ASTM E942-23

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: E942 − 23
Standard Guide for
Investigating the Effects of Helium 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. 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 ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
1.1 This guide provides advice for conducting experiments
mendations issued by the World Trade Organization Technical
to investigate the effects of helium on the properties of metals
Barriers to Trade (TBT) Committee.
where the technique for introducing the helium differs in some
way from the actual mechanism of introduction of helium in
2. Referenced Documents
service. Techniques considered for introducing helium may
2.1 ASTM Standards:
include charged particle implantation, exposure to α-emitting
C859 Terminology Relating to Nuclear Materials
radioisotopes, and tritium decay techniques. Procedures for the
E170 Terminology Relating to Radiation Measurements and
analysis of helium content and helium distribution within the
Dosimetry
specimen are also recommended.
E521 Practice for Investigating the Effects of Neutron Ra-
1.2 Three other methods for introducing helium into irradi-
diation Damage Using Charged-Particle Irradiation
ated materials are not covered in this guide. They are: (1) the
E910 Test Method for Application and Analysis of Helium
enhancement of helium production in nickel-bearing alloys by
Accumulation Fluence Monitors for Reactor Vessel Sur-
spectral tailoring in mixed-spectrum fission reactors, (2) a
veillance
related technique that uses a thin layer of NiAl on the specimen
3. Terminology
surface to inject helium, and (3) isotopic tailoring in both fast
and mixed-spectrum fission reactors. These techniques are
3.1 Descriptions of relevant terms are found in Terminology
described in Refs (1-6). Dual ion beam techniques (7) for
C859 and Terminology E170.
simultaneously implanting helium and generating displace-
4. Significance and Use
ment damage are also not included here. This latter method is
discussed in Practice E521.
4.1 Helium is introduced into metals as a consequence of
nuclear reactions, such as (n, α), or by the injection of helium
1.3 In addition to helium, hydrogen is also produced in
into metals from the plasma in fusion reactors. The character-
many materials by nuclear transmutation. In some cases it
ization of the effect of helium on the properties of metals using
appears to act synergistically with helium (8-10). The specific
direct irradiation methods may be impractical because of the
impact of hydrogen is not addressed in this guide.
time required to perform the irradiation or the lack of a
1.4 The values stated in SI units are to be regarded as
radiation facility, as in the case of the fusion reactor. Simula-
standard. No other units of measurement are included in this
tion techniques can accelerate the research by identifying and
standard.
isolating major effects caused by the presence of helium. The
1.5 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, health, and environmental practices and deter-
during irradiation and other irradiation effects, so care must be
mine the applicability of regulatory limitations prior to use.
exercised to ensure that the effects being studied are a suitable
1.6 This international standard was developed in accor-
approximation of the real effect. By way of illustration, details
dance with internationally recognized principles on standard-
of helium introduction, especially the implantation
temperature, may determine the subsequent distribution of the
This guide is under the jurisdiction of ASTM Committee E10 on Nuclear helium (that is, dispersed atomistically, in small clusters in
Technology and Applications and is the direct responsibility of Subcommittee
bubbles, etc.).
E10.05 on Nuclear Radiation Metrology.
Current edition approved June 1, 2023. Published July 2023. Originally approved
in 1983. Last previous edition approved in 2016 as E942 – 16. DOI: 10.1520/ For referenced ASTM standards, visit the ASTM website, www.astm.org, or
E0942-23. contact ASTM Customer Service at service@astm.org. 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 guide. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E942 − 23
5. Techniques for Introducing Helium therefore much variety exists in the approach to solving each
problem. The general literature should be consulted for de-
5.1 Implantation of Helium Using Charged Particle Accel-
tailed information (16-20). Paragraphs 5.1.3 – 5.1.3.4 provide
erators:
comments on the major components of the helium implantation
5.1.1 Summary of Method—Charged particle accelerators
apparatus.
are designed to deliver well defined, intense beams of mono-
5.1.3.1 Accelerator—Cyclotrons or other accelerators are
energetic particles on a target. They thus provide a convenient,
used for helium implantation experiments because they are
rapid, and relatively inexpensive means of introducing large
well suited to accelerate light ions to the high potentials
concentrations of helium into thin specimens. An energetic
required for implantation. Typical cyclotron operating charac-
alpha particle impinging on a target loses energy by exciting or
teristics are 20 to 80 MeV with a beam current of 20 μA at the
ionizing the target atoms, or both, and by inelastic collisions
source. It should be noted, however, that the usable beam
with the target atom nuclei. Particle ranges for a variety of
current delivered to the specimen is limited by the ability to
materials can be obtained from tabulated range tables (10-14)
remove heat from the specimens which restricts beam currents
or calculated using a Monte Carlo code such as SRIM (15).
to a limit of 4 to 5 μA. A beam-rastering system is the most
5.1.1.1 To obtain a uniform concentration of helium through
practical method for moving the beam across the sample
the thickness of a sample, it is necessary to vary the energy of
surface to uniformly implant helium over large areas of the
the incident beam, rock the sample (6), or, more commonly, to
specimen.
