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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
ORNL-P-2434
CONF-660906-19
(Submitted for presentation at the International Conference on Nuclear Physics,
Gatlinburg, Tennessee -- September 12-17, 1966)
MACTC CFSTI PRICES
MASTER
SP 2 2 1966
HC. 84.00), MN SO
Differential Scattering of Neutrons from Oxygen and Intermediate States
J. L. Fowler and C. H. Johnson
Oak Ridge National Laboratory
Oak Ridge, Tennessee
Abstract
The explanation of the broad structure in the total neutron cross
section of dºo between 3 and 4 MeV is of importance to the understanding
of intermediate states. At 3.77 MeV we find under a 17 keV flo reson-
ance a 3 kev dz/2 resonance interfering with a large dzia phase shift,
and thus remove a difficulty in resonance parameter assignments at
the peak.

RELEASED FOR ANNOUNCEMENT
UU
IN NUCLEAR SCIENCE ABSTRACTS
LEGAL NOTICE
This report was prepared as an account of Government sponsored work. Neither the United
States, por the Commission, nor any person acung on behalf of the Commission:
A. Makes any warianty or representation, expressed or implied, with respect to the accu-
racy, v.o. npleteness, or usefulness of the information contained in this report, or that the use
of any i formation, apparatus, method, or process disclosed in this report may not Infringe
privately owned rights; or
B. Assumes any liabilities with respect to the use of, or for damages resulung from the
use of any information, apparatus, method, or process disclosed in this report,
As used in the above, “person acting on behalf of the Commission" includes any en-
ployee or contractor of the Commission, or employee of such contractor, to the extent that
such employee or contractor of the Commission, or employee of such contractor prepares,
disseminates, or provides access to, any information pursuant to die employment or contract
with the Commission, or his employment with such contractor.
Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
Session VIII
(Submitted for presentation at the International Conference on Nuclear Physics,
Gatlinburg, Tennessee -- September 12-17, 1966)
Differential Scattering of Neutrons from Oxygen and Intermediate States*
J. L. Fowler and C. H. Johnson
Oak Ridge National Laboratory
Oak Ridge, Tennessee
The resonance structure of neutron scattering from +0 between 3 and
4 MeV is of particular interest in connection with explanations of inter-
mediate states. There are several resonances of rather large reduced
widths., The narrow peak at 3.77 Mev, however, has created a problem
in the phase shift analysis of scattering data in this energy region.
Fossan, et al," from total cross section measurements, found the height of
this peak corresponded to a J = 5/2 resonance of 25 keV width. But in 100
proton scattering, the'F level whose excitation corresponds closely to
this one is a J = 7/2- resonance.+
We have remeasured the total cross section in good geometry with
5 keV resolutions as shown on the right hand side of Fig. 1. Assuming
the 3.77 MeV peak is a single resonance and correcting for the experimental
energy resolution of 5 keV, we find I = 19 keV, which is slightly less
than, but consistent with, th: 22 keV reported by Walton, et al,' and the
25 kev found by Fossan, et al.3 With a J = 5/2- assignment, we got a
reasonable fit to an angular distribution at the 3.77 Mev peak.? However,
as Lister and Sayre? have pointed out, we cannot fit angular distributions
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"Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
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Session VIII
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on either side of the resonance with a consistent set of phase shifts
and a 5/2- or 5/2+ assignment.
In a recent analysis we have arrived
at a solution to this problem. There is a narrow, ~3 keV, J = 3/2+
resonance under a ~17 keV J = 7/2- resonance. The large dal background
phase shift, Fig. 2, makes the narrow dzia resonance show up as a dip,
thus reducing the peak height in the total cross section. An attempt
to make a similar fit with a narrow s, 1 resonance was unsuccessful.
The left hand side of Fig. 1 shows the results of angular distribu-
tion of neutron scattering measured in the vicinity of the 3.77 MeV peak.
We used a liquid oxygen sample contained in a 1 mil wall Dewar flask de-
signed to minimize background scattering. The outer wall, which was
under atmospheric pressure, was 10 mils of stainless steel. The
scatterer was supported in the center of a shielded cavity about a
meter square anů 0.6 m deep.' An identical enpty Dewar flask, placed
in the neutron beam, allowed background measurement. We detected neutrons
scattered from the sample in a stilbene crystal from which pulses due to
gamma rays were depressed by pulse-shape discrimination. Since we measured
the direct flux by moving the crystal into the neutron beam, we were able
to obtain absolute differential cross sections. Our results have been
corrected for the effects of spurious neutron backgrounds, self-attenuation
and multiple scattering in the sample, finite angular resolution, and the
energy dependence of the detector efficiency.
The solid line through the angular distribution points is the result
of a least square phase shift fit to the data, with the phase shifts
- 3 -
which give the fit shown in the figure. We adjusted the resonance
parameters of the narrow dalo resonance and the folo resonance so as
to reproduce the total cross section shown as the solid line on the
right hand side of Fig. 1. While as it is evident we obtain a
reasonable fit to all the experimental information with the resonance
parameters listed beside the total cross section curve, it is possible
that further adjustment may improve the overall fit.
Figure 2 shows our phase shifts as a function of neutron energy, -
and includes results at lower energy from a previous analysis.' We
estimate the uncertainties in these phase shifts by a procedure described
in the literature.
The very narrow do lo resonance at 3.770 MeV
corresponds to a similar narrow do lo resonance in the vicinity of the
7/2- resonance in 177.4 This analysis shows two relatively broad dala
resonances, one at 3.40 MeV ~500 keV wide, and another at 4.191 MeV
70 keV wide. Also, there are two p resonances: a Pale 500 keV resonance
at 3.75 MeV, and a 100 keV P, lo resonance at 4.00 MeV. The broad dalo
resonance corresponds to ~1/2 of the single particle limit. There is,
of course, already a dz/2 resonance with a very large fraction of the
single particle limit at 1 MeV. The Palo resonance at 3.75 MeV is about
1/5 of the single particle limit, the pain one is 1/20 this limit, the
fr/2 one at 3.77 is about 1/10 of the limit.
Since near zero energy the hole states of 170 have odd parity and
the single particle states even parity, intermediate states arising from
two-particle one-hole excitations should have odd parity. But as we see
.4.
above, the state in the region 3 - 4 MeV with largest reduced width
(the 3.40 MeV state) has even parity. Recently G. E. Brown and
A. M. Green have described the low lying even parity spectrum of 100
and to in terms of mixing the usual states in the spherical shell
model with deformed states obtained by exciting particles out of a
deformed core. In their model, the rather wide 1/2+ and 3/2+ states
in 17 (Fig. 2) are interpreted as three-particle two-hole and five-
particle four-hole excitations mixed with shell model states. They
obtain reasonable agreement. The fact that the relative values of the
reduced widths of the dais, Palo, and Iris states near 3.5 MeV in Fig. 2
are in roughly the same ratio as those of the dz/2, P3/2, and fy/2 states
near zero energy in the 160 plus a neutron system may be a clue to the
explanation of the to spectrum in the 3 - 4 Mev energy region.
-.5
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References
1. J. L. Fowler and C. H. Johnson, Bull. Am. Phys. Soc. 10, 261 (1965).
2. D. Lister and A. Sayres, Phys. Rev. 143, 745 (1966).
3. D. B. Fossan, R. L. Walker, W. E. Wilson, and H. H. Barschall,
Phys. Rev. 123, 209 (1961).
4. S. P. Salisbury and H. T. Richards, Phys. Rev. 126, 2147 (1962).
5. R. B. Walton, J. D. Clement, and F. Boreli, Phys. Rev. 107, 1067
(1957).
6. J. L. Fowler, to be published in Phys. Rev., July 1966.
7. J. L. Fowler and H. o. Cohn, Phys. Rev. 109, 89 (1958).
8. G. E. Brown and A. M. Green, Nucl. Phys. 75, 401 (1966).
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Figure Captions
Fig. 1
+90 differential neutron cross sections in the vicinity
of the 3.77 MeV resonance.
Fig. 2
to neutron scattering phase shifts as a function of neutron
energy.
ORNL-OWO 16-1204
En - 3.850 :0.015
Bor -106
8pin + 32°, 89* * +124•
80 +148°,809 -6°
817 RESONANCE

