Bands and Coulomb effects in 50 Cr F. Brandolini, 1 J. Sanchez-Solano, 2 S. M. Lenzi, 1 N. H. Medina, 3 A. Poves, 2 C. A. Ur, 1, * D. Bazzacco, 1 G. De Angelis, 4 M. De Poli, 4 E. Farnea, 4 A. Gadea, 4 D. R. Napoli, 4 and C. Rossi-Alvarez 1 1 Dipartimento di Fisica and INFN, Padova, Italy 2 Departamento de Fisica Teorica, Universidad Autonoma, Cantablanco, Madrid, Spain 3 Instituto de Fı ´sica, Universidade de Sa ˜ o Paulo, Sa ˜ o Paulo, Brazil 4 INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy ~Received 11 December 2001; published 2 August 2002! Experimental evidence for the coexistence of states with different K p value was found in 50 Cr. The band- crossing of the K 50 1 ground state band with a K 510 1 one is confirmed. Large scale shell model calculations could explain all of the observed experimental features and in particular the known experimental Coulomb energy differences in the mirror pair 50 Fe- 50 Cr. DOI: 10.1103/PhysRevC.66.021302 PACS number~s!: 21.10.Tg, 23.20.Lv, 27.40.1z A wealth of experimental information on the structure of nuclei in the middle of the 1 f 7/2 shell has been recently col- lected at LNL @1–6#. The attention was mainly focused on the yrast sequence of states of both natural and unnatural parities up to the band termination in the 1 f 7/2 n and 1 d 3/2 21 ^ 1 f 7/2 n 11 configuration space, respectively. In order to inves- tigate rotational collectivity and single particle properties through experimental electromagnetic moments, lifetimes were deduced with the Doppler shift attenuation method ~DSAM! for many levels. Large prolate deformation is pro- duced in most ground state ~g.s.! bands, which decreases approaching band termination. Large scale shell model ~LSSM! calculations for natural parity states were systemati- cally made in the full pf configuration space, getting in gen- eral an excellent agreement with the experimental findings @7–9#. The unnatural parity sidebands were described by ex- tending the pf space to include a nucleon-hole in the 1 d 3/2 orbital. In the present work 50 Cr is further investigated @2,3#, be- cause of the peculiarity in this region, that the g.s. band shows evidence of bandcrossing, which is mainly based on the presence of two close-lying 10 1 levels. Different inter- pretations were given for the side band: in Ref. @9# it was suggested to be oblate, while in Ref. @10# to be a high-K prolate one. This question gained more interest recently, be- cause the backbending was shown to be correlated with a discontinuity of Coulomb energy difference ~CED! in the mirror pair 50 Fe- 50 Cr @11#. In this context, rotational align- ment ~RAL!, described by the cranked shell model ~CSM!, was recently proposed @12# as a further explanation of the observed backbending in 50 Fe- 50 Cr, following a previous suggestion @13#. It will be shown that prolate strongly- coupled Nilsson configurations, i.e., deformation alignment ~DAL!, can explain most of the observed features and that the yrast 10 1 level can be approximately described as a K 510 1 state, due to the simultaneous excitation of a proton and a neutron from the @ 321# 3/2 2 to the @ 312# 5/2 2 orbital and from the @ 312# 5/2 2 to the @ 303# 7/2 2 one, respectively. This was already suggested by the authors in Ref. @14#, but additional arguments will be given here. The reaction 28 Si( 28 Si, a 2 p ) 50 Cr was performed at the GASP spectrometer of LNL, using a target of 0.8 mg/cm 2 backed with 15 mg/cm 2 of Au, at the bombarding energy of 115 MeV. Experimental data for 50 Cr, obtained from the same experiment @3#, are now extended to some nonyrast states. In Fig. 1 the level scheme shows the up-to-date infor- mation for the low-lying levels up to the 14 1 band terminat- ing state in the 1 f 7/2 space. Only transitions from levels rel- evant for the present discussion are shown. The levels shown on the leftmost part of the figure were taken from Ref. @15#. The K quantum number is assigned on the basis of arguments presented in the following. The experimental properties of positive parity levels of interest are summarized in Table I. The level at 3324 keV was known to have I p 54 1 and to decay mostly to the yrast 4 1 state, with a lifetime of t 50.14(3) ps @15#. The level at 3825 keV was reported in Ref. @15# as (4,5,6) 1 . In the present work an upper limit of 1 ps for its lifetime is estimated by a broad gating on the 541 keV feed- ing transition from the 5 2 level at 4366 keV, whose lifetime is determined to be 2.0~5! ps. Both values have been ob- tained in the present work with a DSAM analysis of the 662 and 541 keV lines, with the procedure described in greater detail in Ref. @3#. From the quoted limit, the squared mixing ratio d 2 is estimated to be smaller than a few percent. The angular distribution of the 662 keV transition to the 6 1 state, when gated by the feeding 541 keV one, is forward-peaked as expected for a pure D I 50 M 1 transition, so that the final assignment to the level at 3286 keV is I p 56 1 . The 4 1 level at 3324 keV is fed by the 3875 keV one with a 551 keV line. The 3875 keV level resulted to have a life- time of 0.9~3! ps from the analysis of the 712 keV branch so that its decay scheme allows to assign I p 55 1 . The level at 3792 keV, having a lifetime of 13~2! ps, has been previously suggested to be 4 2 @3#. This is in analogy to 46 Ti, where the yrast 3 2 level is interpreted as the band head of a K p 53 2 band due to the parallel coupling of a proton in the @ 202# 3/2 1 Nilsson orbital with one in the @ 321# 3/2 2 . In the present case the first available orbital is the @ 312# 5/2 2 *On leave from NIPNE Bucharest, Romania. RAPID COMMUNICATIONS PHYSICAL REVIEW C 66, 021302~R!~2002! 0556-2813/2002/66~2!/021302~5!/$20.00 ©2002 The American Physical Society 66 021302-1