldbeusedtopurify
capability of at least 10 A with a stability of <10 A/h.
the helium gas sample before it is admitted into the mass
Output from the electrometer can be monitored directly via a
spectrometer, and to maintain a high vacuum in the mass
stripchartrecorderordigitallyaveragedforreal-timecomputer
spectrometer while it is being operated in the static mode. The
analysis. The mass resolving power of the mass spectrometer
getters could consist, for example, of a liquid-nitrogen-cooled
itself should be a minimum of 50 with a mass scanning range
charcoal trap, followed by, but separated from, a nonevapo-
from 2 to 50 amu. Mass scanning capability is useful in
checking for possible interfering background gases. In rable alloy getter (such as the SAES GT-50). Another alloy
gettershouldbepermanentlyattachedtothemassspectrometer
addition, the entire system should be bakeable to 300°C.
itself to maintain the vacuum while the instrument is isolated
10.2.2 Vacuum System—To minimize the time necessary to
from its ion pump during sample analysis.
pump away gas samples between analyses, a multiple vacuum
systemconsistingofseveralindependentsubsystemsshouldbe 10.2.5 Spike System—A network of accurately calibrated
3 4
used. Rapid pumpout can best be accomplished, especially in volumeswhichdispensesknownquantitiesof Heand He,for
E910 − 18
calibration and for isotope dilution purposes, should be avail- 10.3.2 Atypicalprocedureistoallowthegastoexpandinto
able. For convenience, this network can be attached directly to the liquid-nitrogen-cooled charcoal getter, after which the
the mass spectrometer line. The size and required accuracy of connecting all-metal valve is closed. The gas thus trapped
3 4
the Heand Hespikesmustbedeterminedinconjunctionwith (between 1 and 10% of the total, depending on the size of the
the characteristics of the mass spectrometer and the analysis furnace assembly used) is sufficient for the mass spectrometric
linestoallowforabsoluteheliummeasurementsintherangeof determination of the isotopic composition. After about 20 s,
10 18
10 to 10 atoms of helium to an accuracy of 1 to 2%. Glass this aliquot of gas is permitted to expand into the getter
stopcocks should be used throughout the spike system rather enclosure. Finally, the gas is allowed into the mass spectrom-
than stainless steel valves, mainly because the stopcocks etervolumewhichisisolatedfromitsionpump.Itstaysinthis
provide a more positive and reliable barrier through which volume until the isotopic ratio measurements are complete.
−7
helium has little chance of passing unnoticed.Another impor- The small amount of helium admitted is usually about 10 cc
tantadvantageoverstainlesssteelvalvesistheeasewithwhich STP,whichdoesnotdeleteriouslyaffectthemassspectrometer
the volumes between the stopcocks may be calibrated. Helium vacuum.
absorption on vacuum grease is negligible. Although most of
10.3.3 Gas samples from milligram-size specimens whose
the spike system, including all the stopcocks, can be made of
heliumconcentrationsareabove0.1appmaresufficientlylarge
borosilicate glass, the volumes which are used for long-term
thataverysmallpermeationordesorptionof Heintothemass
storage of helium must be made either from aluminosilicate
spectrometercanbeignored.Forsmallersamples,thisconstant
glass (Corning Type 1720) , which is relatively impervious to
leak becomes perceptible, and eventually it sets the detection
4 3
helium, or from stainless steel.
limit of the instrument.Thus, in all analyses, the He/ He ratio
10.2.6 The spiking systems should include, in addition to
is carefully examined for systematic increase; and, if such an
3 4
various sized He and He spikes, a standard spike mixture of
increase is found, the ratio is measured against time and
3 4
both He and He. This mixture is required for calibration of
extrapolated to the exact time the sample was admitted to the
the relative sensitivity of the mass spectrometer for masses 3
mass spectrometer volume. The ratio that is obtained is the
3 4
and 4. Further, the separate He and He spikes can be used to
helium isotopic ratio at the time the sample was introduced,
provideadditionalcombinationsofthetwoheliumisotopesfor
whichdoesnotaccountfor Heleakageintothesamplelineor
further verification of the relative sensitivity, for verifying that
furnace. By taking a second and third aliquot of gas from this
the individual spike systems are dispensing the expected
samplefurnace,andanalyzingthemasdescribedabove,results
3 4
amountsof Heand He,andtocrosscheckthecalibrationand
can be extrapolated to give the true amount of He that was
linearity of the mass spectrometer system as a whole. Addi-
released from the sample. This can be done with negligible
tional calibration of the system should also be accomplished
uncertainty introduced as a result of the extrapolation, except
using an independent standard source of helium concentration.
for the case of extremely small samples of helium.
3 3
Standard helium gas mixtures can be obtained from the U.S.
10.3.3.1 HAFMs that Contain He—In a few cases, He is
BureauofMines.Alternatively,air,whichhasaknownhelium
also present in irradiated HAFMs. If so, it must be accounted
concentration (5.24 appm), can be used (9).
for in the mass spectrometric analysis because it would not be
10.3 Analysis Procedure—After estimating the approximate distinguished from the He “spike.” This isotope is rarely
formed directly by nuclear reactions, but usually occurs as the
helium concentration in the HAFM sample, and after deter-
miningitsmass,thesampleisloadedintooneofthevaporizing result of decaying tritium. In the case of LiF HAFMs, tritium
is formed every time a helium atom is generated, so He can
systems attached to the mass spectrometer (see 10.2.3). After
suitable vacuum pumping (usually overnight), the samples are become significant after a few month’s decay. Very few other
HAFMreactionsproducetritium,butthisgascanpassthrough
ready for analysis. Immediately before the heating operation
and the release of the sample gas, an appropriately sized spike many metals with ease, and consequently HAFMs that have
not themselves generated any tritium can still contain this gas
of Heisadded.Unlessotherreleasedgasesinterfere,complete
mixingoftheisotopesoccursinafewseconds.Fromthispoint and its He daughter, just from being in a reactor core
environment. In order to measure both helium isotopes
on, it does not matter what fraction of gas is used for the
4 3
analysis because only the ratio He/ He needs to be deter- simultaneously,therefore,aslightlymodifiedmassspectromet-
ric procedure is employed. A small known fraction of the
mined.
