Precise DSAM lifetime measurements in 48Cr and 50Cr as a test of large scale shell model calculations

Precise DSAM lifetime values have been obtained for the positive parity yrast bands of Cr-48 and Cr-50, populated with the reaction Si-28 + Si-28 at 115 MeV bombarding energy using a An backed target. The obtained precision relies on the well-known stopping power at high velocity and on the use of DSAM analysis procedures which avoid the sidefeeding contribution. The deduced B(E2) and B(M1) values are in very good agreement with those predicted by full fp shell model (SM) calculations. A new procedure named narrow gate on transition below has been applied for the analysis of the lifetimes. Clear evidence was found in both nuclei for a low-lying negative parity band which has been successfully described by SM in the fp shell extended to include a hole in the 1d(3/2) orbital. Also in this case the derived B(E2) and B(M1) values are in good agreement with the SM calculations. (C) 1998 Elsevier Science B.V.