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60
MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
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ORNE-B-2026
(Cont-660303-44) |
1966
(Submitted for Proceedings of the Conference on Neutron Cross Section
Technology -- Meeting in Washington, D. C., March 22-24, 1966)
APR 7
CFSTI PRICES
H.C. $1.00;MN: 50
Differential Elastic Scattering of Neutrons from Nitrogen
J. L. Fowler, C. H. Johnson, and R. L. Kernell
Oak Ridge National Laboratory
Oak Ridge, Tennessee
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In 1955 Fowler and Johnson? published a paper, "Differential Elastic
Scattering Cross section for Neutrons on Nitrogen," in which they made
angular momentum and parity assignments for some 6 resonances. Bartholo-
mew, Brown, Gove, Litherland, and Paul, in studying gamma ray angular
distributions from the 14c(p,)+n reaction, assigned even parit, for the
1.401 MeV and 1.595 MeV resonances in disagreement with the assignment
from the neutron scattering experiment. Since apparatus had already
been developed at the Oak Ridge National Laboratory to measure differen-
tial scattering from liquid oxygen, * we decided to remeasure the differ-
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ential scattering from nitrogen and extend the results to higher energies.
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In a shielded neutron scattering chamber, neutrons from the T(p,n) re-
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action produced by bombarding tritium gas in a cell with protons from
en
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the 5.5 M Van de Graaff are collimated by a double tapered slot through
a paraffin shield. These neutrons are incident upon a liquid scatter-
ing sample contained in a Dewar flask. We detect scattered neutrons
Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
Oak Ridge Graduate Fellow from the University of Tennessee.
2J. L. Fowler and C. H. Johnson, Phys. Rev. 98, 728 (1955).
45. L. Fowler and
SG. A. Bartholomew, F. Brown, H. E. Gove, A. E. Litherland, and E. B. Paul,
Can. J. Phys. 33, 441 (1955).
4J. L. Fowler and C. H. Johnson, Bull. Am. Phys. Soc. 10, 261 (1965).
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with a stilbene crystal froin which pulses due to gamma rays are sup-
pressed by pulse shape discrimination. Since we calibrated the detect-
ing crystal in the incident neutron flux at oº, we can calculate the
absolute differential cross section.
We have measured differential cross sections at 9 energies between
1.595 MeV and 3.207 MeV. Figure 1 shows our new results at 1.595 MeV.
Here we plot the center of mass differential cross sections as a
function of the cosine of the center of mass angle. The curve is calcu-
lated from the phase shifts and resonance parameters given in the earlier
paper,? and listed in the upper right hand corner. it is corrected for
the energy resolution of this work. We have assumed the s and p potential
phase shifts are the same in the two-channel spin states. As can be seen,
there is rather good agreement, particularly since there is no normaliza-
tion -- the differential cross sections are absolute. If this were a
J = 5/2+ resonance the theoretical curve would peak in both the forward
and backward directions as is indicated in Fig. 2 which is reproduced
from reference 2. In Fig. 2 the arrows indicate the energy at which the
measurements were made relative to the resonances in the total cross section
of nitrogen. Above 1.54 MeV the differential scattering reported in refer-
ence 2 was measured by observing the nitrogen nuclear recoil spectra
obtained with a carefully designed proportional counter. At the non-
resonant energy of 1.68 MeV, Fig. 3, the differential cross sections are
in agreement with those shown in Fig. 2.
Figure 4 shows our recent measurements on the differential cross
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LEGAL NOTICE. .
w This report mao prosarid u an account of Government sponsored work. Neither the United
Statu, nor the Commission, nor any person acting on behalf of Commandons
A. Makes my warranty or representation, express or implied, with respect to the accu...
racy, completeness, or usefulness of the information contained in the report, or that the we.
of may laformation, apparatus, method, or proceedincloned in the report may not intringo.
privately owind rigates or
B. Asums may labution with respect to the un of, or for dinegu remedies from the
w of ar taformation, apparatu, method, or procon decloud la this report. : . .
As und in the above, porinon sotto on behalf of the Communion" includes may e .
ploym or contractor of the Commission, or employw of much contractor, to the extend their
mnoh snoployme or contractor of the Commission, or employee of much coatructor pronarne,
dienominatot, or provide socou to, way Information pursuant to me employment or pontract
with the Commission, or Me employment with small contractor.
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sections at 1.779 MeV. Here, too, we agree with the 1955 assignment;
this is a d resonance. The difference between even parity levels and
odd parity ones, as is evidenced by the curves at 1.779 MeV and 2.25 MeV
(Fig. 2) is so marked that it is extremely difficult to see how we can
be mistaken with regard to parity assignments.
Figure 5 shows the non-resonant angular distribution at 2.00 MeV
with a least squares Legendre polynomial fit which includes terms up
through Pg. At 2.25 MeV (Fig. 2) there is a 70 keV wide resonance with
large (n,p) and (nga) widths. The new experimenta). angular distribution
(Fig. 6) measured with 15 keV resolution is also in agreement with the
1955 results, showing this is an odd parity level.
Furthermore, measurements on the
reactions confirm
several of our assignments. Lee and Schiffer? have measured the angular
distribution of protons at a number of resonance levels of +%n. Their
angular distributions at somewhat higher excitation than that correspond-
ing to our 1.595 MeV state show interference effects consistent with our
5/2- assignment for this state. The analysis of Lee and Schiffer agrees
with our assignment of .5/2+ for the 1.779 MeV resonance and 3/2- for the
2.25 MeV one.
We have extended our differential cross section measurements to
3.20 MeV neutron energy, and have found two more resonances, one less than
20 keV wide at 2.95 MeV (Fig. 9) and another of the order of 70 keV wide
at 3.20 MeV (Fig. 10). Both are characterized by forward and back scattering
'L. L. Lee and J. P. Schiffer, Phys. Rev. 115, 160 (1959).
of neutrons, indicating even parity. The lower one is very likely the
5/2+ resonance seen by Lee and Schiffer in the 'B(a,p)+4c reaction.”
Figures 7 and 8 show the differential cross sections at two non-resonant
energies. For Figs. 3 through 20 the solid lines are the results of a
least square Legendre polynomial fit including terms up through Pg
Since our more careful recent angular distributions conform the
earlier neutron scattering measurements,' and since in every case in which
Lee and Schiffers look at the same resonance we do with the 11(a, p)24c
reaction they agree with our assignment, we feel that the Chalk River
assignments must be in error..
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abrenoverat
r
ORNL-DWG 65-10486R
1.595 +0.015 MeV
j=572; b=1
=22 kev; n/p = 0.988
do=-840
8,=-40
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olbarns/steradian)
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1.0
0.8
0.6
0.4
0.2
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Fig. 1
Differential cross section of neutrons scattered from
nitrogen at 1.595 MeV. Solid line calculated from phase
shifts and resonance parameters listed in figure.
ORNL-LR-DWG 2518

