Energy levels of 57Co

1.E.1 : 2.A.1

Nuclear Physics A108 (1968) 529--534; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

ENERGY LEVELS OF 57Co NOEL BOUCHARDt

and B I B I A N A (~UJEC

D~partement de Physique, Universitd Laval, Quebec, Canada Received 26 October 1967 Abstract: The reaction S4Fe(:~, p)57Co is used to study the energy levels of sTCo up to 4 MeV excitation with 25 keV resolution. Several new levels are found, and it is p r o p o s e d that the levels at 1.22 MeV and 1.68 MeV have spin and parity ~z and 1~ , respectively.

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N U C L E A R R E A C T I O N S 54Fe(~,p), E = 10.5 and 11 MeV; measured a(Eo, 0). 57Co deduced levels. Enriched target.

Targets of 54Fe (90-98 ~ enriched) evaporated on gold backings were bombarded with 10.5 and 11 MeV e-particles from the Laval University van de Graaff. The outgoing protons were magnetically analysed and focussed along a 25 cm photographic plate (Kodak NTB, 100pm). The plates were covered with aluminium foils to absorb the scattered e-particles. A (3He, d) experiment demonstrated that the amount of 56Fe in the targets was negligibly small because not even the strongest peaks of the reaction 56Fe(3He, d)57Co were observed. Measurements were obtained at several angles. Fig. 1 shows a few typical spectra. The only peaks due to contaminants are the protons from the 14N(e, p)170 ground state transition and protons from the elastic scattering of e-particles on hydrogen. The peaks from tZc and 1°O do not appear because of too low Q-values. The energy levels of 57Co deduced from the observed proton groups are listed in table 1. Included also are the data from other reactions, i.e. 56Fe(3He, d)57Co [ref. 1)], 56Fe(p ' ~7)57Co [ref. 2)], S8Ni(t ' c057Co [ref. 3)] and from B-disintegration of 57Ni [refs. 4, 5)]. All levels observed from those reactions were also observed in the present work except for the 1.46 MeV and 1.58 MeV states reported by Bakhru and Preiss 4) from //-disintegration. Piluso et al. 5), however, did not observe these two levels from /3-disintegration. We found new levels at excitation energies of 2.51, 2.55, 3.39, 3.76, 3.83 and 3.96 MeV. The levels at 1.22 and 1.68 MeV, which were observed previously only with the 56Ni(t, e)57Co reaction, appear in this work very clearly. Also the levels at 2.79 and 3.11 MeV, which were previously observed 2) only with the 54Fe(p, 7) reaction, are seen. Furthermore, this work indicates that the levels at 1.75, 1.90 and 2.30 MeV are probably doublets with about 20 keV separation and thus explains the contradictions in J~ assignments to these levels. The 1.75 MeV level appeared as a 3- state in the t Partly his thesis for the M.Sc. degree. 529

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57Co ENERGY LEVELS

53 1

56Fe(3He, d) and 58Ni(t, e) reactions and as a ~- state in the S6Fe(p, 7;0 angular correlation measurements. Similarly, the 1.90 MeV level appeared as a ~- state in the TABLE 1 Energy levels (in MeV) o f 5rCo ~4Fe(ct, p)

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0 1.218 1.369

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Fig. 2. Characteristics of the ~7Co levels as observed by various reactions in the present and previous [refs. 1-5)] work. Theoretical predictions by Vervier 5) of the levels with the ground state configurations (lf~)-l(2pff) 2 are also presented. With the reactions ~SFe(2He, d) and 58Ni(t, ~) the levels are, as a rule, represented by their transition strengths ( 2 J + 1)C2S and S, respectively. For the levels with unidentified angular m o m e n t u m transfer, however, their observation only is indicated by a short line. With the ~"Fe(2He, d) reaction the /-value of the transferred proton is given, leaving, in general, two possibilities, J = lJ-½, for the spin of the state. Spin assignments consistent with all of the measurements are reproduced in the centre graph, which represents levels from the present work. The levels drawn with a heavy line are suggested (see text) to have high ( > 5) spins.

