Photoneutrons from 9Be

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Nuclear Physics 76 (1966) 377--384; (~) North-Holland Publishing Co., Amsterdam

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to be reproduced by photoprint or microfilm without written permission from the publisher

PHOTONEUTRONS FROM 9Be M. N. THOMPSON t and J. M. TAYLOR tt

School of Physics, University of Melbourne, Australia tit Received 18 August 1965 A measurement of the photoneutron spectrum from 9Be was made using a knock-on proton spectrometer consisting of a stilbene crystal, and incorporating pulse shape discrimination. The resulting spectrum is interpreted in terms of states in 9Be and confirms the existence of several doubtful levels and postulates the presence of at least one new level.

Abstract:

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N U C L E A R REACTIONS aBe(y, n), E = 3-17 MeV; measured E n. aBe, deduced levels.

1. Introduction An examination of the available nuclear data for 9Be, for excitations between 4 and 15 MeV shows a lack of information in this region. Indeed consideration of data compilations by Ajzenberg-Selove and Lauritsen shows major differences in level assignments made in 1959 in ref. 1) and 1962 in ref. 2). This region is not readily investigated by means of particle reactions, but a careful measurement of the photoneutron breakup of 9Be should reveal information on levels with spins and parity ½+, ~+ and ~+. A study of the spectrum of photoneutrons emitted in the reaction 9Be+ ~ ---, n+aBe --, n + 4 H e + 4 H e should allow levels in 9Be to be assigned corresponding to discrete neutron groups observed. Neutrons emitted as a result of the direct three-body decay lead to a continuous neutron spectrum. The decay to 5He+4He with subsequent neutron decay of 5He also leads to an essentially continuous spectrum in the region of observation, because the ground and first excited states of 5He are very broad.

2. Experimental Apparatus The experimental arrangement is shown in fig. 1 where the 3.8 cm diam. solid beryllium metal target is placed in the collimated bremsstrahlung beam from the betatron at the University of Melbourne. The energy was set to 17 MeV and the t Present address, Physics Department, University of Illinois, Urbana, Illinois, USA. tt Present address, Department of Medical Biophysics, University of Toronto, Ontario, Canada. tit Work supported by the United States Army Research Office. 377

M. N. THOMPSON AND J. M. TAYLOR

378

beam was hardened by 30.5 cm of graphite which reduced the number of photons with energies less than 3 MeV by several orders of magnitude. This reduced the problem of v-ray pulse pile up in the detector. In addition a 7.6 cm thick block of paraffin was placed in the beam before the 1.8 m thick concrete wall in order to scatter out any neutrons from the beam which otherwise may have been scattered into the detector by the beryllium target. This detector consisted of a 5.1 cm diameter by 2.5 cm thick stilbene crystal in which the recoil protons were produced and detected, the pulses being recorded by an R C L 512-channel pulse-height analyser. As discussed in detail by several authors (refs. 3,4) the use of such a detector introduces complications, since the recoil proton

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spectrum from a mono-energetic neutron group incident on the detector has a continuous distribution with the proton being equally likely to have an energy from zero up to that of the neutron. Unfortunately the ideal shape of this spectrum will be altered slightly by distortions dependent on the size and shape of the detector. These occur because of protons which escape from the scintillator before depositing all their energy, or conversely because some protons will deposit more than the energy expected due to multiple collisions with either hydrogen or carbon nuclei. According to the analysis of Brock and Anderson 3) the maximum net result of the corrections for a detector of the size and shape used is a 3 ~ inclease in the derived neutron spectrum at 10 MeV. This error is much smaller than the combined experimental errors in the spectrum at this energy. In addition the relation between recoil proton energy and pulse-height is not linear 5, 6 ), however a satisfactory empirical relationship reported by Brodsky 7) was used in conjunction with the spectra from standard v-ray sources to calibrate channel number against proton energy. This method is substantiated by the close correspond-

