ASTM E798-16

Standard Practice for Conducting Irradiations at Accelerator-Based Neutron Sources

ABSTRACT
This practice covers procedures for irradiations at accelerator-based neutron sources. The discussion focuses on nearly monoenergetic 14-MeV neutrons from the deuterium-tritium T(d,n) interaction, and broad spectrum neutrons from stopping deuterium beams in thick beryllium or lithium targets. However, most of the recommendations also apply to other types of accelerator-based sources, including spallation neutron sources. The procedures to be considered include methods for characterizing the accelerator beam and target, the irradiated sample, and the neutron flux and spectrum, as well as procedures for recording and reporting irradiation data.
SCOPE
1.1 This practice covers procedures for irradiations at accelerator-based neutron sources. The discussion focuses on two types of sources, namely nearly monoenergetic 14-MeV neutrons from the deuterium-tritium T(d,n) interaction, and broad spectrum neutrons from stopping deuterium beams in thick beryllium or lithium targets. However, most of the recommendations also apply to other types of accelerator-based sources, including spallation neutron sources (1).2 Interest in spallation sources has increased recently due to their development of high-power, high-flux sources for neutron scattering and their proposed use for transmutation of fission reactor waste (2).  
1.2 Many of the experiments conducted using such neutron sources are intended to provide a simulation of irradiation in another neutron spectrum, for example, that from a DT fusion reaction. The word simulation is used here in a broad sense to imply an approximation of the relevant neutron irradiation environment. The degree of conformity can range from poor to nearly exact. In general, the intent of these experiments is to establish the fundamental relationships between irradiation or material parameters and the material response. The extrapolation of data from such experiments requires that the differences in neutron spectra be considered.  
1.3 The procedures to be considered include methods for characterizing the accelerator beam and target, the irradiated sample, and the neutron flux (fluence rate) and spectrum, as well as procedures for recording and reporting irradiation data.  
1.4 Other experimental problems, such as temperature control, are not included.  
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.6 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 and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Published
• Publication Date: 30-Sep-2016
• ICS: 27.120.10 - Reactor engineering
• Technical Committee: E10 - Nuclear Technology and Applications
• Drafting Committee: E10.07 - Radiation Dosimetry for Radiation Effects on Materials and Devices

Relations

• Refers: ASTM C859-24 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM E265-15(2020) - Standard Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32
• Effective Date: 01-Jul-2020
• Refers: ASTM E393-19 - Standard Test Method for Measuring Reaction Rates by Analysis of Barium-140 From Fission Dosimeters
• Effective Date: 01-Nov-2019
• 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 E263-18 - Standard Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Iron
• Effective Date: 01-Dec-2018
• Refers: ASTM E910-18 - Standard Test Method for Application and Analysis of Helium Accumulation Fluence Monitors for Reactor Vessel Surveillance
• Effective Date: 01-Feb-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 E261-15 - Standard Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
• Effective Date: 01-Jun-2015
• Refers: ASTM E265-15 - Standard Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32
• Effective Date: 01-Jun-2015
• Refers: ASTM E170-15 - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 15-Mar-2015
• Refers: ASTM E170-14a - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 15-Oct-2014
• Refers: ASTM E170-14 - Standard Terminology Relating to Radiation Measurements and Dosimetry
• Effective Date: 01-Sep-2014

Overview

ASTM E798-16 - Standard Practice for Conducting Irradiations at Accelerator-Based Neutron Sources provides comprehensive procedures and best practices for performing irradiations using accelerator-based neutron sources. This standard is highly relevant for laboratories, research institutions, and industries engaging in neutron irradiation experiments, particularly where simulation of neutron fields from fusion reactions and other advanced nuclear applications is required.

The document specifically addresses both nearly monoenergetic 14-MeV neutrons generated from the deuterium-tritium (T(d,n)) reaction and broad-spectrum neutron fields produced by stopping deuterium beams in thick beryllium or lithium targets. Additionally, the recommendations generally apply to other accelerator-driven neutron sources, including modern spallation neutron sources now used in a variety of scientific and industrial contexts.

Key Topics

ASTM E798-16 covers several essential topics for ensuring accurate, consistent, and reliable irradiation results at accelerator-based neutron facilities:

• Neutron Source Characterization
  • Detailed attention to both T(d,n) sources and Be or Li(d,n) sources.
  • Focus on understanding neutron yield, energy spectrum, and spatial distribution.
• Beam and Target Properties
  • Emphasizes critical roles of beam current, energy, spatial stability, and target condition.
  • Discussion of physical and chemical properties of targets affecting neutron production and flux stability.
• Neutron Field Measurement
  • Procedures for measuring and monitoring neutron flux (fluence rate), spectral gradients, and field uniformity.
  • Use of both active and passive dosimetry techniques for in-situ monitoring.
• Sample Positioning and Irradiation Geometry
  • Guidance on careful positioning of samples and dosimeters to ensure uniform exposure and accurate dosimetric data.
• System Instabilities and Recordkeeping
  • Methods for detecting and documenting beam/target instabilities, spatial misalignments, and experimental perturbations.
  • Emphasis on robust data recording and reporting protocols.
• Data Reporting and Standardization
  • Step-by-step recommendations for logging irradiation parameters, sample locations, and environmental factors.
  • Ensures reproducibility and enables comparison of results across different facilities and experiments.

Applications

The procedures outlined in ASTM E798-16 have valuable applications in:

• Nuclear Materials Research
  • Simulating irradiation environments similar to those in fusion or fission reactors for materials testing and development.
• Neutron Scattering Facilities
  • Supporting high-power, high-flux neutron sources used in fundamental and applied research.
• Radiation Damage Studies
  • Assessing the effects of neutron exposure on structural materials, detector components, and advanced alloys.
• Nuclear Waste Transmutation
  • Facilitating investigations into the use of spallation neutron sources for reducing radioactive waste from fission reactors.
• Calibration and Testing
  • Providing protocols for calibrating detectors, monitoring beam/target conditions, and validating neutron field characteristics.
• International Research Collaboration
  • Offering a common reference for cross-institutional experiments and benchmarking studies.

Related Standards

ASTM E798-16 should be used in conjunction with several other standards to ensure comprehensive coverage of terminology, measurement protocols, and dosimetry practices:

• ASTM C859 – Terminology Relating to Nuclear Materials
• ASTM E170 – Terminology Relating to Radiation Measurements and Dosimetry
• ASTM E181 – Test Methods for Detector Calibration and Analysis of Radionuclides
• ASTM E261 – Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
• ASTM E854 – Test Method for Application and Analysis of Solid State Track Recorder (SSTR) Monitors for Reactor Surveillance
• ASTM E910 – Test Method for Application and Analysis of Helium Accumulation Fluence Monitors

By adhering to ASTM E798-16 and the related standards, organizations and researchers can promote standardization, achieve accurate neutron field simulations, and enhance the reproducibility and reliability of results from accelerator-based neutron irradiation experiments. This is essential for advancing nuclear science, engineering, and material development activities worldwide.

