ASTM E942-96(2011)
(Guide)Standard Guide for Simulation of Helium Effects in Irradiated Metals
Standard Guide for Simulation of Helium Effects in Irradiated Metals
ABSTRACT
This guide presents the simulation procedure which would provide advice for conducting experiments to investigate the effects of helium on the properties of irradiated metals where the technique for introducing the helium differs in someway from the actual mechanism of introduction of helium in service. Simulation techniques considered for introducing helium shall include charged particle implantation, exposure to α-emitting radioisotopes, and tritium decay techniques. Procedures for the analysis of helium content and helium distribution within the specimen are also recommended. The two other methods for introducing helium into irradiated materials namely, the enhancement of helium production in nickel-bearing alloys by spectral tailoring in mixed-spectrum fission reactors, and the isotopic tailoring in both fast and mixed-spectrum fission reactors, are not covered in this guide. Dual ion beam techniques for simultaneously implanting helium and generating displacement damage are also not included here.
SIGNIFICANCE AND USE
Helium is introduced into metals as a consequence of nuclear reactions, such as (n, α), or by the injection of helium into metals from the plasma in fusion reactors. The characterization of the effect of helium on the properties of metals using direct irradiation methods may be impractical because of the time required to perform the irradiation or the lack of a radiation facility, as in the case of the fusion reactor. Simulation techniques can accelerate the research by identifying and isolating major effects caused by the presence of helium. The word simulation is used here in a broad sense to imply an approximation of the relevant irradiation environment. There are many complex interactions between the helium produced during irradiation and other irradiation effects, so care must be exercised to ensure that the effects being studied are a suitable approximation of the real effect. By way of illustration, details of helium introduction, especially the implantation temperature, may determine the subsequent distribution of the helium (that is, dispersed atomistically, in small clusters in bubbles, etc.)
SCOPE
1.1 This guide provides advice for conducting experiments to investigate the effects of helium on the properties of metals where the technique for introducing the helium differs in some way from the actual mechanism of introduction of helium in service. Simulation techniques considered for introducing helium shall include charged particle implantation, exposure to α-emitting radioisotopes, and tritium decay techniques. Procedures for the analysis of helium content and helium distribution within the specimen are also recommended.
1.2 Two other methods for introducing helium into irradiated materials are not covered in this guide. They are the enhancement of helium production in nickel-bearing alloys by spectral tailoring in mixed-spectrum fission reactors, and isotopic tailoring in both fast and mixed-spectrum fission reactors. These techniques are described in Refs (1-5). Dual ion beam techniques (6) for simultaneously implanting helium and generating displacement damage are also not included here. This latter method is discussed in Practice E521.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 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.
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Designation: E942 − 96 (Reapproved 2011)
Standard Guide for
Simulation of Helium Effects in Irradiated Metals
This standard is issued under the fixed designation E942; 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 guide provides advice for conducting experiments
E521Practice for Neutron Radiation Damage Simulation by
to investigate the effects of helium on the properties of metals
Charged-Particle Irradiation
where the technique for introducing the helium differs in some
E706MasterMatrixforLight-WaterReactorPressureVessel
way from the actual mechanism of introduction of helium in
Surveillance Standards, E 706(0) (Withdrawn 2011)
service. Simulation techniques considered for introducing he-
E910Test Method for Application and Analysis of Helium
lium shall include charged particle implantation, exposure to
Accumulation Fluence Monitors for Reactor Vessel
α-emitting radioisotopes, and tritium decay techniques. Proce-
Surveillance, E706 (IIIC)
duresfortheanalysisofheliumcontentandheliumdistribution
within the specimen are also recommended.
3. Terminology
1.2 Two other methods for introducing helium into irradi-
3.1 DescriptionsofrelevanttermsarefoundinTerminology
ated materials are not covered in this guide. They are the
C859 and Terminology E170.
enhancement of helium production in nickel-bearing alloys by
spectral tailoring in mixed-spectrum fission reactors, and
4. Significance and Use
isotopic tailoring in both fast and mixed-spectrum fission
4.1 Helium is introduced into metals as a consequence of
reactors. These techniques are described in Refs (1-5). Dual
nuclear reactions, such as (n, α), or by the injection of helium
ion beam techniques (6) for simultaneously implanting helium
into metals from the plasma in fusion reactors. The character-
and generating displacement damage are also not included
izationoftheeffectofheliumonthepropertiesofmetalsusing
here. This latter method is discussed in Practice E521.
direct irradiation methods may be impractical because of the
1.3 The values stated in SI units are to be regarded as
time required to perform the irradiation or the lack of a
standard. No other units of measurement are included in this radiation facility, as in the case of the fusion reactor. Simula-
standard.
tion techniques can accelerate the research by identifying and
isolating major effects caused by the presence of helium. The
1.4 This standard does not purport to address all of the
word simulation is used here in a broad sense to imply an
safety concerns, if any, associated with its use. It is the
approximation of the relevant irradiation environment. There
responsibility of the user of this standard to establish appro-
are many complex interactions between the helium produced
priate safety and health practices and determine the applica-
during irradiation and other irradiation effects, so care must be
bility of regulatory limitations prior to use.
exercised to ensure that the effects being studied are a suitable
approximation of the real effect. By way of illustration, details
2. Referenced Documents
of helium introduction, especially the implantation
2.1 ASTM Standards:
temperature, may determine the subsequent distribution of the
C859Terminology Relating to Nuclear Materials
helium (that is, dispersed atomistically, in small clusters in
bubbles, etc.)