PHOTONEUTRONS

379

ence of the energies of neutron groups in the spectrum from an A m - a - B e source, found using this spectrometer 8) and those found by Geiger 9) and other recent measurements. Pulse shape discrimination (PSD) using the method developed by Owen lo) was used to reject pulses due to y-rays scattered into the detector by the target. Also incorporated in the PSD system was a time gate as used by Johnson x~). In order that satisfactory y-ray discrimination occurs it was necessary in addition to harden the bremsstrahlung beam, to lengthen the beam burst to 350/~s, and to insert 2.5 cm of lead in front of the detector. Under these conditions neutrons of energies down to 2 MeV could be detected while rejecting all y-rays. The introduction of the lead absorber in front of the detector did not produce any spulious neutron groups. Its effect being to scatter about 40 ~ of the incident neutrons, equal numbers being scattered elastically and inelastically. Those scattered elastically suffer negligible energy loss, whilst the rest emerge from the lead with an evaporation spectrum peaked at 0.7 MeV (ref. 12)). These neutrons will increase the low-energy content of the spectrum, but will not produce discrete groups. Additional shielding against room background neutrons and scattered y-rays was provided by surrounding the detector and the photomultiplier tube by 5.1 cm of lead, and the whole assembly by 15.2 cm of paraffin on all sides except the front. General shielding was provided by a 1.8 m thick wall of brick, concrete and borax between the betatron and the experimental area. The RCL analyser was gated to accept neutron pulses only during the time of the betatron beam burst. 3. Data Collection

Three separate determinations of the recoil proton spectrum were made with data being collected in continuous periods of from 24 to 96 h. Each spectrum was consistent within the statistics. A neutron count rate of about 2 per sec was possible without falsely triggering the analyser with y-ray pulses piling up during the beam burst. The neutron background was determined by collecting data under similar conditions to the above but with the beryllium target removed. It was found to be negligible, falling from 3 ~o of the total counts at 3 MeV to essentially zero at 6 MeV. In order to check that this measurement represented the true background, and that no additional neutrons were scattered into the detector from the beam by the target, the beryllium target was replaced by a carbon rod of the same neutron scattering power. Because the maximum bremsstrahlung energy was below the 12C(y, n)11C threshold any increase in the measured neutron yield above the background would indicate the presence of neutrons scattered from the beam. No such increase was found. The recoil proton spectrum is shown in fig. 2, and clear changes of slope indicate the presence of several neutron groups above 8 MeV. Unfolding of the neutron spectrum from this recoil proton data was performed in overlapping intervals of 0.25 MeV using the method previously detailed 8). Fig. 3 shows the resulting neutron spec-

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t r u m and the curve drawn through the points indicates neutron groups at the following energies (in MeV): 10.9, 9.9, 8.75, 8.0, 7.0, 6.0, 4.7, 3.6. Before proceeding to discuss the interpretation of this spectrum, it is worth noting that the possibility that structure was introduced by the inefficiency of the PSD system was investigated and discounted. The efficiency of the PSD system was checked regularly under experimental conditions, and in addition, data were taken without PSD which yielded a smooth pulse-height spectrum, thus indicating that scattered y-rays did not produce the observed structure. An additional complication was introduced by scattering of neutrons before leaving the target. Some 26 ~ of all neutrons in the energy range of interest interacted with the 9Be either by elastic or inelastic scattering, or by the (n, 2n) process. It is not possible to account for neutron groups of the intensity observed in terms of these processes.

4. Interpretation For excitations in 9Be up to 17 MeV as in this experiment, the final state of the 8Be nucleus must be either the ground state or the first excited state at 2.90 MeV. Thus each observed neutron group has two possible originating states in 9Be, these being listed in table 1. TABLE 1 Energies of states in ~Be (MeV) giving rise to the neutron groups observed, assuming decay to g.s. and first excited state of 8Be respectively Neutron energy 10.9 9.9 8.75 8.0 7.0 6.0 4.7 3.6

Emission to g.s.

