ASTM E481-97
(Test Method)Standard Test Method for Measuring Neutron Fluence Rate by Radioactivation of Cobalt and Silver
Standard Test Method for Measuring Neutron Fluence Rate by Radioactivation of Cobalt and Silver
SCOPE
1.1 This test method covers a suitable means of obtaining the thermal neutron fluence rate, or fluence, in well moderated nuclear reactor environments where the use of cadmium, as a thermal neutron shield as described in Method E262, is undesirable because of potential spectrum perturbations or of temperatures above the melting point of cadmium.
1.2 This test method describes a means of measuring a Westcott neutron fluence rate (Note 1) by activation of cobalt- and silver-foil monitors (see Terminology E170). The reaction 59 Co(n,[gamma]) 60 Co results in a well-defined gamma emitter having a half-life of 5.271 years (1). The reaction 109 Ag(n, [gamma] ) 110m Ag results in a nuclide with a complex decay scheme which is well known and having a half-life of 249.85 days (1). Both cobalt and silver are available either in very pure form or alloyed with other metals such as aluminum. A reference source of cobalt in aluminum alloy to serve as a neutron fluence rate monitor wire standard is available from the National Bureau of Standards as Standard Reference Material 953. The competing activities from neutron activation of other isotopes are eliminated, for the most part, by waiting for the short-lived products to die out before counting. With suitable techniques, thermal neutron fluence rate in the range from 10 cm -2 [dot]s -1 to 3 X 10 15 cm -2 [dot]s -1 can be measured. For this method to be applicable, the reactor must be well moderated and be well represented by a Maxwellian low-energy distribution and an (1/E) epithermal distribution. These conditions are usually met in positions surrounded by hydrogenous moderator without nearby strongly absorbing materials. Otherwise the true spectrum must be calculated to obtain effective activation cross sections over all energies. Note 1-Westcott fluence rate = v0[integral] [infinity] 0 n(v)dv .
1.3 The values stated in SI units are to be regarded as the standard.
1.4 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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Standards Content (Sample)
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: E 481 – 97
Standard Test Method for
Measuring Neutron Fluence Rates by Radioactivation of
Cobalt and Silver
This standard is issued under the fixed designation E 481; 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 1.3 The values stated in SI units are to be regarded as the
standard.
1.1 This test method covers a suitable means of obtaining
1.4 This standard does not purport to address all of the
the thermal neutron fluence rate, or fluence, in well moderated
safety concerns, if any, associated with its use. It is the
nuclear reactor environments where the use of cadmium, as a
responsibility of the user of this standard to establish appro-
thermal neutron shield as described in Method E 262, is
priate safety and health practices and determine the applica-
undesirable because of potential spectrum perturbations or of
bility of regulatory limitations prior to use.
temperatures above the melting point of cadmium.
1.2 This test method describes a means of measuring a
2. Referenced Documents
Westcott neutron fluence rate (Note 1) by activation of cobalt-
2.1 ASTM Standards:
and silver-foil monitors (See Terminology E 170). The
59 60 E 170 Terminology Relating to Radiation Measurements
reaction Co(n, ) Co results in a well-defined gamma emit-
g
2 and Dosimetry
ter having a half-life of 1925.5 days (1). The
109 110m E 177 Practice for Use of the Terms Precision and Bias in
reaction Ag(n,g˙) Ag results in a nuclide with a complex
ASTM Test Methods
decay scheme which is well known and having a half-life of
E 181 Test Methods for Detector Calibration and Analysis
249.76 days (2). Both cobalt and silver are available either in
of Radionuclides
very pure form or alloyed with other metals such as aluminum.
