Study of the yrast band in the doubly odd156Tm

Abstract

The yrast band in the doubly odd 156Tm nucleus was studied through 144Sm (19F,2p5n)156Tm reaction at beam energy of 105MeV. Several high-spin states of 156Tm were identified and the highest spin of the band with configuration 7r7/2"[523] @vl/2+ [660] could be built up to spin 2511. The level structure shows the onset of a non-or weak collectivity which generally appears at neutron number of 87 in neutron-deficient rare-earth nuclei. PACS: 23.20.Lv, 27.70.+q Nuclei in the neutron-deficient rare-earth region have been of considerable interest during the past few years. One reason is that nuclei in the light rare-earth region can hold the highest angular momentum possible without fissioning, and therefore are attractive for studies of highspin states in nuclei. In transitional region, nuclei with neutron number N>88 generally exhibit strong collectivity due to their prolate shape whereas those with N<86 more appears as aligned single-particle configuration with noncollective rotation. In this region one may also expect the coexistence of different nuclear shapes and study shape evolution, described by collective and non-collective rotational models. The doubly odd nucleus, 156Tm with N=87, makes a good candidate for such studies.The present note reports on an experiment performed with five HpGe-BGO Compton-Suppressed spectrometers and a 14 elements ball of BGO detectors. The levels of 156Tm were populated by using the 144Sm(19F,2p5n) 156Tm fusion-evaporation reaction with a 105MeV 19F beam provided by the HI-13 tandem accelerator of China Institute of Atomic Energy(CIAE), and a 1.0mg/cm 2 144Sm target. Single 3,-ray spectra were collected at beam energies of 85, 90, 95, 100, and 105MeV for excitation function measurement. A )'-),-BGO coincidence experiment was performed at beam energy of 105MeV and beam current of 10nA. A total of 107 events, each with at least two Compton-Suppressed HpGe detectors and two BGO detectors to be required in coincidence, were recorded for off-line analysis. Besides 156Tm, three other nuclei were produced in this experiment with appreciable intensity from 4n, p3n and 2p3n reactions (159Lu and 158, 159yb), resulting in accidental degeneracies with a number of Vrays in 156Tm. However, much information on the contaminant lines from 159Lu(4n channel) was known from earlier studies[I], as well as the level structures of 158, 159yb well studied by other laboratories [2,3].Before our investigation, a few levels had been identified in 156Tm [4]. From our experiment, a partial level scheme of 156"I'm was established as shown in figure 1 and a coincidence-spectrum gated by 595 keV is shown in figure 2.The level ordering was made by considering the ),-ray intensity balance obtained by gating on the V-7-BGO coincidence matrix, and with additional information obtained from excitation functions. The results may be considered preliminary since no definite spins and parities can be assigned to the levels. However, certain clues to the band structures can be obtained from systematic inform...