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: E798 − 16
Standard Practice for
Conducting Irradiations at Accelerator-Based Neutron
Sources
This standard is issued under the fixed designation E798; 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 1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This practice covers procedures for irradiations at
responsibility of the user of this standard to establish appro-
accelerator-based neutron sources. The discussion focuses on
priate safety, health, and environmental practices and deter-
two types of sources, namely nearly monoenergetic 14-MeV
mine the applicability of regulatory limitations prior to use.
neutrons from the deuterium-tritium T(d,n) interaction, and
1.7 This international standard was developed in accor-
broad spectrum neutrons from stopping deuterium beams in
dance with internationally recognized principles on standard-
thick beryllium or lithium targets. However, most of the
ization established in the Decision on Principles for the
recommendations also apply to other types of accelerator-
2 Development of International Standards, Guides and Recom-
based sources, including spallation neutron sources (1). Inter-
mendations issued by the World Trade Organization Technical
est in spallation sources has increased recently due to their
Barriers to Trade (TBT) Committee.
development of high-power, high-flux sources for neutron
scattering and their proposed use for transmutation of fission
2. Referenced Documents
reactor waste (2).
2.1 ASTM Standards:
1.2 Many of the experiments conducted using such neutron
C859Terminology Relating to Nuclear Materials
sources are intended to provide a simulation of irradiation in
E170Terminology Relating to Radiation Measurements and
another neutron spectrum, for example, that from a DT fusion
Dosimetry
reaction. The word simulation is used here in a broad sense to
E181Test Methods for Detector Calibration andAnalysis of
imply an approximation of the relevant neutron irradiation
Radionuclides
environment.Thedegreeofconformitycanrangefrompoorto
E261Practice for Determining Neutron Fluence, Fluence
nearly exact. In general, the intent of these experiments is to
Rate, and Spectra by Radioactivation Techniques
establish the fundamental relationships between irradiation or
E263Test Method for Measuring Fast-Neutron Reaction
material parameters and the material response. The extrapola-
Rates by Radioactivation of Iron
tionofdatafromsuchexperimentsrequiresthatthedifferences
E264Test Method for Measuring Fast-Neutron Reaction
in neutron spectra be considered.
Rates by Radioactivation of Nickel
1.3 The procedures to be considered include methods for
E265Test Method for Measuring Reaction Rates and Fast-
characterizing the accelerator beam and target, the irradiated
Neutron Fluences by Radioactivation of Sulfur-32
sample, and the neutron flux (fluence rate) and spectrum, as
E266Test Method for Measuring Fast-Neutron Reaction
wellasproceduresforrecordingandreportingirradiationdata.
Rates by Radioactivation of Aluminum
1.4 Other experimental problems, such as temperature E393Test Method for Measuring Reaction Rates byAnaly-
sis of Barium-140 From Fission Dosimeters
control, are not included.
E854Test Method for Application and Analysis of Solid
1.5 The values stated in SI units are to be regarded as
State Track Recorder (SSTR) Monitors for Reactor Sur-
standard. No other units of measurement are included in this
veillance
standard.
E910Test Method for Application and Analysis of Helium
Accumulation Fluence Monitors for Reactor Vessel Sur-
veillance
This practice is under the jurisdiction of ASTM Committee E10 on Nuclear
Technology and Applications and is the direct responsibility of Subcommittee
E10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.
Current edition approved Oct. 1, 2016. Published December 2016. Originally
approved in 1981. Last previous edition approved in 2009 as E798–96(2009). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/E0798-16. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
The boldface numbers in parentheses refer to a list of references at the end of Standards volume information, refer to the standard’s Document Summary page on
this practice. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E798 − 16
3. Terminology neutrons/s, respectively. Although these facilities have been
shut down, they were the most intense sources of 14 MeV
3.1 DescriptionsofrelevanttermsarefoundinTerminology
neutronsbuilttodateforresearchpurposes.Theyarediscussed
C859 and Terminology E170.
here because of their relevance to any future neutron sources.
4. Summary of Existing and Proposed Facilities Their characteristics are summarized in Table 1. A discussion
of similar sources can be found in Ref (8). The deuteron beam
4.1 T(d,n) Sources:
energy was 400 keV and the target was a copper-zirconium
4.1.1 Neutronsareproducedbythehighlyexoergicreaction
alloy (or copper with dispersed alumina) vapor-plated with
d+t → n+α. The total nuclear energy released is 17.589
tritium-occluded titanium. The beam spot size was about 10
MeV, resulting in about a 14.8-MeV neutron and a 2.8-MeV
mm in diameter. In addition to being rotated, the target also
alpha particle at low deuterium beam energies (3). The
wasrockedeveryfewhoursandthedeuteronbeamcurrentwas
deuteron energy (generally 150 to 400 keV) is chosen to
increased slowly in an attempt to maintain a constant flux in
maximize the neutron yield (for a particular target configura-
spite of tritium burn-up in the target. Samples could be placed
tion) from the resonance in the d-t cross section near 100 keV.
as close as 2.5 to 4.0 mm from the region of maximum d-t
The number of neutrons emitted as a function of angle (θ)
13 2
interaction resulting in a typical flux of 10 n/cm ·s over a
between the neutron direction and the incident deuteron beam
is very nearly isotropic in the center-of-mass system. At a small sample. The neutron fields were well characterized by a
deuteron energy of 400 keV in the laboratory system, the variety of methods and the absolute fluence could be routinely
neutronfluxintheforwarddirectionisabout14%greaterthan determined to 67%. Calculated neutron flux contours for
in the backward direction, while the corresponding neutron RTNS-II are shown in Fig. 1.
energy decreases from 15.6 to 13.8 MeV (4). In practice, the
4.2 Be or Li(d,n) Sources (9):
neutron field also depends on the gradual loss of the target
4.2.1 When a high-energy (typically 30- to 40-MeV) deu-
material and the tritium deposition profile. Detailed calcula-
teron beam is stopped in a beryllium (or lithium) target, a
tions should then be made for a specific facility.
continuous spectrum of neutrons is produced extending from
4.1.2 The flux seen at a point (r, θ, z) in cylindrical
thermal energies to about 4 MeV (15 MeV for lithium) above
coordinates from a uniform T(d,n) source of diameter a is
the incident deuteron energy (see Figs. 2-4). In existing
given by the following (5):
facilities,cyclotronswithdeuteronbeamintensitiesof20to40
4 2 2 1/2 2
Y k 14r z 1k
~ ! 13
µA provide neutron source strengths in the range of 10 n/s,
φ r, θ, z 5 ln (1)
~ ! H J
2 2
4πa 2z
using solid beryllium targets with water cooling. A more
intense source (>10 n/s) is now being designed employing
where:
2 2 2 2
liquid lithium targets. In the remainder of this document the
k = a + z − r , and
term Be(d,n) source is meant as a generic term including
