Production cross-sections and neutron polarization in (p,n) reactions at 143 MeV

Nuclear Physics 41 (1963) 177--184; @ North-Holland Publishing Co., Amsterdam 2.B

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Not to be reproduced by photopriat or microfilm without written permission from the publisher

PRODUCTION CROSS-SECTIONS AND NEUTRON POLARIZATION IN (p,n) REACTIONS AT 143 MeV P. H. BOWEN, G. C. COX, G. B. HUXTABLE, J. P. SCANLON, J. J. THRESHER. t

Atomic Energy Research Establishment, Harwell, Berks., England A. L A N G S F O R D tt

Clarendon Laboratory, Oxford, England and

H. APPEL ttt

University of Mainz, Germany Received 14 September 1962 Abstract: Measurements have been made of the energy spectra and polarizations of neutrons emitted at a laboratory angle of 45 ° from deuterium, lithium and aluminium bombarded by protons of 143 MeV. The results are consistent with the view that neutron production from deuterium proceeds mainly by single quasi-free proton-neutron scattering but that multiple scattering effects predominate in heavier nuclei.

1. Introduction Measurements have been reported 1) in which the production of polarized neutron beams was investigated using a neutron time-of-flight spectrometer in conjunction with the Harwell synchrocyclotron. Neutrons were produced by bombarding thick targets of lithium deuteride and aluminium with protons of 143 MeV. Interpretation of these results was hampered by the broad effective energy spectrum of the incident protons resulting from their large energy loss in the targets. However, it appeared that neutron production from deuterium could be explained largely in terms of single quasi-free p-n scattering but production from heavier elements required some additional mechanism to explain the observed sign of polarization which, for neutrons emitted at 45 °, was opposite to that observed for free p-n scattering. In the present experiment the interaction energy was defined more precisely, the thickness of the targets being reduced so that the proton energy loss in passing through them was only 3 MeV. Measurements were made of the polarization and intensity of neutrons with energies in excess of 16 MeV emitted at a laboratory angle of 45 ° from targets of lithium hydride, lithium deuteride and aluminium, results for deuterium being obtained by a difference method. t N o w at the National Institute for Research in Nuclear Science, Harwell, Berks., England. tt N o w at the Atomic Energy Research Establishment, Harwell, Berks, England. ttt N o w at the Technische Hochschule Karlsruhe, Karlsruhe, Germany. 177 March 1963

178

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2. Experimental Procedure Neutron polarizations were determined by measuring the asymmetries in the scattering of neutrons at a small angle from uranium, a method which has been discussed in ref. 1). With the exception of the targets, which could be interchanged remotely, the apparatus was the same as that used previously, the electronic arrangement being that described in the appendix of ref. 1). Neutron spectra were measured as a function of time-of-flight (and hence energy) using a scintillation counter whose detection efficiency had been measured as a function of energy in a separate experiment 2). The background of neutrons which could be detected on removing the targets was found to be negligibly small. Corrections to the spectra were made for absorption and scattering losses in the air in the flight path. To obtain the spectrum of neutrons from deuterium, an accurate comparison of the spectra from lithium hydride and lithium deuteride was necessary. Accordingly a number of measurements each of two minutes duration were made alternately for the two targets keeping the operating conditions of the cyclotron as uniform as possible. The variation in the numbers of neutrons detected in the two-minute periods indicated an overall accuracy of 4-1 ~o in the relative normalization of the two spectra. 3. Results The relative cross-sections for neutron production at 45 ° from the interaction of I43 MeV protons with deuterium, lithium and aluminium are presented in fig. 1. The errors shown are those arising from counting statistics and from the uncertainty in determining the variation in the detection efficiency of the counter as a function of energy. The cross-sections for deuterium and lithium have a relative error of 4-1 ~ while the corresponding uncertainty in comparing them with the production cross-section for aluminium is _+ 15 9/o owing to possible variations in beam deflection conditions. An estimate was made of the circulating proton current and of the fraction which struck the target, in order to obtain approximate values of the absolute production cross-sections. There is an estimated error of + 50 ~ in making this normalization but within these limits the results shown on the arbitrary cross-section scale in fig. 1 may be interpreted as the absolute production cross-section in/~b, sr- t . MeV- 1. The values obtained for the polarizations of neutrons from lithium and aluminium between 16 and 125 MeV, divided into eleven energy bands, are shown in fig. 2. The corresponding energy resolutions are indicated by the broken lines shown at the foot of the figure. The results for deuterium, evaluated from the difference in counting rates measured for the lithium deuteride and lithium hydride targets, contain relatively large random errors and have therefore been divided into only three energy bands. The central band extends from 45 to 75 MeV and contains the broad peak in the neutron spectrum. The other two bands cover the ranges from 16 to 45 MeV and from 75 to 100 MeV.

