High-Spin Spectroscopy in the Gamma-Soft Mass a = 130 Region.
Abstract
High spin states in the odd-proton nuclei ^{133}Pr, ^{135,137 }Pm and ^{131}La, the odd-neutron nucleus ^{135} Nd and the doubly-odd nuclei ^{134 }Pr and ^{136}Pm were studied following heavy-ion fusion-evaporation reactions using beams provided by the Stony Brook Tandem-LINAC accelerators. Gamma -rays produced following these reactions were measured using an array of four BGO suppressed Ge detectors. Experiments performed included excitation functions, angular distribution measurements and gamma-gamma coincidences. In addition in several cases internal conversion electrons were measured using a mini-orange electron spectrometer, which was constructed as part of this dissertation. Collective rotational bands were populated in all of these nuclei. For the odd-proton nuclei the yrast band, based on a decoupled h_{11/2 } proton orbital, exhibits a typical DeltaI = 2 structure with a very large signature splitting ~500 keV. Backbends, due to the alignment of the second and third h _{11/2} protons, are observed in these bands. Sidebands, based on a different proton orbital, the g_{7/2} orbital, and a pair of aligned h_{11/2} protons are also observed. In ^{133 }Pr a low crossing frequency hbar omega = 0.28 MeV has been extracted from the sideband for the first proton alignment. This frequency is the lowest observed in this mass region. The yrast band of the odd-neutron nucleus ^{135}Nd is based on a strongly coupled h_{11/2} neutron orbital. Both signatures of this band are observed with strong DeltaI = 1 M1 transitions linking the two signature components. At a frequency hbaromega = 0.32 MeV both signature components backbend, due to the alignment of the first pair of h_{11/2} protons. The moderate signature splitting of ~150 keV at low frequencies disappears after this alignment. The doubly-odd nuclei exhibit yrast bands based on the pi h_{11/2} otimes nu h_{11/2} structure with a signature splitting of ~ 50 keV caused by the odd-neutron orbital. This is smaller than in ^{135}Nd because of the influence of the valence h_ {11/2} proton. The experimental crossing frequencies and signature splittings are compared with cranked shell model predictions as a function of nuclear triaxiality. Generally the odd -proton nuclei were found to possess prolate or near prolate collective shapes while odd neutron and double-odd nuclei possess more triaxial shapes. A collective oblate band based on the pi h_{11/2} otimes (nu h_{11/2})^2 configuration is observed in ^{131 }La. Here the prolate collective core is driven to an oblate, but still collectively rotating shape by the rotational alignment of a pair of upper midshell h _{11/2} neutrons.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1987
- Bibcode:
- 1987PhDT.......149B
- Keywords:
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- Physics: Nuclear