States in 9Be from 10B(t, α)9Be

Nuclear Physics A l l 6 (1968) 481--488; (~) North-Holland Publishin9 Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

STATES IN 9Be F R O M I°B(t, at)gBe t F. A J Z E N B E R G - S E L O V E a n d R. D. W A R D A S K I

Haeerfi~rd Colleqe, Hacerfi)rd, Pennsyh'ania and R. M I D D L E T O N

Unicersit)" of Penn.sylcania, Philadelphia, Pennsyh'ania Received 7 June 1968 Abstract: At E t =~ 12.9 MeV, c~-groups from the ~°B(t, ~)gBe reaction have been observed to the ground state of~Be and to excited states at E~ = 1.7, 2.43, 3.02 ! 0.05 (F' =- 320 =60 keV), 11.26_-4_-(I.04 (l" -= 530_:_70 keV) and, possibly, at 14.5 MeV (1" ~ 800 keVI. The T -- ~ state at E~ = 14.39 MeV was seen, but the intensities o f the c~-groups are vcry low, typically 5 % o f the intensity o f the group to the 2.43 MeV state. The angular distributions of the c~-particles to the 2.43 MeV state are sharply peaked forward and backward. The ground state 7-particles are not peaked in the backward direction. Angular distributions o f 7-particles from the 'Be(t, ~)~Li reaction to 8Li (0, 0.98, 2.26, 6.53) are also reported.

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NUCLEAR REACTIONS ~B(t, ct), 'Be(t,~), E : 12.9MeV; measured ,r(E,,O).

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I. Introduction Fig. 1 displays the current information on the levels of the A = 9 isobars. This figure includes recent information derived from the 9Be(e, e')9Be *, 7Li(3He, p)9Be, 9Be(d, d')gBc * and 10B(p, d)gB reactions ~- 5) as well as the work summarized earlier o). The energy levels of the T~ = J, members of the A = 9 quartet are characterized by three even-parity states closely related to aBe in its ground and first excited states [in 9Be, at Ex = 1.67, 3.06 and 4.70 MeV with J~ = !l+ , Q)+ and (~)+, respectively] and by two odd-parity states [in 9Be, at Ex = 2.43 and 6.66 MeV] with J" = ! - and 7- which, on the collective model, are members of the ground state K = 3- band. The ~ - member of that band is expected to occur 7) at Ex ,-~ 11 MeV, but it has not yet been identified. At higher excitation energies, three broad states are located 2, 0) at Ex = 11.29, 11.81 and 13.78 MeV in 9Be and at 11.62, 12.06 and 14.01 MeV in 9B. In °Be, the population of all three of these states has been observed with certainty only in the reaction 2) VLi(3He ' p)gBe" Inelastic scattering (of protons and of alpha particles) appears to populate only the 11.3 MeV state. On the other hand, the 11.8 MeV state is reported in the 7Li(6Li, ct)gBe reaction s). The 7Li(3He, n)gB reaction o) populates the 12.06 and 14.01 MeV levels, while the 1°B(3He, ~)gB reaction l o) goes • This w o r k has been supported by the N a t i o n a l Science F o u n d a t i o n . 481 August 1968

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only to the I 1.62 MeV state. The nucleus 9C decays 11) by an allowed transition to the 12.06 MeV state, which subsequently decays by proton emission to aBe (g.s.) and to aBe (2.9) with nearly equal intensities. Assuming J~ --- ~- for 9C, the ! 2.06 MeV state probably has J~ = ½-. The first T = :~ states analogous to the ground states of 9Li and 9C, have been identified in the mirror reactions 7Li(3He, p)gBe and 7Li(aHe, n)gB at 14.392 MeV in 9Be and at 14.670 MeV in 9B. The first excited state of 9Li at Ex = 2.691 MeV appears to have its analogue in the 16.973 MeV state in 9Be, whose width as determined in the 7Li(d, ~)gBe reaction is found 12) to be less than 0.47 keV. Another sharp state (F = 41 _+4 keY) is observed 2) at E~ = 16.671 MeV in the ?Li(aHe, p)qBe reaction. Barker 13) has suggested that there are two T = ~ states at E x ~ 17 MeV in 9Be corresponding to the known 9Li state at E~ = 2.69 MeV, taken to have J~ = ~- and to an, as yet unobserved, J~ -- ½- state. The reaction l°B(t, ~t)gBe, QB = 13.227 MeV, had been studied tg-J6) prior to this work only at energies below E t -- 3.3 MeV. It appeared interesting to study this reaction at higher energies to find the 9Be states which would be populated and also to continue our determination of (t, ~t) angular distributions in light nuclei already reported 17) for the 7Li(t, ct)6He, 12C(t, ~t) 1tB and 160(t, :x) 15N reactions.

