ASTM E942-96(2003)
(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 -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 E 521.
1.3 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 2003)
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 E521 Practice for Neutron Radiation Damage Simulation
by Charged-Particle Irradiation
1.1 This guide provides advice for conducting experiments
E706 Master Matrix for Light-Water Reactor Pressure Ves-
to investigate the effects of helium on the properties of metals
sel Surveillance Standards, E 706(0)
where the technique for introducing the helium differs in some
E910 Test Method for Application and Analysis of Helium
way from the actual mechanism of introduction of helium in
Accumulation Fluence Monitors for Reactor Vessel Sur-
service. Simulation techniques considered for introducing he-
veillance, E706 (IIIC)
lium shall include charged particle implantation, exposure to
a-emitting radioisotopes, and tritium decay techniques. Proce-
3. Terminology
duresfortheanalysisofheliumcontentandheliumdistribution
3.1 DescriptionsofrelevanttermsarefoundinTerminology
within the specimen are also recommended.
C859 and Terminology E170.
1.2 Two other methods for introducing helium into irradi-
ated materials are not covered in this guide. They are the
4. Significance and Use
enhancement of helium production in nickel-bearing alloys by
4.1 Helium is introduced into metals as a consequence of
spectral tailoring in mixed-spectrum fission reactors, and
nuclear reactions, such as (n, a), or by the injection of helium
isotopic tailoring in both fast and mixed-spectrum fission
2 into metals from the plasma in fusion reactors. The character-
reactors. These techniques are described in Refs (1-5). Dual
izationoftheeffectofheliumonthepropertiesofmetalsusing
ion beam techniques (6) for simultaneously implanting helium
direct irradiation methods may be impractical because of the
and generating displacement damage are also not included
time required to perform the irradiation or the lack of a
here. This latter method is discussed in Practice E521.
radiation facility, as in the case of the fusion reactor. Simula-
1.3 This standard does not purport to address all of the
tion techniques can accelerate the research by identifying and
safety concerns, if any, associated with its use. It is the
isolating major effects caused by the presence of helium. The
responsibility of the user of this standard to establish appro-
word simulation is used here in a broad sense to imply an
priate safety and health practices and determine the applica-
approximation of the relevant irradiation environment. There
bility of regulatory limitations prior to use.
are many complex interactions between the helium produced
2. Referenced Documents during irradiation and other irradiation effects, so care must be
3 exercised to ensure that the effects being studied are a suitable
2.1 ASTM Standards:
approximation of the real effect. By way of illustration, details
C859 Terminology Relating to Nuclear Materials
of helium introduction, especially the implantation tempera-
E170 TerminologyRelatingtoRadiationMeasurementsand
ture, may determine the subsequent distribution of the helium
Dosimetry
(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 Applications and is the direct responsibility of Subcommittee
E10.08 on Procedures for Neutron Radiation Damage Simulation.
5.1 Implantation of Helium Using Charged Particle Accel-
Current edition approved July 10, 2003. Published July 2003. Originally
erators:
approved in 1983. Last previous edition approved in 1996 as E942–96. DOI:
5.1.1 Summary of Method—Charged particle accelerators
10.1520/E0942-96R03.
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
rapid, and relatively inexpensive means of introducing large
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
concentrations of helium into thin specimens. An energetic
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. alphaparticleimpingingonatargetlosesenergybyexcitingor
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E942–96 (2003)
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.
Uniformity of 610% is recommended for all material experi-
5.1.2.1 One of the primary reasons for interest in helium
implantation is to simulate the effects resulting from the ments.Distributingheliumovermorelimiteddepthranges(as,
for example, when it is only required to spread helium about
production of helium by transmutation reactions in nuclear
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—Apparatus for helium implantation is
facility and to avoid handling the specimens until the radioac-
usually 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
temperature would occur at an initial rate of about 1300 K·s
5.1.3.1 Accelerator—Cyclotrons or other accelerators are
used for helium implantation experiments because they are and would reach a value of about 2000 K.Techniques used for
specimen cooling will depend on whether the implantation is
well suited to accelerate light ions to the high potentials
required for implantation. Typical Cyclotron operating charac- performed in air or in vacuum and on the physical character-
teristics are 20 to 80 MeV with a beam current of 20 µAat the isticsofthespecimen.Conductivecoolingwitheitherairoran
source. It should be noted, however, that the usable beam inertgasmaybeusedifimplantsarenotperformedinvacuum.
E942–96 (2003)
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
secondary electrons emitted from the target by a negatively
beexercisedtoensurethatmetallurgicalreactionsdonotoccur
between the bonding material and the specimen as a conse- biased aperture just ahead of the specimen, and to collect
electronsknockedoutoftheexitsurfaceofthedegraderfoilby
quence of the beam heating, and that hot spots do not develop
as a consequence of debonding from thermal expansion of the collecting them on a positively charged aperture placed down-
stream from the beam degrader.
specimen. Silver conductive paint has been used successfully
5.1.4.3 Following irradiation the specimens and specimen
as a bonding agent where the temperature rise is minimal.
holderwillhavehighlevelsofinducedactivityandprecautions
Aluminum is recommended in preference to copper for con-
mustbeexercisedinhandlingandstorageofthespecimensand
struction of the target holder because of the high levels of
target holder. Most of this activity is shortlived and decays
radioactivity induced in copper.
within a day. The induced activity can be used advantageously
5.1.3.4 Faraday Cup and Charge Integration System—A
to check the uniformity of the implant by standard autoradio-
Faraday cup should be used to measure the beam current
graphic techniques.
delivered to the target. A600 mm long by 50 mm diameter
5.1.5 Calculation and Interpretation of Results—Theranges
aluminumtubeclosedononeendmakesasatisfactoryFaraday
of energetic particles in solid media have been calculated
cup. An electron suppressor aperture insulated from the Fara-
(7-12) for a number of materials. The range increases with
day cup and positively charged is necessary to collect the
increasing energy and is affected by target parameters such as
electrons emitted from the degrader foils so as to give accurate
electron density, atomic density, and atomic mass. Ranges are
beam current readings. Beam current density and beam profile
−2
stated in units of mg·cm , which, when divided by the
can be determined by reading the current passed by a series of
−3
physical density of the target material, in g·cm gives a
aperturesofcalibratedsizethatcanbeplacedinthebeam.The
distance in tens of µm. The total range is defined as the total
targetholderassemblymustbeinsulatedfromitssurroundings,
path length from the point of entry at the target surface to the
and deionized (low conductivity) water must be used for
point at which the particle comes to rest. The projected range
cooling purposes to permit an integration of current delivered
or penetration depth is defined as the projection of the total
to the target and thereby accurately measure the total helium
range along the normal to the entry face of the target, and is
implan
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