Y = the total source strength.
Li(d,n) sources, whether solid or liquid targets.
For z>>a and r =0 (on beam axis) this reduces to Y/4πz ,
4.2.2 Neutrons are produced by several competing nuclear
as expected for a point source. The available irradiation
reaction mechanisms. The most important one for radiation
volume at maximum flux is usually small. For a sample placed
damage studies is the direct, stripping reaction since it pro-
close to the target, the flux will decrease very rapidly with
duces almost all of the high-energy neutrons. When the
increasingradialdistanceoffthebeamaxis.However,sincethe
incidentdeuteronpassesclosetoatargetnucleus,theprotonis
neutron energy is nearly constant, this drop in flux is relatively
captured and the neutron tends to continue on in a forward
easy to measure by foil activation techniques.
direction. The high energy neutrons are thus preferentially
4.1.3 Other existing sources, such as Cockroft-Walton type
emitted in the direction of the incident deuteron beam.
accelerators, are similar in nature although the available
However, as the deuterons slow down in the target, lower
neutron source strengths are much lower.
energy neutrons will be produced with angular distributions
4.1.4 Rotating Target Neutron Source (RTNS) I and II
(5-7)—RTNS I and II, which formerly were operated at the that are much less forward peaked. Furthermore, when the
residual nucleus is left in an excited state, the angular effects
Lawrence Livermore National Laboratory, provided 14 MeV
12 13
neutron source strengths of about 6×10 and 4×10 are also much less pronounced.These latter two effects tend to
TABLE 1 Characteristics of T(d,n) and Be or Li(d,n) Neutron Sources
Experimental
Source Maximum Flux
Volume for
Facility Availability Beam Target Strength, at Sample,
Maximum
n/s n/cm ·s
Flux, cm
12 12
RTNS I No longer available 400 keV d t 6 × 10 >10 0.2
13 13
RTNS II No longer available 400 keV d t 4 × 10 >10 0.2
>10 5.0
A 13 12
Existing Be or Li(d,n) U.C. Davis Cyclotron 30–40 MeV d Solid Be or Li ;10 >10 ;1.0
16 15
Proposed Li(d,n) Conceptual design (9) 30–40 MeV d Liquid Li 3 × 10 >10 10.0
>10 600.0
A
This is the only existing facility that has been well characterized and is readily available, although other facilities can be used.
E798 − 16
very sharply peaked in the forward direction (see Fig. 2).
Materialsstudiesforwhichthemaximumtotalneutronfluence
is desired are usually conducted close to the target and may
subtendalargerangeofforwardangles(forexample,0to60°).
This practice primarily will be concerned with this close-
geometry situation since it is the most difficult to handle
properly.
4.2.5 Other factors can also influence the neutron field
during a particular irradiation, especially beam and target
characteristics,aswellastheperturbinginfluenceofsurround-
ing materials. At present, these facilities have not been com-
pletely characterized for routine use. In particular, some
uncertainties exist, especially at low (<2 MeV) and high (<30
MeV) neutron energies, since these regions are either difficult
to measure with existing techniques, or the required nuclear
data are insufficient. In these cases, neutron dosimetry data
should be reported directly to allow reanalysis as procedures
and nuclear data improve in the future.
4.2.6 Existing Sources:
4.2.6.1 Whereas virtually any deuteron accelerator with
reasonable energy and intensity can be used as a neutron
source, only two facilities have been used routinely for
materials effects irradiations, namely the cyclotrons at the
University of California at Davis (10) and at Oak Ridge
NOTE 1—Flux contours assume a symmetric, Gaussian beam profile.
National Laboratory (11, 12).Typical flux-spectra obtained are
Figure from Ref. (5).
shown in Figs. 2-4 (9, 11, 13), and typical characteristics are
FIG. 1 Flux Contours for RTNS II
listed in Table 1.
4.2.6.2 Since the neutron flux and spectral gradients are so
steep,experimentersarefacedwiththeproblemofnonuniform
irradiations over their samples unless specimen sizes are
severely limited. Alternatively, the field gradients may be
moderated by deliberately moving or enlarging the beam spot
on the target.This technique will result in a lower total fluence
as well as a lower average neutron energy for a small-size
sample on the beam axis, although larger samples will not be
so severely affected and may in fact show an overall improve-
ment in average fluence and neutron energy.
4.2.6.3 At present, the neutron field can be determined
reasonably well at existing facilities.The flux-spectrum can be
measured to within 610 to 30% in the 2- to 30-MeV energy
region where about 90% of neutron damage is initiated
NOTE1—Neutronspectraasafunctionofenergyandanglefor Be(d,n)
source at ORNL, E =40 MeV. (Data from Ref (8).)
(assuming E =30 to 40 MeV). Highly accurate (610%)
d
d
FIG. 2 Neutron Spectra as a Function of Energy and Angle from
time-of-flight spectrometry has been used to study the field far
the Forward Direction of the Deuteron Beam
fromthesource,exceptfortheenergyregionbelowafewMeV
(11). However, close geometry irradiations must rely on
decrease the average neutron energy at angles other than 0° in passive dosimetry with larger errors due to uncertainties in the
the direction of the beam. nuclear cross sections, especially above 30 MeV (12).
4.2.3 Neutrons can also be produced by compound nuclear 4.2.7 Conceptual Design for Li(d,n) Source (9, 14)—A
reactions in which the entire deuteron is captured by the target conceptualdesignforafusionmaterialsirradiationfacilitywas
nucleus and neutrons are subsequently evaporated. Neutrons done at the Hanford Engineering Development Laboratory
are preferentially emitted with energies less than a few MeV (HEDL). The design consisted of a high-current (100-mA)
and the angular distribution approaches isotropy at neutron deuteron accelerator and a liquid lithium target. This was
energiesbelow1MeV.Neutronsalsoareproducedbydeuteron expected to produce a neutron source strength of about
break-up, in which the deuteron simply breaks apart in the 3×10 n/s (14).The designs called for a wide-area beam spot
Coulombfieldofthenucleus,althoughthiseffectisverysmall on the target (for example, 3 by 1 cm), thereby moderating the
for low-Z materials. steep neutron field gradients in close geometry. Neutron fluxes
15 2
4.2.4 The neutron spectrum thus depends very strongly on up to 10 n/cm ·s could be produced over a volume of several
the angle from the incident deuteron direction, and the flux is cubic centimetres, allowing much larger samples than with
E798 − 16
NOTE 1—The maximum occurs at about 40% of the deuteron energy. (Data from Ref (6).)
FIG. 3 Li(d,n) Spectra at 0° as a Function of Deuteron Energy
present sources. This facility would thus have a higher flux of Facilities with much higher charged particles such as the
high-energy neutrons over a larger volume than any available Intense Pulsed Neutron Source (IPNS) (17) and the Los
accelerator source.Asubsequent design that took advantage of
Alamos Meson Physics Facility (LAMPF) (18, 19) have also
improvements in accelerator technology is discussed in Ref
been used. For example, the IPNS neutron flux spectrum is
(15). More recently, similar technology has been assessed in
shown in Fig. 5 (17). A new irradiation facility was brought
the design and fabrication of prototypic components for an
on-lineattheLAMPFinthe1980s (19).Theprimaryobjective
International Fusion Materials Irradiation Facility (16).
