ASTM E261-98

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 261 – 98
Standard Practice for
Determining Neutron Fluence, Fluence Rate, and Spectra by
Radioactivation Techniques
This standard is issued under the fixed designation E 261; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 393 Test Method for Measuring Reaction Rates by Analy-
sis of Barium-140 from Fission Dosimeters
1.1 This practice describes the general procedures for the
E 418 Method for Measuring Fast-Neutron Flux by Track-
determination of neutron fluence rate, fluence, and energy
Etch Technique
spectra from the radioactivity that is induced in a detector
E 481 Test Method for Measuring Neutron Fluence Rate by
specimen.
Radioactivation of Cobalt and Silver
1.2 The practice is directed toward the determination of
E 523 Test Method for Measuring Fast-Neutron Reaction
these quantities in connection with radiation effects on mate-
Rates by Radioactivation of Copper
rials.
E 526 Test Method for Measuring Fast-Neutron Reaction
1.3 For application of these techniques to reactor vessel
Rates by Radioactivation of Titanium
surveillance, see also Test Methods E 1005.
E 704 Test Method for Measuring Reaction Rates by Ra-
1.4 This standard does not purport to address all of the
dioactivation of Uranium-238
safety concerns, if any, associated with its use. It is the
E 705 Test Method for Measuring Reaction Rates by Ra-
responsibility of the user of this standard to establish appro-
dioactivation of Neptunium-237
priate safety and health practices and determine the applica-
E 844 Guide for Sensor Set Design and Irradiation for
bility of regulatory limitations prior to use.
Reactor Surveillance, E 706(IIC)
NOTE 1—Detailed methods for individual detectors are given in the
E 854 Test Method for Application and Analysis of Solid
following ASTM test methods: E 262, E 263, E 264, E 265, E 266, E 343,
State Track Recorder (SSTR) Monitors for Reactor Sur-
E 393, E 418, E 481, E 523, E 526, E 704, E 705, and E 854.
veillance, E 706(IIIB)
E 944 Guide for Application of Neutron Spectrum Adjust-
2. Referenced Documents
ment Methods in Reactor Surveillance, (IIA)
2.1 ASTM Standards:
E 1005 Test Method for Application and Analysis of Radio-
E 170 Terminology Relating to Radiation Measurements
2 metric Monitors for Reactor Vessel Surveillance, E 706
and Dosimetry
(IIIA)
E 181 Test Methods for Detector Calibration and Analysis
2 E 1018 Guide for Application of ASTM Evaluated Cross
of Radionuclides
Section Data File, Matrix E 706 (IIB)
E 262 Test Method for Determining Thermal Neutron Re-
2 2.2 ISO Standard:
action and Fluence Rates by Radioactivation Techniques
Guide in the Expression of Uncertainty in Measurement
E 263 Test Method for Measuring Fast-Neutron Reaction
Rates by Radioactivation of Iron
3. Terminology
E 264 Test Method for Measuring Fast-Neutron Reaction
3.1 Descriptions of terms relating to dosimetry are found in
Rates by Radioactivation of Nickel
Terminology E 170.
E 265 Test Method for Measuring Reaction Rates and
Fast-Neutron Fluences by Radioactivation of Sulfur-32
4. Summary of Practice
E 266 Test Method for Measuring Fast-Neutron Reaction
4.1 A sample containing a known amount of the nuclide to
Rates by Radioactivation of Aluminum
be activated is placed in the neutron field. The sample is
E 343 Test Method for Measuring Reaction Rates by Analy-
removed after a measured period of time and the induced
sis of Molybdenum-99 Radioactivity from Fission Dosim-
activity is determined.
eters
5. Significance and Use
This practice is under the jurisdiction of ASTM Committee E-10 on Nuclear
5.1 Transmutation Processes—The effect on materials of
Technology and Applications and is the direct responsibility of Subcommittee
bombardment by neutrons depends on the energy of the
E10.05 on Nuclear Radiation Metrology.
Current edition approved Dec. 10, 1996. Published February 1997. Originally
published as E 261 – 65 T. Last previous edition E 261 – 96.
