The possible existence of four new states in Be8

Nuclear Physics 54 (1964) 148--154; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

THE POSSIBLE EXISTENCE OF FOUR NEW STATES IN Be s F. W. PROSSER, Jr. and F. L. WILSON The University o f Kansas, Lawrence, Kansas t Received 19 November 1963 Abstract: The existence of four, previously unknown, excited states of Bes has recently been proposed by Cavallaro et aL from their study of the LF(p, 7)Be s and LiT(p, y~)He a reactions. A systematic search has been made of the or-particle and y-.ray spectra produced by proton bombardment of

Li and of the Li~(p, y)Bes differential cross section for evidence to support this proposal and none has been found. An attempt to find ¢¢-ycoincidences for states in Bes above the broad, 2.9 MeV state was also unsuccessful. Alternate interpretations of their results, primarily in terms of target contamination, are given. N U C L E A R REACTION LiV(p,~ty), (p, ~), Ep ~ 0.4-1.8 MeV; measured ct-, y-spectrum, coy-coin. Bes deduced levels. Natural target.

1. Introduction The existence of four new excited states of Be s was postulated in a recent paper by Cavallaro et al. 1), in a study of the reactions induced by the interactions of protons with Li 7. These new states were assigned energies of 7.56, 13.91, 17.9 and 18.0 MeV, the latter two of which are in the compound nucleus at proton bombarding energies of 720 and 870 keV, respectively. Since the work in progress in our laboratory on the LiT(p, y)Be a reaction 2) and recently reported on the Bl°(d, ~)Be s reaction a) did not appear to confirm the existence of these four levels, a more detailed search for them was undertaken. Also, the presence of the lower of these states, in particular, would have significant effect on the theoretical description of the Be 8 system. The states at 7.56 and 13.91 MeV were attributed to the LiT(p, y~)He ¢ reaction by Cavallaro et al., based on the presence in the ~-particle spectrum of groups with energies of 3.8 and 6.9 MeV, respectively. The compound states were seen as resonances in the yield of high-energy y rays and of 3.8 MeV ~ particles, Therefore, the present investigation was divided into three parts, an analysis of the particle spectrum, a careful search of the excitation curve for the emission of high-energy ? radiation and of the y-ray spectrum for the presence of the lower energy y-rays and an attempt to find ~-y coincidences.

2. Alpha-Particle Spectrum The particles produced in the interaction of protons with Li targets were detected with an R C A silicon junction detector with a surface area of 20 m m 2. The pulses from the detector were amplified with a Hamner Model N-357 preamplifier and * Work supported in part by the U. S. Atomic Energy Commission. 148

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recorded with a T M C Model 404 multichannel analyser. In order to observe the lower energy 0(-particle groups, no foil was used to stop the elastically scattered protons. This necessitated the use of a small solid angle (2.7 x 10 -4 sr) to prevent pileup and dead-time difficulties with the multichannel analyser. The targets were prepared by the evaporation of natural Li metal on to thin carbon foils. The carbon foils were made by cracking CH a I on heated Ni strips and peeling off the layer of carbon; those used in this experiment ranged from 70 to 150/tg/cm 2. I

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The lithium metal oxidizes, even in vacuum, and reacts with the surface layers of the carbon foil. The resulting target appears to consist primarily of Li 2 COa. A typical spectrum, obtained at 90 ° lab angle, is shown in fig. 1. The plane of the target was set at 60 ° relative to the beam direction and the target oriented so that the beam passed through the carbon foil before striking the target and the emerging reaction products reached the detector without penetrating the foil. The carbon foil was determined to have an areal density of 130_+ 15/~g/cm 2 by measuring the energy displacement of the LiT(p, n) threshold between the front and back of the target. The target and foil were weighed together following the experiment and found to have an areal density of 196___28/~g/cm 2 and therefore a net density of 6 6 _ 32 #g/cm 2 for the target alone. The bombarding energy was calculated to be 829 keV, on the basis

