The total neutron cross section of O18 from 2.45 to 19.0 MeV

Nuclear Physics 64 (1965) 343--348; (~) North-Holland Publishinfl Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E T O T A L N E U T R O N C R O S S S E C T I O N O F O Is

F R O M 2.45 T O 19.0 M e V S. R. SALISBURY, D. B. FOSSAN and F. J. VAUGHN Lockheed Missiles and Space Company Research Laboratories Palo Alto, California t Received 9 October 1964 Abstract: The total neutron cross section of 018 has been measured from 2.45 to 8.50 MeV and from 10.6 to 19.0 MeV by a transmission experiment. The neutrons were produced using the D(d, n) He ~, Beg(u, n)C12, Nl~(d, n)O 16 and T(d, n)He4 reactions in the appropriate energy intervals. The scattering sample consisted of water enriched to 97 % in 018 enclosed in thin-walled stainless steel cylinders with a total length of 3 cm. In the energy range up to 8.50 MeV measurements were made with an energy resolution of 35-60 keV; considerable resonance structure was observed. The cross section in the higher energy region, observed with an energy resolution of 100-300 keV, was found to be a more slowly-varying function of energy. E I

I

NUCLEAR REACTION OlS(n), E = 2.45-1.90 MeV; measured anT(E). Enriched target.

1. Introduction I n the preceding paper x) total n e u t r o n cross section m e a s u r e m e n t s have been rep o r t e d for n e u t r o n energies u p to 2.47 MeV, which c o r r e s p o n d to a n excitation energy of 6.30 M e V i n the 019 c o m p o u n d nucleus. T o t a l cross sections at higher energies can provide a d d i t i o n a l i n f o r m a t i o n a b o u t levels i n 019 a n d m a y exhibit statistical effects in the energy region where overlapping o f levels occurs. Therefore the measurem e n t s have been extended to the n e u t r o n energy regions f r o m 2.45 to 8.50 MeV a n d f r o m 10.6 to 19.0 MeV. These m e a s u r e m e n t s are described i n the present article. Reference will be m a d e to the preceding paper for aspects o f the experiments which were the same i n b o t h sets o f m e a s u r e m e n t s . T o t a l cross section results o f O 1s have previously been reported 2) for the n e u t r o n energy range f r o m 2.82 to 4.17 MeV. A p o r t i o n of the excitation energy region examined in the present experiment has also been studied u s i n g the OlS(d, p ) O 19 react i o n 3). Recently a n extensive series o f m e a s u r e m e n t s o f a n g u l a r distributions o f elastically scattered n e u t r o n s for n e u t r o n energies u p to 2.702 MeV has been reported 4). The discussion of the present results will include considerations o f the pert i n e n t data f r o m these previous experiments. t This work was supported by the Lockheed Independent Research Programme and the U.S. Atomic Energy Commission. 343

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S. R. SALISBURY et aL

2. Experimental Procedures Cross sections were obtained from measurements of the neutron transmission of the H 2 0 t s samples described in the preceding paper 1). A scattering sample of length 3 cm was usually employed. At the higher neutron energies, even this length, which was the maximum available, was insufficient to produce a transmission of the optim u m magnitude. A polyethylene shadow cone was used to measure the effect of scattered neutrons at energies below about 3 MeV, at higher neutron energies a copper shadow cone was employed. For the source reactions involving the use of a gas cell, frequent measurements were made with helium in the cell to measure the effect of neutrons produced in the region of the cell but not in the gas itself. As at lower neutron energies, periodic measurements of the known carbon cross section 5) were made as a check on the overall operation of the system. In order to perform the transmission measurements over the desired neutron energy range using a 3.5 MeV Van de Graaff accelerator, it was necessary to use several source reactions. The reactions used are given in table 1, which also lists the energy intervals covered, the neutron energy spreads, the type of target employed with each reaction and the angles at which the measurements were made. The neutron energy spreads listed are the magnitudes of the energy intervals within which approximately 90 70 of the neutrons are included. The energy gap from 8.5 to 10.6 MeV resulted from the difficulty in producing mono-energetic neutrons of these energies within the limitations imposed by the Van de Graaff beam energy and the available source reactions. When gas targets were used, the target material was contained in a small gas cell 1 cm long, separated from the beam pipe by a thin Ni foil. The solid Be target consisted of a thin layer of Be evaporated onto a Ta backing. TABLE 1 N e u t r o n source reactions u s e d in the 0