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
from beam energy degradation of different thicknesses of procedure offers the additional advantages that range straggling
material. The uniformity of the implant depends on the number increases with energy, thus producing a broader depth profile,
of superpositions. Charged particle beams have dimensions of and the angular divergence of the beam increases as a conse-
the order of a few millimetres so that some means of translating quence of the electronic energy loss process, thus increasing
the specimen in the beam or of rastering the beam across the the spot size and reducing the localized beam heating nonuni-
specimen must be employed to uniformly implant specimens of formly. The beam energy degrader requires that a known
the size required for tensile or creep tests. The rate of helium thickness of material be placed in front of the beam with
deposition is usually limited by the heat removal rate from the provisions for remotely changing the thickness and for removal
specimens and the limits on temperature rise for a given
of heat from the beam energy degrader. Acceptable methods
experiment. Care must be exercised that phase transformations include a rotating stepped or wedged wheel, a movable wedge,
or annealing of microstructural components do not result from
or a stack of foils. Beam degrader materials can be beryllium,
beam heating. aluminum, or graphite. The wedge or rotating tapered wheel
5.1.2 Limitations—One of the major limitations of the designs provide a continuous change in energy deposition, so
as to provide a uniform distribution of helium in the specimen
technique is that the thickness of a specimen that can be
implanted with helium is limited to the range of the most but introduce the additional complexity of moving parts and
energetic alpha particle beam available (or twice the range if cooling of thick sections of material. The stacked foil designs
the specimen is implanted from both sides). Thus a stainless are simpler, can be cooled adequately by an air jet, and have
steel tensile specimen is limited to 1.2 mm thickness using a well-calibrated thickness. The design must be selected on the
70-MeV beam to implant the specimen from both sides. This basis of experiment purpose and facility flexibility. Concentra-
limiting thickness is greater for light elements such as alumi-
tions of helium uniform to within 65 % can be achieved by
num and less for heavier elements such as molybdenum. superposition of the depth profiles produced by 25-μm incre-
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 investigate the effects resulting from the
ments for both concentration and spatial uniformity including
production of helium by transmutation reactions in nuclear
any increase in spot size from angular divergence. Distributing
reactors. It should be appreciated that the property changes in
helium over more limited depth ranges (as, for example, when
irradiated metals result from complex interactions between the
it is only required to spread helium about the peak region of
helium atoms and the radiation damage produced during the
heavy ion damage, in specimens that will be examined by
irradiation in ways that are not fully understood. Implantation
transmission electron microscopy) can be done by cycling the
of energetic alpha particles does produce atomic
energy of the helium-implanting accelerator (19) in place of
displacements, but in a manner atypical of most neutron
degrader techniques.
irradiations. The displacement rate is generally higher than that
in fast reactor, but the ratio of helium atoms to displaced atoms
5.1.3.3 Specimen Holder—The essential features of the
is some 10 times greater for implantation of stainless steel
specimen holder are provisions for accurately placing the
with a 50-MeV alpha beam.
specimen in the beam and for cooling the specimens. Addi-
5.1.3 Apparatus—Apparatus for helium implantation is usu- tional features may include systems for handling and irradiat-
ally custom designed and built at each research center and ing large numbers of specimens to improve the efficiency of the
E942 − 23
facility and to avoid handling the specimens until the radioac- of the mean energy of the emergent particles from the degrader
tivity induced during the implantation has had an opportunity using a detector placed directly in the beam line behind the
degrader.
to decay. Some method of specimen cooling is essential since
5.1.4.1 The uniformity of the fluence rate on the surface of
a degraded, singly charged beam of average energy of 20 MeV
the specimen must be determined for the implant conditions
and current of 5 μA striking a 1-cm nickel target, 0.025 cm
and for each degrader thickness. This is easily done prior to
thick, deposits 100 W of heat into a mass of 0.22 g. Assuming
implantation using a small-diameter aperture that can be
only radiative heat loss to the surroundings, the resulting rise in
−1 moved into the centerline of the particle beam to compare the
temperature would occur at an initial rate of about 1300 K·s
fluence rate on the axis to the average fluence rate on the
and would reach a value of about 2000 K. Techniques used for
specimen. The Faraday cup is placed behind this small aperture
specimen cooling will depend on whether the implantation is
to measure the current, and the ratio of peak current density on
performed in air or in vacuum and on the physical character-
the specimen to the average current density can then be
istics of the specimen. Conductive cooling with either air or an
determined for each degrader thickness since the ratio of the
inert gas may be used if implants are not performed in vacuum.