10.3.1 The removal of unwanted gases released during the helium gas released from the HAFM is analyzed for isotopic
contentbefore,ratherthanafter,theadditionofthespike.After
vaporization of the sample is accomplished while the helium
3 4 3
passes by the getters. The most important aspect of the the He content is measured with respect to the He, the He
spike is added to the remainder of the gas sample, and the
operation is to make sure that as little helium as possible from
all other sources contaminates the sample gas and changes the altered isotopic ratio is measured to provide absolute concen-
4 3 3
tration. Once it has been established that the He content in a
sample-plus-spike He/ He ratio before it is measured. This
set of HAFMs is negligible compared with the added He
means that the purification should be done quickly.
spike, this modified procedure is no longer required.
The sole source of supply of the apparatus known to the committee at this time
11. Irradiation Guidelines
is Corning Glass Works, Corning, NY 14831. If you are aware of alternative
suppliers, please provide this information to ASTM International Headquarters.
11.1 SelectionofHAFMSensorMaterial—Thereareseveral
Your comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend. factorstobeconsideredintheselectionofHAFMmaterialsfor
E910 − 18
reactor vessel surveillance. Of primary importance is the because of the relatively high thermal neutron fluxes at typical
desired energy coverage. Since the HAFM method is closely reactor surveillance locations.
tied to the radiometric foil dosimetry method, the HAFM
11.2 Experimental Considerations—In order to reduce the
sensor materials should be chosen to complement the various
possibility of external helium contamination, HAFMs should
multiple foils present. As discussed earlier, some RM and
beirradiatedinanon-heliumatmosphereifpossible.Ifthermal
metallurgical materials can provide data for both methods
heat sinking is required to prevent HAFM overheating, argon,
simultaneously. Examples of this double utilization include
or preferably neon which has a higher thermal conductivity,
46 46 54 54 58 58
using the Ti(n,p) Sc, Fe(n,p) Mn, Ni(n,p) Co,
may be used to surround the HAFMs. If the HAFM must be
58 55 59 60 63 60 109
Ni(n,α) Fe, Co(n,γ) Co, Cu(n,α) Co, and Ag(n,
placed in a helium environment, the resulting surface helium
110m
γ) Agreactionsforradiometricdeterminations,whileatthe
can be removed by post-irradiation surface etching (generally
same time using the natural Ti, Fe, Ni, and Cu, and the alloys
<;10µm).Theeffectivenessofthisproceduremaybeverified
Al-0.1%Co and Al-0.1%Ag for helium accumulation. Beryl-
by the analysis of empty “blank’’ irradiated capsules, with and
lium has proven to be a useful dosimeter for low fluence
without the etching step.
applications, for example in reactor cavity locations. The
11.2.1 If it is feasible, duplicate HAFM capsules of each
Be(n,totalHe)crosssectionissufficientlylargesoastoresult
type should be irradiated at each desired location. This will
inmeasurableheliumlevelsinthelowappbrange.Theneutron
yieldameasureoftheHAFMreproducibilityandalsoimprove
energythresholdforheliumgenerationinberylliumisapproxi-
thefinalstatistics.UnencapsulatedHAFMs,suchasbarewires
mately 2 MeV.
of elements or alloys, generally do not require duplication
11.1.1 Also to be considered are the masses of the various since one piece is usually sufficient to provide duplicate or
HAFM sensor materials. Because of the relatively large range triplicate analyses. In extreme cases where knowledge of the
of helium production cross sections for the various HAFM reproducibility is essential, encapsulated small crystals or
materials, each material must be assessed for its total helium crystalline powder can be removed from the HAFM capsule
after irradiation and analyzed as separate lots. Inclusion of one
production in the particular irradiation environment. With the
standard HAFM capsule dimensions described earlier, HAFM or more empty “blank’’ HAFM capsules in each irradiation
materialmasscanrangefromabout0.1to10mg.Forverylow environment is necessary to verify the contribution to the total
fluence applications, slightly thinner walled capsules can be helium from the capsule itself.
employed to increase internal volume and maximize sensor
12. Calculation
material mass.
12.1 The total helium concentration, H, in an irradiated
11.1.2 For lower energy neutron fields, the nonthreshold
6 10 HAFM is calculated as follows:
HAFM materials, Li and B, required alloying in order to
reduce their effective nuclear density and subsequent self- H 5 N/MS (1)
shielding/flux depression corrections. Corrections are never-
where:
theless usually required to account for material burnup. The
N = total number of helium atoms measured in the HAFM,
effective energy range of the non-threshold HAFM (and
radiometric) materials can be changed by placing thermal
M = HAFM mass, (g), and
neutron shields such as boron (B C), gadolinium, or cadmium
S = HAFM nuclear density, (atoms/g).
around the set of HAFMs.
12.2 Theincidentneutronfluence,φt,maybeobtainedfrom
11.1.3 Consideration must also be given to the total helium
the total helium concentration, H, as follows:
produced in the encapsulating material itself. Vanadium is
often used, but for some very low (n,α) cross section sensor φt 5 H/σ¯ (2)
materials, the relative contribution from the vanadium can
where:
become significant. To this end, empty “blank’’ HAFM cap-
φ = neutron fluence rate (n/cm · s),
sules should be included in order to determine the helium
t = irradiation time (s), and
contribution from the encapsulating material.
σ¯ = spectrum averaged HAFM cross section (cm ).