2. Experimental procedures and results 2.1. EXPERIMENTAL SETUP A self-supported 1o B ( ~ 95 9/ooenriched) target, ~ 50 pg/cm 2 thick, was bombarded by 12.92 MeV tritons accelerated in the Aldermaston tandem Van de Graaff. The resultant alphas were detected with 25 ~Lm thick Ilford K-I nuclear plates placed at 24 angles relative to the incident triton beam. The magnetic field setting in the multi-gap was I 1.744 kG. The exposure was of 1000/~C. Fig. 2 shows a typical or-particle spectrum at 0 = 20 °. States in 9Be, 1°Be, 11B and 15N are superimposed on a continuous background of or-particles presumably from one or more of the reactions t ° B + t --* ~t+aBe+ n, t ° B + t ~ 2~t+ 5He or l ° B + t -~ 3~t + n [E~ (max) ~ 22.7, 21.8 and 22.8 MeV, respectively, at 0 = 20~]. An increase in the intensity of the or-particle background near E~ = 9 MeV corresponds to the onset of the ~ ° B + t ~ c t + p + a L i and t ° B + t ~ ~ + d + T L i reactions. 2.2. THE LOW EXCITED STATES OF 9Be In the region E~ < l0 MeV, alpha-particle groups were observed corresponding to the ground state of 9Be and to the first three excited states. Fig. 3 shows the data relating to these three excited states at ten angles (127.5 ° < 0 < 175 °, lab). At more forward angles the groups corresponding to the two even-parity states [9Be*(1.67, 3.06)] are low in intensity relative to the ~- state at 2.43 MeV. The "edge" of the first excited state occurs at E~ = 1.683_+0.025 MeV; the " p e a k s " of the groups corresponding to this state occur at E~ = 1.752_+0.030 MeV. The energy of the third

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e x c i t e d state is f o u n d to be 3.024-0.05 M e V , F = 3 2 0 + 6 0 keV. T h e s e values are all in g o o d a g r e e m e n t w i t h p r e v i o u s values 3, 6). N o e v i d e n c e was o b s e r v e d at a n y angles for ~ - g r o u p s to the w e l l - k n o w n states at E~ = 4.70 a n d 6.66 M e V . T h i s is n o t surprising, since b o t h states are v e r y b r o a d (0.7 2

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Fig, 3. Spectra corresponding to 1.5 < Ex < 3.5 MeV at 107.5 ° "~ 0 , . z 175o. The two broad states (groups 1 and 3) are the even-parity states at Ex = 1.67 and 3.06 MeV; the sharp groups (groups 2) correspond to the ~- level at 2.43 MeV. The ordinate scale gives the average number of 0~-tracks recorded cvery mm in a 215 t~m wide "bin" of the emulsion.

a n d 1.3 M e V , respectively), and the s h a r p e r o f the t w o p r o b a b l y i n v o l v e s an l = 2 transfer. 2.3. THE T - - .~ STATES OFgBc WITH 11 < E~. < 15 MeV A l p h a - p a r t i c l e g r o u p s are o b s e r v e d at five angles (0 = 20%50 °) c o r r e s p o n d i n g to a 9Be state at E x = 11.26__+0.04 M e V , F = 5 3 0 + 7 0 keV. T h e s e results are in g o o d a c c o r d w i t h t h o s e o f C o c k e 2), w h o f o u n d the state to be at 11.294-0.03

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F = 620_+70 keV. The previously reported 2) states at E~ = 11.81 and 13.78 MeV were not observed in this reaction. In the mirror nucleus 9B, the mirror reaction I°B(aHe, ~t)gB also only feeds 9) the lowest of the three states corresponding to 9Be* (l 1.28, I !.81, 13.78), i.e. the state at E~ = 11.62 MeV. This suggests that the I i.28 MeV state in aBe is the analogue to the 11.62 MeV state in 9B. If J~ = ~-- as has been suggested by Barker t a), 9Be. (l !.28 MeV) would be reached by 1 = 1 in l°B(t, ~t)qBe. The positon decay of 9C suggests that the 12.06 MeV state in 9B has J~ = ½-. If we assume that the I 1.82 MeV state in 9Be is its analogue, such a J ~ assignment would be consistent with it not having been observed in this reaction; l = 3 would be required to reach a ½- state in the t 0B(t, ct)gBe reaction (and of course in the ~°B(3He, ~t)gB reaction). No comments can be made at this time about the failure to observe the 13.78 MeV state, since there is no experimental information on its character; its width 2) of 590+60 keV would not inherently preclude its observation in this reaction. At 20 ° and at 27.5 °, there is some evidence for a broad hump which may correspond to a state in 9Be at E~ ~ 14.5 MeV, F ~ 0.8 MeV. This structure has not been reported in any other reaction. There is some evidence 4' 5) however, from the t o B(p, d)gB reaction, for a 9B state at Ex ~ 14.9 MeV, F ~ 1.4 MeV. Altogether, then, there is some evidence for ten T = ½ excited states of 9Be below E~ = 14.6 MeV. Theoretical calculations by Barker t a) and by Kurath 7) suggest I l and 14 such states, respectively. 2.4. THE T ~ ~ STATES OF 9Be If isospin were perfectly conserved, the 10B(t, ~t)gBe reaction should not populate T = ~ states. As might be expected, therefore, the alpha groups to the ~ - , T = ~, state at E x = 14.39 MeV were observed at several forward angles but with sharply reduced intensities, typically ~ 5 9/0 of the intensity of the group to the 2.43 MeV state. The second T = ~ state at Ex = 16.97 MeV was not observed; if it were the analogue o f a J~ = ½-state of 9Li, its production would be inhibited both by T-forbiddenness and by the necessity for I = 3 waves. Recently Cocke 2) has reported a state at E~ = 16.671 MeV, F = 41 + 4 keV, of unknown character. It appears to be populated quite strongly in this reaction also (0 = 27.5°-50°), but the presence of many groups due to the 160(t, Ct)1SN reaction to several excited states of tSN makes it impossible to obtain quantitative results concerning 9Be* (16.67 MeV). The T = ~, J~ = ~- state discussed by Barker t3) would be reached by I = 1 in l°B(t, ct)gBe and by l = 0 in 7Li(aHe, p)gBe. 2.5. ANGULAR DISTRIBUTIONS TO 9Be (0) AND 9Be (2.4 MeV) Fig. of 9Be not be alphas