area of small aperture to the total implant area is known. An
Water cooling is a more effective method of heat removal and
alternative is the use of a commercially available beam profile
permits higher current densities to be used on thick tensile
monitor.
specimens. The specimens may be bonded to a cooled support
5.1.4.2 The total charge deposited on the specimen by the
block or may be in direct contact with the coolant. Care must
incident alpha particles must be measured. Precautions must be
be exercised to ensure that metallurgical reactions do not occur
taken to minimize leakage currents through the cooling water
between the bonding material and the specimen as a conse-
by the use of low conductivity water, to suppress collection of
quence of the beam heating, and that hot spots do not develop
secondary electrons emitted from the target by a negatively
as a consequence of debonding from thermal expansion of the
biased aperture just ahead of the specimen, and to collect
specimen. Silver conductive paint has been used successfully
electrons knocked out of the exit surface of the degrader foil by
as a bonding agent where the temperature rise is minimal.
collecting them on a positively charged aperture placed down-
Aluminum is recommended in preference to copper for con- stream from the beam degrader.
struction of the target holder because of the high levels of 5.1.4.3 Following irradiation the specimens and specimen
radioactivity induced in copper. holder will have high levels of induced activity and precautions
must be exercised in handling and storage of the specimens and
5.1.3.4 Faraday Cup and Charge Integration System—A
target holder. Most of this activity is short-lived and decays
Faraday cup should be used to measure the beam current
within a day. The induced activity can be used advantageously
delivered to the target. A 600 mm long by 50 mm diameter
to check the uniformity of the implant by standard autoradio-
aluminum tube closed on one end makes a satisfactory Faraday
graphic techniques.
cup. An electron suppressor aperture insulated from the Fara-
5.1.5 Calculation and Interpretation of Results—The ranges
day cup and positively charged is necessary to collect the
of energetic particles in solid media have been calculated
electrons emitted from the degrader foils so as to give accurate
(10-15) for a number of materials. The range increases with
beam current readings. Beam current density and beam profile
increasing energy and is affected by target parameters such as
can be determined by reading the current passed by a series of
electron density, atomic density, and atomic mass. Ranges are
apertures of calibrated size that can be placed in the beam. The
−2
stated in units of mg·cm , which, when divided by the
target holder assembly must be insulated from its surroundings, −3
physical density of the target material, in g·cm gives a
and deionized (low conductivity) water must be used for
distance in tens of μm. The total range is defined as the total
cooling purposes to permit an integration of current delivered
path length from the point of entry at the target surface to the
to the target and thereby accurately measure the total helium
point at which the particle comes to rest. The projected range
implanted independent of fluctuations in the beam current. A
or penetration depth is defined as the projection of the total
negatively biased aperture must be placed between the target
range along the normal to the entry face of the target, and is
holder and the degrader foils to suppress secondary electrons
therefore a sensitive function of the angle of incidence of the α
emitted from the target that would give erroneously high values
particle at the target surface. The concentration of helium in
of total charge deposited on the specimen.
parts per million is defined as the ratio of the number density
of helium nuclei to the number density of host material times
5.1.4 Procedure—Prior to the actual implantation of helium
10 :
in a specimen, certain standardization and calibration proce-
dures should be performed. The temperature rise to be expected
C 5 M /M × 10 (1)
~ !
ppm He H
from beam heating and the intended specimen cooling mode
M 5 N ρ /A (2)
H 0 H H
must be measured. Such measurements can be performed on
where:
dummy specimens using a thermocouple embedded in the
N = Avogadro’s number,
sample behind the beam spot or with an infrared pyrometer
A = gram molecular weight of host material, and
capable of reading the surface temperature of an area the size H
−3
ρ = its density, g·cm .
H
of the beam spot. The thickness of the beam energy degrader
must be accurately measured to determine the depth of the
5.1.5.1 The quantity M (helium density) is a function of
He
helium implant. This can be determined from a measurement the range as given by the range-straggling formula. This
E942 − 23
expression has been normalized to a unit particle fluence rate months. The distribution of helium in the foil is controlled by
since the total area under a normal distribution curve is equal the energy of the particle and the extent of shielding by the
to σ2π. If N is the total number of particles incident on the source material, and therefore is nonuniform. The source
T
surface per unit area (fluence) then: geometry is a thin sheet that conforms to the surface of the
material to be implanted. The sources represent a potential
¯
N ~R 2 R!
