Intermediate structure in the photoexcitation ofSem77,

Abstract

Continuing the systematic investigation into the photoexcitation of isomers over wide mass and energy ranges, the production of Se, " Br, and '"Ba was studied with the bremsstrahlung facility at the superconducting Darmstadt linear accelerator. These isomers have half-lives on the order of seconds. Excitation functions were measured for the (y, y ) reactions populating the metastable states for energies of 2 -7 MeV and the important intermediate states were identified. Nuclear structure calculations with the quasiparticle-phonon model for Br and the particle-(hole-) core coupling approach for " Ba gave satisfactory descriptions for the strength and position of the dominant mediating levels. Admixtures of fragmented outershell single-particle strength shifted to low energies were identified as essential features of the wave functions of those states. Intermediate states in Se displayed very large strengths compared to other isomers in the same mass region, providing further support for the correlation between integrated cross sections and ground state deformations recently discovered in the A = 160 -200 mass region. Such an enhancement would considerably improve the feasibility of a gamma-ray laser based on the sudden deexcitation of isomeric populations in deformed nuclei.PACS number(s): 25.20. Lj, 27.50.+e, 27.60.+j, 21.90.+f

Population of isomers in the decay of the giant dipole resonance

“…Recently, essential progress in isomer photoexcitation has been achieved by using bremsstrahlung radiation at low values of an end-point energy [6][7][8][9]. The isomer yield in spherical nuclei as a function of the end-point energy was found to have a linear dependence in the energy interval of about one MeV, the line then breaks, and another linear dependence for one MeV takes place.…”

“…The application of the model in the case of isomeric states is described in detail in Refs. [6][7][8][9]. This model makes use of a separable form of the residual interaction which enables the diagonalization of the model Hamiltonian in a large dimensional configuration space.…”

“…This effect received its theoretical interpretation as follows: A level or a group of closely lying levels with very specific properties are located at the line break. The wave function of these excited states includes configurations which allow their intensive excitation from the ground state while some other configurations are responsible for a cascade decay from these levels at intermediate energies which lead to the isomeric state [6][7][8][9]. Usually the maximum value of the end-point energy in these bremsstrahlung experiments is 4-4.5 MeV.…”

“…For this reason, a microscopic calculation should be applied to explain the A-dependence of the isomer yield which is experimentally observed. For that purpose, we employ the Quasiparticle Phonon Model (QPM) which has already been successfully applied to the interpretation of the isomer population from the intermediate states at lower energies [6][7][8][9].…”

“…Considerable interest in the structure of the states at intermediate excitation energy in atomic nuclei has arisen recently [1][2][3][4][5][6][7][8][9][10]. The states are excited via different nuclear reactions and the deexcitation populates the low-lying excited states that have a simple structure [1].…”