ASTM E262-97
(Test Method)Standard Method for Determining Thermal Neutron Reaction and Fluence Rates by Radioactivation Techniques
Standard Method for Determining Thermal Neutron Reaction and Fluence Rates by Radioactivation Techniques
SCOPE
1.1 The purpose of this method is to define a general procedure for determining an unknown thermal neutron-fluence rate by neutron activation techniques. It is not practicable to describe completely a technique applicable to the large number of experimental situations that require the measurement of a thermal-neutron fluence rate. Therefore, this method is presented so that the user may adapt to his particular situation the fundamental procedures of the following techniques:
1.1.1 Absolute counting technique using pure cobalt, pure gold, or cobalt-aluminum or gold-aluminum alloy,
1.1.2 Standard foil technique using pure gold, or gold-aluminum alloy, and
1.1.3 Secondary standard foil techniques using pure indium, indium-aluminum alloy, and dysprosium-aluminum alloy.
1.2 The techniques presented are limited to measurements at room temperatures. However, special problems when making thermal-neutron fluence rate measurements in high-temperature environments are discussed in 8.2. For those circumstances where the use of cadmium as a thermal shield is undesirable because of potential spectrum perturbations or of temperatures above the melting point of cadmium, the method described in Test Method E481 can be used in some cases. Alternatively, gadolinium filters may be used instead of cadmium. For high temperature applications in which aluminum alloys are unsuitable, other alloys such as cobalt-nickel or cobalt-vanadium have been used.
1.3 Table 1 indicates the useful neutron-fluence ranges for each detector material.
1.4 The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are for information only.
1.5 This standard may involve hazardous materials, operations and equipment. This standard does not purport to address all of the safety problems 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: E 262 – 97
Standard Test Method for
Determining Thermal Neutron Reaction and Fluence Rates
by Radioactivation Techniques
This standard is issued under the fixed designation E 262; 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.
TABLE 1 Useful Neutron Fluence Ranges of Foil Material
1. Scope
’ Useful Range
1.1 The purpose of this method is to define a general
Foil Material Form
(neutrons/cm )
procedure for determining an unknown thermal neutron-
3 12
Indium pure or alloyed with 10 to 10
fluence rate by neutron activation techniques. It is not practi-
aluminum
7 14
cable to describe completely a technique applicable to the large
Gold pure or alloyed with 10 to 10
aluminum
number of experimental situations that require the measure-
3 10
Dysprosium pure or alloyed with 10 to 10
ment of a thermal-neutron fluence rate. Therefore, this method
aluminum
14 20
is presented so that the user may adapt to his particular Cobalt pure or alloyed with 10 to 10
aluminum
situation the fundamental procedures of the following tech-
niques.
1.1.1 Absolute counting technique using pure cobalt, pure
ASTM Test Methods
gold, or cobalt-aluminum or gold-aluminum alloy.
E 181 Test Methods for Detector Calibration and Analysis
1.1.2 Standard foil technique using pure gold, or gold-
of Radionuclides
aluminum alloy, and
E 261 Practice for Determining Neutron Fluence Rate, Flu-
1.1.3 Secondary standard foil techniques using pure indium,
ence, and Spectra by Radioactivation Techniques
indium-aluminum alloy, and dysprosium-aluminum alloy.
E 481 Test Method for Measuring Neutron Fluence Rate by
1.2 The techniques presented are limited to measurements at
Radioactivation of Cobalt and Silver
room temperatures. However, special problems when making
thermal-neutron fluence rate measurements in high-
3. Significance and Use
temperature environments are discussed in 8.2. For those
3.1 This method can be extended to use any material that
circumstances where the use of cadmium as a thermal shield is
has the necessary nuclear and activation properties that suit the
undesirable because of potential spectrum perturbations or of
experimenter’s particular situation. No attempt has been made
temperatures above the melting point of cadmium, the method
to fully describe the myriad problems of absolute counting
described in Test Method E 481 can be used in some cases.
techniques, neutron-fluence depression, and thick-foil self-
Alternatively, gadolinium filters may be used instead of cad-
shielding. It is assumed that the experimenter will refer to
mium. For high temperature applications in which aluminum
existing literature on these subjects. This method does offer a
alloys are unsuitable, other alloys such as cobalt-nickel or
referee method (the standard gold foil irradiation at National
cobalt-vanadium have been used.