T
health and contamination hazard, and therefore require han-
M 5 exp2 (3)
He
2σ
=
σ 2π
dling in a glovebox facility with suitable shielding. The
technique offers an inexpensive, simple method for implanting
The peak number density which occurs at the mean range
helium if surface implantation with a nonuniform profile is
¯
(R = R) is:
acceptable.
=
M 5 N /σ 2π (4) 5.2.2 Limitation—The major limitation of the technique is
He T
the depth to which helium can be implanted. The α-particles
therefore:
from usable sources have energies between 4 and 8 MeV and
C 5 N A /~σ=2π N ρ !·10 (5) for a 6-MeV α-particle, the maximum penetration depth is
ppm T H 0 H
about 30 μm in aluminum, about 12 μm in nickel, and about
Or solving for N will give the total number of alpha
T
20 μm in zirconium. The helium concentration profile will be
particles required to obtain a peak concentration of C :
ppm
nonuniform, varying from 0 helium just beyond the maximum
range of the α-particles at normal incidence to some maximum
N 5 C N ρ σ=2π/A ·10 (6)
T ppm 0 H H
value. Thickness of the source will affect the concentration
−19
Since the alpha particle carries a charge of 3.2 × 10
profile if it is less than the self-absorption thickness.
coulombs, the total charge in coulombs delivered to the
5.2.3 Apparatus:
specimen per unit area is:
5.2.3.1 Source—Practical alpha sources are those unstable
225 isotopes that decay and will give a target helium concentration
=
Q 5 3.2 × 10 C N ρ σ 2π/A (7)
ppm 0 H H
of the order of 10 to 100 appm in a period of one to two
5.1.5.2 A uniform helium depth profile can be approximated
months. Table 1 provides the recommended nuclear data for
by injecting a sequence of helium layers whose mean range
most of the practical sources that are recommended for use in
differs by the full-width-half-maximum of the range straggling
this application (24).
distribution (FWHM = 2.35 σ). Under these conditions, the
5.2.3.2 Of these, Pu represents the upper limit of half-life
midpoint concentration will be equal to the peak concentration,
consistent with reasonable implantation time, and Cm rep-
whereas the summed peak concentration will be increased by
resents a lower limit of half-life below which consumption of
12 %. This increase is due to a 6 % contribution from the tail
the source may be undesirable. Some of these isotopes are also
of each of the adjacent peaks.
subject to spontaneous fission, creating neutrons and fission
5.1.6 Report—Information to be reported for helium im-
products, and some are sources of high [gamma] activity. All α
plantation experiments should include the estimated helium
sources are potential health hazards due to the toxic nature of
concentration and its distribution in the material, the energy of
ingested particles. Safety requirements dictate that these
the alpha particles employed, method for degrading the energy,
sources be handled in a glovebox, and some may require
beam current on the target, temperature rise, and total charge
special licensing similar to that for handling of Pu. Metallic α
implanted.
sources are extremely reactive with oxygen and with most
5.2 Implantation of Helium Using α-Emitting Radioiso-
topes:
A less flexible variant of this method is the examination of a microstructure in
5.2.1 Summary of Method—The emission of α-particles
238 244 208 242
the helium “halos” generated around any naturally occurring boron-containing
during the radioactive decay of Pu, Cm, Po, and Cm
particles in metals (21). Boron has been deliberately introduced (22, 23), but this can
can be used to implant helium concentrations of 10 to 100
introduce chemical alterations of the matrix or other alloy phases. These variants
appm in the surface layer of specimens in periods of one to two also entail studying the effects of lithium on microstructural development (22).
A
TABLE 1 Recommended Nuclear Data for Relevant Alpha Sources
Spontaneous
Alpha (α) Emissions Gamma (γ) Transitions
Source Half-Life Fission Branching
Energy (MeV) Probability (%) Energy (MeV) Probability (%)
Ratio (%)
Pu 87.74 (3) a 5.49903 (20) 71.04 (6) 0.099852 (3) 0.00735 (8) 1.85 (5) E-7
5.4563 (2) 28.85 (6) 0.043498 (1) 0.0397 (8)
Cm 18.11 (3) a 5.80477 (5) 76.7 (4) 0.042824 (8) 0.0258 (7) 1.36 (1) E-4
5.76265 (5) 23.3 (4) 0.098860 (13) 0.00136 (9)
Po 2.898 (2) a 5.1149 (14) 99.9958 (4) 0.29181 (5) 0.00227 —
4.220 (15) 0.00024 (7) 0.57013 (7) 0.00138 (17)
Cm 162.86 (8) d 6.11272 (8) 74.06 (7) 0.04408 (3) 0.0330 (7) 6.36 (14) E-6
6.06937 (9) 25.94 (7) 0.10192 (4) 0.00251 (14)
A 208
Nuclear data from Ref (24), except Po that is taken from Ref (25).
