1.1
This practice covers a suitable means of obtaining the thermal neutron fluence rate, or fluence, in nuclear reactor environments where the use of cadmium, as a thermal neutron shield as described in Test Method
E262
, is undesirable for reasons such as potential spectrum perturbations or due to temperatures above the melting point of cadmium.
1.2
The reaction
59
Co(
n,γ
)
60
Co results in a well-defined gamma emitter having a half-life of 5.2711 years
2
(8)
3
(
1
)
.
4
The reaction
109
Ag(n,γ)
110m
Ag results in a nuclide with a well-known, complex decay scheme with a half-life of 249.78 (2) 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 Institute of Standards and Technology (NIST) as Standard Reference Material (SRM) 953.
5
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
8
cm
−2
·s
−1
to 3 × 10
15
cm
−2
·s
−1
can be measured. Two calculational practices are described in Section
9
for the determination of neutron fluence rates. The practice described in
9.3
may be used in all cases. This practice describes a means of measuring a Westcott neutron fluence rate in
9.2
(
Note 1
) by activation of cobalt- and silver-foil monitors (see Terminology
E170
). For the Wescott Neutron Fluence Convention method to be applicable, the measurement location must be well moderated and be well represented by a Maxwellian low-energy distribution and an (1/
E
) epithermal distribution. These conditions are usually only met in positions surrounded by hydrogenous moderator without nearby strongly multiplying or absorbing materials.
Note 1:
Westcott fluence rate
1.3
The values stated in SI units are to be regarded as the standard, except in the case of nuclear data where the source referenced units are retained in order to preserve the integrity of the referenced uncertainty values.
1.4
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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.5
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.
====== Significance And Use ======
3.1
This practice uses one monitor (cobalt) with a nearly 1/
v
absorption cross-section curve and a second monitor (silver) with a large resonance peak so that its resonance integral is large compared to its thermal cross section. The pertinent data for these two reactions are given in
Table 1
. The equations are based on the Westcott formalism (
(
2
,
3
)
and Practice
E261
) and determine a Westcott 2200 m/s neutron fluence rate
nv
0
and the Westcott epithermal index parameter
. References
(
4-
6
)
contain a general discussion of the two-reaction test method. In this practice, the absolute activities of both cobalt and silver monitors are determined. This differs from the test method in the references wherein only one absolute activity is determined.
(A)
The numbers in parentheses following given values are the uncertainty in the last digit(s) of the value; 0.729 (8) means 0.729 ± 0.008, 70.8(1) means 70.8 ± 0.1.
(B)
The decay constant, λ, is defined as ln(2) / t
1/2
with units of sec
–1
, where t
1/2
is the nuclide half-life in seconds.
(C)
Calculated using
Eq 10
.
(D)
In
Fig. 1
, Θ = 4E
r
kT/AΓ
2
= 0.2 corresponds to the value for
109
Ag for
T
= 293 K,
∑
r
= N
0
σ
r,max,T=0K
σ
r,max,T=0K
= 31138.03 barn at 5.19 eV
(
13
)
. The value of σ
r,max,T=0K
= 31138.03 barns is calculated using the Breit-Wigner single-level resonance formula
where the
109
Ag atomic mass is A = 108.9047558 amu
(
14
)
, the ENDF/B-VIII.0 (MAT = 4731)
(
13
)
resonance parameters are: resonance total width Γ = 0.1427333 eV, formation neutron width Γ
n
= 0.0127333 eV, and radiative/decay width Γ
γ
= 0.13 eV, with a resonance spin J=1, and the statistical spin factor
where s
1
=
1
/
2
and s
2
=
1
/
2
are the spins of the two particles (neutron and
109
Ag ground state
(
15
)
) forming resonance.
3.2
The advantages of this approach are the elimination of four 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 cadmium; and (
4
) the potential for high dose-rate encountered when handling activated cadmium. In addition, the reactivity changes accompanying the rapid insertion and removal of cadmium may prohibit the use of the cadmium-ratio method. Self-shielding corrections are only important if the concentrations of cobalt and silver are large, but may be neglected for diluted alloys (<1 %). Studies indicate that the accuracy of the two-reaction method for determination of thermal neutron fluence is comparable to the cadmium-ratio method
(
16
)
.
3.3
The long half-lives of the two monitors permit the determination of fluence for long-term monitoring.
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