11.1.4 Encapsulation materials other than vanadium with
12.2.1 Eq 2 assumes negligible burnup of the helium
significantly lower threshold (n,α) cross s
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E910 − 07 (Reapproved 2013) E910 − 18
Standard Test Method for
Application and Analysis of Helium Accumulation Fluence
Monitors for Reactor Vessel Surveillance, E706
(IIIC)Surveillance
This standard is issued under the fixed designation E910; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method describes the concept and use of helium accumulation for neutron fluence dosimetry for reactor vessel
surveillance. Although this test method is directed toward applications in vessel surveillance, the concepts and techniques are
equally applicable to the general field of neutron dosimetry. The various applications of this test method for reactor vessel
surveillance are as follows:
1.1.1 Helium accumulation fluence monitor (HAFM) capsules,
1.1.2 Unencapsulated, or cadmium or gadolinium covered, radiometric monitors (RM) and HAFM wires for helium analysis,
1.1.3 Charpy test block samples for helium accumulation, and
1.1.4 Reactor vessel (RV) wall samples for helium accumulation.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C859 Terminology Relating to Nuclear Materials
E170 Terminology Relating to Radiation Measurements and Dosimetry
E244 Test Method for Atom Percent Fission in Uranium and Plutonium Fuel (Mass Spectrometric Method) (Withdrawn 2001)
E261 Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
E482 Guide for Application of Neutron Transport Methods for Reactor Vessel Surveillance
E706 Master Matrix for Light-Water Reactor Pressure Vessel Surveillance Standards
E844 Guide for Sensor Set Design and Irradiation for Reactor Surveillance
E853 Practice for Analysis and Interpretation of Light-Water Reactor Surveillance Results
E854 Test Method for Application and Analysis of Solid State Track Recorder (SSTR) Monitors for Reactor Surveillance
E900 Guide for Predicting Radiation-Induced Transition Temperature Shift in Reactor Vessel Materials
E944 Guide for Application of Neutron Spectrum Adjustment Methods in Reactor Surveillance
E1005 Test Method for Application and Analysis of Radiometric Monitors for Reactor Vessel Surveillance
E1018 Guide for Application of ASTM Evaluated Cross Section Data File
E2005 Guide for Benchmark Testing of Reactor Dosimetry in Standard and Reference Neutron Fields
This test method is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applicationsand is the direct responsibility of Subcommittee E10.05 on
Nuclear Radiation Metrology.
Current edition approved Jan. 1, 2013Feb. 1, 2018. Published January 2013March 2018. Originally approved in 1982. Last previous edition approved in 20072013 as
E910 – 07.E910 – 07 (2013). DOI: 10.1520/E0910-07R13.10.1520/E0910-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website. The roman numeral-alphabetical designation at the end of some of the titles
indicates that a brief description of this standard may be found in Matrix E706.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E910 − 18
3. Terminology
3.1 Definitions—For definition of terms used in this test method, refer to TerminologyTerminologies C859 and E170. For terms
not defined therein, reference may be made to other published glossaries.
4. Summary of the HAFM Test Method
4.1 Helium accumulation fluence monitors (HAFMs) are passive neutron dosimeters that have a measured reaction product that
is helium. The monitors are placed in the reactor locations of interest, and the helium generated through (n,α) reactions accumulates
and is retained in the HAFM (or HAFM capsule) until the time of removal, perhaps many years later. The helium is then measured
very precisely by high-sensitivity gas mass spectrometry (1, 2). The neutron fluence is then directly obtained by dividing the
measured helium concentration by the spectrum-averaged cross section. Competing helium producing reactions, such as (γ,α) do
not, except for Be(γ,α), affect the HAFM results. The range of helium concentrations that can be accurately measured in irradiated
−14 −1
HAFMs extends from 10 to 10 atom fraction. This range permits the HAFMs to be tested in low fluence environments yet
to work equally well for high fluence situations.
4.2 Typically, HAFMs are either individual small solid samples, such as wire segments (3) or miniature encapsulated samples
of small crystals of powder (4), as shown in Fig. 1. As with radiometric dosimetry, different materials are used to provide different
energy sensitivity ranges. Encapsulation is necessary for those HAFM materials and reactor environment combinations where
sample melting, sample contamination, or loss of generated helium could possibly occur. Additionally, encapsulation generally
facilitates the handling and identification of the HAFM both prior to and following irradiation. The contents of HAFM capsules
typically range from 0.1 to 10 mg.
4.3 Following irradiation, encapsulated HAFMs are cleaned and identified in preparation for helium analysis. Helium analysis
is then accomplished by vaporizing both the capsule and its contents and analyzing the helium in the resulting gases in a high
4 4 3
sensitivity mass spectrometer system (5). The amount of He is determined by measuring the He-to- He isotopic ratio in the
sample gases subsequent to the addition of an accurately calibrated amount of He “spike.” Unencapsulated HAFMs, for example,
pure element wires, are usually etched to remove a predetermined layer of outer material before helium analysis (3). This
eliminates corrections for both cross contamination between samples and α-recoil into or out of the sample during the irradiation.
4.4 The He concentration in the HAFM, in general terms, is proportional to the incident neutron fluence. Consideration must,
however, be made for such factors as HAFM material burnup, neutron self-shielding and flux depression, α-recoil, and neutron
gradients. Corrections for these effects are discussed more fully in Section 13. Generally, they total less than 5 % of the measured
helium concentration. Since the individual corrections are usually known to within 50 %, the total error from these corrections
amounts to ≤2 %. Sources of uncertainty also lie in the HAFM material mass, isotopic composition, and mass spectrometric helium
analysis. As indicated in Section 13, however, these uncertainties generally contribute less than 1 % of the total uncertainty for
routine analyses.
4.5 Applying the above corrections to the measured HAFM helium concentration, the total incident neutron fluence (over the
energy range of sensitivity of the HAFM) can be obtained directly from a knowledge of the spectrum-integrated total helium
production cross section for the particular irradiation environment. At the present time, the uncertainty in the derived neutron
fluence is mainly due to uncertainty in the spectrum-integrated cross section of the HAFM sensor material rather than the combined
uncertainties in the helium determination process. This situation is expected to improve as the cross sections are more accurately
measured, integrally tested in benchmark facilities (6), and reevaluated.
FIG. 1 Helium Accumulation Fluence Monitor Capsule
See Dictionary of Scientific Terms, 3rd Edition, Sybil P. Parker, Ed., McGraw Hill, Inc.
The boldface numbers in parentheses refer to the list of references appended to this test method.
E910 − 18
5. Significance and Use
5.1 The HAFM test method is one of several available passive neutron dosimetry techniques (see, for example, Test Methods
E854 and E1005). This test method can be used in combination with other dosimetry methods, or, if sufficient data are available
from different HAFM sensor materials, as an alternative dosimetry test method. The HAFM method yields a direct measurement
of total helium production in an irradiated sample. Absolute neutron fluence can then be inferred from this, assuming the
appropriate spectrum integrated total helium production cross section. Alternatively, a calibration of the composite neutron
detection efficiency for the HAFM method may be obtained by exposure in a benchmark neutron field where the fluence and
spectrum averaged cross section are both known (see MatrixGuide E706E2005 IIE).).