E 262 Test Method for Determining Thermal Neutron Re-
A reference source of cobalt in aluminum alloy to serve as a
action and Fluence Rates by Radioactivation Techniques
neutron fluence rate monitor wire standard is available from the
National Institute of Standards and Technology (NIST) as
3. Significance and Use
Standard Reference Material 953. The competing activities
3.1 The pertinent data for these two reactions are given in
from neutron activation of other isotopes are eliminated, for the
Table 1. This test method uses one monitor (cobalt) with a
most part, by waiting for the short-lived products to die out
nearly 1/v absorption cross-section curve and a second monitor
before counting. With suitable techniques, thermal neutron
9 −2 −1 15 −2 (silver) with a large resonance peak so that its resonance
fluence rate in the range from 10 cm ·s to 3 3 10 cm
−1
integral is large compared to the thermal cross section. The
·s can be measured. For this method to be applicable, the
equations are based on the Westcott formalism (3, 4) and
reactor must be well moderated and be well represented by a
determine a Westcott 2200 m/s neutron fluence rate nv and the
Maxwellian low-energy distribution and an (1/E) epithermal
Westcott epithermal index parameter r T/T . References 5,
=
distribution. These conditions are usually met in positions
6, and 7 contain a general discussion of the two-reaction test
surrounded by hydrogenous moderator without nearby strongly
method. In this test method, the absolute activities of both
absorbing materials. Otherwise the true spectrum must be
cobalt and silver monitors are determined. This differs from the
calculated to obtain effective activation cross sections over all
test method in the references wherein only one absolute
energies.
activity is determined.
‘
NOTE 1—Westcott fluence rate 5 v * n~v!dv.
0 0 3.2 The advantages of this test method are the elimination of
three difficulties associated with the use of cadmium: (1) the
perturbation of the field by the cadmium; (2) the inexact
cadmium cut-off energy; (3) the low melting temperature of
This test method is under the jurisdiction of ASTM Committee E-10 on Nuclear
cadmium. In addition, the reactivity changes accompanying the
Technology and Applications and is the direct responsibility of Subcommittee
rapid insertion and removal of cadmium may prohibit the use
E10.05 on Nuclear Radiation Metrology.
Current edition approved June 10, 1997. Published May 1998. Originally of the cadmium-ratio method. However, the self-shielding
published as E 481 – 73 T. Last previous edition E 481 – 86 (1991).
The boldface numbers in parentheses refer to references listed at the end of this
test method.
3 4
Standard Reference Material 953 is available from National Institute of Annual Book of ASTM Standards, Vol 12.02.
Standards and Technology, U.S. Dept. of Commerce, Washington, DC 20234. Annual Book of ASTM Standards, Vol 14.02.
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 481
TABLE 1 Recommended Constants
60 110m
Cobalt ( Co) Silver ( Ag)
Symbol Parameter
A A
Value Reference Value Reference
t Half-life 1925.5 (5) days (1) 249.76 (20) days (2)
1/2
59 109
A Abundance of parent isotope 100 % ( Co) (14) 48.161 (7) % ( Ag) (14)
D Mass excess of target isotope (scaled to −61.645 MeV (14) −87.339 MeV (14)
D [ C] 5 0)
B
(1 amu 5 931.494MeV)
590 C,D
s Absorption 2200 m/s cross section for target Co and 37.233 b 6 0.16 % 91.0 b6 1% (16)
a
Ag
60 110m C,D
s 2200 m/s cross section for formation of Co and Ag 37.233 b 6 0.16 % 4.7 b 6 4% (16)
S Correction factor which describes the departure of the 1.69 (7) 18.53 (7)
59 60 109 110m
cross section from the 1/v law in the epithermal [ Co(n,g) Co] [ Ag(n,g) Ag]
region 17.10
109 110m+110g
[ Ag(n,g) Ag]
E
I Resonance Integral 75.421 b 6 0.77 % (15), 66 b (16)
59 60 109 110m
[ Co(n,g) Co] [ Ag(n,g) Ag]
s Effective absorption cross section for product nuclide 2b (11) 82 b (13)
(reactor spectrum)
G Thermal neutron self-shielding factor Table 3 (12) > 1 − 4/3 R( (5)
th B
F
G8 Resonance neutron self-shielding factor Table 3 (12) Fig. 1 (5)
res
g Correction factor which describes the departure of the 1.0 (3) See Table 4 (3)
cross section from 1/v law in thermal region
A
The numbers in parenthesis following given values is the uncertainty in the last digit(s) of the value; 0.729 (8) means 0.729 6 0.008, 70.8(1) means 70.8 6 0.1.
B
Isotopic masses may be calculated as A + 931.494/D, where A is the atomic mass number.