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4. Discussion 4.1. N E U T R O N

ENERGY

SPECTRA

The neutron spectrum from deuterium, which extends from the experimental lower limit of 16 MeV to 100 MeV, has a broad but well defined peak centred at 58 + 2 MeV. It would seem likely that the major contribution to this peak arises from single quasi-free p-n scattering, but that an underlying continuum of neutrons from double scattering processes, involving two separate nucleon-nucleon collisions within the deuteron, is present at all energies. The position of the peak is approximately 12 MeV lower than would be expected for free p-n scattering. A comparable displacement, for a proton energy of 84 MeV, was observed by Hoffman and Strauch 3) who found that the displacement increases steadily with increasing scattering angle. A possible explanation is that as the momentum transfer increases the "spectator" proton plays a larger part in the interaction and so carries away more of the available energy. A value for the neutron production cross-section has been found by integrating over all neutron energies, the contribution from neutrons with energies less than 16 MeV being estimated by extrapolating the spectrum shape to zero energy. In view of the 50~o absolute normalization error, the uncertainty involved in making the extrapolation is negligible by comparison. A value o f 4.5+2.3 mb/sr was obtained, and is to be compared with the corresponding laboratory cross-section for free n-p scattering of 7.3 mb/sr. For heavier elements, the contribution from double and higher order collisions within the nucleus would be expected to be more pronounced and this is consistent with the observed neutron spectra for lithium and aluminium, both of which have a continuous distribution of neutrons up to approximately 140 MeV. No peaks are observed though there is some structure in the spectrum from lithium at about 60 MeV. This may be attributable to a relatively small effect from single scattering which is masked by the continuum. Values for the production cross-sections were found as described previously for deuterium and were 11 and 33 mb/sr for lithium and aluminium, each with a + 50 ~ absolute normalization error but having a relative accuracy of + 15 ~ . Measurements of cross-sections and energy spectra for the inelastic scattering of 150 MeV protons from carbon at 30 ° and 60 ° have been made by Radvanyi and G6nin 4). Though comparison with the present results can only be made in general terms because of the different elements and scattering angles used in the two experiments, one interesting feature emerges. Whereas the integrated proton cross-sections, which are 75 and 25 mb/sr for 30 ° and 60 ° respectively, are comparable with the neutron values, there is a marked difference between the neutron and proton spectra. The former, both for lithium and for aluminium, exhibit a marked rise towards low neutron energies, while the proton spectra for carbon are peaked, with a fall towards low energies.

(13, n) REACTIONS AT 148 Mev 4.2. N E U T R O N

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POLARIZATION

The polarization of neutrons emitted at a laboratory angle O, as a result of a single quasi-free scattering of 143 MeV protons by neutrons bound in nuclei, would be expected to be similar to that resulting from a free p-n scattering at the corresponding centre-of-mass collision angle (180-20)% A curve of the polarization of the recoil neutron in free p-n scattering as a function of centre-of-mass scattering angle is shown in fig. 3 and is based on measurements of the scattering of polarized neutrons by hydrogen at 128 MeV s) and 140 MeV 6). Donaldson and Bradner 7) have measured the left-right asymmetry of neutrons emitted from carbon and beryllium bombarded with 285 MeV polarized protons. By confining the kinematics of the process to Polarisation

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correspond to a single quasi-free p-n scattering in the nucleus, the asymmetry was found to be in close agreement with that observed in free p-n scattering. For a scattering angle of 45 ° and an incident proton energy of 143 MeV (the conditions obtaining in the present experiment) a negative neutron polarization with a magnitude of 0.17 is predicted if the effects of refraction at the nuclear surface and the nucleon momentum distribution within the nucleus are ignored. (Squires s) has concluded that the effect of the nuclear spin orbit potential should be small.) In the present experiment, however, the scattered particles were not defined in this way and neutrons resulting from double and higher order nucleon-nucleon collisions, with energies from 16 MeV up to the maximum value permitted from kinematic considerations, could be detected. A detailed calculation of polarization as a function of energy for such neutrons would be extremely complicated and has not, to the authors' knowledge, been attempted. However, Squires s) has shown semi-quantitatively that the high energy neutrons emitted at wide angles as a result of multiple nucleon-nucleon interactions can have the opposite sign of polarization to that given by a single quasi-free p-n scattering through the same angles. In particular he concluded that neutrons of approximately 100 MeV, emitted at 55 ° from beryllium bombarded by 165 MeV protons should have a polarization of about +0.25, in agreement with the observations of Stafford, Tornabene and Whitehead 9). His arguments are readily applicable to the present results. For a double scattering process, maximum phase space is available when the scattering angles in the two interactions are approximately equal. Thus, for neutrons of 100 MeV emitted at a