Emission to 2.90 MeV state

14.0 12.7 11.4 10.6 9.6 8.4 7.1 5.7

16.9 15.6 14.4 13.5 12.5 11.3 9.9 8.6

The present experiment yields no information which would allow a choice to be made between the two possible states in 9Be. The following interpretation is designed to make a choice consistent with evidence available from other experiments. Extensive reference will be made to a measurement of the photoneutron cross section of 9Be leading to either the ground state of aBe or the first excited state, by Bertozzi et al. 13). Although in general the resolution of this work is not equal to that reported here, the overall behaviour of the two cross sections together with the spectrum reported here

382

M. N . T H O M P S O N A N D J. M . T A Y L O R

provide some useful information. The cross sections will be referred to as the g.s. measurement, and the first excited state measurement of Bertozzi. Each neutron group will be discussed separately and the evidence for the level assignment will be tabulated with the assignment. The 10.9 MeV neutron 9roup may be identified with the very sharp level at 14.39 MeV found by Lynch et al. 14), in their study of the reaction 7Li(3He, p)9Be. Such an identification would suggest incidentally that the spectrometer calibration is low by about 2 ~ in this energy region. Further evidence of neutron emission from this excitation region is found in the cross section measurements of Seward et al. 15), and the g.s. measurement of Bertozzi, both of which indicate considerable cross section in this region. The 9.9 MeV neutron 9roup most likely indicates a 9Be state at 12.7 MeV decaying by neutron emission to the g.s. of aBe. The cross section measurement of Seward et al., indicates a large resonance here while the g.s. measurement by Bertozzi shows an increase of the cross section in this region. Thies et al. 16), failed to find structure here, but it is noteworthy that evidence for structure near 12.5 MeV can be seen in the published yield curve and less harsh smoothing would possibly reveal this in the cross section. On the other hand the presence of a state at 15.6 MeV decaying to the first excited state of 8Be would not be inconsistent with the results of Bertozzi. However, because of the width of the first excited state of aBe, see refs. 17,18), it would be expected that the neutron group corresponding to a decay to this state would be broader than that observed at 9.9 MeV. The 8.75 MeV neutron 9roup indicates a level at 11.4 MeV decaying by neutron emission to the g.s. of 8Be. Such a state is consistent with the cross section peak found at this energy by Thies et al. 16), and is indicated by inelastic proton scattering measurements 19, 20). The ground state measurement by Bertozzi shows considerable cross section in this region. The 7.9 MeV neutron 9roup has a width and energy consistent with a state at 13.4 MeV decaying by neutron emission to the first excited state of aBe. This is in agreement with the observations by both Thies et al. 16) and Seward 15) of a large cross section resonance in this region. Yet there is little indication of structure at this energy in the first excited state measurement of Bertozzi. I f on the other hand we assume that this neutron group indicates a state at 10.6 MeV decaying to the g.s. of SBe we get better agreement with the measurement of Bertozzi et al. However, this would leave the peak observed by Thies and Seward unexplained, and would postulate the existence of a broad state in 9Be in the region of 10.6 MeV, for which evidence has previously not been found. The 7.0 MeV neutron 9roup must represent a transition to the ground state of aBe since its width is narrower than that reported for the first excited state of aBe. It thus indicates a level at 9.6 MeV which is not inconsistent with the g.s. measurement of Bertozzi. Almqvist et al. 21) have reported a broad state near 9.2 MeV and it is possible