Institute of Standards and Technology (NIST) to aid the
1.3 Table 1 indicates the useful neutron-fluence ranges for
experimenter when he is in doubt of his ability to measure an
each detector material.
absolute thermal fluence rate.
1.4 This standard does not purport to address all of the
3.2 The standard foil technique uses a set of foils that are as
safety concerns, if any, associated with its use. It is the
nearly identical as possible in shape and mass. The foils are
responsibility of the user of this standard to establish appro-
fabricated from any material that activates by an (n, g)
priate safety and health practices and determine the applica-
reaction, preferably having a cross section approximately
bility of regulatory limitations prior to use.
inversely proportional to neutron speed in the thermal energy
2. Referenced Documents range. Some of the foils are irradiated in a known neutron field
(at NIST) or other standards laboratory). The foils are counted
2.1 ASTM Standards:
in a fixed geometry on a stable radiation-detecting instrument.
E 177 Practice for Use of the Terms Precision and Bias in
The neutron induced reaction rate of the foils is computed from
the counting data, and the ratio of the known neutron fluence
This method is under the jurisdiction of ASTM Committee E-10 on Nuclear
Technology and Applications and is the direct responsibility of Subcommittee
E10.05 on Nuclear Radiation Metrology.
Current edition approved June 10, 1997. Published August 1998. Originally Annual Book of ASTM Standards, Vol 14.02.
published as E 262 – 65 T. Last previous edition E 262 – 86 (1991). 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 262
rate to the computed reaction rate is determined. For any given ~R ! 5 n gs v 5 gs f (3)
s 0 th 0 0 0 0
foil, neutron energy spectrum, and counting set-up, this ratio is
where g is a correction factor that accounts for the departures
a constant. Other foils from the identical set can now be
from the ideal 1/v detector cross section in the thermal energy
exposed to an unknown neutron field. The magnitude of the
range. The same factor appears in the Westcott convention Ref
fluence rate in the unknown field can be obtained by comparing
(1), and is usually referred to as the Westcott g factor. g
the reaction rates as determined from the counting data from
depends on the neutron temperature, T, and is defined as
the unknown and reference field, with proper corrections to
follows:
account for spectral differences between the two fields (see
3 3/2 2
1 ‘ 4 v T v T
0 0
Section 4). One important feature of this technique is that it
g 5 · exp – s~v!dv
S D S D F S D S DG
* 1/2
v s v T v T
0 0 p 0 0
eliminates the need for absolute counting.
(4)
4. Theory 4.2.3 If the thermal neutron spectrum truly follows the
Maxwellian distribution and if the neutron temperature is
4.1 1/v Cross Sections—It is not possible using radioactiva-
known, it is possible to calculate the true thermal neutron
tion techniques to determine the true thermal neutron fluence
fluence rate by multiplying the conventional (equivalent 2200
rate, without making some assumptions about the spectral
m/s) thermal fluence rate by the factor
shapes of both the thermal and epithermal components of the
1/2
neutron density. For most purposes, however, the information
v¯ 4T
5 (5)
S D
v pT
required is only that needed to make calculations of activation
0 0
and other reaction rates for various materials exposed to the
where v¯ is the Maxwellian mean speed for neutron tempera-
neutron field. For reactions in which the cross section varies
ture T, and T is the standard temperature of 293.4°K. This
inversely as the neutron speed (1/v cross sections) the reaction
conversion is most often unnecessary and is usually not made
rates are proportional to the total neutron density and do not
because the temperature T may be unknown. Naturally, it is
depend on the spectrum shape. Many radioactivation detectors
essential when reporting results to be absolutely clear whether
have reaction cross sections in the thermal energy range which
the true thermal fluence rate or the equivalent 2200 m/s thermal
approximate to 1/v cross sections (1/v detectors). Departures
fluence rate or the equivalent 2200 m/s total (Westcott) fluence
from the 1/v shape can be accounted for by means of correction
rate is used. If the true thermal fluence rate is used, then its
factors.
