Coincidence summing in γ-ray spectra and determination of the intensity of weak crossover γ transitions

The intensities of peaks formed in γ-ray spectra by summing the pulses due to coincident cascade γ transitions in radioactive nuclides are used to determine the intensities of weak crossover γ-radiative transitions. A procedure for measuring γ-ray spectra and calculating the intensities of crossover γ transitions is proposed. The dead time of the measuring facility and a decrease in the intensity of cascade γ-ray peaks, caused by their coincidence with the photopeak and the Compton distribution due to the γ-transition-partner in the cascade, are taken into account. This procedure has been tested by measuring γ spectra of 209Tl, 208Tl, 207Bi, and 44Sc decays. It is shown that the proposed technique can be used to observe crossover γ transitions with an intensity as high as ∼5×10−5 per decay. The upper limits on the intensities of crossover γ transitions in decays of 208Tl and 207Bi nuclei have been determined for the first time.