4 displays the angular distributions of the or-particles to the ~ - ground state and to the ~- state at E x = 2.43 MeV. Unfortunately, the experiment could repeated to complete the forward part of the distribution of the ground-state to see if the difference in the behavior of the two distributions at backward

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angles would be repeated in the forward direction. Both transitions should involve 1 = I. In the region in which both distributions are available, the detailed structure does not reproduce, although secondary m a x i m a appear at the same angles. It is clear from the large backward peak observed for the 2.43 MeV state that a simple p r o t o n pick-up mechanism does not hold. At this time, as on a previous occasion iv), the needed parameters (i.e. well depths etc.) for sophisticated theoretical calculations of the distributions are not available. 2.6. ANGULAR DISTRIBUTIONS IN THE 9Be(t, 00SLi REACTION Since there is so little i n f o r m a t i o n available on (t, a) reactions, it seems useful here to present a n g u l a r distributions obtained is) earlier but not published t. The 9Be (t, ~t)8Li reaction feeds the J = 2 + g r o u n d state of 8Li as well as the well-known excited states at 0.975 (J~ = l +) and 2.26 MeV ( J " = 3+). In addition, the reaction goes to an excited state at Ex = 6.53_+0.02 MeV, F < 40 keV, whose character is unknown. Fig. 5 shows the a n g u l a r distributions of the a-particles to these four states. The lowest three should involve I = 1 and crude similarities in the distributions (particularly involving a secondary m a x i m u m at 0 ~ 120 °) are apparent. The distribution to the 6.53 MeV state is rather featureless and in particular does not involve a forward m a x i m u m ; this suggests l > 1. A laTge /-transfer is consistent with the n a r r o w width of that state. The authors are very grateful to H. M a r c h a n t and to the operating stall" of the A l d e r m a s t o n T a n d e m for their assistance in m a k i n g the exposure a n d to Professor T. Lauritsen and to Dr. C. L. Cocke for a very helpful discussion. t We are indebted to J. W. Watson for permission to publish these results obtained in collaboration with him.

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

H. G. Clerc, K. J. Wetzel and E. Spamer, Phys. Lett. 20 (1966) 667 C. L. Cooke, Nucl. Phys. All0 0968) 321 J. J. Kroepfl and C. P. Browne, Nucl. Phys. AI08 0968) 289 B. L. Kull and E. Kashy, Bull. Am. Phys. Soc. 12 (1967) 484 D. Bachelier, M. Bernas, i. Brissaud, C. Dctraz and P. Radvanyi, J. de Phys. CI-51 (1966) T. Lauritsen and F. Ajzcnbcrg-Selovc, Nucl. Phys. 78 (1966) I D. Kurath, private communication R. A. Mcndelson, E. Norbcck and R. R. Carlson, Phys. Rev. 135 (1964) Bl319 F. S. Dietrich, Nucl. Phys. 69 (1965) 49 T. R. Fisher and W. Whaling, Bull. Am. Phys. Soc. 8 (1963) 598 J. C. Hardy, R. 1. Verrall, R. Barton and R. E. Bell, Phys. Rev. Left. 14 (1965) 376 J. B. Woods and D. H. Wilkinson, Nucl. Phys. 61 (1965) 661 F. C. Barker, Nucl. Phys. 83 (1966) 418 E. Almqvist, K. W. Allen and C. B. Bigham, Phys. Rev. 99 0955) 631A H. D. Holmgren, L. M. Cameron and R. L. Johnston, Nucl. Phys. 48 0963) 1 R. Seltz, C. Gerardin, M. Wery and D. Magnac-Valette, J. de Phys. C1-148 (1966) F. Ajzenberg-Selovc, J. W. Watson and R. Middleton, Phys. Rcv. 139 (1965) B592 J. W. Watson, F. Ajzenbcrg-Selove and R. Middleton, Phys. Lctt. 18 (1965) 302