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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963
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will be commission, or ble employment with suck coatractor,

Leve: spacing and s-Wave Neutron Strength Function of the Odd-Z Odd-N Nuclide
138,
of
the Forgon
J. A. HARVEY and G. G. SLAUGHTER
Oak Ridge National Laboratory
Oak Ridge, Tennessee, USA
Although a vast amount of information is available on the level spacings
and s-wave neutron strength functions of heavy nuclides, little is known about
the neutron cross sections of the heavy odd-2 odd-N target nuclides. There are
only a few stable odd-2. odd-N isotopes and these occur in nature in very low
abundar.ce compared to the oud-Z even-N isotopes. Enriched samples are needed
un order to be able to assign resonances to these odd-2 odd-N nuclides. Often
more resonances in a low abundant isotope can be observed with a small enriched
sample than with a large sample of natural enrichment. In natural lanthanum,
15° La is abundant to only 0.089%. The enriched sample available for these
neutron transmission measurements was a 1.17-gram sample of La, Oz enriched to
7.03% in *°La. The oxide powder was packed into a small sample holder with
a cross section 0.050" x 1.063" which resulted in a sample thickness of
0.01220 x 20*24 atoms of Lil per cm. .
Transmission measurements from 0.2 to 2000 eV in 4 different energy
ranges were made with the ORR fast chopper time-of-flight neutron spectrometer
using 3F, proportional counters at the 45 meter flight station. The energy
resolution varied from 1 to 2% in energy below 50 eV and was 60 nanoseconds
per meter above 50 eV. The open beam counting rate was 0.5 counts per minute.
per 0.5 usecond channel and the room background was ~ 10% above 50 eV and less
below so ev. The backgrounds were determined 'oy the technique of "b).acked-out
resonances", such as the 132-eV cobalt resonance. The sample was alternated
L'ATENT CLEARANCE OBTAINED. RELEASE TO
THE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IN T HE CIVING SECTION.
NDIO AC-138..::1
ONNL - AEC - OFFICIAL
ORNI - AC-DEFICIA!
LORNI - AEC - OFFICIAL
with the background sample in about & l-hour cycle with each transmission
measurement requiring a few days to obtain sufficient counts. Transmission
data vs time of flight sbtained with 0.5 usecond channel widths are shown in
Figure 1 from 450 to 160 eV and from 110 to 60 eV. The statistical accuracies
of the transmission points are a 4.5% and the resolution 18 ~ 5.7 channels..
ine transmission data have been analyzed using the area analysis programtid
with a value for r, of 0.21 eV. The parameters for the 10 resonances assigned
to +5°La are listed in Table I. The large resonance in Figure 1 at 72.3 eV
and another resonance at 1200 eV are due to ->12. The resonances at 3.08
and 21.1 eV have been reported previouslyted from measurements with a b la
sampie enriched to only 0.597%.
From the parameters of the resonances up to 160 eV and a 40% correc-
tion for "missed" resonances, a value of 46 £ 15 eľ was obtained for the
average level spacing per spin state. Since the spin of 15° La 16 5, the
average level spacing refers to the spacings of levels with spins 9/2 and 11/2
with odd parity: From ühe parameters of the 8 resonances up to 280 eV and a
16% correction for "missed" resonances,& value of (1.0 t 0.5) x 10^* was
obtained for the s-wave neutron strength function. This value is somewhat
lower than theoretical predictions24] which range from 2 to 3 x 10"" for this
mass number.
The radiation width of the 3.08-eV resonance was determined to let
0.099 + 0.006 eV from shape analysis of the transmission data of the 150
sample. The error includes contributions due to the uncertainties in the
resolution and the Doppler broadening. This value of r, 18 in good agreement
. with the value of 0.11 0.01 eV obtained using a natural lanthanum sample.
mento mis maith and the best
a
Admi
!
indisso-35! INGO
-2-
ORNI - AEC - OFFICIAL
.
TABLE I
Parameters of Resonances of 138La
ORNI - ACC - OFFICIAL
E. (ev)
2870 (mv)*
3.08 $ 0.03
21.1 10.1
66.7 + 0.3
89.2 $ 0.4
130.4 $ 0.7
201 & 2
237 $ 2
261 53
300 $ 6
308 16
0.73 + 0.03
0.96 $ 0.05
2.2 10.3
2.5 t 0.4
4.8 11.1
4 1 2
13 15
14 54
+
+
a awn
+
17
6 6
+
24
18
*BPO = 28 (g/VE. (in ev).
1]
REFERENCES
S. E. Atta and J. A Harvey, ORNL-3205 (1961) and Addendum: Phys. Div.
Ana. Prog. Rept. Jan. 31, 1963, ORNL-3425, page 46.
R. C. Block, G. G. Slaughter and J. A. Harvey, Phys. Div. Ann. Prog.
Rept. March 10, 1959, ORNL-2728, page 26.
37 T. Fuketa and J. A. Harvey, Nuclear Instruments and Methods 33, 107
(1965).
247 D. M. Chase, L. Wilets and A. R. Edmonds, Phys. Rev. 110, 1080 (1958);
F. Perey and B. Buck, Nuclear Physics 32, 353 (1962).
Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
ORNI - AEC - OFFICIAL
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Fis. 1. Transmission vs. time of flight.
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END
DATE FILMED
18/ 27/65







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