of this facility is to study the basic aspects of radiation effects
as produced by medium energy protons and neutrons that are
4.3 Other Sources:
born through spallation reactions as the protons interact with
4.3.1 There have been other accelerator-based neutron
the target nuclei. Another objective is to study radiation
sources available, generally having lower neutron energy and
flux. Most are used for medical or nuclear research applica- damage to structural and detector materials used with accel-
erators. A description of the facility is given in Ref (20). The
tions. Van de Graaffs and cyclotrons have also been used with
3 7 7
other nuclear reactions such as d(d,n) He and Li(n,p) Be. availableneutronfluxandspectrumaredescribedbytheresults
E798 − 16
4.3.3 The procedures recommended in this work also apply
to these other sources and should be used where applicable.
However, the experimenter should always be aware of the
possibility of additional problems due to peculiarities of
individual sources.
5. Characterization of Irradiation Environments
5.1 Scope—The methods used to define the flux, fluence,
and spectra precisely in accelerator environments are signifi-
cantly different from those used in reactor environments. The
reason for this difference is that, whereas reactors generally
producestablefieldswithgentlegradients,acceleratorstendto
produce fields with very sharp spatial flux and spectral
gradients, which may vary over short time intervals and may
not scale linearly with beam current. For example, small
changes in accelerator tuning can move the spatial location of
the neutron source relative to the irradiated sample, thereby
changing the flux and spectrum. Consequently, it is critically
important to follow well established and well calibrated
procedures in order to measure adequately the irradiation
Note—Neutron fluence contours measured at the Univer-
sity of California with Davis Cyclotron Be(d,n), E =30
d exposure parameters. Otherwise, it will be impossible to
MeV. (Data from Ref (10).)
correctly calculate damage parameters such as DPA or to
FIG. 4a Neutron Fluence Contours
correlate materials effects measured at different facilities.
5.2 System Parameters—In the following section, it is
importanttodistinguishbetweenT(d,n)(14-MeV)sourcesand
broad spectrum Be(d,n) sources. Whereas both types of
sources exhibit strong flux gradients, only the broad-spectrum
sources exhibit significant spectral gradients. Consequently, in
the following subsections it should be understood that refer-
ences to flux measurement refer to both facilities, whereas
references to spectral measurement refer only to the Be(d,n)
sources.
5.2.1 Beam Characterization—It is important to realize that
virtuallyanychangeintheacceleratorbeamwillproducesome
alteration of the neutron field. Two classes of instabilities can
be defined according to whether they affect only the neutron
NOTE 1—Forward (0°), thick target neutron yield above 2 MeV from
9 flux or the neutron spectrum as well. Whereas the flux may
the Be(d,n) reaction as a function of deuteron energy.
vary independently of the spectrum, spectral changes always
FIG. 4 b Forward (0°), Thick Target Neutron Yield
imply a change in flux. Flux changes are usually easy to
measure and to account for in calculating total exposure or
of calculations (21) and foil activation measurements (22).
damage rates (see 5.3). However, spectral changes are much
Radiation damage parameters for the facility have also been
harder to measure or to account for in subsequent calculations.
calculated (23). In the case of facilities such as LAMPF and
For example, if the spectrum changes significantly even once
IPNS, the dosimetry and damage analysis must take into
duringalongrun,thenactivatedfoilswithshorthalf-livesmay
accountthepresenceofveryhigh-energyneutrons(>40MeV),
indicate an average spectrum that is quite different from that
aswellasasmallfluxofchargedparticles.TheLAMPFisnow
indicated by foils with long half-lives. Furthermore, it may be
known as the Los Alamos Neutron Science Center (LAN-
impossible to account for this difference unless great care is
SCE).
exercised to record the pertinent beam information, namely
4.3.2 Modern spallation neutron sources have also been
beam current, beam energy, and spatial alignment.
used for irradiation experiments. For example, the Swiss
5.2.1.1 Flux Instabilities—The most important sources of
Spallation Neutron Source, SINQ , has a unique SINQ Target
fluxinstabilityarethebeamcurrentandtargetcondition.Ifthe
IrradiationProgram(STIP).TheSTIPhasbeenusedinaseries
beam is well collimated, stable in energy, and stable in spatial
of materials irradiation experiments to investigate the effect
position, then the flux should be directly proportional to the
high damage rates with high helium and hydrogen generation
beam current, neglecting target effects. At solid Be(d,n)
rates (24).
sources, target effects are usually unimportant. However, at
T(d,n) sources, time-dependent changes in the target are the
dominant cause of flux instabilities (6). The beam current
See http://lansce.lanl.gov/.
See http://www.psi.ch/sinq/. should be read using a Faraday cup or well-insulated target
E798 − 16
NOTE 1—Neutron flux spectrum at the Intense Pulsed Neutron Source of ANL with 500 MeV protons and a depleted uranium target. The solid line
is calculated and the dashed is an adjusted spectrum based on radiometric dosimetry. (Data from Ref (12).)
FIG. 5 Neutron Flux Spectrum at the Intense Pulsed Neutron Source
assembly where possible. The current-sensing equipment characterization of an irradiation. Active and passive methods
should be checked for beam leakage, linearity, and long-term ofmeasuringfluxandspectralinstabilitiesarecoveredin5.3.2
stability. The output should then be recorded at regular time and 5.3.3.
intervals. 5.2.2 Target Characterization:
5.2.1.2 Flux and Spectral Instabilities—A change in the 5.2.2.1 Physical characteristics of the target assembly are
beam energy will alter both the flux and spectrum, although also vitally important in determining the neutron field. The
most accelerators have active means of keeping the beam design of the target will strongly influence the field produced
energyconstantwithinrelativelysmallpresetlimits.Itisworth and instabilities in the target can lead to large variations in the
mentioning that beam stability is often linked to beam current flux and spectra. In order to understand these effects, it is
sincebeamcontrolsystemsmayuseslitsorapertureswhichin important to understand neutron production in the target. Well
turn limit the transmission through the machine. Hence, designed targets are thick enough to stop the deuteron beam.
attempts to maximize the beam current may allow a wider Thiscanbecheckedwithanystandardrange-energytablesuch
rangeofparticletrajectories,resultinginalargerenergyspread as Refs (26, 27). However, improper target design may cause
aswellaspoorerspatialdefinition.Theexperimentershouldbe the target to burn up during exposure, leading to drastic
aware of these problems and check that the energy stability, alterations of the neutron field. Such catastrophic failures are
beamcurrentmonitoring,andtargetintegrityareadequate.The easily seen by remote sensing systems (see 5.2.6.2).
mostimportantsourceofspectralinstabilityatbroad-spectrum 5.2.2.2 As the deuteron beam is stopped in the target, it
