A method for determining the neutron multiplicity for gamma rays from (particle, xnγ ) reactions

NUCLEAR INSTRUMENTS

AND METHODS

169 (1980)

173-177;

(~) N O R T H - H O L L A N D

P U B L I S H I N G CO.

A M E T H O D FOR D E T E R M I N I N G T H E N E U T R O N M U L T I P L I C I T Y FOR G A M M A RAYS F R O M (PARTICLE, xnT) REACTIONS* C. A. FIELDS, F. W. N. de BOER, R. A. RISTINEN, L. E. SAMUELSON and P. A. SMITH

Nuclear Physics Laboratory, Department of Physics and Astrophysics, University of Colorado, Boulder, Colorado 80309, USA Received 17 September 1979 A technique is presented for identifying y rays from specific (particle, xny) reactions by measuring the time-of-flight of the outgoing neutrons. Gamma rays associated with different neutron multiplicities are shown to be coincident with different parts of the neutron time-of-flight spectrum.

1. Introduction In the study of 7-ray spectra from neutron-emitting reactions where several reaction channels compete, the identification of individual )'-rays with specific final nuclei is often accomplished by comparing the excitation functions of the unknown )'rays to either statistical model predictions or to the excitation functions of )'-rays known (e.g., from fldecay studies) to belong to the final nucleus in question. For this method to be accurate, single )'-ray spectra must be recorded at several bombarding energies. G a m m a - g a m m a coincidence is useful as an identification method when a partial level scheme is known, but transitions which are not in prompt coincidence with the known 7-rays are of course missed. In this paper, we show that )'-rays from (~, 2 n)') reactions, for example, can be uniquely separated from those produced by competing reactions in the A = 100-200 region by demanding )'-ray coincidence with a selected region of the time-of-flight spectrum of the outgoing neutrons. Only one bombarding energy is necessary, and no knowledge of the level structure of the final nuclei is needed. Moreover, the analysis of the data is very straightforward. 2. Experimental To illustrate the method, several targets were bombarded with a 35.1 MeV ~z-particle beam from the University of Colorado cyclotron. Neutron energies were measured by time-of-flight (TOF) techniques. A single 20.3 cm (diameter)×5.1 cm NE224 liquid scintillator was used to detect outgoing neutrons. This detector was placed 63.5 cm from the target at 40 ° relative to the beam direction. * Work supported in part by the US Department of Energy.

Neutron flight times were measured relative to the cyclotron radio frequency, and n-)' pulse shape discrimination was used to gate out almost all the ),-ray signals from the scintillator. The electronic set-up for the TOF measurements is described in detail in ref. 1. G a m m a rays were detected in a 60 cm 3 Ge(Li) detector placed 3 cm from the target at 90 ° relative to the beam direction (130 ° relative to the scintillator). Only events in which both a neutron and a )'-ray were detected within a 65 ns time window were accepted. This coincidence time window was set on the output of a time-to-amplitude converter (TAC) which was started by the detection of a neutron and stopped by a )'-ray signal. True and random coincidence spectra were simultaneously recorded in event mode on magnetic tape. 3. Results and discussion The data for each target were analyzed in two different ways. The spectra of neutrons coincident with 7-rays from specific reactions were examined first, then the 7-ray spectra coincident with different parts of the neutron spectrum were displayed. In all cases the coincidence spectra were corrected for random events. The 7-ray gates were corrected for Compton background, and the neutron gates were corrected for the time-uncorrelated background in the neutron detector. Experiments were performed on the following targets: l°4pd, 12°Sn, 15°Nd, 165H0 and 2°spb. It was found that gates set on )'-rays known to be from different reactions on a given target revealed distinctly different neutron spectra. However, the spectra of neutrons coincident with )'-rays from, e.g., the 15°Nd(o:',2n)')152Sm and l°4pd(~z, 2n)')l°6Cd reactions at E~=35.1 MeV had very different

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Fig. 2. (a) A neutron time-of-flight spectrum gated by all ~, rays from the t°4pd(~x, xn~) reactions. The sharp peak at the right is due to ? rays from the target detected in the scintillator. (b), (c), (d): gamma ray spectra from l°4pd(a~,xn),) reactions gated by the regions of the neutron spectrum shown in (a). Prominent transitions in each final nucleus are identified by their energies in keV and the

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c . A . FIELDS et al.

