The reaction Ne22(d, n)Na23

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Nuclear Physics 54 (1964) 497--504; ( ~ North-Holland Publishing Co., Amsterdam

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

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T H E R E A C T I O N Ne22(d, n)Na 2s

E. B. PAUL Physics Dept., University of Manchester

and J. H. MONTAGUE Atomic Energy Research Establishment, Harweli

Received 1 January 1964 Abstract: The relative yields and angular distributions of neutron groups from Nen(d, n)Na2a have been measured. The results give strong support to a spin and parity assignment of ½+for the 2.39 MeV level in Na ~, and suggest ~+ or {t+ for the 2.98 MeV level.

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NUCLEAR REACTION Ne2~(d,n), Ed = 3 MeV; measured n-spectrum d, n(0). Na 2s deduced levels, at, z~, !. Enriched target.

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1. Introduction

The low lying levels of Na 23 have beert studied by a variety of reactions. The level scheme is shown in fig. 1. The level positions have been most accurately established by inelastic proton scattering 1). The spin and parity of the ground state 2) is ½+. The first excited state at 0.439 MeV has a spin and parity 3) of ~+. The second excited state 7, ~9) at 2.08 MeV probably has a spin and parity < ~+; the fact that it shows a ~,-ray decay branch to the ground state favours ~+. The third excited state at 2.39 MeV has in various experiments 5, 7, 12, 19) had an assignment of either ½+ or a high spin. F r o m proton capture states formed in the reaction Ne22(p, 7)Na 23 it seems to be fed rather infrequently. The fourth and fifth excited states lie at 2.64 and 2.71 MeV. The former state was produced with a high cross section in the reaction Mg24(p, 2p)Na 23 at 150 MeV 13), and was therefore interpreted as being a ½+ hole state formed by raising one member of the filled level (corresponding to Ne ~° ground state) to pair off with the odd particle in the ½+ state. The 2.71 MeV level does not decay to the ground state and hence has been tentatively assigned 9+, though ~+ is not excluded. The spin and parity of the sixth excited state at 2.98 MeV have not been definitely assigned, although ~+ seems probable, No higher levels were considered in this work. A number of attempts at theoretical interpretation of these levels have been made 14-17). These have all been based on applications of the Nilsson model of single particle states in a deformed potential well, a model which has worked quite successfully in the case of masses 19, 20 and 25. In all trials, mixing of different values of K - i.e. different bands - had to be assumed to arrive at the correct spacing for the low lying first excited state. Paul and Montague 17) assumed that all the states could be accounted for by promotion of the odd particle to successive Nilsson levels 497

£'98

E, B. PAUL AND 1. H. MONTAGUE

and by rotational states based on these intrinsic states. Clegg and Foley 14), on the other hand, ignored promotion states but assumed the 2.64 MeV state and the ground state were the ½+ hole state and the ½+ particle state respectively; they could account for all states except the 2.39 MeV and 2.98 MeV states. The present work is an attempt to throw more light on the spins and parities of the five states between 2.08 MeV and 2.98 MeV, where some of the evidence is still ambiguous, by making a study of the angular distributions of neutron groups from the Ne22(d, n)Na 23 reaction (Q = 6.56 MeV).

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Fig. 1. Levels of N a " below 3 MeV. The figure also shows the transitions leading to the neutron groups no to ne that have been observed in the present study of the Nell(d, n ) N a " reaction; these are not drawn to scale.