sources is the movement of the beam on the target (at T(d,n) interactswiththetritiumorberyllium,asdiscussedpreviously.
sources this will only significantly affect the flux). Collimation ForT(d,n)sourcestheprimarycauseofconcernistheburn-out
aperatures are generally used to define the beam size and andboil-offoftritiumandslowbuild-upofdeuterium(seeFig.
location.Itisagainimportanttonotethatattemptstomaximize 6). The former causes a reduction in flux but no significant
thebeamcurrentandhencethefluxmayresultinunacceptably difference in the geometric source specification. The latter can
large variations in beam spot size and location on the target. lead to neutron production from the d(d,n) He reaction, al-
The collimation system should thus be analyzed to predict the though this contribution is generally negligible since massive
maximum possible variations. This can be translated into exposures are required to build up significant deuterium in the
flux/spectral information by examining measured angular dis- targetandtheneutronproductioncrosssectionismuchsmaller
tribution data. For example, at a deuteron beam energy of 30 than from tritium. At the RTNS, these effects were well
MeV on a Be target, the total flux falls a factor of two as the understood. Remote neutron detectors were used to continu-
angle from the beam axis changes from 0° to only 10° (25).At ously monitor the target condition and the target was then
a close irradiation distance of about 0.5 cm, this would slowly rocked in position in an attempt to maintain a nearly
correspondtoachangeinthebeamspotlocationofonly1mm. constant neutron flux. The experimenter could thus obtain an
Beam spatial alignment and stability are thus crucial to the accurate time history of the neutron exposure.
E798 − 16
NOTE 1—Depth profiles of tritium in new (solid line) and used (dashed line) targets. Deuterium accumulated in the target is also shown. (Data from
Ref (4).)
FIG. 6 Depth Profiles of Tritium in New and Used Targets at the RTNS I
5.2.2.3 More complex target problems are encountered at 5.2.3.1 A major problem in determining the flux and spec-
Be(d,n) facilities.The amount of material that backs the active trum seen by an irradiated sample is that it is often difficult to
Be region as well as the surrounding support material will determineinadvancethepreciselocationofthesamplerelative
attenuateorscattertheneutrons,probablyaccountingforsome tothesource.Forthisreason,passiveinsitudosimetersshould
differences in the low-energy neutron flux reported at different be included with all close-geometry irradiations. For example,
facilities. On the other hand, backing material cannot be too changes in position of less than 1 mm can easily change the
thin or high energy protons from (d,p) reactions may escape flux at existing Be(d,n) sources by as much as a factor of two
from the target and irradiate the specimen. The lifetime of a when samples are placed within 0.5 cm of the target. Careful
beryllium target is not well established, although experience at measurements of sample and dosimeter locations should thus
U. C. Davis indicates that they should be able to withstand be made to ensure adequate information for complete dosim-
deuteron exposures of at least 200 C/cm . However, if target etric analysis.
cooling is inadequate, the beryllium may evaporate or melt 5.2.3.2 Techniques such as autoradiographs are very useful
within a matter of minutes. Such failures are readily apparent in determining the position of the sample relative to the beam
by sudden changes in the neutron flux.Amore serious concern and, if done prior to irradiation, can ensure maximum fluence
istheslowerosionoftheberylliumsincethisleadstoagradual in the samples (see 5.2.6.1).
change in the location of the source in the beryllium and may 5.2.4 Other Perturbation Effects:
produce perturbations in the flux and spectrum at close 5.2.4.1 Experimental equipment and sample materials may
geometries. Passive in situ dosimetry should be able to themselves perturb the neutron field through attenuation and
integrate over such changes, although shorter-lived dosimetry scattering effects. A particularly important example has been
materials may have to be replaced during very long irradia- found with organic materials, which can greatly increase the
tions. thermal or epithermal flux. In close geometries, attenuation
5.2.3 Sample Positioning: effects have been found to be the most important since
E798 − 16
scattered fluxes (for example, room return) are generally very short-lived activities to die out, leaving the longer-lived
small compared to the primary flux. As an example, metallic activities, which are representative of the entire irradiation
dosimetry packages measuring 7 mm in total thickness were rather than just the last few minutes or hours.
found to produce total attenuations of about 15% in Be(d,n)
5.2.6.2 Active Methods of System Measurement—Inaddition
fields (28). Multiple-specimen irradiations thus require passive
tomonitoringbeamcurrentandenergy,itisalsorecommended
dosimeters at many locations, preferably with each group of
that some active method be used to measure the neutron flux.
specimens, in order to map these small perturbation effects.
A remote detector will not be sensitive to small position
changes but will detect any significant changes in beam
5.2.4.2 It has also been suggested that materials may be
intensity or energy as well as target degradation effects (see
deliberately placed near the target to tailor the spectrum. For
example, a uranium shell may be used near a T(d,n) source to 5.3.2). Many accelerators also have other active beam sensing
devices,suchas x-yscanners,whichcanbeusedtomonitorthe
simulatemorecloselyafirstwallfusionreactorspectrum.Such
facilities should be well documented and may require special spatialpositionofthebeam.Itisalsooftenpossibletoposition
small detectors or fission chambers at back angles in close
procedures.
proximity to the target in order to monitor small changes in
5.2.4.3 At existing Be(d,n) facilities there are occasional
beamlocation.Allsuchdevicesshouldbeusedwherepractical
changes in the source configuration since the facilities are not
to provide the best record possible concerning the time history
dedicated solely to materials irradiations. For example, differ-
of the actual neutron field.
ent beam lines may be used and massive, extraneous equip-
5.2.6.3 Passive Methods of System Measurement—Themost
ment and shielding may be repositioned. Such changes may
accurate method of determining the irradiation received by a
produce changes in the neutron field, especially at low ener-
sample is through the use of passive in situ monitors such as
gies. Therefore experimenters cannot assume that the neutron
multiple-foil activations, helium accumulation fluence moni-
flux-spectrum will necessarily be identical to previous mea-
tors (HAFM) (29), or solid-state track recorders (SSTR) (30).
surements at that facility.
The latter two stable product monitors will not be discussed in
5.2.5 Beam Rastering Techniques—Thesharpfluxandspec-
detail in this practice since they involve specialized techniques
tral gradients produced at Be(d,n) sources can be moderated
and they are described in Test Method E854 and Specification
somewhat by deliberately moving the beam on the target in a
E910. However, the basic procedures involved in using these
prescribed raster pattern, although this may lower the total
devices during an irradiation are the same as for the f
...

