A study of high spin states in the transitional nucleus90Mo

Abstract

The reaction 58Ni(36Ar, 4p)9~has been studied at a beam energy of 149 MeV. A detector array consisting of the OSIRIS spectrometer (12 Compton-suppressed Ge detectors), four charged-particle AE detectors and seven NE-213 neutron detectors at the VICKSI accelerator in Berlin has been used to measure the gamma radiation in 77-and particle-77-coincidence mode. An additional Ge detector was placed at 162 ~ to the beam direction to provide information on DCO ratios. The level scheme of 9~ has been extended up to an excitation energy of about 12 MeV and probable spin 23ta. Some 70 transitions and 40 levels have been newly identified. Spin assignments have been proposed on the basis of measured DCO ratios. Shell model calculations in a model space consisting of the proton lf5/2, 2p3/2, 2pl/2, and 1 g9/2 orbits and the neutron 2pl/2, 1 g9/2, 1 g7/2, 2 d5/2, 2d3/a, and 3 Sl/2 orbits with some truncation were made for states above 9h and the predicted structure of these states is discussed.

Identification and structure of high spin particle-hole states in89Mo

“…Most of the other transitions show RDco~0.5 which is the expected value for a stretched dipole transition. As Kabadiyski et al [2] have shown in their studies on 9~ with the same experimental setup, a A I= 0 dipole transition should have RDco,~ 1.0, which causes difficulties in separating them from stretched E2 transitions. These authors also stated that the DCO-ratios for this setup tend to decrease for small transition energies (Ey < 300 keV) because of different timing properties of the Ge detectors.…”

“…Our recent detailed investigations on the eveneven nuclei SSMo and 9~ [1,2] have, indeed, given evidence that the positive-parity yrast states can be surprisingly well explained within this restricted shell model space, up to the highest spins observed, e.g. 24 h in 88Mo [ 1 ].…”

“…Gallagher et al [8] established a 190 ms isomer at 387 keV to which they assigned I ~ = 1/2-. The results of in-beam measurements for the high spin states in the N = 48 isotones 88Zr and 9~ have been explained by the shell model [2,10,11], but as both g9/2 protons and neutrons are active, different seniority schemes have been proposed to become yrast in the two isotones. Via Pauli blocking, the unpaired g9/2 neutron in 89Mo clearly affects the possible shell model multiplets: among the (3 qp) states, the 3 V (g9/2)-multiplet reaches up to spin 21/2 h, while the rt (g9/2)21, ' (g9/2)-1 multiplet terminates at spin 25/2 h. A study of the odd-A nucleus 89Mo may therefore allow us to differentiate between proton alignment which was predicted to be yrast in the N= 46 isotones 86Zr [ 12], and neutron alignment active in the N = 48 isotones 88Zr and 9~…”

“…For a given pair of coincident transitions, the DCO-ratio is defined as Ey < 300 keV, we find two separate groups of experimental values. Those with RDco~ 1.0 can be identified as E2 transitions on the basis of known data [2]. Most of the other transitions show RDco~0.5 which is the expected value for a stretched dipole transition.…”

“…The target was located in the center of the OSIRIS spectrometer [14], with a ring of six Compton-suppressed Ge detectors of about 25% efficiency located at 65 ~ to the beam and another six detectors at 115 ~ An additional large-volume Ge detector without Compton-suppression was placed at 162 ~ to the beam direction in order to search for Doppler-broadened lineshapes of short-lived transitions and to measure DCO-ratios (see below). Details of the accumulation and analysis of the ~y coincidence data have been described in our recent ss'9~ and 89Tc papers [1,2,15].…”