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). Interest in spallation sources has increased recently due to 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 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 simulations 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 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
Historical
Publication Date
31-Jul-2009
Current Stage
Ref Project

Relations

Buy Standard

Standard
ASTM E798-96(2009) - Standard Practice for Conducting Irradiations at Accelerator-Based Neutron Sources
English language
13 pages
sale 15% off
Preview
sale 15% off
Preview

Standards Content (Sample)


NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E798 − 96 (Reapproved 2009)
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 responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
1.1 This practice covers procedures for irradiations at
bility of regulatory limitations prior to use.
accelerator-based neutron sources. The discussion focuses on
two types of sources, namely nearly monoenergetic 14-MeV
2. Referenced Documents
neutrons from the deuterium-tritium T(d,n) interaction, and
2.1 ASTM Standards:
broad spectrum neutrons from stopping deuterium beams in
C859Terminology Relating to Nuclear Materials
thick beryllium or lithium targets. However, most of the
E170Terminology Relating to Radiation Measurements and
recommendations also apply to other types of accelerator-
Dosimetry
based sources, including spallation neutron sources (1). Inter-
E181Test Methods for Detector Calibration andAnalysis of
est in spallation sources has increased recently due to their
Radionuclides
proposed use for transmutation of fission reactor waste (2).
E261Practice for Determining Neutron Fluence, Fluence
1.2 Many of the experiments conducted using such neutron
Rate, and Spectra by Radioactivation Techniques
sources are intended to simulate irradiation in another neutron
E263Test Method for Measuring Fast-Neutron Reaction
spectrum, for example, that from a DT fusion reaction. The
Rates by Radioactivation of Iron
word simulation is used here in a broad sense to imply an
E264Test Method for Measuring Fast-Neutron Reaction
approximationoftherelevantneutronirradiationenvironment.
Rates by Radioactivation of Nickel
The degree of conformity can range from poor to nearly exact.
E265Test Method for Measuring Reaction Rates and Fast-
In general, the intent of these simulations is to establish the
Neutron Fluences by Radioactivation of Sulfur-32
fundamental relationships between irradiation or material pa-
E266Test Method for Measuring Fast-Neutron Reaction
rameters and the material response. The extrapolation of data
Rates by Radioactivation of Aluminum
from such experiments requires that the differences in neutron
E393Test Method for Measuring Reaction Rates byAnaly-
spectra be considered.
sis of Barium-140 From Fission Dosimeters
1.3 The procedures to be considered include methods for
E854Test Method for Application and Analysis of Solid
characterizing the accelerator beam and target, the irradiated State Track Recorder (SSTR) Monitors for Reactor
sample, and the neutron flux and spectrum, as well as proce-
Surveillance, E706(IIIB)
dures for recording and reporting irradiation data. E910Test Method for Application and Analysis of Helium
Accumulation Fluence Monitors for Reactor Vessel
1.4 Other experimental problems, such as temperature
Surveillance, E706 (IIIC)
control, are not included.
1.5 The values stated in SI units are to be regarded as
3. Terminology
standard. No other units of measurement are included in this
3.1 DescriptionsofrelevanttermsarefoundinTerminology
standard.
C859 and Terminology E170.
1.6 This standard does not purport to address all of the
4. Summary of Existing and Proposed Facilities
safety concerns, if any, associated with its use. It is the
4.1 T(d,n) Sources:
4.1.1 Neutronsareproducedbythehighlyexoergicreaction
This practice is under the jurisdiction of ASTM Committee E10 on Nuclear
Technology and Applicationsand is the direct responsibility of Subcommittee d+t → n+α. The total nuclear energy released is 17.589
E10.08 on Procedures for Neutron Radiation Damage Simulation.
Current edition approved Aug. 1, 2009. Published September 2009. Originally
approved in 1981. Last previous edition approved in 2003 as E798–96(2003). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/E0798-96R09. 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 practice. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E798 − 96 (2009)
