K-capture probability in the decay of139Ce(137.5d)

The K-capture probability (P _K) in the decay of¹³⁹Ce (137.5d) has been determined from the analysis of the K x-ray gamma ray sum-peak observed with a Ge (Li) x-ray detector. Furthermore,P _K is estimated independently from the measurement of the K x-ray intensity in the decay, using the known conversion coefficients of the 165.8 keV transition. Using the weighted average ofP _K measured as above, the mass difference between¹³⁹Ce and¹³⁹La is found to be 273 _?13 ⁺²¹ keV.