On the? ? decay of the 2s isomer in98Y and the existence of an intruder band in98Zr

Abstract

The 7 radiation from the I1-decay of the high-spin isomer in 9Sy has 4390 been studied at the separator JOSEF. A main goal was to get insight into the spins of levels in 98Zr which have been interpreted as 3773 members of an intruder band based on the 02 + state at 854 keV. The measured Y-7 angular-correlations lead to the assignment of spins of 4 and 3 h to the 2491 and 1844 keV levels, respectively, and Fig. 1. This interpretation of the levels of 9SZr is appealing since it may mean that the onset of deformation at A -I00 proceeds more smoothly than would be concluded from the rapid drop of the 21 + energies from 1223 to 213 keV between 98Zr and l~176 The yrast states of the intruder bands in the Zr isotopes with N<60 might [4,5] become gradually compressed with increasing neutron number (filling of the vgT/2 subshell) and, thus, smoothly connect to the ground-state bands of the deformed isotopes m~ and m2Zr.Because of the importance of this interpretation of the levels of 98Zr it is indispensable to put it on as solid grounds as possible. In order to get information on spins, 7-7 angular-correlation measurements have been undertaken at the fission-product separator JOSEF, which allows the study of the IP decay of the high-spin isomer of 98y [2]. The corresponding decay scheme [6], see Fig. 2, contains the 1844 and 2491 keV levels, which have been interpreted [4] as the I=4 + and 6 + members of the intruder band, cf. Fig. 1. Thus, the "skeleton" of this band could be checked.The procedure for 7-7 angular-correlation studies at JOSEF consisted in a separation of the 98y parents, their deposition on a plastic tape with the use of an air jet, the transport to the center of a goniometer table in startstop mode and in a measurement of the y radiation with a three-detector system [7]. Three Ge(Li) detectors with volumes of 136, 145 and 170 cm 3 were positioned at distances of 7 cm from the sample. Two of the