value must be accompanied by the associated temperature
4.2 Fluence Conventions:
value.
4.2.1 The purpose of a fluence convention (formerly called“
4.3 Epithermal Neutrons—In order to determine the effects
flux convention”) is to describe a neutron field in terms of a
of epithermal neutrons, that are invariably present together
few parameters that can be conveniently used to calculate
with thermal neutrons, cadmium covered foil irradiations are
reaction rates. The best known fluence conventions relating to
made. It is important to realize that some epithermal neutrons
thermal neutron fields are the Westcott convention (1) and the
can have energies below the effective cadmium cut-off energy,
Stoughton and Halperin convention (2). Both make use of the
E . The lowest energy of epithermal neutrons is usually taken
cd
concept of an equivalent 2200 m/s fluence rate, that is equal to
to be equal to 5kT (where k is Boltzmann’s constant) which is
the product of the neutron density and the standard speed, v ,
equal to 0.13eV for room temperature (293°K) neutrons (1),
equal to 2200 m/s which is the most probable speed of
though 4 kT has been recommended for some reactors (3).In
Maxwellian thermal neutrons when the characteristic tempera-
order to correct for these, it is necessary to make some
ture is 293.4°K. In the Westcott convention, it is the total
assumption about the epithermal neutron spectrum shape, and
neutron density (thermal plus epithermal) which is multiplied
the assumption made in Refs 1 and 2 is that the epithermal
by v to form the “Westcott flux”, but in the Stoughton and
neutron fluence rate per unit energy is proportional to 1/E:
Halperin convention, the conventional fluence rate is the
f ~E!5f /E,E $ 5kT (6)
e e
product of the Maxwellian thermal neutron density and v . The
latter convention is the one followed in this method:
where f is an epithermal fluence parameter equal to the
e
fluence rate per unit energy, f (E), at 1 eV. This assumption is
f 5 n v (1)
e
0 th 0
usually adequate for the purpose of correcting thermal neutron
where f is the equivalent 2200 m/s thermal fluence rate and
fluence rate measurements for epithermal neutrons at energies
n represents the thermal neutron density, which is propor-
th
below the cadmium cut-off. To represent the epithermal fluence
tional to the reaction rate per atom in a 1/v detector exposed to
more correctly, however, many authors have shown that the use
thermal neutrons:
(1+a)
of a 1/E spectrum shape is preferable, where a is an
~R ! 5 n s v 5s f (2)
s 0 th 0 0 0 0 empirical parameter. Refs (4-10).
4.2.2 (R ) represents only that part of the reaction rate that 4.4 Resonance Integral:
s 0
is induced by thermal neutrons, which have the Maxwellian
4.4.1 The resonance integral for an ideal dilute detector is
spectrum shape. s is the 2200 m/s cross section. For a non-1/v
defined as follows:
detector Eq 2 needs to be replaced by:
‘ dE
I 5 s~E! (7)
*
E
E
cd
4.4.2 The cadmium cut-off energy is taken to be 0.55eV for
The boldface numbers in parentheses refer to the list of references appended to
this method. a cylindrical cadmium box of wall thickness 1 mm. (11). The
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 262
TABLE 3 Resonance Self-Shielding Data for Cobalt Foils
data needed to correct for epithermal neutron reactions in the
(Reference (30))
methods described are the values of I /gs for each reaction
0 0
Foil Thickness
(see Table 2). These values, taken from Refs (26-28), are based
G8
res
G
res
(132 eV)
(in.) (cm)
on integral measurements.