En = 3.780 : 0.018
8577-1350
Boy+f*, date + 82
889 RESON., doo -4•
87 - RESONANCE
RESONANCES
Bu
12 -0.0015
{ POT. -53+ 300 (E-3.770]•
LEO- 3.772
IIIIIII
[Tasco 75
POT. - 0
En * 3.769
B
3.90
Ex 3.770 : 0.018
Boy -138•
8ply -11°, dan +76
81RESON., 884 -9
87= RESONANCE
3.88
3.86
3.84
En (MeV)
LLLLLLLL
o ($) (barns i
3.76
3.74
En = 3.767 + 0.017
854 -1390
804 -12°, dass +78°
80% RESON., 88% -13°
817 RESONANCE
3.72
3.70
17
6
5
4
3
2
1
0
og (barns)
En = 3.7600.018
80m =-144.
8p4-12°, 804+ 750
8172 = RESON., 804 -17
81% = RESONANCE
E, - 3.700 + 0.014
A
doua-130
1
84 =-21°, 804* + 720
884-46°, 80 -10°
8p7p = RESONANCE
لنلللننناه
0.8 Q4 0-04 -0.8
cos $ (C.M.)
FIG. 1
ORNL-OWG 64-10113R2
1.00
1.32
2.37
3.40 3.7573.77 -4.00
-FOWLER AND COHN-
—JOHNSON AND FOWLER-
883/2-
PHASE SHIFT (deg)
8p%
Los 12
R
1.0
1.5
3.0
3.5
4.0
2.0 2.5
En (Mev)
Fig. 2
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END
DATE FILMED
10/21 /66







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