Abstract
The position spectrum emitted in the decay of Dy155 to Tb155 (T12 = 10.2 h) has been studied by means of a large double-focusing beta ray spectrometer and an intermediate image beta ray spectrometer. Two components in the positon spectrum were observed with the end-point energies 850±6 and 1075±8 keV. By positon-gamma coincidence measurements the 850 keV positon branch was found to feed the 227 keV level in Tb155. The total decay energy of Dy155 was thus deduced to be 2099±6 keV and this value is compared with that obtained from a semi-empirical atomic mass formula and with the available decay energies of the other odd-mass dysprosium isotopes. The K/β+ ratio for the beta transition to the 227 keV level was found to be 44±5, in close agreement with the theoretical value expected for an allowed transition. The log ft values for the beta transitions to the 227 keV and the ground state of Tb155 were found to be 6.1 and 7.7, respectively. The spin, parity and the symptotic quantum numbers of the ground state of Dy155 are discussed. For this state the most probable classification is 32− [521].
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