shapes. It was therefore difficult to identify unambiguously 7-rays with a specific reaction by the shape of the corresponding neutron distribution. This difficulty was worsened by the relatively small number of counts in any given 7-ray peak, which limited the statistical quality of the coincident neutron spectrum. In all cases, however, the centroid of the neutron distribution coincident with 7-rays from the (o~,2 n7) reaction fell at higher energy than those of the neutron spectra from the competing (~z,3 nT) and (~z,4 nT) reactions. Gates set on the high energy section of the neutron spectrum consistently enhanced the y-rays from the (~z,2 n7) reaction over those from all other reactions. Figs. 1 and 2 show, respectively, results for the ~5°Nd(~z,xnT) and l°4pd(~z,xnT) reactions. Each target was bombarded for approximately 12 h with a 1 nA beam. In each figure, part (a) is the neutron TOF spectrum coincident with all 7-rays, part (b) the 7-ray spectrum coincident with the neutron gate labelled " b " , part (c) the 7-ray spectrum coincident with neutron gate " c " , and part (d) the 7-ray spectrum coincident with neutron gate " d " . In each case, the 7-ray spectrum coincident with gate " c " is very similar to the total coincident 7-ray spectrum (not shown). The most intense 7rays produced by the (cz, 2 nT) reaction can, in each I00

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case, be unambiguously identified by comparing the spectra coincident with gates " c " and " d " to see which transitions are enhanced by gate " d " . Similar results were obtained for all other targets tested. A more detailed analysis of the data is possible, and is illustrated for reactions on ~5°Nd and ]°4pd in figs. 3 and 4, respectively. The areas of prominent yrast y-ray peaks from each reaction were extracted from the gated spectra shown in figs. 1 and 2, and divided by the number of neutron counts in the relevant gate. These normalized areas were then plotted against the average energies of the gating neutrons. The possible effects of different angular distributions were ignored in this analysis. The y-rays from different reactions show distinctly different dependence on neutron energy. Although this technique is applied most easily for the very intense, low-lying yrast transitions, it is not restricted to them. In the 15°Nd(~z,2nT)152Sm data, for instance, the yrast y-rays could be identified up to J = 14 (N3 MeV). Members of at least three rotational bands were observed in the J65Ho(1,2ny)167Tm gated spectra. The method is not sensitive to transitions from states with lifetimes much greater than the width of the n-7 coincidence window.

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Fig. 3. Gamma-ray counts per gating neutron plotted against the average energy of the gating neutrons for 15°Nd(ac, xn7) reactions at 35.1 MeV. The data for each 7 ray are labelled by the transition energy in keV. The absolute normalizations of the curves are arbitrary, and the lines are drawn only to connect the data points. Curves for different reactions are seen to have different shapes.

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Fig. 4. Gamma-ray counts per gating neutron plotted against the average energy of the gating neutrons for l°4pd(~,xny) reactions at 35.1 MeV. The data for each 7 ray are labelled by the transition energy in keV. The absolute normalizations of the curves are arbitrary, and the lines are drawn only to connect the data points. Curves for different reactions are seen to have different shapes.

NEUTRON MULTIPLICITY

4. Conclusions The data presented in this work demonstrate that this n-y coincidence technique applies to (a~,xny) reactions on targets in the A = 100-200 region. Although no targets with A ~<100 were tested, the data for l°4pd, shown in figs. 2 and 4, suggest that this technique can successfully separate y-rays from the (7, 2 n~,) reaction from those due to the (a~, pny) reaction, which becomes prominent as the target Z decreases. As seen in fig. 4, the (a~,pny) reaction does not produce a large number of high-energy neutrons, as does the (7, 2ny) reaction. The outgoing proton must be sufficiently energetic to escape the Coulomb barrier and thus on the average must carry off more energy than does the neutron. Gamma rays from the (a~,xpy) and (~, pny) reactions could also be selected by measuring the y-ray spectrum coincident with a charged-particle detector. Results similar to those presented above for (~z,xny) reactions have also been obtained at this laboratory for (3He, xny) reactions at 24.5 and

1"77

33.8 MeV. Gamma rays from the 15°Nd(3He, 3ny) 15°Sm reaction could be separated from those from the competing (3He, 4ny) and (3He, 5ny) reactions by gating on high-energy neutrons. Data taken elsewhere, such as that reported by Ejiri et al., suggest that similar methods could be used to identify yrays from (p,xny) reactions with 4~