150

F . W . PROSSER, JR. AND F. L. WILSON

of a beam energy of 868 keV and the thickness of 39 keV for the carbon foil. The data of Cavellaro et al. indicate that both new groups of a particles should be present at this energy. Only the lower of these can be seen in fig. 1. In fact all attempts to see the 6.9 MeV group, including the use of a thick brass target backing as employed by them, failed until the target was exposed to the fumes of HF. Then the group appeared strongly in the spectrum. This evidence, together with the agreement in energy and in the general shape of the excitation curve reported for this a-particle group by Cavallaro et aL, appeared to require the assignment of this group to the F 1 9 ( p , a ) O 16 reaction. TABLE 1 Cross sections for the particle groups in fig. 1 Ea (MeV)

Reaction

Present values

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0.19 b) 31 0.050 b) 5.6

~r(90° lab) (mb/sr) Previous values 0.30 b,e) 49 a) 9.8 e)

Adjusted s) 0.32 52 0.084 9.4

a) Present values adjusted to agree with the previous results for the LiT(p,g)He4and LiS(p, HeS)He' reactions to correct for the large uncertainty in target thickness. b) These values have been corrected for the emission of two ~ particles/reaction. c) See ref. ~). a) See ref. s). e) See ref. e). It has been suggested 4) that the energy and excitation curve for the 3.8 MeV a-particle group resembled that of the OlS(p, a ) N 15 reaction 5). The cross sections for the a-particle groups seen in fig. 1 are listed in table 1, together with other measurements reported for these cross sections. The target has been assumed to consist of Li 2 CO3 and the cross section for the group at 3.76 MeV has been calculated for the alternate assumptions of its originating from either Li 7 or 018. Because of the large uncertainty in the target weight, the cross sections are also shown normalized to agree with the published values of the Li6(p, He3)He 4 reaction 6) and the LiT(p, a)He 4 reaction 7). When this correction is made the cross section for the 3.76 MeV group is seen to agree well with the accepted value for the OlS(p, a)N 15 reaction. Further evidence for this assignment has been found by Miller s), who reports that the variation of a-particle energy with angle agrees well with an atomic weight for the target nucleus between 17 and 20 and that the excitation curve for this group agrees quite well with that expected for this reaction.

3. Gamma-Ray Measurements The excitation curve for the yield of ~ rays to the ground state and first-excited state of Be s from the reaction 2) LiT(p, y)Be s is shown in fig. 2. It is evident that there is

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152

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no evidence for resonances at either 720 or 870 keV with intensities at all comparable to those reported by Cavallaro et al. In addition it should be noted that the cross section 2, 9) for this reaction at 840 keV is about 14 #b/sr. Since the cross section for the 3.76-MeV ~ particle group is 84 #b/sr, as shown in table 1, under the assumption that it arises from the Li7(p, 7~)He 4 reaction, a 7 ray with an energy of 10.4 MeV I

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Fig. 4. T h e coincidence a n d non-coincldence spectra o b t a i n e d for the reaction F19(p, cc7)O 16. T h e bias o f the particle d e t e c t o r w a s set t o reject t h e •-particle g r o u p s l e a d i n g to the states in 016 a b o v e that at 6.12 M e V a n d the 7 r a y s f r o m t h e s e h i g h e r states are clearly a b s e n t f r o m the c o i n c i d e n c e s p e c t r u m , w h e n the c h a n c e c o i n c i d e n c e s are subtracted.

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(or several ? rays with this total energy) should be present in the ?-ray spectrum with an intensity six times that o f the higher energy ? rays. A typical spectrum is shown in fig. 3, where it is apparent that no such T ray is present with an intensity greater than 1 ~ o f the sum o f the higher energy 7 rays.