5.2 HAFMs have the advantage of producing an end product, helium, which is stable, making the HAFM method very attractive
for both short-term and long-term fluence measurements without requiring time-dependent corrections for decay. HAFMs are
therefore ideal passive, time-integrating fluence monitors. Additionally, the burnout of the daughter product, helium, is negligible.
5.2.1 Many of the HAFM materials can be irradiated in the form of unencapsulated wire segments (see 1.1.2). These segments
can easily be fabricated by cutting from a standard inventoried material lot. The advantage is that encapsulation, with its associated
costs, is not necessary. In several cases, unencapsulated wires such as Fe, Ni, Al/Co, and Cu, which are already included in the
standard radiometric (RM) dosimetry sets (Table 1) can be used for both radiometric and helium accumulation dosimetry. After
radiometric counting, the samples are later vaporized for helium measurement.
5.3 The HAFM method is complementary to RM and solid state track recorder (SSTR) foils, and has been used as an integral
part of the multiple foil method. The HAFM method follows essentially the same principle as the RM foil technique, which has
been used successfully for accurate neutron dosimetry for the past 20 to 25 years. dosimetry. Various HAFM sensor materials exist
10 6
which have significantly different neutron energy sensitivities from each other. HAFMs containing B and Li have been used
routinely in LMFBR applications in conjunction with RM foils. The resulting data are entirely compatible with existing adjustment
methods for radiometric foil neutron dosimetry (refer to MethodGuide E944 ).
5.4 An application for the HAFM method lies in the direct analysis of pressure vessel wall scrapings or Charpy block
surveillance samples. Measurements of the helium production in these materials can provide in situ integral information on the
neutron fluence spectrum. This application can provide dosimetry information at critical positions where conventional dosimeter
placement is difficult if not impossible. Analyses must first be conducted to determine the boron, lithium, and other component
concentrations, and their homogeneities, so that their possible contributions to the total helium production can be determined.
Boron (and lithium) can be determined by converting a fraction of the boron to helium with a known thermal neutron exposure.
Measurements of the helium in the material before and after the exposure will enable a determination of the boron content (7).
Boron level down to less than 1 wt. ppm can be obtained in this manner.
TABLE 1 Neutron Characteristics of Candidate HAFM Materials for Reactor Vessel Surveillance
Fission Neutron Spectrum
Principal Helium Producing Thermal Neutron Cross
HAFM Sensor Material
90 % Response
A
Reaction Section, (b)
Cross Section, (mb)
A
Range, (MeV)
Li Li(n,α)T 942 457 0.167–5.66
9 6 6
Be Be(n,α) He ;ra Li . 284 2.5–7.3
10 7
B B(n,α) Li 3838 494 0.066–5.25
10 7
B B(n,α) Li 3838 494 0.066–5.25
14 11
N N(n,α) B . 86.2 1.7–5.7
14 11
N N(n,α) B . 86.2 1.7–5.7
19 16
F F(n,α) N . 27.6 3.7–9.7
B 27 24
Al Al(n,α) Na . 0.903 6.47–11.9
B 27 24
Al Al(n,α) Na . 0.903 6.47–11.9
32 29
S S(n,α) Si . . .
32 29
S S(n,α) Si . . .
35 32
Cl Cl(n,α) P . ;13 (Cl) 2.6–8.3
35 32
Cl Cl(n,α) P . ;13 (Cl) 2.6–8.3
B 47 44
Ti Ti(n,α) Ca . 0.634 (Ti) 6.5–12.8
B 47 44
Ti Ti(n,α) Ca . 0.634 (Ti) 6.5–12.8
B 56 53
Fe Fe(n,α) Cr . 0.395 (Fe) 5.2–11.9
B 56 53
Fe Fe(n,α) Cr . 0.395 (Fe) 5.2–11.9
B 58 55
Ni Ni(n,α) Fe . 5.58 (Ni) 3.9–10.1
B 58 55
Ni Ni(n,α) Fe . 5.58 (Ni) 3.9–10.1
B 63 60
Cu Cu(n,α) Co . 0.330 4.74–11.1
B 63 60
Cu Cu(n,α) Co . 0.330 4.74–11.1
316-SS
Helium Production Largely
56 58
from Fe and Ni
PV Steel
J
Charpy Block
A 235
Evaluated U fission neutron spectrum averaged helium production cross section and energy range in which 90 % of the reactions occur. All values are obtained from
ENDF/B-V Gas Production Dosimetry File data. Bracketed terms indicate cross section is for naturally occurring element.
B
Often included in dosimetry sets as a radiometric monitor, either as a pure element foil or wire or, in the case of aluminum, as an allaying material for other elements.
E910 − 18
−14
5.5 By careful selection of the appropriate HAFM sensor material and its mass, helium concentrations ranging from ;10 to
−1 12 27 2
10 atom fraction can be generated and measured. In terms of fluence, this represents a range of roughly 10 to 10 n/cm .
Fluence (>1 MeV) values that may be encountered during routine surveillance testing are expected to range from ;3 × 10 to
20 2
2 × 10 n/cm , which is well within the range of the HAFM technique.
5.6 The analysis of HAFMs requires an absolute determination of the helium content. The analysis system specified in this test
method incorporates a specialized mass spectrometer in conjunction with an accurately calibrated helium spiking system. Helium
determination is by isotope dilution with subsequent isotope ratio measurement. The fact that the helium is stable makes the
monitors permanent with the helium analysis able to be conducted at a later time, often without the inconvenience in handling
caused by induced radioactivity. Such systems for analysis exist, and additional analysis facilities could be reproduced, should that
be required. In this respect, therefore, the analytical requirements are similar to other ASTM test methods (compare with Test
Method methods.E244).
6. Apparatus
6.1 High-Sensitivity Gas Mass Spectrometer System, capable of vaporizing both unencapsulated and encapsulated HAFM
materials and analyzing the resulting total helium content is required. A description of a suitable system is contained in Ref (5).
6.2 Analytical Microbalance for Accurate Weighing of HAFM Samples, minimum specifications: 200-mg capacity with an
absolute accuracy of 60.5 μg. Working standard masses must be traceable to appropriate national or international mass standards.
Additionally, a general purpose balance with a capacity of at least 200 g and an accuracy of 0.1 mg is required for weighing larger
specimens.