'57Co E N E R G Y

LEVELS

533

58Ni(t, c0 reaction and as a s - state in the/j-decay and 56Fe(p, YT) measurements. The 2.13 MeV level appeared as a ~-- state in the 56Ni(3He, d) work and as ~+ state in the 56Fe(p, 77) correlation measurements. The probable ~+ J~ assignment for this same level in the 58Ni(t, ~) work could be consistent with the 3 - assignment due to the similarity of angular distributions for 3 + and ~ - states. For the 1.90 MeV doublet we conclude from a comparison of the excitation energies obtained from the 58Ni(t, ~) reaction with those from 56Fe(p, 77) and/J-disintegration that the lower level has J~ = ½- and the higher has J~ = ~-. In fig. 2, data are combined on 57Co levels obtained in different reactions. By noting in which of the reactions a level was seen and how strongly it was excited, we get some additional information about its spin, about consistency and reliability of different spin assignments and about the configurations of the level. In the 56Fe(3He, d)57Co stripping reaction, the levels with J~ = ~ - , ½- and 2s.are favoured due to the transfer of a proton into the unoccupied 2p~, 2p~ and lf_~ orbits. In the 58Ni(t, ~)SVCo pick-up reaction, the levels with J~ = ~ - , -~+ and ½+ are favoured due to the pick-up of a proton from the occupied lt~, ld~ and 2s~ orbits. The//-disintegration of 57Ni(J~ = 3 - ) into 57C0 is allowed and accordingly populates only the levels with J~ - l - 3 - and ~-. The proton capture reaction 56Fe(p, 7) is not expected to populate levels with high spin. The situation is opposite with the 54Fe(c~, p) reaction. While the cross sections to the individual levels in the stripping and pick-up reactions vary within a factor of 100, and, moreover, many levels are not seen at all, in the 54Fe(~, p) reaction all of the levels are observed, and the cross sections do not vary more than within a factor of three. We may conclude, that the 54F'e(o~, p) reaction at 11 MeV bombarding energy proceeds mainly via compound-nucleus formation, such that all levels with spins up to about 15 -~are excited. And we may reasonably believe that the levels observed only in the present work have spins higher than ~. We now discuss the low-lying levels in terms of the shell model and tentatively assign spins to the 1.22 and 1.68 MeV levels. The ground-state configuration, Qrf~)-z (vp~) z, with one proton hole in the f~ orbit and two neutrons in the 2p~ orbit gives a J~ = ~ - state with seniority v = 1 and states J~ -- - 2 a_- , 2 5- ~ ~2 ~ o2 ' ]2 ~ - and seniority v = 3. These levels were calculated by Vervier 6) (fig. 2) and recently by McGrory 7) 2 who, for the two neutrons, allowed the p~ and f2 configurations in addition to the p~ configuration. The uncertainties in predictions were estimated 7) to 200 keV. Both calculations were performed with very similar results on a series of nuclei with N = 30 and Z < 28. In 5VCo the differences between the two predictions are unessential, except for the ~-- and { - states. The experimental levels in general agree with the predictions. The agreement is especially good for 55Mn, where all levels up to 2 MeV are accounted for within 200 keV, and is especially bad for 57Co. Here, more levels exist than are predicted. The single-particle character of the 3 - state at 1.37 MeV and the existence of the ½state at 1.50 MeV show that at least the configuration (#f~)-2 (Trp~)(vp~)2 is present

534

N. BOUCHARD

A N D B. ~ U J E C

at low energy and consequently should be included in calculations. This would introduce new levels and would through mixing transplace the levels of the ground-state configuration. The experimental spectrum, however, agrees with predictions based only on the ground-state configuration, except for the mentioned ~- and ½- states and for one of the two ~- states. The ground state has J~ = ½-, the levels with J~ = ~-, ~-, 7-are at correct energies, the gap between seniority v = 1 and seniority v = 3 levels is reproduced. The two levels with yet unassigned spins at 1.22 and 1.68 MeV very probably have J~ > ~ because they were unobserved in the fl-decay, in the (p, V) and in the (3He, d) reaction, and were extremely weakly excited in the (t, ct) reaction. Consequently, these two levels are candidates for the missing ~- and ~ - levels and are the only candidates below 2.4 MeV. Vervier predicted these two levels at about 1.6 MeV, while McGrory predicted the ~- level at 1.20 MeV, and the ~ - level at 1.75 MeV. As McGrory's predictions in general represent improvement over Vervier's predictions, we tentatively assign J~ = ~- to the 1.22 MeV level and J~ = ~ - - to the 1.68 MeV level. References 1) 2) 3) 4) 5) 6) 7)

B. Rosner and C. H. Holbrow, Phys. Rev. 154 (1967) 1080 L. A. August, C. R. Gossett and P. A. Treado, Phys. Rev. 142 (1966) 664 A. G. Blair and D. D. Armstrong, Phys. Rev. 151 (1966) 930 H. Bakhru and I. L. Preiss, Phys. Rev. 154 (1967) 1091 C. J. Piluso, D. O. Wells and D. K. McDaniels, Nuclear Physics 77 (1966) 193 J. Vervier, Nuclear Physics 78 (1966) 497 J. B. McGrory, Phys. Rev. 160 (1967) 915