Investigation Of Different Possible Excitation Modes In Neutron-Rich As-78

The yrast and nonyrast level structure of the odd-odd As-78 nucleus has been investigated using the standard gamma ray spectroscopic techniques. The excited states of As-78 were populated through Ge-76(alpha, pn) fusion evaporation reaction at the incident beam energies of 30, 35, and 40 MeV. The deexciting gamma rays were detected with the Indian National Gamma Array spectrometer composed of Clover and low-energy photon spectrometer detectors. In addition to the extension of the previously known yrast positive-parity band up to E-x approximate to 2.7 MeV with J = 11h, an excited dipole band-like structure based on the 7((-)), 1243-keV state has also newly been established. Several weak transitions feeding the low-lying nonyrast states have newly been identified and placed in the level scheme. A highly irregular sequence of levels has been observed at the low-excitation regime suggesting the dominance of single-particle excitations. The experimental results have been compared with the predicted results from the large-scale shell-model calculations. The calculated results indicate an enhanced contribution from the intruder pi(1g(9/2)) orbital to the wave function of the positive-parity yrast excited states with J >= 7h. This enhanced contribution is supposed to provide subsequent mild collectivity for the concerned states. The nature of collectivity has been extracted from the total Routhian surface calculations. Evidences for the possible persistence of the stapler like mechanism prevailing in the positive- and negative-parity level sequences are presented.