Gamma deexcitation of the Ta-180m isomer

Vibrational states built on the K-pi = 9(-) isomer and on the ground state (K-pi = 1(+)) in Ta-180 are calculated within the quasiparticle-phonon nucl ear model using the Hf-178 nucleus as a core. A procedure for calculating t he rates of K-allowed gamma -ray transitions from vibrational states built on the isomer to those built on the ground state is presented. The probabil ities of two-step processes consisting of a dipole excitation of the isomer and successive El and E2 transitions from them to vibrational states built oil the ground state of the Ta-180 nucleus are calculated. Two-step transi tions from the isomer to vibrational states below 2.7 MeV and to the vibrat ional states built on the ground state appear to be very weak. There are ma ny El transitions from the vibrational states built on the isomer to the vi brational states built oil the ground state. They are weak and cannot be re sponsible for the strong deexcitation of Ta-180m in the relevant (gamma,gam ma ') reaction. A decisive role is played by collective E2 transitions from dipole excitations in several excitation energy intervals ranging between 2.7 and 4.0 MeV. These highly intense K-allowed two-step gamma -ray transit ions can be responsible for the strong deexcitation of the Ta-180m state in the (gamma,gamma ') reactions. (C) 2001 MAIK "Nauka/Interperiodica".