2. Experimental Arrangement A pulsed deuteron beam of 3 MeV energy was provided by the Harwell "Ibis" Van de Graaff accelerator. This beam is terminal pulsed and magnetically bunched to produce at the target deuteron bursts of width about 1 ns and of peak intensity about 2 mA, at a repetition rate of 1 MHz. The beam was collimated by a 2 mm aperture in a gold disc located just in front of the target, and then entered a gas target cell though a 2 . 5 / l m Ni foil window. The cell was generally 1 cm long and was filled with neon gas at a pressure of 300 m m Hg. After passing through the gas the deuteron beam was stopped in the end of the cell which was 0.13 mm platinum sheet. A simple gas handling system enabled the cell either to be evacuated, to be filled with natural Ne or to be filled with enriched Ne 22 gas. A hand piston pump was used to withdraw and store the enriched gas. The enriched gas used was > 98

THE REACTION Ne~(d, n)Na~a

499

Ne22; one sample was obtained from Oak Ridge National Laboratory, and another from Yale University through the courtesy of Professor D. A. Bromley and Mr. A. J. Howard, Jr. The neutron groups of interest had energies between 6.6 and 9.5 MeV at 0 °. The various groups were resolved using the time-of-flight of the neutrons. A block diagram of the time-of-flight measuring system is shown in fig. 2. The neutron detector has been described by Adams et al. zs). It consisted of a 5 cm thick plastic scintillator 12.5 cm in diameter, which was viewed by two 5 cm diameter 56 AVP photomultipliers. F r o m each photomultiplier two pulses were taken - a fast timing pulse from the anode and a linear pulse from one of the dynodes. The two linear pulses were added together and the sum pulse fed, after amplification, to a single-channel ', PICKUP /

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Fig. 3. Time-of-flight spectra for n e u t r o n s f r o m t h e Ne~2(d, n ) N a 2a reaction, for angles o f emission in the l a b o r a t o r y s y s t e m o f 0 ° a n d 28 ° respectively. T h e spectra are t a k e n at a n incident d e u t e r o n energy o f 2.75 MeV, a n d a n e u t r o n flight p a t h o f 15 m . T h e t i m e dispersion is 0.24 n s / c h a n n e l , a n d b o t h spectra are t a k e n for the s a m e n u m b e r o f incident deuterons. T h e arrows identify t h e n e u t r o n groups no to n6 leading respectively to t h e g r o u n d state a n d to t h e first ~ix excited states o f N a ~8. A n

THE REACTION Ne~(d, n)Na la

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The neutron detector was enclosed in a cylindrical shield consisting of 5 cm of Pb surrounded by 18 cm of Li2 CO3-1oaded paraffin wax. Gamma-ray background was further reduced by inserting 3 mm of Pb between the detector and the gas target. The background due to scattered neutrons was kept low by locating the gas target and the detector 4 m from the concrete floor in an open hangar. Most of the runs were taken at a flight distance of 15 m. The overall time resolution was about 2 ns. Sources of time spread were: (I) the deuteron burst length which was about 0.7-1.0 ns; (2) the time for traversal of the deuteron beam through the gas cell, i.e. about 0.6 us; (3) a time spread of about 1.0 ns resulting from the energy spread introduced by the slowing down of the deuterons in the gas; (4) the detector thickness, which contributed about 0.7 ns spread.

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Fig. 4. Resolution o f the combined group (n~+nj) into two separate groups, as discussed in the text. The spectra s h o w n are abstracted f r o m the complete time spectra o f fig. 1, but have h a d the r a n d o m b a c k g r o u n d subtracted. The e r r o r bars c o r r e s p o n d to standard deviations.

All the runs were taken at a deuteron bombarding energy of 3.0 MeV. After taking account of the window thickness and the length of the gas cell, the effective bombarding energy was 2.75 MeV. Spectra were taken at nine angles between 0 ° and 65 ° in the centre-of-mass system, and at 153°; each spectrum took approximately one hour to accumulate. Fig. 3 shows two typical time spectra taken at laboratory angles of 0 ° and 28 ° respectively. In such time spectra it was possible to identify neutron groups leading to the ground state and to the first six excited states of Na23; these groups are labelled no to n6 respectively, and the corresponding transitions are shown schematically on fig. 1. The yield of each neutron group was obtained by integrating the