This guide is under the jurisdiction of ASTM Committee E10 on Nuclear
5. Techniques for Introducing Helium
Technology and Applicationsand is the direct responsibility of Subcommittee
5.1 Implantation of Helium Using Charged Particle Accel-
E10.08 on Procedures for Neutron Radiation Damage Simulation.
Current edition approved June 15, 2011. Published July 2011. Originally
erators:
approvedin1983.Lastpreviouseditionapprovedin2003asE942–96(2003).DOI:
5.1.1 Summary of Method—Charged particle accelerators
10.1520/E0942-96R11.
aredesignedtodeliverwelldefined,intensebeamsofmonoen-
The boldface numbers in parentheses refer to a list of references at the end of
this guide. ergetic particles on a target. They thus provide a convenient,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E942 − 96 (2011)
rapid, and relatively inexpensive means of introducing large required for implantation. Typical Cyclotron operating charac-
concentrations of helium into thin specimens. An energetic teristics are 20 to 80 MeV with a beam current of 20 µAat the
alphaparticleimpingingonatargetlosesenergybyexcitingor
source. It should be noted, however, that the usable beam
ionizing the target atoms, or both, and by inelastic collisions current delivered to the specimen is limited by the ability to
with the target atom nuclei. Particle ranges for a variety of
remove heat from the specimens which restricts beam currents
materials can be obtained from tabulated range tables (7-11). to a limit of 4 to 5 µA. A beam-rastering system is the most
practical method for moving the beam across the sample
5.1.1.1 Toobtainauniformconcentrationofheliumthrough
surface to uniformly implant helium over large areas of the
the thickness of a sample, it is necessary to vary the energy of
specimen.
theincidentbeam,rockthesample (12),or,morecommonly,to
degrade the energy of the beam by interposing a thin sheet or
5.1.3.2 Beam Energy Degrader—The most efficient proce-
wedge of material ahead of the target. The range of monoen-
dure for implanting helium with an accelerator, because of the
ergetic particles is described by a Gaussian distribution around
time involved in changing the energy, is to operate the
the mean range. This range straggling provides a means of
accelerator at the maximum energy and to control the depth of
implanting uniform concentrations through the thickness of a
the helium implant by degrading the beam energy. This
specimen by superimposing the Gaussian profiles that result
procedureofferstheadditionaladvantagesthatrangestraggling
from beam energy degradation of different thicknesses of
increases with energy, thus producing a broader depth profile,
material.Theuniformityoftheimplantdependsonthenumber
and the angular divergence of the beam increases as a conse-
of superpositions. Charged particle beams have dimensions of
quence of the electronic energy loss process, thus increasing
theorderofafewmillimetressothatsomemeansoftranslating
the spot size and reducing the localized beam heating. The
the specimen in the beam or of rastering the beam across the
beam energy degrader requires that a known thickness of
specimenmustbeemployedtouniformlyimplantspecimensof
material be placed in front of the beam with provisions for
the size required for tensile or creep tests. The rate of helium
remotely changing the thickness and for removal of heat from
deposition is usually limited by the heat removal rate from the
the beam energy degrader. Acceptable methods include a
specimens and the limits on temperature rise for a given
rotatingsteppedorwedgedwheel,amovablewedge,orastack
experiment. Care must be exercised that phase transformations
of foils. Beam degrader materials can be beryllium, aluminum,
or annealing of microstructural components do not result from
or graphite. The wedge or rotating tapered wheel designs
beam heating.
provide a continuous change in energy deposition, so as to
5.1.2 Limitations—One of the major limitations of the
provide a uniform distribution of helium in the specimen but
technique is that the thickness of a specimen that can be
introduce the additional complexity of moving parts and
implanted with helium is limited to the range of the most
cooling of thick sections of material. The stacked foil designs
energetic alpha particle beam available (or twice the range if
are simpler, can be cooled adequately by an air jet, and have
the specimen is implanted from both sides). Thus a stainless
well calibrated thickness. The design must be selected on the
steel tensile specimen is limited to 1.2 mm thickness using a
basis of experiment purpose and facility flexibility. Concentra-
70-MeV beam to implant the specimen from both sides. This
tions of helium uniform to within 65% can be achieved by
limiting thickness is greater for light elements such as alumi-
superposition of the depth profiles produced by 25-µm incre-
num and less for heavier elements such as molybdenum.
ments in the thickness of aluminum beam degrader foils.