2 3
Annual Book of ASTM Standards, Vol 12.02. Discontinued—see 1984 Annual Book of ASTM Standards, Vol 12.02.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 261
neutrons; therefore, it is important that the energy distribution period is calculated as follows:
of the neutron fluence, as well as the total fluence, be
A5lD/ 1 2 exp 2l t !! exp 2l t ! (1)
@~ ~ ~ #
c w
determined.
where:
6. Counting Apparatus
l5 decay constant for the radioactive nuclide,
6.1 A number of instruments are used to determine the
t 5 time interval for counting,
c
disintegration rate of the radioactive product of the neutron-
t 5 time elapsed between the end of the irradiation period
w
induced reaction. These include the scintillation counters, and the start of the counting period, and
ionization chambers, proportional counters, Geiger tubes, and D 5 number of disintegrations (net number of counts
solid state detectors. Recommendations of counters for particu- corrected for background, random and true coinci-
lar applications are given in General Methods E 181. dence losses, efficiency of the counting system, and
fraction of the sample counted) in the interval t .
c
7. Requirements for Activation-Detector Materials
9.1.1 If, as is often the case, the counting period is short
7.1 The general considerations concerning the suitability of
compared to the half-life ( 5 0.693/l) of the radioactive
a material for use as an activation detector are found in Guide
nuclide, the activity is well approximated as follows:
E 844.
A 5 D/@t exp ~2l t !# (2)
c w
7.2 The amounts of fissionable material needed for fission
9.2 For irradiations at constant fluence rate, the saturation
threshold detectors are rather small and the availability of the
material is limited. Licenses from the U.S. Nuclear Regulatory activity, A , is calculated as follows:
s
Commission are required for possession.
A 5 A/ 1 2 exp2l8t ! (3)
~
s i
7.3 A detailed description of procedures for the use of
where:
fission threshold detectors is given in Test Methods E 343,
t 5 exposure duration, and
E 393, and E 854, and Guide E 844. i
l8 5 effective decay constant during the irradiation.
8. Irradiation Procedures
NOTE 2—The saturation activity corresponds to the number of disinte-
8.1 The irradiations are carried out in two general ways
grations per foil per unit time for the steady-state condition in which the
depending upon whether the instantaneous fluence rate or the
rate of production of the radioactive nuclide is equal to the rate of loss by
fluence is being determined. For fluence rate, irradiate the
radioactive decay and transmutation.
detector for a short period at sufficiently low power that
9.2.1 The effective decay constant, which may be a function
handling difficulties and shielding requirements are minimized.
of time, is related to the decay constant as follows:
Then extrapolate the resulting fluence rate value to the value
‘
anticipated for full reactor power. This technique is sometimes
l85l1 s ~E!f~E! dE (4)
* a
used for the fluence mapping of reactors (1, 2).
8.2 The determination of fluence is most often required in
where:
experiments on radiation effects on materials. Irradiate the
s (E) 5 neutron absorption cross section for the product
a
detectors for the same duration as the experiment at a position
nuclide, and
in the reactor where, as closely as possible, they will experi-
f(E) 5 neutron fluence rate per unit energy.
ence the same fluence, or will bracket the fluence of the
9.2.2 Application of the effective decay constant for irradia-
position of interest. When feasible, place the detectors in the
tions under varying fluence rates is discussed in this section
experiment capsule. In this case, long-term irradiations are
and in the detailed methods for individual detectors.
often required.
9.3 The reaction rate is calculated as follows:
8.3 It is desirable, but not required, that the neutron detector
R 5 A l8/Nl (5)
s s
be irradiated during the entire time period considered and that
a measurable part of the activity generated during the initial
where:
period of irradiation be present in the detector at the end of the N 5 number of target nuclei in the detector at time of
irradiation. Therefore, the effective half-life, t8 - 0.693/l8,of irradiation.
1/2
the reaction product should not be much less than the total
9.3.1 The number of target nuclei can often be assumed to
elapsed time from the initial exposure to the final shutdown.
be equal to N , the number prior to irradiation.
o
8.4 As mentioned in 9.11 and 9.12, the use of cadmium-
N 5 N Fm/M (6)
o A
shielded detectors is convenient in separating contributions to
the measured activity from thermal and epithermal neutrons. where:
N 5 Avogadro’s number
Also, cadmium-shielding is helpful in reducing activities due
A
23 −1
5 6.022 3 10 mole ,
to impurities and the loss of the activated nuclide by thermal-
F 5 atom fraction of the target nuclide in the target
neutron absorption. The recommended thicknesses of cadmium
element,
is 1 mm. When bare and cadmium-shielded samples are placed
m 5 mass of target element, g, and
in the same vicinity, take care to avoid partial shielding of the
M 5 atomic mass of the target element.
bare detectors by the cadmium-shielded ones.