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: E798 − 96 (Reapproved 2009) E798 − 16
Standard Practice for
Conducting Irradiations at Accelerator-Based Neutron
Sources
This standard is issued under the fixed designation E798; 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 practice covers procedures for irradiations at accelerator-based neutron sources. The discussion focuses on two types
of sources, namely nearly monoenergetic 14-MeV neutrons from the deuterium-tritium T(d,n) interaction, and broad spectrum
neutrons from stopping deuterium beams in thick beryllium or lithium targets. However, most of the recommendations also apply
to other types of accelerator-based sources, including spallation neutron sources (1). Interest in spallation sources has increased
recently due to their development of high-power, high-flux sources for neutron scattering and their proposed use for transmutation
of fission reactor waste (2).
1.2 Many of the experiments conducted using such neutron sources are intended to simulate provide a simulation of irradiation
in another neutron spectrum, for example, that from a DT fusion reaction. The word simulation is used here in a broad sense to
imply an approximation of the relevant neutron irradiation environment. The degree of conformity can range from poor to nearly
exact. In general, the intent of these simulationsexperiments is to establish the fundamental relationships between irradiation or
material parameters and the material response. The extrapolation of data from such experiments requires that the differences in
neutron spectra be considered.
1.3 The procedures to be considered include methods for characterizing the accelerator beam and target, the irradiated sample,
and the neutron flux (fluence rate) and spectrum, as well as procedures for recording and reporting irradiation data.
1.4 Other experimental problems, such as temperature control, are not included.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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 and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
C859 Terminology Relating to Nuclear Materials
E170 Terminology Relating to Radiation Measurements and Dosimetry
E181 Test Methods for Detector Calibration and Analysis of Radionuclides
E261 Practice for Determining Neutron Fluence, Fluence Rate, and Spectra by Radioactivation Techniques
E263 Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Iron
E264 Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Nickel
E265 Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32
E266 Test Method for Measuring Fast-Neutron Reaction Rates by Radioactivation of Aluminum
E393 Test Method for Measuring Reaction Rates by Analysis of Barium-140 From Fission Dosimeters
E854 Test Method for Application and Analysis of Solid State Track Recorder (SSTR) Monitors for Reactor Surveillance,
E706(IIIB)
This practice is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applicationsand is the direct responsibility of Subcommittee E10.08 on
Procedures for Neutron Radiation Damage Simulation.
Current edition approved Aug. 1, 2009Oct. 1, 2016. Published September 2009December 2016. Originally approved in 1981. Last previous edition approved in 20032009
as E798 – 96 (2003).(2009). DOI: 10.1520/E0798-96R09.10.1520/E0798-16.
The boldface numbers in parentheses refer to a list of references at the end of this practice.
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.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E798 − 16
E910 Test Method for Application and Analysis of Helium Accumulation Fluence Monitors for Reactor Vessel Surveillance,
E706 (IIIC)
3. Terminology
3.1 Descriptions of relevant terms are found in Terminology C859 and Terminology E170.
4. Summary of Existing and Proposed Facilities
4.1 T(d,n) Sources:
4.1.1 Neutrons are produced by the highly exoergic reaction d + t → n + α. The total nuclear energy released is 17.589 MeV,
resulting in about a 14.8-MeV neutron and a 2.8-MeV alpha particle at low deuterium beam energies (3). The deuteron energy
(generally 150 to 400 keV) is chosen to maximize the neutron yield (for a particular target configuration) from the resonance in
the d-t cross section near 100 keV. The number of neutrons emitted as a function of angle (θ) between the neutron direction and
the incident deuteron beam is very nearly isotropic in the center-of-mass system. At a deuteron energy of 400 keV in the laboratory
system, the neutron flux in the forward direction is about 14 % greater than in the backward direction, while the corresponding
neutron energy decreases from 15.6 to 13.8 MeV (4). In practice, the neutron field also depends on the gradual loss of the target
material and the tritium deposition profile. Detailed calculations should then be made for a specific facility.
4.1.2 The flux seen at a point (r, θ, z) in cylindrical coordinates from a uniform T(d,n) source of diameter a is given by the
following (5):
4 2 2 1/2 2
Y k 14r z 1k
~ !
φ r, θ, z 5 ln (1)
~ ! H J
2 2
4πa 2z
where:
2 2 2 2
k = a + z − r , and
Y = the total source strength.
For z >> a and r = 0 (on beam axis) this reduces to Y/4πz , as expected for a point source. The available irradiation volume
at maximum flux is usually small. For a sample placed close to the target, the flux will decrease very rapidly with increasing radial
distance off the beam axis. However, since the neutron energy is nearly constant, this drop in flux is relatively easy to measure
by foil activation techniques.
4.1.3 Other existing sources, such as Cockroft-Walton type accelerators, are similar in nature although the available neutron
source strengths are much lower.
4.1.4 Rotating Target Neutron Source (RTNS) I and II (5-7)—RTNS I and II, which formerly were operated at the Lawrence
12 13
Livermore National Laboratory, provided 14 MeV neutron source strengths of about 6 × 10 and 4 × 10 neutrons/s, respectively.
Although these facilities have been shut down, they were the most intense sources of 14 MeV neutrons built to date for research
purposes. They are discussed here because of their relevance to any future neutron sources. Their characteristics are summarized
in Table 1. A discussion of similar sources can be found in Ref (8). The deuteron beam energy was 400 keV and the target was
a copper-zirconium alloy (or copper with dispersed alumina) vapor-plated with tritium-occluded titanium. The beam spot size was
about 10 mm in diameter. In addition to being rotated, the target also was rocked every few hours and the deuteron beam current
was increased slowly in an attempt to maintain a constant flux in spite of tritium burn-up in the target. Samples could be placed
13 2
as close as 2.5 to 4.0 mm from the region of maximum d-t interaction resulting in a typical flux of 10 n/cm ·s over a small sample.
The neutron fields were well characterized by a variety of methods and the absolute fluence could be routinely determined to
67 %. Calculated neutron flux contours for RTNS-II are shown in Fig. 1.
4.2 Be or Li(d,n) Sources (9):
4.2.1 When a high-energy (typically 30- to 40-MeV) deuteron beam is stopped in a beryllium (or lithium) target, a continuous
spectrum of neutrons is produced extending from thermal energies to about 4 MeV (15 MeV for lithium) above the incident
deuteron energy (see Figs. 2-4). In existing facilities, cyclotrons with deuteron beam intensities of 20 to 40 μA provide neutron
13 16
source strengths in the range of 10 n/s, using solid beryllium targets with water cooling. A more intense source (>10 n/s) is now
TABLE 1 Characteristics of T(d,n) and Be or Li(d,n) Neutron Sources
Experimental
Source Maximum Flux
Volume for
Facility Availability Beam Target Strength, at Sample,
Maximum
n/s n/cm ·s
Flux, cm
12 12
RTNS I No longer available 400 keV d t 6 × 10 >10 0.2
13 13
RTNS II No longer available 400 keV d t 4 × 10 >10 0.2
>10 5.0
A 13 12
Existing Be or Li(d,n) U.C. Davis Cyclotron 30–40 MeV d Solid Be or Li ;10 >10 ;1.0
16 15
Proposed Li(d,n) Conceptual design (9) 30–40 MeV d Liquid Li 3 × 10 >10 10.0
>10 600.0
A
This is the only existing facility that has been well characterized and is readily available, although other facilities can be used.
E798 − 16
NOTE 1—Flux contours assume a symmetric, Gaussian beam profile. Figure from Ref. (5).(5).
FIG. 1 Flux Contours for RTNS II
NOTE 1—Neutron spectra as a function of energy and angle for Be(d,n) source at ORNL, E = 40 MeV. (Data from Ref (8).(8).)
d
FIG. 2 Neutron Spectra as a Function of Energy and Angle from the Forward Direction of the Deuteron Beam
being designed employing liquid lithium targets. In the remainder of this document the term Be(d,n) source is meant as a generic
term including Li(d,n) sources, whether solid or liquid targets.
4.2.2 Neutrons are produced by several competing nuclear reaction mechanisms. The most important one for radiation damage
studies is the direct, stripping reaction since it produces almost all of the high-energy neutrons. When the incident deuteron passes
close to a target nucleus, the proton is captured and the neutron tends to continue on in a forward direction. The high energy
neutrons are thus preferentially emitted in the direction of the incident deuteron beam. However, as the deuterons slow down in
the target, lower energy neutrons will be produced with angular distributions that are much less forward peaked. Furthermore,
when the residual nucleus is left in an excited state, the angular effects are also much less pronounced. These latter two effects tend
to decrease the average neutron energy at angles other than 0° in the direction of the beam.
4.2.3 Neutrons can also be produced by compound nuclear reactions in which the entire deuteron is captured by the target
nucleus and neutrons are subsequently evaporated. Neutrons are preferentially emitted with energies less than a few MeV and the
angular distribution approaches isotropy at neutron energies below 1 MeV. Neutrons also are produced by deuteron break-up, in
which the deuteron simply breaks apart in the Coulomb field of the nucleus, although this effect is very small for low-Z materials.
4.2.4 The neutron spectrum thus depends very strongly on the angle from the incident deuteron direction, and the flux is very
sharply peaked in the forward direction (see Fig. 2). Materials studies for which the maximum total neutron fluence is desired are