4.5 Reaction Rate:
0.0004 0.001018 0.8264 0.864
0.0010 0.02254 0.7000 0.765
4.5.1 The reaction rate per atom, for an isotope exposed to
0.0025 0.00635 0.5470 0.645
a mixed thermal and epithermal neutron field is given by:
0.0050 0.0127 0.4395 0.561
0.0075 0.01905 0.3831 0.517
R 5f gs 1f gs @f 1 w8 /g 1 I /gs # (8)
s 0 0 e 0 1 0 0
0.010 0.0254 0.3476 0.489
f is a function that describes the epithermal activation of a 0.015 0.0381 0.3028 0.454
0.020 0.0508 0.2744 0.432
1/v detector in the energy range 5kT to E :
cd
1/2
E
cd kT dE
f 5 (9)
S D
1 *
E E
5kT
gs
G 5 G8 1 ~1– G8 ! 0.429 (14)
res res res
I
4.5.2 For E equal to 0.55eV and T equal to 293.4°K, f 5 0
cd 0 1
0.468. w8 in Eq 8 is a function which accounts for departure of
4.7 Fluence Depression Factors—Thermal fluence depres-
the cross section from the 1/v law in the energy range 5kT to
sion is an additional perturbation that occurs when an absorber
E :
is surrounded by a moderator. Because the effects are sensitive
cd
1/2 to the details of individual situations, it is not possible to
1 E kT dE
cd
w8 5 s~E!– gs (10)
F S D G
* 0
provide correction factors here. References (12-20) describe
s E E
5kT
these effects. The problem is avoided when foils are exposed in
Some values of w8 for T equal 293.4°K are given in Table 2.
cavities of vary large volume compared to the detector volume.
4.5.3 For a cadmium covered foil, the reaction rate is given
In other cases, a rough guide is that the external perturbation
as:
effect is usually less than the thermal self-shielding effect, and
R 5f I (11)
s,Cd e 0
much less when the hydrogenous moderator is absent.
4.5.4 This can be used to eliminate the unknown epithermal
5. Apparatus
fluence rate parameter, f , from Eq 8. After rearrangement, one
e
5.1 Radiation-Detection Instruments:
obtains an expression for the saturation activity due to thermal
5.1.1 The radiation detectors that may be used in neutron
neutrons only:
activation techniques are described in the Standard Methods,
gs s w8
0 0
f gs 5 ~R ! 5 R – R 1 1 f 1 (12) E 181. In addition, or as an alternative, a calibration high-
S D
0 0 s 0 s s,Cd 1
I I
0 0
pressure ionization chamber may be used. Details for its
4.6 Neutron Self-Shielding:
construction and calibration may be found in Ref (21).
4.6.1 Unless extremely thin or dilute alloy materials are
5.2 Precision Punch:
used, all of the measurement methods are subject to the effects
5.2.1 A precision punch is required to fabricate a set of
of neutron self-shielding. The modified version of Eq 12 which
identical foils for the standard foil technique. The punch must
takes into account both a thermal self-shielding factor G , and
th
cut foils that have smooth edges. Since finding such a punch
an epithermal self shielding factor G is:
res
commercially available is difficult, it is recommended that the
~R !
punch be custom made. It is possible to have several dies made
s 0
f gs 5 (13)
0 0
G
th to fit one punch so that a variety of foil sizes can be obtained.
Normally, foil diameters are 12.7 mm (0.500 in.) or less. The
1 gs s w8
0 0
5 R – R 1 1 f 1
F s s,Cd S 1 DG
precision punch is one of the most important items in the
G G I G I
th res 0 res 0
standard foil technique particularly if the counting technique
4.6.2 Values of the self-shieldi
...
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