9.3.2 Calculations of the isotopic concentration after irra-
9. Calculation
diation is discussed in 9.6.6 and in the detailed methods for
9.1 The activity of the sample, A, at the end of the exposure individual detectors.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 261
9.4 The neutron fluence rate, f, is calculated as follows:
f(E) 5 differential neutron fluence rate, that is the fluence
per unit energy per unit time for neutrons with
f5 R /s¯ (7)
s
energies between E and E +dE.
where:
9.6.3 The production rate of a radioactive nuclide is related
s¯ 5 the spectral weighted neutron activation cross section.
to the reaction rate by the following equation:
dn/dt 5 NR 2 nl8 (11)
s
9.4.1 Cross sections should be processed from an appropri-
9.6.4 Solution of Eq 11, for the case where R and N are
s
ate cross-section library that includes covariance data. Guide
constant, yields the following expression for the activity of a
E 1018 provides information and recommendations on how to
foil:
select the cross section library. The International Reactor
Dosimetry File (IRDF-90) (38) is one good source for cross
A 5 ~l/l8!NR ~1 2 ~exp2l8t!! (12)
s
sections. The SNLRML cross section compendium (25) pro-
9.6.5 The saturation activity of a foil is defined as the
vides a processed fine-group representation of recommended
activity when dn/dt 5 0; thus Eq 11 yields the following
dosimetry cross sections and covariance matrices.
relationship for the saturation activity:
9.4.2 If spectral-averaged cross-section or spectrum data are
A 5 ~l/l8!NR (13)
s s
not available, one of the alternative procedures discussed in
9.6.6 The isotopic content of the target nuclide may be
9.10 to 9.13 may be used to calculate an approximate neutron
reduced during the irradiation by more than one transmutation
fluence rate from the saturation activity.
process and it may be increased by transmutation of other
9.5 The neutron fluence, F, is related to the time varying
nuclides so that the rate of change of the number of target
differential neutron fluence rate f(E,t) by the following expres-
nuclei with time is described by a number of terms:
sion:
n m
‘ t
dN/dt 5 N ~R 1 R ! 2 N R (14)
F5 f ~E,t! dt dE (8) ( (
s i j j
* *
0 t t 5 1 j 5 1
where: where:
t −t 5 duration of the irradiation period. i 5 discrete transmutation path for removal of the target
2 1
isotope, and
9.5.1 Long irradiations usually involve operation at various
j 5 discrete transmutation reaction whereby the target iso-
power levels, and changes in isotopic content of the system;
tope is produced from isotope N and each of the R and
under such conditions f(E, t) can show large variations with j i
R terms could be calculated from equations similar to
j
time.
Eq 10, using the appropriate cross sections.
9.5.2 It is usual to assume, however, that the neutron fluence
9.6.6.1 The R term may predominate and, if R is constant,
s s
rate is directly proportional to reactor power; under these
Eq 14 can be solved as N 5 N exp (− R t). The change in the
o s
conditions, the fluence can be well approximated by:
target composition may be negligible and N may be approxi-
n
f
mated by N .
o
F5 P t (9)
S D
( i i
P
i 5 1
9.6.7 During irradiation, the effective decay rate is increased
by transmutations of the product isotope (see Eq 4).
where:
9.7 Long Term Irradiations:
f/P 5 average value of the neutron fluence rate, f,ata
9.7.1 Long irradiations for materials testing programs and
reference power level, P,
th
reactor pressure vessel surveillance are common. Long irradia-
t 5 duration of the i operating period during which the
i
tions usually involve operation at various power levels, includ-
reactor operated at approximately constant power,
ing extended zero-power periods; thus, appropriate corrections
and
must be made for depletion of the target nuclide, decay and
P 5 reactor power level during that operating period.
i
burnout of the radioactive nuclide, and variations in neutron
9.5.2.1 Alternate methods include measuring the power
fluence rate. Multiple irradiations and nuclide burnup must also
generation rate in a fraction of the reactor volume adjacent to
be considered in short-irradiation calculations where reaction-
the volume of interest.
product half-lives are relatively short and nuclide cross sec-
9.6 Transmutation Processes:
tions are high.
9.6.1 The neutron fluence rate spectrum, f(E), can be
9.7.2 The total irradiation period can be divided into a
determined by computer calculations using neutron transport
continuous series of periods during each of which f(E)is
codes, and adjustment techniques using radioactivation data th
essentially constant. Then the activity generated during the i
from multiple foil irradiations.
irradiation period is:
9.6.2 The reaction rate is related to the fluence rate by the
A 5 @lN ~R /l8! #~1 2 exp ~2l8 t !! (15)
i i s i i i
following equation:
‘
where:
R 5 s~E!f ~E!dE (10)
s *
0 N 5 number of
...