E798 − 16
NOTE 1—The maximum occurs at about 40% of the deuteron energy. (Data from Ref (6).(6).)
FIG. 3 Li(d,n) Spectra at 0° as a Function of Deuteron Energy
usually conducted close to the target and may subtend a large range of forward angles (for example, 0 to 60°). This practice
primarily will be concerned with this close-geometry situation since it is the most difficult to handle properly.
4.2.5 Other factors can also influence the neutron field during a particular irradiation, especially beam and target characteristics,
as well as the perturbing influence of surrounding materials. At present, these facilities have not been completely characterized for
routine use. In particular, some uncertainties exist, especially at low (<2 MeV) and high (<30 MeV) neutron energies, since these
regions are either difficult to measure with existing techniques, or the required nuclear data are insufficient. In these cases, neutron
dosimetry data should be reported directly to allow reanalysis as procedures and nuclear data improve in the future.
4.2.6 Existing Sources:
4.2.6.1 Whereas virtually any deuteron accelerator with reasonable energy and intensity can be used as a neutron source, only
two facilities have been used routinely for materials effects irradiations, namely the cyclotrons at the University of California at
Davis (10) and at Oak Ridge National Laboratory (11, 12). Typical flux-spectra obtained are shown in Figs. 2-4 (9, 11, 13), and
typical characteristics are listed in Table 1.
4.2.6.2 Since the neutron flux and spectral gradients are so steep, experimenters are faced with the problem of nonuniform
irradiations over their samples unless specimen sizes are severely limited. Alternatively, the field gradients may be moderated by
E798 − 16
Note—Neutron fluence contours measured at the Univer-
sity of California with Davis Cyclotron Be(d,n), E = 30
d
MeV. (Data from Ref (10).)
Note—Neutron fluence contours measured at the Univer-
sity of California with Davis Cyclotron Be(d,n), E = 30
d
MeV. (Data from Ref (10).)
FIG. 4a Neutron Fluence Contours
NOTE 1—Forward (0°), thick target neutron yield above 2 MeV from the Be(d,n) reaction as a function of deuteron energy.
FIG. 4 b Forward (0°), Thick Target Neutron Yield
deliberately moving or enlarging the beam spot on the target. This technique will result in a lower total fluence as well as a lower
average neutron energy for a small-size sample on the beam axis, although larger samples will not be so severely affected and may
in fact show an overall improvement in average fluence and neutron energy.
4.2.6.3 At present, the neutron field can be determined reasonably well at existing facilities. The flux-spectrum can be measured
to within 610 to 30 % in the 2- to 30-MeV energy region where about 90 % of neutron damage is initiated (assuming E = 30
d
to 40 MeV). Highly accurate (610 %) time-of-flight spectrometry has been used to study the field far from the source, except for
the energy region below a few MeV (11). However, close geometry irradiations must rely on passive dosimetry with larger errors
due to uncertainties in the nuclear cross sections, especially above 30 MeV (12).
4.2.7 Conceptual Design for Li(d,n) Source (9, 14)—A conceptual design for a fusion materials irradiation facility was done at
the Hanford Engineering Development Laboratory (HEDL). The design consisted of a high-current (100-mA) deuteron accelerator
and a liquid lithium target. This was expected to produce a neutron source strength of about 3 × 10 n/s (14). The designs called
for a wide-area beam spot on the target (for example, 3 by 1 cm), thereby moderating the steep neutron field gradients in close
15 2
geometry. Neutron fluxes up to 10 n/cm ·s could be produced over a volume of several cubic centimetres, allowing much larger
samples than with present sources. This facility would thus have a higher flux of high-energy neutrons over a larger volume than
any available accelerator source. A more recent subsequent design that takestook advantage of improvements in accelerator
technology is discussed in Ref (15). More recently, similar technology has been assessed in the design and fabrication of prototypic
components for an International Fusion Materials Irradiation Facility (16).
E798 − 16
4.3 Other Sources:
4.3.1 There are manyhave been other accelerator-based neutron sources available, generally having lower neutron energy and
flux. Most are used for medical or nuclear research applications. Van de Graaffs and cyclotrons have also been used with many
3 7 7
other nuclear reactions such as d(d,n) He and Li(n,p) Be. Facilities with much higher charged particles such as the Intense Pulsed
Neutron Source (IPNS) (1617) and the Los Alamos Meson Physics Facility (LAMPF) (1718, 1819) have also been used. For
example, the IPNS neutron flux spectrum is shown in Fig. 5 (1617). A new irradiation facility has been was brought on-line at the
LAMPF in the 1980s (1819). The primary objective of this facility is to study the basic aspects of radiation effects as produced
by medium energy protons and neutrons that are born through spallation reactions as the protons interact with the target nuclei.
Another objective is to study radiation damage to structural and detector materials used with accelerators. A description of the
facility is given in Ref (1920). The available neutron flux and spectrum are described by the results of calculations (2021) and foil
activation measurements (2122). Radiation damage parameters for the facility have also been calculated (2223). In the case of
facilities such as LAMPF and IPNS, the dosimetry and damage analysis must take into account the presence of very high-energy
neutrons (>40 MeV), as well as a small flux of charged particles. The LAMPF is now known as the Los Alamos Neutron Science
Center (LANSCE).
4.3.2 Modern spallation neutron sources have also been used for irradiation experiments. For example, the Swiss Spallation
Neutron Source, SINQ , has a unique SINQ Target Irradiation Program (STIP). The STIP has been used in a series of materials
irradiation experiments to investigate the effect high damage rates with high helium and hydrogen generation rates (24).
4.3.3 The procedures recommended in this work also apply to these other sources and should be used where applicable.
However, the experimenter should always be aware of the possibility of additional problems due to peculiarities of individual
sources.
5. Characterization of Irradiation Environments
5.1 Scope—The methods used to define the flux, fluence, and spectra precisely in accelerator environments are significantly
different from those used in reactor environments. The reason for this difference is that, whereas reactors generally produce stable
fields with gentle gradients, accelerators tend to produce fields with very sharp spatial flux and spectral gradients, which may vary
over short time intervals and may not scale linearly with beam current. For example, small changes in accelerator tuning can move
the spatial location of the neutron source relative to the irradiated sample, thereby changing the flux and spectrum. Consequently,
it is critically important to follow well established and well calibrated procedures in order to measure adequately the irradiation
See http://lansce.lanl.gov/.
See http://www.psi.ch/sinq/.
NOTE 1—Neutron flux spectrum at the Intense Pulsed Neutron Source of ANL with 500 MeV protons and a depleted uranium target. The solid line
is calculated and the dashed is an adjusted spectrum based on radiometric dosimetry. (Data from Ref (12).(12).)
FIG. 5 Neutron Flux Spectrum at the Intense Pulsed Neutron Source
E798 − 16
exposure parameters. Otherwise, it will be impossible to correctly calculate damage parameters such as DPA or to correlate
materials effects measured at different facilities.
5.2 System Parameters—In the following section, it is important to distinguish between T(d,n) (14-MeV) sources and broad
spectrum Be(d,n) sources. Whereas both types of sources exhibit strong flux gradients, only the broad-spectrum sources exhibit
significant spectral gradients. Consequently, in the following subsections it should be understood that references to flux
measurement refer to both facilities, whereas references to spectral measurement refer only to the Be(d,n) sources.
5.2.1 Beam Characterization—It is important to realize that virtually any change in the accelerator beam will produce some
alteration of the neutron field. Two classes of instabilities can be defined according to whether they affect only the neutron flux
or the neutron spectrum as well. Whereas the flux may vary independently of the spectrum, spectral changes always imply a change
in flux. Flux changes are usually easy to measure and to account for in calculating total exposure or damage rates (see 5.3).
However, spectral changes are much harder to measure or to account for in subsequent calculations. For example, if the spectrum
changes significantly even once during a long run, then activated foils with short half-lives may indicate an average spectrum that
is quite different from that indicated by foils with long half-lives. Furthermore, it may be impossible to account for this difference
unless great care is exercised to record the pertinent beam information, namely beam current, beam energy, and spatial alignment.
5.2.1.1 Flux Instabilities—The most important sources of flux instability are the beam current and target condition. If the beam
is well collimated, stable in energy, and stable in spatial position, then the flux should be directly proportional to the beam current,
neglecting target effects. At solid Be(d,n) sources, target effects are usually unimportant. However, at T(d,n) sources,
time-dependent changes in the target are the dominant cause of flux instabilities (6). The beam current should be read using a
Faraday cup or well insulated well-insulated target assembly where possible. The current-sensing equipment should be checked
for beam leakage, linearity, and long-term stability. The output should then be recorded at regular time intervals.
5.2.1.2 Flux and Spectral Instabilities—A change in the beam energy will alter both the flux and spectrum, although most
