Precise measurements of the 24997Bk ground state half-life and the β--decay end-point energy

The half-life of the Bk-249 ground state was determined by means of gamma-ray spectroscopy, following for 728 days the growth of its alpha-decaying daughter nuclide Cf-249. Using a chemically purified source containing Bk-249 and Cs-137 nuclides, gamma-ray singles measurements were carried out using a 25% coaxial Ge detector. The areas of the strongest 333.37- and 388.17-keV gamma-ray peaks, produced in the a decay of Cf-249, and the 661.66-keV peak, produced in the beta(-) decay of Cs-137, were determined. The measured activity of the latter was used to account for geometrical and dead-time corrections to the efficiency of the spectrometer, thus minimizing the systematic uncertainties associated with long-time, gamma-ray counting measurements. Using the growth with time of the ratio of the 388.17- and 661.66-keV gamma-ray peaks, a value of T-1/2 = 327.2 +/- 0.3 d (the uncertainty quoted is one standard deviation, 1 sigma) for the half-life of the Bk-249 ground state was determined. The beta(-)-decay end-point energy of Bk-249 was measured with a passivated implanted planar silicon detector to be 123.6 +/- 0.4 keV.