182

P . H . BOWEN et aL

laboratory scattering angle of 45 ° for a bombarding energy of 143 MeV, the most likely individual scattering angles are both approximately 22½° (lab.). At the corresponding barycentric angle (135 ° for p-n scattering, 45 ° for identical particles) the sign of polarization is opposite to that of a single scattering through 90 ° (e.m.) as shown in fig. 3. Taking into acccount the various possible combinations of p-p, p-n, n-p and n-n interactions and the possible range of values of the depolarization parameter in the second scattering, the above arguments predict positive polarization with a magnitude of about 0.2. The polarization for lower energy neutrons resulting from double collisions is much more difficult to predict. The individual scattering processes must, on average, take place through larger angles, a further complication being that as the energy is reduced, the two consecutive scatterings will tend to become increasingly noncoplanar. However, as the individual scattering angles increase the polarization would be expected to reduce in value and could become negative (see fig. 3). In addition the effects of triple and higher order scattering will become more important at low energies. The general features of the experimentally determined polarizations are in agreement with these predictions. For deuterium, the average value of -0.17-t-0.06 obtained in the energy band from 45 to 75 MeV is consistent with single quasi-free p-n scattering. The relatively small number of neutrons observed above 75 MeV show a significantly positive polarization, consistent with a double collision process. For lithium and aluminium, double interactions are relatively more probable, and the measured polarizations do not become significantly negative at any energy, though the values fall rapidly below 70 MeV. This reduction may be attributed to the contribution from single quasi-free scattering, but would also result from double interactions occurring with large individual scattering angles. The values of about +0.2 obtained above 70 MeV are consistent with the arguments given by Squires. The polarizations of the higher energy neutrons from lithium and aluminium are comparable with those observed by Carpenter and Wilson l o), who have not, however, made extensive measurements at angles and energies where neutrons from quasi-free proton-neutron scattering would be expected to have a negative polarization. The authors wish to thank Dr. A. M. Lane, Dr. R. J. N. Phillips, Mr. B. Rose and Dr. E. J. Squires for valuable discussions and the members of the cyclotron crew for their willing cooperation. One of them (A. L.) wishes to thank the D.S.I.R. for the award of a Research Studentship. The stay of H. A. in England was made possible by a grant from the German Ministry of Atomic Affairs. References I) P. H. Bowen, G. C. Cox, G. B. Hu×table, J. P. Scanlon, J. J. T h r e s h e r a n d A. Langsford, Nucl. Instr. 15 (1962) 31

(p, n) REACTIONS AT 143 Mev

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2) P. H. Bowen, G. C. Cox, G. B. Huxtable, A. Langsford, J. P. Scanlon, G. H. Stafford and J. J. Thresher, Nucl. Instr., in the press 3) J. A. Hoffman and K. Strauch, Phys. Rev. 90 (1953) 449 4) P. Radvanyi and J. G~.nin, J. de Phys. et le Rad 21 (1960) 322 5J R. K. Hobble and D. Miller, Phys. Rev. 12.0 (1960) 2201 6) G. H. Stafford and C. Whitehead, Proe. Phys. Soe. 79 (1962) 430 7) R. G. Donaldson and H. Bradner, Phys. Rev. 99 (1955) 892 8) E. J. Squires, Proc. Phys. Soe. A72 (1958) 433 9) G. H. Stafford, S. Tornabene and C. Whitehead, Phys. Rev. 106 (1957) 831 10) S. G. Carpenter and R. Wilson, Phys. Rev. 113 (1959) 650

Nuclear Physics 41 (1963) 184--191; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

POSITON-DECAY

O F Te t21 A N D

THE EXCITED

STATES

O F Sb 12t

R. B H A T T A C H A R Y Y A and S. S H A S T R Y

Saha Institute of Nuclear Physics, Calcutta Received 20 July 1962 Abstract: A/~+ group from the Te lzx nucleus has been found to have an end energy ~ 260 keV. The highest excited state has been found to be at 1130 keV which is found to decay with a half life o f 11.0 ns to the ground state by the emission o f a 1130 keV gamma-ray and a 1060-70 keV cascade. The Y-7' directional correlations have been measured for the possible cascades and a decay scheme has been proposed.

1. Introduction

The decay-schemes of Te 12xrn and Te 12t have been investigated previously 1, 2). The spins and parities of the excited states of Te 121 are quite well-established, but this is not so for the excited states of Sb 121. From a consideration of the l o g f t values ( ~ 8), Gupta 3) assigned an even-parity to the 1200 keV state. Gupta also found this 1200 keV state as the highest excited state of Sb ~21 and showed that it decays only through the 1130-70 keV cascade to the ground state. However, the alternative possibility of having the highest excited state at 1130 keV as reported earlier 1) decaying through a 1060-70 keV cascade, and also by a 1130 keV direct transition, has not been ruled out conclusively. The assumption of pure electron capture decay from Te 121m and Te ~21 feeding into the 1130 and 575 keV states does not fit in with the observed intensities 3) of 70, 506 and 1130 keV gamma-rays. If a weak parallel /3+-decay from Te 121 is considered, the annihilation radiation may account for the increased intensity in the region of the 506 keV gamma-ray. The present investigation was carried out in order to remeasure the energy of the highest excited state, to follow the sequence of states in Sb 12~ with the assignment of the multipolarities and spins of these states and to search for a possible /3 +emission from the Te t21 nucleus. 2. Source

The We 121m and T e 121 w e r e prepared by the enriched S b 121 (d, 2n) reaction in the Philips cyclotron. To 15 mg of the irradiated sample, 10 mg of spectroscopically pure Te powder was added as carrier. Following dissolution in boiling concentrated H2SO4, a 10 % crystalline stannous chloride solution was added until black Te precipitation was complete. The precipitate was filtred and subjected to thorough washing. Four cc of concentrated HCI was then added to the precipitate to dissolve 184