11236
2.36 Mev
TTTTTTT 2.5
N
o. 2.25 Mey Tu=2, 11
AS ORI
J="id=1
T TS
SEY2 OR 32
2.3
2.07 Mev
1796 Mev
J=512,1=2
DIFFERENTIAL CROSS SECTION (barns/steradian).
NEUTRON ENERGY (Mev)
1.779 Mev
20
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1756
Ni Bolo
5=%
Ts=/
U
1.682 Mev
1
LUUIUIUI
14 3 2 0
(barns)
9.0
06 0.2 -0.2 -0.6 -1.0
COS $
Fig. 2
Differential cross sections of neutrons scattered from
nitrogen from 1.682 to 2.360 MeV.
ORNL-DWG 65-10489R
1.68
0.015 MeV
o(barns/steradian)
1.0
0.8
0.6
0.4
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2
0
-0.2 -0.4 -0.6 -0.8
-1.0

COS $
Fig. 3
Differential cross section of neutrons scattered from
nitrogen at 1.68 MeV.
ORNL-DWG 66-947
1.779 + 0.021 MeV
o (barns/steradian)
1.0
0.8
0.6
0.4
0.2
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COS$
-0.2
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-1.0

Fig. 4
Differential cross section of neutrons scattered from
nitrogen at 1779 MeV.
ORNL-DWG 65-10487R
2.000 10.015 MeV
o(barns/steradian)
1.0
0.8
0.6
0.4
0.2
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cos $
-0.2
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Fig. 5
Differential cross section of neutrons scattered from
nitrogen at 2.00 MeV.
ORNL-DWG 65-10488R
2.250 +0.015 MeV
o(barns/steradian)
1,0
0.8
0.6
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0.2
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cos $
-0.2
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4.0

Fig. 6
Differential cross section of neutrons scattered from
nitrogen at 2.250 MeV.
ORNL-DWG 66-948
2.540 + 0.018 MeV
o(barns/steradian)
1:0
0.8
0.6
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Differential cross section of neutrons scattered from
nitrogen at 2.54 MeV.
Fig. 7
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ORNL-DWG 66-949
2.742 + 0.017 MeV
o(barns/steradian)
1.0
0.8
0.6
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0.4 0.2 0 -0.2 -0.4
cos
differentiate contre section of ne
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Fig. 8
Fig. 8
Differential cross section of neutrons scattered from
nitrogen at 2.74 MeV.
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ORNL-DWG 66-950
IN
2.950 + 0.015 MeV
o (barns/steradian)
1.0
0.8
0.6
0.4
0.2
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-0.2 -0.4 -0.6 -0.8 -10

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Fig. 9
Differential cross section of neutrons scattered from
nitrogen at 2.950 MeV.
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ORNL-DWG 66-951
stort
3.207 MeV
o (barns/steradian)
4.0
0.8
0.6
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0.2
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COS $
-0.2 -0.4 -0.6 -0.8 -1.0

Fig. 1
Differential cross section of neutrons scattered from
nitrogen at 3.207 MeV.
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END
DATE FILMED
5 / 27 / 66
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