E942 − 23
3 3
other elements, so their use in metallic form requires some active decay of tritium by the reaction T → He 1 β half
~
1 2 1
form of protective atmosphere or a cladding envelope. The 2life of 12.34 years!, and then heating the specimen in vacuum to
source strength is reduced if cladding is used to protect the remove the remaining tritium. The method offers the advantage
surface. The reactivity of the metals used for sources also limits of introducing helium into bulk specimens and into specimens
their use to temperatures below 500 °C. In the form of oxides with unusual contours.
they are more stable and can be used unshielded and at higher
5.3.2 Limitations—The distribution of helium in a specimen
temperatures. However, it is recommended that even oxide
may be influenced by segregation or trapping of the tritium at
sources should be clad or confined to minimize contamination
internal sinks or by the formation of tritides. The use of this
of targets by spallation and to reduce health hazards.
technique must be accompanied by characterization of the
5.2.4 Procedure—An example of the use of α sources for
sample to ensure that a homogeneous distribution of helium
244 244
implantation is given in Ref (26). A source of CmO +
has been achieved. An inherent characteristic of the technique
Cm O was evaporated on a 25.4-mm diameter titanium disk
2 3
for simulating the effects of transmutation-produced helium in
substrate to a thickness of 3 to 4 mg/cm . The target was placed
neutron-irradiated specimens is the absence of radiation dam-
in a recessed aluminum holder covered with a 5-μm thick
age. The mobility of helium may change under irradiation
aluminum cover foil to minimize contamination from the
because of changes in the diffusion mechanism when a
source. All operations were performed in a glovebox. A
steady-state concentration of interstitials and vacancies is
stainless steel spacer ring 25.4 mm in diameter and 1.5 mm
present in the material during irradiation. The ratio of helium to
thick was placed on top of the cover foil, and the source laid
dpa also may influence swelling and mechanical properties.
face down over the ring for the required implantation time. The
The tritium decay method will not duplicate these effects and
ring holds the source away from the aluminum foil, preventing
therefore should not be used in circumstances requiring both
scratches and reaction products from damaging the source.
helium and displacement damage. It might, however, be
5.2.4.1 The use of a source whose thickness is less than the
considered an advantage in separating the effects due to helium
range of α-particles in source material makes possible a
from those of the associated displacement damage. Tritium is a
tailored profile in the target: a plateau preceding a linear
radiological safety hazard, and suitable facilities for handling
decline. The depth of this plateau, at acceptable helium levels,
tritium must be available.
is not likely to exceed half the maximum penetration depth. In
5.3.3 Apparatus—Depending on the method applied, the
the example cited in 5.2.4 (26), a zone 3.5 μm deep below the
tritium charging system must be capable of evacuation to at
surface of a nickel target attained a uniform ;10 atomic ppm
−3
least 10 Pa and capable of containing tritium at overpressures
helium concentration after three days of exposure.
of a few tens of Pa. Elevated temperature capability to at least
Alternatively, two-sided implantation of specimen foils thinner
500 °C is required for the charging system and higher if
than the maximum penetration depth can be used (27). The
outgassing is done in the same system. If the radioactive decay
configuration selected for implantation should be consistent
stage is done at elevated temperatures, a temperature controller
with the intended simulation (peaked distribution or uniform
with a stability of 65 °C for periods of a month also will be
concentration).
required. Provision for measuring the tritium pressure over the
5.2.5 Calculation and Interpretation of Results—The range
specimens with sufficient accuracy to determine changes in
of the α particles should be calculated from range tables using
pressure during the charging stage is required. Outgassing of
the procedures described in 5.1 for implantation using charged
the specimens following the decay period is required and may
particle accelerators. Calculations of the rate of implantation of
be done in either the charging system or another system with
helium into a target and its final concentration must take into
high-vacuum and high-temperature capabilities.
consideration the amount of α-emitter in the source, the age of
5.3.4 Procedure—Several procedures have been used to
the isotope, source thickness, contamination from other
introduce helium into specimens by tritium decay; three will be
α-emitters, source density, and the range of α particles within
mentioned here. The methods typically involve charging the
the source. Some of these factors can be determined by
specimens with tritium at elevated temperature and a final
chemical analyses, by precision weighing, and by radiation
outgassing step, but differ in details such as the level of tritium
counting. It is recommended that the source be calibrated by
overpressure and whether the tritium decay step is carried out
implantation of a stack of 1 μm thick foils, analysis of the
at elevated temperature under a tritium pressure or whether it
helium content of the individual foils, and then fitting the
is done at room temperature with no tritium overpressure.
concentration profile to the calculated source characteristics.