6.3 Laminar flow (optional) clean benches, for use in the preparation of HAFM samples and capsules.
6.4 Stereo microscope, with 7 to 30 magnification, a ;0.1-mm graticule, and an optical illuminator.
6.5 Electron beam welder, with moveable platform stage, for sealing HAFM capsules, minimum specifications: variable beam
power to 0 to 1 kW, variable beam size capable of focusing down to a diameter of 0.5 mm. Controls must also be available to permit
automatic control of beam duration and onset and offset beam power slopes.
6.6 High temperature vacuum furnace for out-gassing HAFM materials, capsules, and mass spectrometer system furnace
−5
components. Minimum specifications: 1000°C at a maximum pressure of 10 Torr.
6.7 Micro-sand blaster/cleaner, for cleaning mass spectrometer vacuum furnace parts.
6.8 X-ray machine, for quality assurance test of HAFM capsules. Minimum specifications; 300 kV, 10 mA, 4-mm spot size with
control of source distance to 1.0 m and exposure time to 5 min.
6.9 General Laboratory Supplies:
6.9.1 Ultrasonic Cleaner—100 to 200 W,
6.9.2 Heat Lamp—250 W, and
6.9.3 Optical Pyrometer—700 to 2000°C.
6.10 Radioactive Material Handling:
6.10.1 Lead shielding,
6.10.2 Portable radioactive (β-γ) counters (0.01 mrem/h to 100 rem/h), and
6.10.3 Radioactive waste disposal capability.
6.11 Reagents and Materials:
6.11.1 Hydrochloric Acid (HCl), (37 %),
6.11.2 Hydrofluoric Acid (HF), (48 %),
6.11.3 Nitric Acid (HNO ), (70 %),
6.11.4 Sulfuric Acid (H SO ), (96 %),
2 4
6.11.5 Acetone [(CH ) CO]—Reagent grade (>99.7 %),
3 2
6.11.6 Alcohol (C H OH)—Pure (200 proof),
2 5
6.11.7 Chloroform (CHCl )—Reagent grade (>99.2 %),
6.11.8 Distilled and Deionized Water, and
6.11.9 Detergent Cleaning Solution (Alconox or equivalent).
7. HAFM Materials
7.1 General Requirements—The general requirements concerning the characteristics of HAFM materials fall into two broad
categories: (1) nuclear properties, and (2) chemical properties. These two categories are discussed separately below.
7.2 Nuclear Properties:
Alconox is a registered trademark of Alconox Inc., 215 Park Ave. South, New York, NY 10003.
E910 − 18
7.2.1 Helium Production Cross Section—Consideration must be made for the energy range or energy sensitivity of the (n,(n,
total helium) cross section of the potential HAFM sensor material. For any given neutron environment, the set of HAFMs or
combination of HAFMs, RM, and SSTR multiple foils must be chosen to cover the entire neutron energy range (refer to Guide
E844). The majority of potential HAFM materials fall into the threshold reaction category. That is, below the threshold energy
(usually in the 1–10 MeV range), these materials produce essentially no helium from neutron reactions. Above this energy,
however, the (n,(n, total helium) cross section generally rises fairly rapidly to a plateau from where it continues to rise relatively
slowly. Generally, the higher the threshold energy, the lower the total cross section. The threshold reaction HAFM isotopes
9 14 19 27 32 35 56 58 63
presently identified as being most suitable for reactor vessel surveillance are Be, N, F, Al, S, Cl, Fe, Ni and Cu (see
Table 1).
6 10
7.2.1.1 The two stable isotopes that have significant nonthreshold helium production cross sections are Li and B. The cross
sections of these two isotopes, which are large and well known, vary inversely with the neutron velocity below about 0.1 MeV.
Above 0.1 MeV, the cross section behavior becomes more irregular, with the Li exhibiting a significant resonance near 0.24 MeV.
7.2.1.2 Other stable isotopes exist which have nonthreshold helium production cross sections, but all are much less than 1 barn
−24 2 59
(10 cm ). Of the radioactive isotopes, Ni, which has a ;12 barn thermal neutron (n,α) cross section, is the only one important
58 59 56
for HAFM neutron dosimetry through the two-stage reaction Ni(n,γ)· Ni(n,α) Fe. Also included in Table 1 are additional
potential HAFM materials which are already included in the standard specified RM foil and metallurgical sets (refer to Matrix
E706) and thus may serve a double purpose (see 11.1). These materials include the natural elements Ti, Fe, Cu, and Ni; stainless
steel dosimetry capsule material, RV steel; and Charpy block metallurgical specimens. Relevant characteristics of the various
HAFM isotopes and materials are listed in Table 1. Aluminum is also often included in RM sets in the form of alloys of Co and
Au.
7.2.2 Activation Cross Sections—Also to be considered in the selection of HAFM materials is their relative activation cross
sections in typical reactor vessel neutron fields. Although activation reactions in general do not interfere with helium production
(exceptions are cases of two-stage reactions as with Ni, and cases where daughter products have contributing (n,α) reactions such
9 6 6
as Be(n,α) He → Li), the resulting radioactive decay contributes to post-irradiation handling and analysis difficulties and, to this
extent, should be minimized.
6 10
7.2.3 Neutron Self-Shielding—High cross section isotopes, such as Li and B, exhibit significant neutron self-shielding and
surface flux depression in thermal and epithermal neutron environments. In order to apply these isotopes to reactor surveillance
dosimetry, dilution of these materials by alloying is required to reduce their effective isotopic concentrations. Suitable alloying
materials for boron and lithium at the 0.1 to 0.5 weight percent level are vanadium, niobium, and aluminum. Additional details
on self-shielding are given in Section 13.
7.2.4 Neutron Screening at Low Energies—An alternate technique, or one that can be used in conjunction with alloying to
reduce neutron self-shielding, is to protect the boron and lithium from low-energy neutrons by covering with appropriate materials.
Cadmium or gadolinium provides a low-energy neutron cutoff of ;0.5 eV. A considerably higher cutoff energy can be achieved
by shielding with boron carbide (B C). For 1 keV neutrons, ;4.5 cm of B C provides ;90 % attentuation. Because of the neutron
4 4
perturbation effects of B C, however, this latter technique would be useful only at ex-vessel surveillance locations.
