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NATIONAL BUREAU OF STANDARDS - 1963
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LEGAL NOTICE
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that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
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-LEGAL NOTICE –
The report me prepared us ua account of Gorenmrat sponsored work. Haltbar the United
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viu the Commission, or wo employment with ouca contractor,
S-Wave Neutron Strength Functions of Isotopes in
the 3-8 Resonance Region 40 <A 70*
R. Wagner, W. M. Good, and D. Paya"
Oak Ridge National Laboratory
Oak Ridge, Tennessee
INTRODUCTION
There is an important class of nuclei which has not so far been
systematically studied in the mass region of the 38 strength function
resonances, viz: the even-even compound nuclides consisting of **ca, *071,
somi, 5+cr, 5iFe, Ni. These nuclides have -4, -2, 0, +2, +4, +6 neutrons
respectively relative to the closed 28 neutron shell. They have not been
studied extensively because of the low relative abundance of the correspond-
ing even-odd target nucles. These nuclides are the object of the present
investigation.
DESCRIPTION OF EXPERIMENT
The pulsed Van de Graaft at ORNL by utilizing the Ll(pa) reaction
and a relatively thick target can be exployed to measure transmissions by
time-of-flight in the energy range 2 keV - 60 keV with an energy precision
Research sponsored by the U. S. Atomic Energy Commission wder contract
with the thai.on Carbide Corporation.
Present address: Paysikalleches Institut der Universitat Basel, Basel
Switzerland.
PP present address: Commissariat A L'Energie Atomi que, Centre d'Etude
Mucleaires de Saclay, Saclay, France.
LENT CLEARANCE OBTAINED. RELEASE TU
THE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IN THE RECOVING SECTION,

TOTAL CROSS section
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ا نسانية
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TOTAL anoss OCT

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1
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TOTAL CROSS SECTION
سم. .. ضه...نننننننننننمتنمتنتنتن نیستمین ::... ...

ممه
د
.مم
امید
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توجه
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هذا ان
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:::::
::::::::
.Fig. 1. Results of total cross section measurements.
2.
of better than 2%. This order of resolution is adequate to obtain considerable
information on nuclides whose level spacings exceed, say, 2 kev, such as light
nuclides and nuclides near closed shells. The instrument is adapted for such
studies, which usually require separated isotopes, because the cource of
neutrons is small (1.e. bright) for which reason it is possible to use small
samples.
RESULIS
Figure 1 exhibits the total cross sections of the nuclides under investi-
gation while Table I summarizes the s-wave strength function and average level
spacing. 1994 represents the compound nucleus with 28 neutrons, and it is
evident that there is a striking change in the general appearance of the cross
section versus energy when the new shell starts filling. A change in level
spacing is expected to occur near ciosed shells. However, the Wigner distri-
bution of nearest neighbor spacings which represents a "repulsion" of neighboring
levels and tends to make for a more nearly uniform distribution is still expected
to maintain a dominating influence which is not evident in either 53cr or 57Fe.
... -- -- - - -- --....
In fact the distributions, Figs. 2
and 3, on reduced width amplitudes and
spacings for 53cr, 5Fe, and vi, show
a tendency not to obey the usual Porter-
Thomas and Wigner distributions. Such
a failure might constitute evidence
Fig. 3. 010
that the levels involved are exceptionally
Fig. 2.
simple configurations rather than the com-
plicated ones implicit in the Porter-Thomas and Wigner distributions. From Table I


CS3 + Fast & Nill
31 INTERVALS
CS3 4 Fg5? nie
34 LEVELS
RVALS/O.
OER O HITERV
NVOER OF WOT
M
iriyen
Histogram of the Distribution of Widths
for the three Nucial Cros. Fo37NIⓇ
Histogram ol Distribullon of Noorust Nolgn.
bor Lovol Sposings for the Three Nuclol Cros
Foo?, NICI,
and Figs. 2 and 3 we conclude that the strength function is not influenced by the
-3-
Table I
Nuclide
(kev)
in units 10-4
(kev)
43ce
HII
HITS
2-60
53cr
2-40
2-36
1.4 + 0.7 3.3 0.6.
2.6 0.9 2.6 t 0.4
2-60
2.74 1.4 5.6 4 1.0
5.1 + 2.6
3.0 + 0.7
3-15
12.4 + 8
2-55
3.7 4 2.6
20 – 3
2-50
2.5 + 0.9 . 204 + 0.6
Summary of 8-wave strength functions and average level spacings
obtained from the measurements of Fig. 1.
anomalies that accompany the closing of a major shell; however, there may be
a breakdown in the statistics of the widths and spacings.
stre
6181
IMiD
The 38 Giant Resonance Region
Enough data are now available to warrant classifying the nuclei in the
region of the 38 giant resonance according as the compound nuclei are even-even,
odd-odd, or even-odd. Figure 4 shows such a classification ! 9111
of the existing s-wave strength function data as obtained i
from individual resonance analysis. The solid curves are
subjective interpretations of the data points and the dashed
curve 18 the caloulation of Buck and Perey. As these data
stand at present, it is not possible to make any convincing
new claims about the mass region around and beyond the 38
resonance. However, there are suggestions in Fig. 4 that
Fig.4. S-Wave Strength
seem to warrant improving statistics and supplying data: Functions of 0-0, E-E,
0-E Nuclides.
that are missing altogether in order to determine whether
or not (a) the Duck-Perey calculation disagrees with the even-even target mualai
elle

S
.
.
for which the theory is intended to apply, (b) there are detectable differences
between the E-E and O-O compound nucleus situatioa; (c) there may be a minimum
in soºjust before mass 90.
1. B. Buck and F. Perey, Phys. Rev. Letters 8, 464 (1962).

END
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