5.1.2.1 One of the primary reasons for interest in helium Uniformity of 610% is recommended for all material experi-
implantation is to simulate the effects resulting from the ments.Distributingheliumovermorelimiteddepthranges(as,
production of helium by transmutation reactions in nuclear for example, when it is only required to spread helium about
reactors. It should be appreciated that the property changes in thepeakregionofheavyiondamage,inspecimensthatwillbe
irradiated metals result from complex interactions between the examinedbytransmissionelectronmicroscopy)canbedoneby
helium atoms and the radiation damage produced during the cyclingtheenergyofthehelium-implantingaccelerator (15)in
irradiation in ways that are not fully understood. Energetic
place of degrader techniques.
alpha particles do produce atomic displacements, but in a
5.1.3.3 Specimen Holder—The essential features of the
manneratypicalofmostneutronirradiations.Thedisplacement
specimen holder are provisions for accurately placing the
rate is generally higher than that in fast reactor, but the ratio of
specimen in the beam and for cooling the specimens. Addi-
helium atoms to displaced atoms is some 10 times greater for
tional features may include systems for handling and irradiat-
implantation of stainless steel with a 50-MeV alpha beam.
inglargenumbersofspecimenstoimprovetheefficiencyofthe
5.1.3 Apparatus—Apparatusforheliumimplantationisusu-
facility and to avoid handling the specimens until the radioac-
ally custom designed and built at each research center and
tivity induced during the implantation has had an opportunity
therefore much variety exists in the approach to solving each
to decay. Some method of specimen cooling is essential since
problem. The general literature should be consulted for de-
adegraded,singlychargedbeamofaverageenergyof20MeV
tailed information (12-16). Paragraphs 5.1.3 – 5.1.3.4 provide
and current of 5 µA striking a 1-cm nickel target, 0.025 cm
commentsonthemajorcomponentsoftheheliumimplantation
thick, deposits 100 W of heat into a mass of 0.22 g.Assuming
apparatus.
onlyradiativeheatlosstothesurroundings,theresultingrisein
−1
5.1.3.1 Accelerator—Cyclotrons or other accelerators are temperature would occur at an initial rate of about 1300 K·s
used for helium implantation experiments because they are and would reach a value of about 2000 K.Techniques used for
well suited to accelerate light ions to the high potentials specimen cooling will depend on whether the implantation is
E942 − 96 (2011)
performed in air or in vacuum and on the physical character- area of small aperture to the total implant area is known. An
isticsofthespecimen.Conductivecoolingwitheitherairoran alternative is the use of a commercially available beam profile
inertgasmaybeusedifimplantsarenotperformedinvacuum. monitor.
Water cooling is a more effective method of heat removal and
5.1.4.2 The total charge deposited on the specimen by the
permits higher current densities to be used on thick tensile
incidentalphaparticlesmustbemeasured.Precautionsmustbe
specimens. The specimens may be bonded to a cooled support
taken to minimize leakage currents through the cooling water
block or may be in direct contact with the coolant. Care must
by the use of low conductivity water, to suppress collection of
beexercisedtoensurethatmetallurgicalreactionsdonotoccur
secondary electrons emitted from the target by a negatively
between the bonding material and the specimen as a conse-
biased aperture just ahead of the specimen, and to collect
quence of the beam heating, and that hot spots do not develop electronsknockedoutoftheexitsurfaceofthedegraderfoilby
as a consequence of debonding from thermal expansion of the
collecting them on a positively charged aperture placed down-
specimen. Silver conductive paint has been used successfully stream from the beam degrader.
as a bonding agent where the temperature rise is minimal.
5.1.4.3 Following irradiation the specimens and specimen
Aluminum is recommended in preference to copper for con-
holderwillhavehighlevelsofinducedactivityandprecautions
struction of the target holder because of the high levels of
mustbeexercisedinhandlingandstorageofthespecimensand
radioactivity induced in copper.
target holder. Most of this activity is shortlived and decays
5.1.3.4 Faraday Cup and Charge Integration System—A within a day. The induced activity can be used advantageously
Faraday cup should be used to measure the beam current to check the uniformity of the implant by standard autoradio-
graphic techniques.
delivered to the target. A600 mm long by 50 mm diameter
aluminumtubeclosedononeendmakesasatisfactoryFaraday
5.1.5 Calculation and Interpretation of Results—Theranges
cup. An electron suppressor aperture insulated from the Fara-
of energetic particles in solid media have been calculated
day cup and positively charged is necessary to collect the
(7-12) for a number of materials. The range increases with
electrons emitted from the degrader foils so as to give accurate
increasing energy and is affected by target parameters such as
beam current readings. Beam current density and beam profile electron density, atomic density, and atomic mass. Ranges are
−2
can be determined by reading the current passed by a series of
stated in units of mg·cm , which, when divided by the
−3
aperturesofcalibratedsizethatcanbeplacedinthebeam.The physical density of the target material, in g·cm gives a
targetholderassemblymustbeinsulatedfromitssurroundings,
distance in tens of µm. The total range is defined as the total
and deionized (low conductivity) water must be used for path length from the point of
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