7.3 Sensor Chemical Properties—Various considerations must be made concerning the chemical properties of the HAFM sensor
6 7 14
materials. Many of the HAFM isotopes, such as Li, Li, N, etc., are conveniently useableusable only in compound form.
6 7
Examples of suitable compounds are LiF, LiF, TiN, and ZrN. In the choice of the most useful compound, consideration must be
given to such factors as: (1) helium production and activation cross sections of the host element (F, Ti, and Zr in the above
examples), (2) homogeneity and stoichiometry of the compound, (3) residual impurities such as boron or lithium, (4) stability and
resistance to decomposition at higher temperatures, (5) alloying potential with the encapsulating material, and (6) melting and
vaporization temperatures, which are important when it comes to releasing the helium for mass spectrometric analysis.
7.4 HAFM Material Encapsulation—Encapsulation is necessary for those HAFM sensor materials and irradiation conditions for
which there is a potential for either contamination, loss of generated helium from α-recoil or diffusion, or loss of sensor material
itself. This includes those HAFM compounds which are in the form of fine powders or crystals, or which may melt at the
temperatures anticipated in the irradiation environment. The encapsulating material must be chosen so as to completely contain
the HAFM sensor and its generated helium, while at the same time having relatively low helium production and activation cross
sections. The former is of importance for total helium production since the entire HAFM sensor plus capsule is later analyzed for
helium. The latter is of importance in minimizing induced radioactivity in the HAFM capsule. Further requirements are that the
encapsulating material must be reasonably durable to withstand handling before and after irradiation and that the material be both
machinable and weldable to facilitate HAFM capsule fabrication. Generally, when it has been determined that the HAFM sensor
material has itself the required helium retention, strength, and chemical inertness, the HAFM is used in the form of a “bare’’ wire
segment without being encapsulated (3).
8. HAFM Material Processing
8.1 HAFM sensor and encapsulating materials must be analyzed for possible residual helium by pre-irradiation analysis of the
various lot materials. In this regard, precautions should be taken to ensure that no helium has been used (as an inert gas) during
any stage of material fabrication.
E910 − 18
8.2 HAFM and encapsulating materials must also be analyzed for thermal neutron helium producing impurities (for example,
6 10
Li and B at sub-ppm levels). As discussed previously, this is most effectively done by helium analysis of a sample of each lot
of material following a thermal neutron irradiation. The concentration and homogeneity of alloys containing low weight contents
of boron and lithium (discussed earlier in 7.2.3) can also be determined in this way.
9. Manufacture of HAFMs
9.1 HAFM Capsules:
9.1.1 Fabrication and X-ray Qualification—As discussed previously, encapsulation of HAFM sensor material is necessary in
those cases where contamination, loss of sensor material, or loss of internally generated helium could occur. A typical HAFM
capsule is shown in Fig. 1. These capsules generally are 6.4-mm long, with outside diameters of 0.9 or 1.3 mm and inside diameters
ranging from 0.5 to 1 mm. To ensure no loss of internally generated helium, capsule walls must have a minimum thickness of 0.17
mm. This is most easily verified by X-ray inspection of each empty capsule from two perpendicular angles. To minimize time and
cost, the capsules may be X-rayed in groups of approximately 100. Various X-ray conditions have been investigated, and from
these tests, it has been determined that optimum capsule definition is obtained by enclosing the capsules in stainless steel
hypodermic tubing during the X-ray procedure. The stainless steel serves both as a convenient holder and aligning material, and
it has the effect of lowering the X-ray exposure to the film at the capsule edge. In this manner, a “sharp’’ material density edge
for the X-rays is achieved, resulting in a well-defined capsule edge. Following the X-ray procedure, either the X-ray negatives or
enlargement prints can be visually scanned using a calibrated magnifier to locate capsules whose central holes are not concentric
and whose minimum wall thicknesses may fall outside the allowable limits. The X-ray negatives or prints should be kept on
permanent file, with some means of identification for later tracing individual capsules back to the X-ray records.
9.1.1.1 In addition to the capsule X-ray number, each HAFM capsule should have an alphanumeric identification code stamped
on the solid base, and as well may have one or two identifying grooves around the circumference. In this manner, individual
capsules or groups of capsules can be identified remotely during post-irradiation hot cell recovery.
9.1.2 HAFM Material Mass—Encapsulated HAFM sensor materials can range in mass from single crystals (for example, B
or LiF) weighing less than 0.1 mg to fine crystalline powders weighing up to 10 mg. In each case, the total HAFM material mass
should be determined using a microbalance and preferably a double substitution weighing scheme, in which the samples are
compared with the working standard masses. Periodic calibration of the working standards must be made relative to appropriate
national or international mass standards. Total mass accuracy, using this technique, is generally better than 60.3 μg. For single
crystals, the mass is best determined prior to loading. For the finer crystalline powders, however, the most reliable and accurate
method of determining the mass is by weighing the HAFM capsule before and after loading.
9.1.3 Capsule Welding—Because of the need to exclude air, with its natural helium content, from the HAFM interior, weld
closure of the capsule top is best accomplished by electron beam under vacuum. This form of welding has the additional advantage
of precise control of weld power and heating zone. TIG welding, an alternate technique, would involve closure under an inert gas
atmosphere which could complicate later helium analysis.
9.1.3.1 After HAFM material loading and prior to capsule welding, thin spacer disks should be placed above the sensor material
to reflect the heat from the weld zone (see Fig. 1). This is followed by partially closing the capsule top to facilitate the weld process.
This can be accomplished either by insertion of a solid plug or by squeezing the top portion of the capsule together. Some gaps
should be left in the capsule top to allow for complete evacuation (or inert gas backfilling) prior to final closure. To further reduce
HAFM sensor material heating during welding, the lower portion of each capsule should be in firm contact with a suitable heat
sink, “chill block.’’block.” The length of the weld zone should be limited to the top ;1 mm of capsule.
9.1.4 Final Capsule Weighing—As an additional aid in pre- and post-irradiation identification, the final welded capsules should
be weighed to an accuracy of at least 610 μg. Therefore, if part of the alphanumeric identification base code becomes unreadable,
capsule identification would still be likely. Additionally, this additional weighing step reveals any possible HAFM material mass
loss during the welding process. In this respect, capsule weighings before and after loading should include the actual spacer disks
and weld cap (if applicable) to be used (see 9.1.3).