C
A 2200 m/s cross section (E 5 0.0253 eV, T 5 20°C) was taken from the sources indicated in Reference (15).
D
Cross section uncertainty data is taken from Reference (16), the cross section comes from the other reference.
E
Cross section uncertainty comes from convariance data provided in the cross section source. The other reference indicates the source of the cross section.
F 2 109
In Fig. 1, Q5 4E kT/AG 5 0.2 corresponds to the value for Ag.
r
corrections remain important unless the concentrations of therefore, an environment in which aluminum would not be
cobalt and silver are small. Studies indicate that the accuracy of adversely affected would be generally satisfactory for the
the two-reaction method is comparable to the cadmium-ratio alloys. However, the low mechanical strength of the monitors
method. requires in many instances that it be encapsulated or shielded
3.3 The long half-lives of the two monitors permit the from physical disturbances by some type of container. Alumi-
determination of fluence for long-term monitoring. num cans or tubing are satisfactory for many cases of interest,
but for hostile environments, stainless steel or vanadium may
4. Apparatus
be preferable. Perturbation due to the presence of the container
must be accounted for, especially in the case of stainless steel.
4.1 NaI(Tl) or Germanium Gamma-Ray Spectrometer (us-
The container should be constructed in such a manner that it
ing a multichannel analyzer)—For the NaI(Tl) technique and
will not create a significant flux perturbation and that it may be
the germanium technique, see Method E 181.
opened easily, especially if the monitors must be removed
4.2 Precision Balance.
4.3 Digital Computer. remotely.
5. Materials and Manufacture 6. Procedure
5.1 The two monitors required for this test method are 6.1 Decide on the size and shape of the monitors to be
cobalt and silver. Although these two materials are available irradiated, taking into consideration the size and shape of the
commercially in very pure form, they have been used (8) irradiation space. The mass and exposure time are parameters
alloyed with aluminum (#1 % cobalt and #1 % silver) to which can be varied to obtain a desired disintegration rate for
minimize the self-shielding effect and to permit insertion into a given neutron fluence rate level. To facilitate the convergence
15 −2 −1
a high thermal-neutron fluence rate (>10 cm s ) facility (7, of the two activity equations for the fluence rate and the
9). Typical alloys contain 0.1 % silver or cobalt in aluminum) epithermal index in Section 7, the concentration of the alloys
see 6.1 and 8.1). should be chosen so that the ratio of the disintegration rates is
5.2 The uncertainties and nonuniformity of alloy concentra- on the order of one.
tions must be established by one or more different test 6.2 Weigh the samples to a precision of 61.0 % (1S %) as
methods. These might include chemical and activation analy- defined in Practice E 177.
sis, or spectrometry. The purity of the aluminum matrix should 6.3 Irradiate the samples for the predetermined time period.
also be established. Record the power level and any changes in power during the
5.3 Whenever possible, the alloys should be tested for irradiation, the time at the beginning and end of the irradiation,
interfering impurities by neutron activation. and the relative position of the monitors in the irradiation
5.4 The method of encapsulating the monitors for irradia- facility.
tion depends upon the characteristics of the facility in which 6.4 A waiting period is necessary between termination of
the measurements are to be made. The monitors have essen- the exposure and start of counting when using Co-Al and
tially the same chemical characteristics as pure aluminum; Ag-Al monitors. This allows the 0.62356 days (1) half-life Na
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 481
TABLE 3 Self-Shielding Factors for Cobalt Wires (12)
which is formed by fast-neutron reactions on Al or by
thermal-neutron captures by Na impurities to decay below Wire Cobalt
Diameter Content, G8 (132 eV) G
res th
levels at which its radiations may cause interferences. It is
in. (mm) (mass %)
sometimes advisable to count the samples periodically and
24 0.050 (1.27) 0.104 1.00 1.00
follow the decay of the portions of the activities due to the Na.
0.050 (1.27) 0.976 0.95 6 0.04 0.99 6 0.01
The length of the waiting period can be reduced by the use of
0.001 (0.03) 100 0.81 6 0.03 0.99 6 0.02
0.005 (0.13) 100 0.52 6 0.02 0.97 6 0.01
a germanium detector.