502

E. B. PAUL AND J. H. MONTAGUE

counts in the corresponding peak. Beam-independent backgrounds were estimated from the level of counts between the peaks, and were subtracted from the raw yields. The yields were then corrected for the variation of the detector efficiency with neutron energy. Runs at different angles were normalized with respect to the total charge collected in the target during the run, checks with a long counter having indicated that this was a valid method of normalization. Uncertainties in this normalization contributed an error of +__7 ~o (standard deviation) to the relative yields at the various angles. It was in general not possible to resolve neutron groups n 4 and n s, leading respectively to the Na 23 states at2.64 MeV and 2.71 MeV. However, the composite peak formed by these two unresolved groups was noticeably wider than would be expected for a peak corresponding to a mono-energetic neutron group; this was particularly true at 0 °, and the effect can be clearly seen in fig. 3. The composite peak for (n,,+ns) was therefore decomposed into two single peaks. The positions of these peaks could be predicted from the known positions of the neighbouring peaks n3 and n 6, and the appropriate shape for a mono-energetic peak could be obtained from the shapes of these same two neighbouring peaks. There were, however, considerable difficulties in making such analyses, and these are reflected in the large statistical errors assigned to the yields of n4 and n 5. Fig. 4 shows the way that this composite peak is resolved for each of the spectra of fig. 3. In each case, the points are plotted with the random background subtracted, and the two solid curves show the fits adopted for n 4 and n5. 3. Results The centre-of-mass angular distributions of the seven neutron groups no to n 6 are shown in fig. 5. The most striking feature is the very pronounced forward peak exhibited by the n 3 angular distribution. This is strong evidence for lp = 0 in the stripping reaction giving this group, and indicates a spirt of ½+ for the corresponding Na 23 state at 2.39 MeV. The remaining angular distributions are not so simple to interpret in terms of conventional stripping theory, but some inferences may still be made from them. For instance, the spins and parities are known for the ground state and 0.439 MeV first excited state of Na 23, and demand lp = 2 in the stripping reactions producing neutron groups n o and nt. These neutron groups have similar angular distributions, characterized by a small peak at forward angles and a relatively high yield at backward angles; it is therefore not unreasonable to associate such angular distributions with lp = 2 stripping. In a similar way it may be argued that the relatively fiat angular distribution shown by neutron group n2 is characteristic of lp = 4 stripping, since the 2.08 MeV state in Na 23, which corresponds to group n2, is most probably ~7 + . On the basis of the foregoing arguments, it might be inferred that lp = 2 in the stripping reaction producing neutron group n6, as the angular distribution of this

THE R E A C T I O N

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group is qualitatively very similar to the angular distributions of no and n t. This would require that the corresponding Na 2a state - that at 2.98 MeV - should be 3 + or ~+. The angular distributions of group n4, leading to the 2.64 MeV state in

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Na 23, is relatively flat, although there is possibly some weak structure at forward angles. The flatness would suggest lp > 3 for the Na 2a level. It may be that this is the 9+ state expected in this region. Group %, corresponding to the 2.71 MeV Na 23 level, is weakly excited and also has an almost flat angular distribution. There is, however, an indication of a weak maximum at 0% and it is believed that this is really

504

E. B. PAUL AND J. H. MONTAGUE

present since it is at 0 ° that the combined peak corresponding to groups n4 and n 5 shows most clearly that it consists of two components. This 0 ° maximum would suggest lp = 0, but the very weak excitation of the level in this reaction indicates that it might be a state of a considerably different character from the others being considered. To summarize, we can rather confidently assign a spirt and parity of ½+ to the 2.39 MeV state and with less certainty suggest ½+ or ~+ for the 2.98 MeV state. Of the two states 2.64 and 2.71 MeV, one of which was strongly excited in the experiment of Foley et aL x3), neither shows a strong excitation in this experiment. There is weak evidence for a ½+ assignment to the 2.71 MeV state, although this is in contradiction with the evidence from the (p, ?) experiments 7, 19). The authors are indebted to Dr. A. T. G. Ferguson and Mr. Martin Adams for permitting them to use the dual-phototube neutron detector, and for their advice and assistance during the course of the experiments. Thanks are also due to Mr. S. Waring and to the IBIS operating crew for the efficient running of the accelerator. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

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