PHOTONUCLEAR REACTIONS ON TC-99, ISOMER EXCITATION AND DEEXCITATION, IMPLICATIONS IN NUCLEOSYNTHESIS

Using bremsstrahlung beams up to 50 MeV, (gamma,xn) and (gamma,gamma') reaction cross sections of Tc-99 were determined for the first time, by means of gamma-spectrometry. Calculations were also carried out in the framework of a neutron evaporation and a statistical gamma-ray cas cade model, respectively. As a result, no deviation from the widely ac cepted picture was experienced in the case of (gamma,xn) reactions. Th e photoexcitation function for Te-99m production, however, while exhib iting a maximum at neutron emission threshold, dropped monotonously fr om about 9 to 50 MeV, in sharp contrast with the long-standing belief in a two-humped excitation function based on earlier reports. Allowed beta-transitions from photoexcited states of Tc-99 to Ru-99 levels may reduce dramatically the terrestrial half-life of Tc-99 in stellar env ironments. Significance of Tc abundance measurements in stars and shel l-model calculations in determining the parameters of the s-process nu cleosynthesis are highlighted.