Similar levels of helium content can be obtained with each
5.2.6 Reporting of Results—Information to be reported
method and in the absence of any obvious factor that would
should include the estimated helium concentration, α source
indicate a preference for one technique over the other, any of
characteristics such as isotope, activity, chemical species,
the methods may be acceptable for tritium (helium) charging.
physical dimensions, cladding, source calibration method, time
5.3.4.1 Method A (28)—The first step in the process in-
of implantation, and the basic assumptions used to calculate the
volves diffusion of tritium into specimen. The specimen is
helium concentration.
placed in a glass vacuum system that is subsequently evacuated
−3
5.3 Tritium Decay Charging:
to less than 10 Pa and is then pressurized with tritium to a
5.3.1 Summary of Method—Helium is introduced into the pressure of 1.5 to 2.0 kPa by heating a uranium tritide bed. The
metal specimen by diffusing tritium into the specimen, accu- section of the system containing the specimen is heated to
mulating the desired concentration of helium from the radio- 475 °C. The tritium pressure change in the system is monitored
E942 − 23
to determine when tritium absorption in the specimen is welding. Both studies require samples that have been exposed
essentially complete. This step usually takes from 2 to 3 h and to tritium gas at high pressures, up to 34 MPa, and tempera-
tures up to 350 °C for two to three weeks. The temperature of
the furnace is then cooled to room temperature. The pressure of
the remaining tritium is measured at room temperature and 350 °C is high enough for tritium to diffuse into ~6-mm thick
compared with the original pressure to determine the amount of sections and obtain a uniform concentration but low enough to
prevent significant changes to the preexisting microstructure.
tritium absorbed by the specimen. This room temperature
pressure is essentially the same as the final high-temperature Tritium diffusion calculations (30) are used to estimate the
amount of dissolved tritium. Helium concentrations in the
pressure. Therefore, it is possible to charge a specific tritium
range of ~1 to 20 appm are used for studies of helium effects
concentration into a given sample by monitoring the pressure
during absorption. The excess tritium remaining in the glass on welding. The tritium gas pressure is chosen based on the
amount of dissolved tritium and decay helium that is required.
system is reabsorbed and stored on the uranium tritide bed. The
For most weld studies, the tritium is off gassed at 350 °C after
second step involves aging of the specimen to allow time for
the desired amount of helium has obtained from tritium decay.
transmutation of the tritium to helium. In Method A, the aging
Following tritium exposure, samples are cooled and may be
step is carried out at room temperature. The tritium decay time
stored in air at for long periods of time (years) at –50 °C. This
is determined from the final helium concentration desired in a
temperature is low enough prevent tritium diffusion while the
given specimen, the tritium concentration charged into the
helium decay product can accumulate in the microstructure.
specimen, and the tritium half-life (12.34 years). A typical
Samples can be dissolved in an acid and tritium content
initial tritium content of 95 000 appm yields a charging rate of
measured, and the helium content is typically measured by
75-appm helium per month. The final step is removal of the
vacuum extraction measurements such as those described in
tritium from the specimen. The specimen is placed in the
6.1.
original glass vacuum system, which again is evacuated to less
−3
than 10 Pa and heated to tritium outgassing temperatures of 5.3.5 Calculations or Interpretation of Results:
875 to 925 °C. The evolved tritium is pumped into a calibrated
5.3.5.1 Computation of Helium Content—The helium con-
volume chamber and pressure measurements are taken to
tent of a tritium charged specimen is estimated from the tritium
dn
determine the amount of tritium recovered. Typical pressure-
half-life using the radioactive decay equation − ⁄dt = λn in the
+
volume measurements show recovery of 96 to 99 % of the
following form:
tritium calculated to be in the samples at the end of the aging
@He # 5 @T # 1 2 exp 2λt (8)
~ !
appm appm
t i
period. The specimen is cooled to room temperature and the
where:
outgassed tritium is reabsorbed on the uranium tritide bed.
5.3.4.2 Method B (29)—The first step in the process again
t = decay time,
[He ] = He content at decay time t in atomic parts per
involves diffusion of tritium into the specimen. The specimen
appm t
is weighed and placed in the charging vessel, the system is million, appm,
[T ] = initial T concentration, appm, and
evacuated to 4 Pa, and heated to 400 °C. A known volume of
appm i
λ = decay rate constant = 0.693 ⁄t ⁄2 ,
tritium is metered into the charging vessel sufficient for that to
be absorbed in the specimen and an equilibrium pressure of
where:
1.33 kPa in the chamber. The charging vessel is valved off, and
t ⁄2 = half-life.