PHOTONEUTRONS

383

that they are one and the same. However the level indicated by this experiment would be less than 600 keV wide and thus much narrower than the state indicated by Almqvist et al. The 6.0 MeV neutron group has a width requiring either a broad level at 8.4 MeV in 9Be decaying to the g.s. of 8Be, or a state at 11.3 MeV decaying to the first excited state. Evidence for the former is slight, but cross section structure at 11.3 MeV found by Thies ~s) and Seward 16) has already been mentioned with regard to the neutron group at 8.75 MeV. It would seem likely that there is a state in 9Be at I 1.3 MeV which decays to both the g.s. and first excited states of 8Be. The 4.7 MeV neutron group suggests a state at about 9.1 MeV decaying to the first excited state of aBe. This would confirm the broad state near this energy reported by Almqvist et al. 21). Certainly such an assignment is consistent with the first excited state measurement by Bertozzi et al. The 3.6 MeV neutron 9roup again has a width such that we might assign it to a decay to the broad first excited state of 8Be. This would require the postulation of a state at 8.6 MeV not previously reported. On the other hand if emission to the g.s. is assumed, a state at 5.7 in 9Be is indicated - the only other evidence for which is found in a slight indication of structure at this energy in the g.s. measurement of Bertozzi. 5. Conclusions

Because the spectrum was measured at only one bremsstrahlung energy it is not possible to conclude with certainty the origin of the neutron groups. Table 2 summarizes the levels in 9Be which seem to be confirmed by the present data. Also given are the decay modes and references to previous observations. TABLE 2

Levels in 9Be Levels assigned energies in M e V error ± 2 0 0 keV 14.0 12.7 11.4 9.6 9.1

Other reports Decay

Ref.

g.s. g.s. g.s. + first g.s. first

14) 15)

Energies

15,16,19,20)

14.4 12.0--12.5 11.3

21)

9.2

In addition, the following four levels are consistent with the observed spectrum but it is not possible to decide on the present information which of each pair of states is the correct one: Levels assigned energies in M e V error ! 2 0 0 keV or or

13.4 10.6 8.6 5.7

Other reports Decay first g.s. first g.s.

Ref.

15,16)

Energies

13.3

384

M.N.

THOMPSON AND J. M. TAYLOR

T h e a u t h o r s a r e i n d e b t e d t o D r . B. M . S p i c e r f o r v a l u a b l e d i s c u s s i o n s a n d h e l p f u l advice.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)

F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1 F. Ajzenberg-Selove and T. Lauritsen, Energy levels in light nuclei, Calif. Inst. Tech. (1962) H, W. Broek and C. E. Anderson, Rev. Sci. Instr. 31 (1960) 1063 Fast neutron physics, ed. by J. B. Marion and J. L. Fowler (lnterscience Publishers, London, 1960) sect. liB G. T. Wright, Proc. Phys. Soc. B69 (1956) 358 B. Brundtfelter, J. Kockum and N. Starfelt, Neutron dosimetry, vol. 2 (International Atomic Energy Agency, Vienna, 1963) p. 351 A. Brodsky, AEC Report (1961) TID 13075 M. N. Thompson and J. M. Taylor, Nucl. Instr, 37 (1965) 305 K. W. Geiger and G. K. Hargrove, Nuclear Physics 53 (1964) 204 R. B. Owen, Nucleonics 17 (1959) 92 F. A. Johnson, Can. J. Phys. 41 (1963) 793 E. R. Graves and L. Rosen, Phys. Rev. 89 (1953) 343 W. Bertozzi et al., Proc. Int. Conf. on Nuclear Structure (University of Toronto Press, 1960)p. 928 B. Lynch, G. M. Grifllths and T. Lauritsen, Nuclear Physics 65 (1965) 641 F, D. Seward, S. C. Fultz, C. P. Jupiter and R. E. Shafer, U C R L 6177 (1960) H. H. Thies, B. M. Spicer and J. E. E. Baglin, Austr. J. Phys, 12 (1959) 21 J. L. Russell, G. L. Phillips and C. W. Reich, Phys. Rev. 104 (1956) 135 W. Waling and C. W. Li, Phys. Rev. 81 (1951) 661 J. Benveniste, R. G. Finke and E. A. Martinelli, Phys. Rev. 101 (1956) 655 K. Strauch and F. Titus, Phys. Rev. 104 (1956) 191 E. Almqvist, K. W. Allen and C. B. Bigham, Phys. Rev. 99 (1955) 631