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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STAMOAROS - 1963
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ORNu-p-304
(Paper to be presented at the International Conference on the Study of
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A
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Session II
(R. B. Weinberg)
Energy Dependence of the 20B(n,a)/2°B{n, ay) Ratio*
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Monforte
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R. L. Macklin and J. H. Gibbons
Oak Ridge National Laboratory
Oak Ridge, Tennessee, USA
Introduction
The-B(n, ay) reaction is a convenient high cross section
standard in the 0.1-100 keV energy range for neutron reaction cross
section measurements [1]. For use as a standard it is usually necessary
to know the relative proportion of the reaction cross section leading
to the ground state versus the first excited state of 'Li. Earlier
studies of the cross section ratio (2-7] have been reported for thermal
and above 200 keV, but no results have been reported for the energy range
between these extremes. The suggestion of a broad resonance near 180
kev in the total cross section (8) calls into question the propriety of
smooth interpolation from thermal to 300 keV and has renewedeateterest in
the ratio measurement [9].
PATENT CLEARANCE OBTAINED." RELEASQUerest to
THE PUBLIC IS APPROVED. PROCEDURES
ARE AN EILE UN THE RISI:VING SECTION,
Research sponsored by the U.S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
Experimental
Two face-to-face silicon surface barrier detectors were used to
detect the two charged particles from the reaction in coincidence. After
experiencing breakage of self supporting foils and detector damage with
°B evaporated directly on the gold surface barrier, we tried “B_H
(10) at 0.1 atmosphere pressure. In this gas the detectors drew high
current and became more noisy. This effect was variable (worse for
some detectors than for others) and two detectors were found which gave
acceptable pulse height spectra for about two weeks. Exposure to air
appeared to reduce the current and noise toward initial values.
Pulsed neutrons were provided at a 500 kc/sec repetition rate by
the 'Li(p,n) reaction at the ORNL 3 MV pulsed Van de Graaff. Pulses from
the two detectors, passed through fast discriminators to eliminate noise
and most Y-ray pulses, operated a 50 ns coincidence unit whose output
started a time-to-pulse-height converter. The bias on the fast discrimi-
nators was set low enough (500 keV) to ensure detection of the minimum
energy (180 degree) lithium recoils. Two time intervals were thus
chosen, one corresponding to fast neutron arrival and the other to slow
and thermalized neutron and other time-independent backgrounds. Summed
detector pulses were stored in sections of a multichannel analyzer corres-
ponding to these two time intervals.
Results and Discussion
A thermal neutron spectrum (obtained with a wax moderator) 18 given'
in figs. 1 and 2. The single counter spectrum (fig. 1) shows peaks
* ORTEC Model 780 SHe Spectrometer.
corresponding to the two alpha groups and the two 'Li groups. The sharp
rise at lower pulse height is related to y rays from the lithium target.
The sum-coincidence spectrum is shown in fig. 2. The resolution is
| quite adequate to measure the areas corresponding to capture leading to
the 'Li ground and first excited states. The average ratio for four runs
was 0.067 + .002 in excellent agreement with recent measurements (5-7).
The thermal spectrum was used routinely to standardize electronic gains
.
and threshold settings.
At (30 + 10) kev (forward angle neutrons from 'Li (p,n) near
threshola) the ratio (0.072 + .003) is barely increased over the thermal
value, in agreement with recent Harwell results [9). At (110 + 15) kev
(0.077 .004) and (160 + 15) keV (0.084 = .005) perceptible increases
are observed. At higher energies the pulse height resolution became
progressively worse (mostly due to longer exposure of the detectors to
B_HG) and difficulty was experienced in determining the ratio. The
experiment was terminated at 700 kev, just below the 'Li(p,n') threshola.
Values obtained are indicated in Fig. 3 where they are compared with
results of other investigators (1-3).
We feel that the results collectively indicate an effect, albeit
small, of the resonance near 530 keV on the cross section ratio. Our
results show that the ratio remains within about five percent of the
thermal value for neutron energies up to about 30 keV, after which it
rises in a slow, continuous fashion. These new values for the cross
section ratio in the 7 < E. < 170 keV range cause a change of less than
1.5 percent (less than 0.5 percent for E< 100 kev) in the shape of
the (nory) cross section assumed for flux calibration in our earlier
work on heavy element capture cross sections [1].
Acknowledgements
Interest in this work was particularly stimulated by the EANDC
Conference at Oxford in 1963. P. D. Miller planned and carried through
much of the preliminary work on this experiment. P. S. Rudolph kindly
provided traces of radon for early calibration or the spectrometer.
References
1. J. H. Gibbons, R. L. Macklin, P. D. Miller, and J. H. Neiler,
Phys. Rev. 122, 182 (1961).
2. B. Petree, C. H. Johnson, and D. W. Miller, Phys. Rev. 83, 1148 (1951).
3. H. Bichsel, W. 18, P. Huber, and A. Stebler, Phys. Rev. 81, 456
(1951).
4. E. A. Davis, F. Gabbard, T. W. Bonner, and R. Bass, Muclear Phys.
27, 448 (1961).
5. S. Malmskog, Physica 29, 987 (1963).
H. F. Brinkmann and D. Greber, Kernenergie 3, Heft 4/60 (1960).
7. J. A. DeJuren and H. Rosenwasser, Phys. Rev. 23, 831 (1954).
8. C. K. Bockelman, D. W. Miller, k. K. Adair, and H. H. Barschall,
Phys. Rev. 84, 69 (1951).
9. M. G. Sowerby and M. C. Moxon also A. K. M. Siddiq and N. Gale in
Harwell Nuclear Physics Division Progress Report AERE-PR/NP7 (1964).
10. H. L. Holsopple and L. E. Scroggie "Synthesis and Infrared
Absorption Spectrum of Diborane-2°B" Report ORNL-TM-1061, February
19, 1965.
Figure Captions
Fig. 1.
Pulse height distribution for thermal neutrons from 'B(n,a,m)'Li
in a single surface barrier counter. Points indicated on the
figure correspond to (a) ground state alphas, (b) alphas to the
first excited state of 'Li, (c) 'Li nuclei from the ground
state reaction, (a) 'Li nuclei from the reaction leading to the
first excited state, (e) upper "edge" of pulses due to gammes
from '(p,p'r), (f) pulse height threshold caused by fast
discriminator gate.
Fig. 2. Summed-coincident pulse height distribution for thermal neutrons
from *°B(n,,, ) L1 in the spectrometer. The two peaks correspond
to the total charged particle reaction energy from the two
reaction branches.
Fig. 3. The energy dependence of the branching ratio, la/09), in the
reaction -°B(n, a)'li, **Li. The solid points are due to the
present studies; open triangles are from ref. 4; open circles
are from ref. 2; the cross is from ref. 3.
ORNL-OWG 65-3934

2000
1500
RELATIVE COUNTS
500
0.5
1.5
2.0
9.0
CHARGED PARTICLE ENERGY (MeV)
ORNL-DWG 65-3935
30,000

25,000
20,000
RELATIVE COUNTS
10,000
5000
1.0
...............000
2.0
CHARGED PARTICLE ENERGY (MeV)
3.0
Fig. 2

ORNL-DWG 65-3936
10B (n, co//'OB in, ap) RATIO
THERMAL VALUE
0.014
10
20
50
500
4000
100 200
En (kev)
Fig. 3
END
DATE FILMED
8 / 27 /65






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