Reaction D(d, n ) H e 3 Bea(~, n ) C lz Nl~(d, n ) O TM T (d, n) He 4

N e u t r o n energy interval (MeV) 2.45- 3.98 3.89- 6.45 6.40- 7.50 7 . 5 0 - 8.50 10.61-12.60 12.40-16.00 15.8- 19.0

TM

cross section m e a s u r e m e n t

N e u t r o n energy spread (MeV) 0.050-0.060 0.048-0.053 0.059-0.065 0.035-0.031 0.130-0.100 0.100-0.180 0.300-0.110

Target

gas solid gas gas

0L (deg) 100, 90, 70 0 100, 80, 60 0 0 150-- 70 0

Neutrons were detected with a stilbene scintillator, 3.8 cm in diameter and 5.0 cm long, which served as a biased proton-recoil detector. G a m m a ray discrimination was achieved by the use of space charge techniques, utilizing the difference in pulse shape

T O T A L N E U T R O N CROSS SECTION

345

between electron- and proton-induced scintillations in the stilbene. The operation o f the discrimination circuit was checked with a pulsed-beam time-of-flight system, by the use of which y-rays are easily isolated from the neutron peaks. To ensure elimination of pulses produced by neutron groups of lower energies, the energy bias of the detector was set with the aid of the time-of-flight system when the Be9(0~, n)C 12 and N 1s (d, n)O 16 source reactions were being used. The actual data were taken using an unpulsed beam. Data were normally taken at energies separated by approximately the neutron energy resolution, although more closely-spaced measurements were made in the region of resonances.

3. Results and Analysis The experimental data were analysed to obtain cross sections by computer calculations, using a code developed by the authors since the work described in the preceding paper was done. Cross sections for O 18, statistical uncertainties in these cross sections, neutron energies and neutron energy spreads were calculated using this code. The O 18 cross sections were obtained by subtracting the contributions of hydrogen and the small amount of 016 in the scattering samples from the total experimental cross sections. Neutron cross sections of hydrogen were obtained from G a m m e l ' s review article 6), while total cross sections of 016 were obtained from the work of Fossan et al. s) and Nereson and Darden 7). As in the preceding paper, the in-scattering correction was found to be negligible relative to the statistical uncertainties. The cross section results are shown in figs. 1 and 2. Fig. 1 includes the results for neutron energies up to 8.5 MeV, while the cross sections at higher energies are shown in fig. 2. Some of the points included in the figures are averages of cross sections measured with an energy interval less than the total neutron energy spread. Statistical uncertainties are indicated periodically on the figures by error bars. Neutron energy spreads are shown by the widths of the triangles near the base line. The energy regions covered by neutrons from the various source reactions are also indicated. The curves drawn in figs. 1 and 2 are intended to show the general behaviour of the cross section. A comparison of the results given in figs. 1 and 2 with those at lower energies presented in the preceding article shows that the errors are in general considerably larger for the present measurements. This was due partially to the fact that transmissions at the higher energies with the scattering sample length available were greater than o p t i m u m because both the 0 18 and hydrogen cross sections decrease with increasing neutron energy. This effect increased with energy, and was the principal one which produced the relatively large uncertainties indicated in the energy region from 10.6 to 19.0 MeV. In addition it was necessary to use thin targets with the D(d, n)He 3 and Be9(~, n)C lz source reactions in order to obtain the neutron energy spreads listed in table 1. The low neutron yield from these thin targets led to relatively low counting rates and hence relatively large statistical errors in the cross section.

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s . R . SALISBURY e t al.

The cross sections shown in fig. 1 indicate the presence of a considerable amount of resonance structure in the energy region up to 8.5 MeV. However, the combination of the relatively large uncertainties in the cross sections and the average neutron energy spread of about 50 keV makes it generally unfeasible to obtain parameters of individual resonances from analysis of the data.

D ( d, n) He3

~

Be9 (a,n) C t2

i

5

4

~3 "

T

2

o

o o

9

o

.[

o

o

o

o

.o

o

1

,A

0

a

A

A

A

,A

A

~

A

E n (MeV)

Fig. 1. Total n e u t r o n cross section o f 018 f r o m 2.45 to 8.50 MeV.