the temperature is maintained at 400 °C. The aging step in
For tritium, t ⁄2 = 12.34 years. The initial tritium content is
Method B is carried out at temperature and under the pressure
either calculated from the experimentally determined tritium
of 1.33 kPa. The time at temperature is determined by the final
uptake during the tritium charging cycle (Method A), or it is
helium concentration desired in the specimens. The tritium is
assumed to be the equilibrium concentration determined from
removed from the specimen by evacuating the system for one
the metal-hydrogen phase diagram at the given tritium charg-
week at 4 Pa. The temperature is held at 400 °C. The charging
ing temperature and pressure (Method B). Calculation of the
vessel is cooled and the specimens are placed in a high-vacuum
helium concentration in a specimen assumes a constant volume
system. The specimens are heated to 550 °C in a vacuum of
−4
tritium charging apparatus and a single, initial tritium gas
about 1.33 × 10 Pa and outgassed for another week. The
charge. The calculation for determining the helium content of
charging vessel is cooled and a small sample (about 0.05 g) is
a specimen after a given number of charging days is given as
removed from the specimen. The sample is dissolved in acid,
follows:
and an analysis for tritium is made. If the tritium level is above
0.3 to 1.0 C /g, the outgassing is repeated until these levels are
@He # 5 @T # 1 2 exp 21.547 × 10 t (9)
i ~ !
appm appm
t i
achieved.
for decay time t measured in days where the moles of tritium
5.3.4.3 Method C (30, 31)—This method has been em-
(as T ) absorbed into the metal specimen are equal to twice the
ployed at the Savannah River National Laboratory for two
moles of tritium gas (as T ) absorbed by the specimen,
kinds of studies on stainless steels and other alloys. The first
determined experimentally by the pressure drop in the constant
kind of study involves measuring the effect of tritium and its
volume charging system. The equations used to calculate the
decay product, helium, on the mechanical and fracture tough-
amount of tritium absorbed in atom parts per million are given
ness properties of the alloy, while the second is for measuring
as follows:
the effects of only the helium decay product on the cracking
properties of the steel at elevated temperature or during T 5 n /W /M × 10 ppm (10)
@ # ~ !
appm T s m
i
E942 − 23
n 5 n M /M (11) 5.4.1 Summary of Method—The specimen size limitations
~ !
T T T T
2 2
inherent in the alpha particle implantation methods described
n 5 ~~ΔP!V/RT! (12)
T
2 V,T
in 5.1 and 5.2 can be bypassed by implanting metal powders
therefore: with a low energy alpha beam and then fabricating specimens
from the powder using powder metallurgy techniques. The
M /M
ΔP V
~ ! T T
@T # 5 H J × 10 ppm (13)
F G method falls conceptually into three steps: (1) ion implantation,
appm i
RT W /M
s m
(2) consolidation, and (3) thermomechanical processing. In the
where:
first step, helium is implanted in the individual particles of
n = number of moles tritium gas absorbed by the
metal powder by ion bombardment. The second step involves
T
specimen, fabricating a bulk solid from the helium-containing powder.
n = number of moles T absorbed by the specimen,
T The third step is intended principally to control the microstruc-
ΔP = experimentally observed pressure drop during tritium
ture of the product and the distribution of helium within it.
charging,
5.4.2 Limitations—The technique is limited by the availabil-
V = charging system volume,
ity of powders in fine sizes and the degree to which the
T = temperature of P measurement,
properties of the powder metallurgy product represent those
R = gas law constant,
fabricated by conventional techniques.
M = molecular weight of tritium gas,
T
M = molecular weight of tritium, 5.4.2.1 The displacement damage produced by the low
T
M = molecular weight of specimen matrix, and
energy implant may not be representative of damage produced
m
W = weight of specimen.
by neutrons and, as with other alpha implant techniques,
s
caution should be exercised in interpreting or extrapolating
5.3.5.2 The Method B technique for charging a specimen
results where both helium content and displacement damage
with helium using the tritium decay method is based on a
influence the effects to be simulated. Consolidation treatments
loading rate of 50 appm per week. The tritium required in the
require high pressures and temperatures near 0.5 T , which
charging process is calculated below for an example using m
may result in formation of small helium bubbles.
niobium. The moles He required per gram of niobium are:
5.4.3 Apparatus—The apparatus required for this technique
26 3
50 × 10 1 mols He
~ !
× 5 5.382 × 10 (14)
includes a linear accelerator capable of accelerating alpha
wk 92.9 g~Nb!/mol g~Nb! wk
particles to energies of 150 keV, electrostatic deflection plates,
The helium generation rate based on a half-life of 12.34
a target chamber, and sample cup capable of rotation to mix the
years is:
powders. Final consolidation requires a hot isostatic press.