accelerators have active means of keeping the beam energy constant within relatively small preset limits. It is worth mentioning
that beam stability is often linked to beam current since beam control systems may use slits or aperaturesapertures which in turn
limit the transmission through the machine. Hence, attempts to maximize the beam current may allow a wider range of particle
trajectories, resulting in a larger energy spread as well as poorer spatial definition. The experimenter should be aware of these
problems and check that the energy stability, beam current monitoring, and target integrity are adequate. The most important source
of spectral instability at broad-spectrum sources is the movement of the beam on the target (at T(d,n) sources this will only
significantly affect the flux). Collimation aperatures are generally used to define the beam size and location. It is again important
to note that attempts to maximize the beam current and hence the flux may result in unacceptably large variations in beam spot
size and location on the target. The collimation system should thus be analyzed to predict the maximum possible variations. This
can be translated into flux/spectral information by examining measured angular distribution data. For example, at a deuteron beam
energy of 30 MeV on a Be target, the total flux falls a factor of two as the angle from the beam axis changes from 0° to only 10°
(2325). At a close irradiation distance of about 0.5 cm, this would correspond to a change in the beam spot location of only 1 mm.
Beam spatial alignment and stability are thus crucial to the characterization of an irradiation. Active and passive methods of
measuring flux and spectral instabilities are covered in 5.3.2 and 5.3.3.
5.2.2 Target Characterization:
5.2.2.1 Physical characteristics of the target assembly are also vitally important in determining the neutron field. The design of
the target will strongly influence the field produced and instabilities in the target can lead to large variations in the flux and spectra.
In order to understand these effects, it is important to understand neutron production in the target. Well designed targets are thick
enough to stop the deuteron beam. This can be checked with any standard range-energy table such as Refs (2426, 2527). However,
improper target design may cause the target to burn up during exposure, leading to drastic alterations of the neutron field. Such
catastrophic failures are easily seen by remote sensing systems (see 5.2.6.2).
5.2.2.2 As the deuteron beam is stopped in the target, it interacts with the tritium or beryllium, as discussed previously. For
T(d,n) sources the primary cause of concern is the burn-out and boil-off of tritium and slow build-up of deuterium (see Fig. 6).
The former causes a reduction in flux but no significant difference in the geometric source specification. The latter can lead to
neutron production from the d(d,n) He reaction, although this contribution is generally negligible since massive exposures are
required to build up significant deuterium in the target and the neutron production cross section is much smaller than from tritium.
At the RTNS, these effects were well understood. Remote neutron detectors were used to continuously monitor the target condition
and the target was then slowly rocked in position in an attempt to maintain a nearly constant neutron flux. The experimenter could
thus obtain an accurate time history of the neutron exposure.
5.2.2.3 More complex target problems are encountered at Be(d,n) facilities. The amount of material that backs the active Be
region as well as the surrounding support material will attenuate or scatter the neutrons, probably accounting for some differences
in the low-energy neutron flux reported at different facilities. On the other hand, backing material cannot be too thin or high energy
protons from (d,p) reactions may escape from the target and irradiate the specimen. The lifetime of a beryllium target is not well
established, although experience at U. C. Davis indicates that they should be able to withstand deuteron exposures of at least 200
C/cm . However, if target cooling is inadequate, the beryllium may evaporate or melt within a matter of minutes. Such failures
are readily apparent by sudden changes in the neutron flux. A more serious concern is the slow erosion of the beryllium since this
leads to a gradual change in the location of the source in the beryllium and may produce perturbations in the flux and spectrum
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NOTE 1—Depth profiles of tritium in new (solid line) and used (dashed line) targets. Deuterium accumulated in the target is also shown. (Data from
Ref (4).(4).)
FIG. 6 Depth Profiles of Tritium in New and Used Targets at the RTNS I
at close geometries. Passive in situ dosimetry should be able to integrate over such changes, although shorter-lived dosimetry
materials may have to be replaced during very long irradiations.
5.2.3 Sample Positioning:
5.2.3.1 A major problem in determining the flux and spectrum seen by an irradiated sample is that it is often difficult to
determine in advance the precise location of the sample relative to the source. For this reason, passive in situ dosimeters should
be included with all close-geometry irradiations. For example, changes in position of less than 1 mm can easily change the flux
at existing Be(d,n) sources by as much as a factor of two when samples are placed within 0.5 cm of the target. Careful
measurements of sample and dosimeter locations should thus be made to ensure adequate information for complete dosimetric
analysis.
5.2.3.2 Techniques such as autoradiographs are very useful in determining the position of the sample relative to the beam and,
if done prior to irradiation, can ensure maximum fluence in the samples (see 5.2.6.1).
5.2.4 Other Perturbation Effects:
5.2.4.1 Experimental equipment and sample materials may themselves perturb the neutron field through attenuation and
scattering effects. A particularly important example has been found with organic materials, which can greatly increase the thermal
or epithermal flux. In close geometries, attenuation effects have been found to be the most important since scattered fluxes (for
example, room return) are generally very small compared to the primary flux. As an example, metallic dosimetry packages
measuring 7 mm in total thickness were found to produce total attenuations of about 15 % in Be(d,n) fields (2628).
Multiple-specimen irradiations thus require passive dosimeters at many locations, preferably with each group of specimens, in
order to map these small perturbation effects.
5.2.4.2 It has also been suggested that materials may be deliberately placed near the target to tailor the spectrum. For example,
a uranium shell may be used near a T(d,n) source to simulate more closely a first wall fusion reactor spectrum. Such facilities
should be well documented and may require special procedures.
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5.2.4.3 At existing Be(d,n) facilities there are occasional changes in the source configuration since the facilities are not
dedicated solely to materials irradiations. For example, different beam lines may be used and massive, extraneous equipment and
shielding may be repositioned. Such changes may produce changes in the neutron field, especially at low energies. Therefore
experimenters cannot assume that the neutron flux-spectrum will necessarily be identical to previous measurements at that facility.
5.2.5 Beam Rastering Techniques—The sharp flux and spectral gradients produced at Be(d,n) sources can be moderated
somewhat by deliberately moving the beam on the target in a prescribed raster pattern, although this may lower the total fluence
in small irradiated samples. The problems of beam and target instabilities are still present and may be further complicated by this
approach if the raster pattern varies in spatial position. The periodic time dependence (for example, 10 Hz) of the neutron flux also
may produce changes in the damage production, especially at elevated temperatures. Passive in situ dosimetry is thus required in
order to determine the average flux and spectrum seen by a sample, and the precise rastering technique should be reported along
with the dosimetry results.
5.2.6 Measurement of System Instabilities—Many of the possible instabilities noted above are routinely measured at most
accelerator facilities, especially beam current and energy. However, the experimenter usually is responsible for positional effects
and the following methods are recommended:
5.2.6.1 Autoradiographs—A very simple means of determining the beam spot size and location on the target (to within< 1within
<1 mm) is to take an autoradiograph. This is done by attaching a sheet of Polaroid film to the target assembly following a brief
irradiation. Alternatively, a thin sheet of metal may be attached to the target, irradiated, and then autoradiographed. The induced
activity in the target usually will be quite high (>1 R) so that exposures of a minute or less will give a clear image of the center
of the activated target assembly material. This should be done after the machine has been tuned and before the specimens are
mounted in order to ensure the best alignment of the specimens. This will not only simplify the irradiation characterization but will
also maximize the fluence seen by the sample. Autoradiographs also should be taken after the irradiation of both the target
assembly and the specimen and dosimetry packages. Due to the very high residual activity, some delay may be required before
such exposures are possible. This delay is desirable in any case, since this allows short-lived activities to die out, leaving the
longer-lived activities, which are representative of the entire irradiation rather than just the last few minutes or hours.
5.2.6.2 Active Methods of System Measurement—In addition to monitoring beam current and energy, it is also recommended that
some active method be used to measure the neutron flux. A remote detector will not be sensitive to small position changes but will
detect any significant changes in beam intensity or energy as well as target degradation effects (see 5.3.2). Many accelerators also
have other active beam sensing devices,
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