10. HAFM Analysis
10.1 Outline of Test Method—Determination of the helium content in HAFM materials is made by vaporizing the materials
4 3
under vacuum. Immediately before the sample is vaporized and the He is released, a precisely-known amount of He is added
( He “spike”). After mixing of the two isotopes, the gas passes over getters that remove unwanted gases, then passes into the mass
4 3
spectrometer volume, which is isolated from its vacuum pump for “static mode” operation. The measurement of the He/ He ratio
and a knowledge of the mass of the HAFM material then produces the helium concentration. A recommended helium analysis
system has been described previously (5). Precautions must be taken to account for He that might already be present in the HAFM
(see 10.3.1).
10.2 Apparatus:
10.2.1 Mass Spectrometer—Magnetic sector mass spectrometer with all-metal tube and an interior volume of about 1 L. The
instrument should have an electron impact ion source, electron multiplier, and an electrometer with current measuring capability
−13 −14
of at least 10 A with a stability of <10 A/h. Output from the electrometer can be monitored directly via a strip chart recorder
E910 − 18
or digitally averaged for real-time computer analysis. The mass resolving power of the mass spectrometer itself should be a
minimum of 50 with a mass scanning range from 2 to 50 amu. Mass scanning capability is useful in checking for possible
interfering background gases. In addition, the entire system should be bakeable to 300°C.
10.2.2 Vacuum System—To minimize the time necessary to pump away gas samples between analyses, a multiple vacuum
system consisting of several independent subsystems should be used. Rapid pumpout can best be accomplished, especially in the
case of helium, when sequential pumping is employed. A rotary pump and then a turbomolecular pump first remove most of the
helium very rapidly. As soon as the lower limit is reached, an ion pump is used to reduce the vacuum to a lower level. Finally,
−9
another ion pump is used only to maintain the mass spectrometer in the 10 Torr range between analyses.
;
10.2.3 Furnaces—Several methods have been successfully used to vaporize HAFM materials. For small samples ~, 2 mg) with
melting temperatures less than ;1800°C, the samples can be readily vaporized in small resistance-heated 0.25-mm diameter
tungsten wire coil baskets (2). Larger samples (>2 mg), including HAFM capsules or samples with melting temperatures above
1800°C, can be vaporized in larger resistance-heated cylindrical graphite crucibles (4.8-mm OD, 20-mm long) (2). Prior to loading,
the tungsten coil baskets and graphite crucibles should be degassed in vacuum by heating to ;1750°C for about 2 min. Vacuum
furnaces have been constructed that contain up to ten individual tungsten coils or graphite crucibles. The design of the vacuum
furnaces must allow vaporization of samples with masses ranging from about 0.5 to 200 mg (the heavier masses are associated
with encapsulated HAFMs). During analysis, the current through the baskets or crucibles is steadily increased until decomposition
of the tungsten or graphite occurs. In this manner, vaporization of the enclosed sample and total helium release is assured. For
maximum sensitivity for very low level samples, the heating can be stopped prior to tungsten or graphite decomposition provided
it can be ascertained that all HAFM sensor material has been vaporized. This reduced heating generally reduces the amount of
helium“ background” released by the furnace itself.
10.2.3.1 A third furnace type has been used to vaporize larger metallic samples with melting points up to ;1200°C (8). This
furnace uses a graphite crucible which is resistance heated and then maintained at a constant temperature of ;2000°C. Samples
are dropped individually by remote means into the heated crucible and vaporized. The fact that the furnace temperature remains
essentially constant during the analysis procedure reduces the uncertainty in the furnace “blank’’—the “blank’’ – the amount of
helium attributable to the furnace itself. This reduced uncertainty has the effect of lowering the effective detection limit of the mass
spectrometer system. Using this technique, samples with masses up to ;1 g can be analyzed, with a resulting helium analysis
8 −14
uncertainty of ;1 × 10 atoms. In copper, this is equivalent to a helium concentration of ;10 atom fraction.
10.2.4 Getters—A system of getters should be used to purify the helium gas sample before it is admitted into the mass
spectrometer, and to maintain a high vacuum in the mass spectrometer while it is being operated in the static mode. The getters
could consist, for example, of a liquid-nitrogen-cooled charcoal trap, followed by, but separated from, a nonevaporable alloy getter
(such as the SAES GT-50). Another alloy getter should be permanently attached to the mass spectrometer itself to maintain the
vacuum while the instrument is isolated from its ion pump during sample analysis.
3 4
10.2.5 Spike System—A network of accurately calibrated volumes which dispenses known quantities of He and He, for
calibration and for isotope dilution purposes, should be available. For convenience, this network can be attached directly to the
3 4
mass spectrometer line. The size and required accuracy of the He and He spikes must be determined in conjunction with the
characteristics of the mass spectrometer and the analysis lines to allow for absolute helium measurements in the range of 10 to
10 atoms of helium to an accuracy of 1 to 2 %. Glass stopcocks should be used throughout the spike system rather than stainless
steel valves, mainly because the stopcocks provide a more positive and reliable barrier through which helium has little chance of
passing unnoticed. Another important advantage over stainless steel valves is the ease with which the volumes between the
stopcocks may be calibrated. Helium absorption on vacuum grease is negligible. Although most of the spike system, including all
the stopcocks, can be made of borosilicate glass, the volumes which are used for long-term storage of helium must be made either
from aluminosilicate glass (Corning Type 1720) , which is relatively impervious to helium, or from stainless steel.
3 4
10.2.6 The spiking systems should include, in addition to various sized He and He spikes, a standard spike mixture of both
3 4
He and He. This mixture is required for calibration of the relative sensitivity of the mass spectrometer for masses 3 and 4. Further,
3 4
the separate He and He spikes can be used to provide additional combinations of the two helium isotopes for further verification
3 4
of the relative sensitivity, for verifying that the individual spike systems are dispensing the expected amounts of He and He, and
to cross check the calibration and linearity of the mass spectrometer system as a whole. Additional calibration of the system should
also be accomplished using an independent standard source of helium concentration. Standard helium gas mixtures can be obtained
from the U.S. Bureau of Mines. Alternatively, air, which has a known helium concentration (5.24 appm), can be used (9).