23 3
1.097 × 10 atoms He /atom T /wk
~ ! ~ ! 5.4.4 Procedure—The procedure involves ion implantation
of the powder, consolidation, and thermomechanical process-
The number of cm of tritium at STP required per gram of
ing. Examples of the utilization of this technique for implant-
niobium are:
ing AISI Type 316 stainless steel and molybdenum are pro-
27 3 3
5.382 × 10 mols He /g Nb /wk × 22428 cm T /mol T
@~ ~ ! ~ ! ! ~ ~ ! ~ !!#
2 vided in Refs (32) and (33). The powder particles used for the
23 3
@ 1.097 × 10 atoms He /atom T /wk
~ ~ ! ~ ! !
implant should have diameters approximately two times the
3 3
× ~2N atoms~T!/mol~T !/N atoms~ He!/mol~ He!!# range of the available helium ions. The particle size distribu-
A 2 A
tion should be determined by X-ray sedimentation analysis
5.50 cm T
5 (15)
using a dilute water suspension of the powders. The suspen-
g Nb
~ !
sions should be ultrasonically dispersed for 30 min prior to
Parameters of the charging system are:
analysis. Separated fines with a mean particle diameter twice
system volume = 128 cm ,
the ion range and with 90 % of the particles having diameters
charging vessel volume = 155 cm ,
less than four times the ion range can be obtained by this
gas fill = 94 % T ,
method. This provides a reasonably uniform distribution of
charging temperature = 673 °K, and
implanted helium atoms because most of the volume of a
spherical particle lies close to its surface.
equilibrium gas pressure = 10 mm.
The total cm of gas required for specimens weighing a total
5.4.4.1 Fine particle size powders are characterized by high
of 40 g are:
chemical activity and tend to absorb relatively large amounts of
oxygen when exposed to air. If not removed, this oxide “skin”
40 g Nb 5.5 cm T
~ ! ~ !
S D
forms an oxide grain boundary phase when the powder is
g Nb 1
~ !
5 235 cm T STP (16)
~ !
3 2 pressed. To reduce the oxygen level, the powder is heat treated
10 mm × 273 K × 155 cm 0.94
3 4
S D
for 8 h at temperatures high enough to react with the oxide
760 mm × 673 K
under slowly flowing dry hydrogen (dew point −60 °C). The
5.3.6 Report—Information to be reported should include the
effluent gas should be monitored for moisture content until the
estimated helium concentration, residual tritium concentration,
moisture level has dropped to the initial level of the source gas.
and pertinent details of the charging sequence.
5.4.4.2 After the hydrogen treatment, the powder in the
closed reaction vessel is transferred to an inert gas glovebox
5.4 Introduction of Helium by Ion Implantation and Hot
Isostatic Pressing of Metal Powders: without exposure to air. The powder is then loaded into the
E942 − 23
implantation cup and transferred to the accelerator, again 6.1.2 Limitations—The main factors that determine the
without exposure to air. detection limits of the method are the background helium level
5.4.4.3 Helium ions are accelerated to 150 keV with a linear from desorption of helium from the mass spectrometer system
accelerator. The desired high-energy species are selected with walls when the crucibles are heated, and permeation of helium
a magnetic mass analyzer. The resulting ion beam is electro- through the walls, joints, and valves of the system. Helium
8 4
statically steered in the vertical and horizontal planes to pass contents of ;1 to 10 × 10 atoms of He have been measured,
through an aperture and electrostatically steered to impinge on which translates to a detection limit of from ;1 to 10 ppt (parts
−12
the target. per trillion, ;1 to 10 × 10 atom fraction), depending on the
5.4.4.4 The powder, lying in the corner of the inclined cup, sample mass.
tumbles and mixes as the cup rotates, thereby exposing all
6.1.3 Apparatus—The procedures described herein were
powder particles to the ion beam. Scattered ions and ions
performed on a custom-built apparatus (34) consisting of a gas
passing through the outer layers of particles provide lower
mass spectrometer, high-vacuum system, high-temperature
energy helium to distribute throughout the powder particle
furnace, and calibrated volume spike system. The mass spec-
volume.
trometer has an all-metal tube with interior volume of approxi-
5.4.4.5 After helium implantation, the powder is sieved
mately 1000 cm , an electron bombardment ion source, a
(down to −400 mesh) to break up aggregates. Analysis for
permanent magnet, and an electron multiplier ion source. The
residual and added gases is done by vacuum fusion extraction
application of the system is discussed in Test Method E910.
of the gases and mass spectrographic analysis. Duplicate
6.1.3.1 A vacuum system capable of maintaining the mass
−7
powder samples are wrapped in platinum (which acts as a
spectrometer pressure as low as 10 Pa between analyses is
fluxing agent) and heated by induction in graphite crucibles.
required. The mass spectrometer must be capable of being
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