~ - - . Nt 5 ( d , n ) 0 t 6 ~ I

z~

T(d,n}He

4

t

2

¢

A 0

A I

t0

i 12

A i

I t4

A I

I 16

o

A I

A I ~8

I

E n ( MeV )

Fig. 2. Total neutron cross section o f 0 zs f r o m 10.6 to 19.0 MeY.

As previously stated, other results have been reported in the lower energy portion of the region observed here. Donoghue et aL 4) have measured angular distributions of elastically scattered neutrons for energies up to 2.702 MeV. The present total cross section results for neutron energies between about 2.5 and 2.7 MeV indicate the presence of a level at a neutron energy of about 2.58 MeV as well as a possible state at

TOTAL

NEUTRON

CROSS

SECTION

347

about 2.63 MeV. However, the reported analysis of Donoghue et al. did not extend to energies above the resonance at 2.445 MeV reported in the preceding paper. Schellenberg et al. 2) have measured the O x8 total cross section from 2.82 to 4.17 MeV with an energy resolution of about 50 keV, approximately the same as that employed here in the same energy interval. Levels of 019 at excitations corresponding to neutron energies up to 3.47 MeV have been observed by Yagi et al. a) using the O 18(d, p)O 19 reaction. ;

I

3 act

2

0

I 3

I 4

En (MeV)

Fig. 3. Comparison of present results with those of previous experiments. A comparison of the results o f Schellenberg et al. and Yagi et al. with those of the present experiment is given in fig. 3. The solid curve and the data points indicate our results, while the broken curve gives those of Schellenberg. The arrows near the base line indicate the position of levels observed in the (d, p) work; only those levels at 3.21 MeV and 3.47 MeV were attributed to O x9 with certainty. A comparison of the present results with those of Yagi et al. indicates that one or more high total cross section points were observed at approximately the energies of the levels seen in the (d, p) work, except for the level at 3.11 MeV, which corresponds in position only to a possible "shoulder" on the total cross section curve. Thus the present total cross section results are consistent with those of Yagi et al. However, considerable discrepancies exist between the present results and those of Schellenberg et al. The general trend of the cross section exhibited by the two sets of measurements agrees reasonably well, but the detailed behaviour is significantly different. The reason for these discrepancies is not known. Schellenberg et al. used a scatterer in the f o r m of a mixture of 46.6 ~o O ~a and 53.4 9/00 x6 gases and subtracted the contribution of the 016 by

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s . R . SALISBURY e t al.

m a k i n g m e a s u r e m e n t s with a sample c o n t a i n i n g 0 16 only. B o t h this p r o c e d u r e a n d the m e t h o d e m p l o y e d in the present experiment, which involves s u b t r a c t i o n o f the h y d r o g e n cross section, have possibilities o f c o n s i d e r a b l e error. The h y d r o g e n cross section s u b t r a c t i o n does, however, have the a d v a n t a g e t h a t the q u a n t i t y s u b t r a c t e d is a s m o o t h l y varying f u n c t i o n o f energy, while the 016 cross section exhibits a considerable a m o u n t o f r e s o n a n c e structure. T h e w o r k o f Ericson s) a n d others has i n d i c a t e d that useful i n f o r m a t i o n m a y sometimes be extracted f r o m o b s e r v a t i o n s o f cross sections in energy regions where overl a p p i n g o f levels occurs. This i n f o r m a t i o n is generally o b t a i n e d b y calculation a n d analysis o f a c o r r e l a t i o n function, defined b y the expression (tr(E)a(E+ e)) -
References

1) F. J. Vaughn, H. A. Grench, W. L. Imhof, J. H. Rowland and M. Walt, Nuclear Physics 64 (1965) 336 2) L. Schellenberg, E. Baumgartner, P. Huber and F. Seller, Helv. Phys. Acta 32 (1959) 357 3) K. Yagi et al., Nuclear Physics 41 (1963) 584 4) T. R. Donoghue, A. F. Behof and S. E. Darden, Nuclear Physics 54 (1964) 33 5) D. B. Fossan, R. L. Walter, W. E. Wilson and H. H. Barschall, Phys. Rev. 123 (1961) 209 6) J. L. Gammel in Fast neutron physics, Vol. 2, ed. by J. B. Marion and J. L. Fowler (Interscience, New York, 1963) section V.T 7) N. Nereson and S. E. Darden, Phys. Rev. 89 (1953) 1775 8) T. Ericson, Ann. of Phys. 23 (1963) 390