MeV, resulting in about a 14.8-MeV neutron and a 2.8-MeV tritium-occluded titanium. The beam spot size was about 10
alpha particle at low deuterium beam energies (3). The mm in diameter. In addition to being rotated, the target also
deuteron energy (generally 150 to 400 keV) is chosen to wasrockedeveryfewhoursandthedeuteronbeamcurrentwas
maximize the neutron yield (for a particular target configura- increased slowly in an attempt to maintain a constant flux in
tion) from the resonance in the d-t cross section near 100 keV.
spite of tritium burn-up in the target. Samples could be placed
The number of neutrons emitted as a function of angle (θ) as close as 2.5 to 4.0 mm from the region of maximum d-t
13 2
between the neutron direction and the incident deuteron beam
interaction resulting in a typical flux of 10 n/cm ·s over a
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
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
that are much less forward peaked. Furthermore, when the
(5-7)—RTNS I and II, which formerly were operated at the
residual nucleus is left in an excited state, the angular effects
Lawrence Livermore National Laboratory, provided 14 MeV
12 13
are also much less pronounced.These latter two effects tend to
neutron source strengths of about 6×10 and 4×10
decrease the average neutron energy at angles other than 0° in
neutrons/s, respectively. Although these facilities have been
the direction of the beam.
shut down, they were the most intense sources of 14 MeV
neutronsbuilttodateforresearchpurposes.Theyarediscussed 4.2.3 Neutrons can also be produced by compound nuclear
here because of their relevance to any future neutron sources. reactions in which the entire deuteron is captured by the target
Their characteristics are summarized in Table 1. A discussion nucleus and neutrons are subsequently evaporated. Neutrons
of similar sources can be found in Ref (8). The deuteron beam are preferentially emitted with energies less than a few MeV
energy was 400 keV and the target was a copper-zirconium and the angular distribution approaches isotropy at neutron
alloy (or copper with dispersed alumina) vapor-plated with energiesbelow1MeV.Neutronsalsoareproducedbydeuteron
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 − 96 (2009)
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
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,experimentersarefacedwiththeproblemofnonuniform
irradiations over their samples unless specimen sizes are
severely limited. Alternatively, the field gradients may be
NOTE 1—Flux contours assume a symmetric, Gaussian beam profile.
moderated by deliberately moving or enlarging the beam spot
Figure from Ref. (5).
on the target.This technique will result in a lower total fluence
FIG. 1 Flux Contours for RTNS II
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
(assuming E =30 to 40 MeV). Highly accurate (610%)
d
time-of-flight spectrometry has been used to study the field far
fromthesource,exceptfortheenergyregionbelowafewMeV
(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).
NOTE1—Neutronspectraasafunctionofenergyandanglefor Be(d,n)
4.2.7 Conceptual Design for Li(d,n) Source (9,14)—A con-
source at ORNL, E =40 MeV. (Data from Ref (8).)
d
ceptual design for a fusion materials irradiation facility was
FIG. 2 Neutron Spectra as a Function of Energy and Angle from
done at the Hanford Engineering Development Laboratory
the Forward Direction of the Deuteron Beam
(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
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
4.2.4 The neutron spectrum thus depends very strongly on
15 2
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
very sharply peaked in the forward direction (see Fig. 2).
present sources. This facility would thus have a higher flux of
Materialsstudiesforwhichthemaximumtotalneutronfluence
high-energy neutrons over a larger volume than any available
is desired are usually conducted close to the target and may
accelerator source. A more recent design that takes advantage
subtendalargerangeofforwardangles(forexample,0to60°).
of improvements in accelerator technology is discussed in Ref
This practice primarily will be concerned with this close-
(15).
geometry situation since it is the most difficult to handle
properly. 4.3 Other Sources:
E798 − 96 (2009)
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
4.3.1 There are many other accelerator-based neutron producedbymediumenergyprotonsandneutrons
...

Questions, Comments and Discussion

Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.