4. C o i n c i d e n c e M e a s u r e m e n t s

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Fig. 5. The coincidence and non-coincidence spectraobtained for the reaction LF(p, 7~)He~. The presence of F 19',contamination in the target is evident and serves to confirm the proper functioning of the apparatus. The bias of the particle detector was set as indicated in fig. 4 and would thus prevent the observation in the coincidence spectrum of transitions to an excitation energy of less than 3.8 MeV in Be 8.

tween 0t particles o f energy greater than 1.8 MeV and any ? rays. The ?-ray detector was placed with its front surface within 4 cm o f the target. The solid angle subtended by the particle detector was limited by an aperture immediately in front o f the detector to 6.6 x 10-4 sr. The targets were prepared as described for the e-particle measurements and oriented in the same way with respect to the beam. The pulses f r o m the ?-ray detector were amplified and recorded by the multichannel analyser, which was gated by pulses f r o m an integral discriminator set to accept all pulses f r o m the particle detector corresponding to an energy o f greater than 1.75 MeV. The low counting

154

F.W.

PROSSER, JR. AND F. L. WILSON

rates both for particles above this energy and for ? rays allowed the use of the gate circuit of the analyser itself as the coincidence circuit, which provided a total resolving time of about 4/~s. The detectors and electronics were checked by using the F19(p, ey~)O16 reaction from a N a F target. The spectra obtained with and without coincidence are shown in fig. 4, as is the chance coincidence spectrum. The correct functioning of the apparatus, as well as the level of the integral discriminator bias, is indicated by the absence in the coincidence spectrum of ?-rays from the 6.92 and 7.12 MeV states in O x6. The energies of the ~-particle groups leading to these two states are 1.6 and 1.4 MeV, respectively, while that of the group leading.to the 6.14 MeV state is 2.2 MeV. The data, obtained in an identical manner with the Li target, are shown in fig. 5. The presence of F 19 contamination is clearly seen, both in the coincidence and noncoincidence spectra, and comparison of these results with those shown in fig. 4 again indicates the proper functioning of the equipment. N o structure other than that attributable to the F19(p, ~ ) O 16 reaction can be seen in the coincidence spectrum. The two counts which appear in this spectrum slightly above the region where a 10.4 MeV ? ray would be expected may, of course, result from chance coincidence, but more probably arise from the presence of the high energy tail of the broad, 2.9 MeV state in Be s.

5. Conclusion N o evidence is found to support the existence of any of the four new states in Be s observed by Cavallaro et aL The two lower states postulated by them appear to have resulted from the presence of target contaminants in their work, while the appearance of the compound states in their work apparently arose from an unfortunate trend in the statistical fluctuations of their data which coincided with the strong resonance in the OlS(p, ~)N ~5 reaction. The authors wish to express their indebtedness to S. S. Hanna for the suggested explanation of the "7.56 MeV state", to P. D. Miller for the information about his measurements of the kinematics and the excitation curve of this same group of particles and to Y. A. Tan of this laboratory for his assistance in the preparation of the targets and in the taking of the data.

References 1) S. Cavallaro, R. Potenza and A. Rubbino, Nuclear Physics 36 (1962) 597 2) R. W. Krone, F. W. Prosser, Jr., and D. J. Schlueter, Conference on Compound Nuclear States, Gatlinburg, Tenn. (1963) 3) K. H. Purser and B. H. Wildenthal, Nuclear Physics 44 (1963) 22 4) S. S. Hanna, private communication 5) H. A. Hill and J. M. Blair, Phys. Rev. 104 (1956) 198 6) J. B. Marion, G. Weber and F. S. Mozer, Phys. Rev. 104 (1956) 1402 7) J. M. Freeman, R. C. Hanna and J. H. Montague, Nuclear Physics 5 (1958) 148; N. P. Heydenburg, C. M. Hudson, D. R. Inglis and W. D. Whitehead, Phys. Rev. 73 (1948) 241 8) P. D. Miller, private communication 9) W. A. Fowler and C. C. Lauritsen, Phys. Rev. 76 (1949) 314