0.010 (0.25) 100 0.42 6 0.02 0.94 6 0.01
6.5 With the gamma-ray spectrometer, analyze the silver
0.015 (0.38) 100 0.38 6 0.01 0.92 6 0.02
110m 60
sample for Ag and the cobalt sample for Co. Obtain the
0.020 (0.51) 100 0.34 6 0.01 0.90 6 0.02
0.025 (0.64) 100 0.32 6 0.01 0.88 6 0.03
net count rate in each full-energy gamma-ray peak of interest,
110m
that is, 657.7623 keV or 884.684 keV for Ag: 1332.501
keV for Co (see Method E 181). See Table 2 for gamma
110m
being measured, can be approximated by the following equa-
radiations of Ag.
tion:
7. Calculation
B . 1 2 ~4/3!~μ R! (2)
a
110m 60
7.1 Calculate the activities of Ag and Co in disintegra-
where:
tions per second.
−1
μ 5 linear absorption coefficient in monitor, cm , and
a
7.2 A Westcott 2200 m/s neutron fluence rate, nv ,or f
0 w
R 5 radius of monitor wire, cm.
and the Westcott epithermal index parameter, r =T/T are
The burnup and decay correction factor is given by:
related to the measured activities of the silver and cobalt
exp~2sˆ f t ! 2 exp@2~l 1sˆ f !t #
a w i 2 2 w i
monitors by the following equation:
F 5 (3)
$@@~l t /f t !1sˆ #2sˆ #2sˆ %f t
2 i w i 2 2 a w i
A 5 N l BFGsˆ f t (1)
0 2 1 w i
where:
where:
sˆ 5 Westcott’s effective absorption cross section for target
a
A 5 measured activity at the end of the exposure
nuclide, cm , and
time, disintegrations/s,
sˆ 5 Westcott’s effective absorption cross section for the
59 109 2
N 5 number of target atoms of Co or Ag at
product nuclide, cm .
start of irradiation,
The self-shielding factor is given by:
l 5 disintegration constant of product nuclide,
−1
s , gG 1 ~r T/T !S G8
=
th 0 0 res
G 5 (4)
B 5 Self-absorption factor of the decay gamma
g 1 ~r T/T !S
=
0 0
ray in the monitor material,
where:
F 5 burnup and decay correction factor,
g 5 correction factor which describes the departure of
G 5 self-shielding factor (see Eq 4, Table 3 and
the cross section from the 1/v law in the thermal
Fig. 1).
region (see Table 4 for silver “g” factors),
sˆ 5 Westcott’s effective absorption cross section
G 5 thermal neutron self-shielding factor,
for production of the product nuclide, cm , th
G8 5 resonance neutron self-shielding factor,
res
f (or nv ) 5 a 2200 m/s neutron fluence rate in which n
w 0
r 5 a measure of the proportion of epithermal neutrons
is the neutron density (including both ther-
in the reactor spectrum,
mal and epithermal neutrons) and t is 2200
i
T 5 neutron temperature, K,
m/s, and
T 5 293.6 K, and
t 5 exposure time.
i
S 5 correction factor which describes the departure of
The self-absorption factor, if not known for the gamma rays
the cross section from the 1/v law in the epithermal
region.
110m
TABLE 2 Gamma Radiations of Ag (2) Although the S values in Table 1 are measured values, S
0 0
A B,A
can be calculated by the following equation:
Energy of Gamma (keV) Relative Emission Probability (%)
1. 657.7622 (21) 110.0 (4)
2 I9 2 I E
0 0 0
2. 884.685 (3) 76.8 (3) S 5 5 2 2g (5)
S ˛ D
s s E
=p 0 =p 0 Cd
3. 937.493 (4) 36.31 (12)
4. 1384.300 (4) 25.66 (8)
where:
5. 763.944 (3) 23.55 (9)
6. 706.682 (3) 17.37 (10) I9 5 resonance integral excess over the 1/v cross section
7. 1505.040 (5) 13.78 (5) 2
value, cm ,
8. 667.6227 (24) 10.94 (8)
s 5 2200 m/s cross-section value, cm ,
9. 818.031 (4) 7.76 (4) 0
I 5 resonance integral,
10.
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