10.3 Analysis Procedure—After estimating the approximate helium concentration in the HAFM sample, and after determining
its mass, the sample is loaded into one of the vaporizing systems attached to the mass spectrometer (see 10.2.3). After suitable
vacuum pumping (usually over night), overnight), the samples are ready for analysis. Immediately before the heating operation and
the release of the sample gas, an appropriately sized spike of He is added. Unless other released gases interfere, complete mixing
The sole source of supply of the apparatus known to the committee at this time is Corning Glass Works, Corning, NY 14831. If you are aware of alternative suppliers,
please provide this information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible technical committee,
which you may attend.
E910 − 18
of the isotopes occurs in a few seconds. From this point on, it does not matter what fraction of gas is used for the analysis because
4 3
only the ratio He/ He needs to be determined.
10.3.1 The removal of unwanted gases released during the vaporization of the sample is accomplished while the helium passes
by the getters. The most important aspect of the operation is to make sure that as little helium as possible from all other sources
4 3
contaminates the sample gas and changes the sample-plus-spike He/ He ratio before it is measured. This means that the
purification should be done quickly.
10.3.2 A typical procedure is to allow the gas to expand into the liquid-nitrogen-cooled charcoal getter, after which the
connecting all-metal valve is closed. The gas thus trapped (between 1 and 10 % of the total, depending on the size of the furnace
assembly used) is sufficient for the mass spectrometric determination of the isotopic composition. After about 20 s, this aliquot of
gas is permitted to expand into the getter enclosure. Finally, the gas is allowed into the mass spectrometer volume which is isolated
from its ion pump. It stays in this volume until the isotopic ratio measurements are complete. The small amount of helium admitted
−7
is usually about 10 cc STP, which does not deleteriously affect the mass spectrometer vacuum.
10.3.3 Gas samples from milligram-size specimens whose helium concentrations are above 0.1 appm are sufficiently large that
a very small permeation or desorption of He into the mass spectrometer can be ignored. For smaller samples, this constant leak
4 3
becomes perceptible, and eventually itsit sets the detection limit of the instrument. Thus, in all analyses, the He/ He ratio is
carefully examined for systematic increase; and, if such an increase is found, the ratio is measured against time and extrapolated
to the exact time the sample was admitted to the mass spectrometer volume. The ratio that is obtained is the helium isotopic ratio
at the time the sample was introduced, which does not account for He leakage into the sample line or furnace. By taking a second
and third aliquot of gas from this sample furnace, and analyzing them as described above, results can be extrapolated to give the
true amount of He that was released from the sample. This can be done with negligible uncertainty introduced as a result of the
extrapolation, except for the case of extremely small samples of helium.
3 3
10.3.3.1 HAFMs that Contain He—In a few cases, He is also present in irradiated HAFMs. If so, it must be accounted for in
the mass spectrometric analysis because it would not be distinguished from the He “spike.” This isotope is rarely formed directly
by nuclear reactions, but usually occurs as the result of decaying tritium. In the case of LiF HAFMs, tritium is formed every time
a helium atom is generated, so He can become significant after a few month’s decay. Very few other HAFM reactions produce
tritium, but this gas can pass through many metals with ease, and consequently HAFMs that have not themselves generated any
tritium can still contain this gas and its He daughter, just from being in a reactor core environment. In order to measure both
helium isotopes simultaneously, therefore, a slightly modified mass spectrometric procedure is employed. A small known fraction
of the helium gas released from the HAFM is analyzed for isotopic content before, rather than after, the addition of the spike. After
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the He content is measured with respect to the He, the He spike is added to the remainder of the gas sample, and the altered
isotopic ratio is measured to provide absolute concentration. Once it has been established that the He content in a set of HAFMs
is negligible compared with the added He spike, this modified procedure is no longer required.
11. Irradiation Guidelines
11.1 Selection of HAFM Sensor Material—There are several factors to be considered in the selection of HAFM materials for
reactor vessel surveillance. Of primary importance is the desired energy coverage. Since the HAFM method is closely tied to the
radiometric foil dosimetry method, the HAFM sensor materials should be chosen to complement the various multiple foils present.
As discussed earlier, some RM and metallurgical materials can provide data for both methods simultaneously. Examples of this
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double utilization include using the Ti(n,p) Sc, Fe(n,p) Mn, Ni(n,p) Co, Ni(n,α) Fe, Co(n,γ) Co, Cu(n,α) Co,
109 110m
and Ag(n,γ) Ag reactions for radiometric determinations, while at the same time using the natural Ti, Fe, Ni, and Cu, and
the alloys Al-0.1 %Co and Al-0.1 %Ag for helium accumulation. Beryllium has proven to be a useful dosimeter for low fluence
applications, for example in reactor cavity locations. The Be(n, total He) cross section is sufficiently large so as to result in
measurable helium levels in the low appb range. The neutron energy threshold for helium generation in beryllium is approximately
2 MeV.
11.1.1 Also to be considered are the masses of the various HAFM sensor materials. Because of the relatively large range of
helium production cross sections for the various HAFM materials, each material must be assessed for its total helium production
in the particular irradiation environment. With the standard HAFM capsule dimensions described earlier, HAFM material mass can
range from about 0.1 to 10 mg. For very low fluence applications, slightly thinner walled capsules can be employed to increase
internal volume and maximize sensor material mass.
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11.1.2 For lower energy neutron fields, the nonthreshold HAFM materials, Li and B, required alloying in order to reduce their
effective nuclear density and subsequent self-shielding/flux depression corrections. Corrections are nevertheless usually required
to account for material burnup. The effective energy range of the non-threshold HAFM (and radiometric) materials can be changed
by placing thermal neutron shields such as boron (B C), gadolinium, or cadmium around the set of HAFMs.
11.1.3 Consideration must also be given to the total helium produced in the encapsulating material itself. Vanadium is often
used, but for some very low (n,α) cross section sensor materials, the relative contribution from the vanadium can become
significant. To this end, empty “blank’’ HAFM capsules should be included in order to determine the helium contribution from
the encapsulating material.
11.1.4 Encapsulation materials other than vanadium with significantly lower threshold (n,α) cross sections are available. These
include platinum, gold, and alloys of these two elements. Of these, the gold-platinum alloys have advantages because (1) the alloys
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