Low energy total cross section of $sup 36$Ar Page: 2 of 3
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THE 10. ENERGY TOTAL CROSS SECTION OF 3Are
S. F. Hughabghob and B. A. Magurno
Brookhaven National Laboratory
Upton. New York 11973
To compare the predictions of the valence model with measured partial radiative
widths of Ar-36 an accurate knowledge of the bound-level parameters is required. This
is achieved by carrying out a Breit-Wigmer parmeter fit to the total cross section of
Ar-36 mesured by Chrien et al and renormalised to the recommended values of the thermal
capture and scattering cross sections. The result is as follows:
t . -10 keV, : - 92.3 eV, T - 1.26 eV
(paraters of bound level of Ar-36)
The purpose of the present study Is to determine
the resonance parameters of the s-wave neutron bound
state of Ar-36 in the light of recent cross sections
measurements. Accurate knowledge of the reduced
neutron and total radiative widths is an important
requirement for the calculation of the valence capture
component in Ar-36. The calculation can then be com-
pared with experimental capture spectre ats. This is
carried out in a separate contribution to this
Previously. Chrien, Jain and Palavsky(S)eWasured
the total cross section of Ar-36 in the energy range
from 0.1 eV to about 10 keV. From chase esasurements.
the authors(a derived parameters for a bound level on
a basis of a scattering radius of R*- 4.5 lfuia'.
Total cross section measurement by Ranshaw a at a
neutron energy of 0.076 eV showed that * . 77*9b. A
determination of the thermal capture cross action of
Ar-36 by McMurtrie and Crawford(S)gave 0- - 6.5*1.0b,
based on an isotopic abundance of 0.307% for Ar-36.
Shan this value is normalized to an abundance of 0.34%
for Ar-36, one obtains O. - 5.9*0.9b. This is in
agreepept with a value of 4.80.5b reported by Von
Will. In addition, a comparison of th thermal
capture grctra measured by Von Wille and Hardell.
and Beer favors a thermal capture cross section of
5.Ob for Ar-36. Thermal scattering moasuramen t by
Krohn and Rinao and later by Andriesse e atl gav
0 - 73.710.4b and 742b respectively. In this evalu-
ation, thermal capture, scattering, and total cross
section values of 5.0k0.5b, 73.7*0.4b, and 78.7*0.6b
respectively were adopted for Ar-36. The latter value
is u 9ized to renormalize the total cross section
Analysts. Results. and Discussion
To calculate the total cross section of Ar-36, a
knowledge of the nuclear scattering radius R' is an
additional requirement, This is achieved by deriving
R' -from Ar-40 data by two methods:
(1) A Breit-Wigner multilevel para ric fit of
the total cross section of natural argon (99.59.
Ar-40) gave R' - 2.8 Em.
(2) A coherent chattering amplitude for Ar-40,
acah = 1.83*0.05 fm, and use of the relation:
acal l ' - 2.34
with the aid of the resonance parameters of Ar-40 as
*Research supported by U.S. Energy Research and
reported in BNL-325 (1973) implied R' - 2 6 1.. This
is based on the reasonable assumption that bound level
contributions In Ar-40 are insignificant. This is
essentially justified in vi'w of the good agreement
between this result and that derived by methad(l).
It is interesting to note that a value of R - 2.6
is consistent with a minimum in R' in this mass region
as obtained from optical model calculations.
By constraining the thermal capture, scattering.
and total cross sections to the adopted values
mentioned previously, various combinations for the
parameters of the bound level are derived. Fig. 1
shows the calculated Breit-Wigner total cross sections
for three such sets illustrated in Table 1.
set 0 so (kov) r* (OV) I'(ev)
I - a 73-82 1.01
2 -10 92.27 1.26
-15 138.4 1.9
As described in Fig. 1, the best fit to the
experimental data particularly at high neutron energies
is achieved for the case of set 2. In addition, with
these parassters, the Wescott g factors for capture
and scattering were calculated. The results are:
for capture, g = 1.0016
for scattering, g - 1.1128
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Mughabghab, S.F. & Magurno, B.A. Low energy total cross section of $sup 36$Ar, article, March 1, 1975; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc1017654/m1/2/: accessed November 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.