SPONTANEOUS-FISSION HALF-LIVES OF PLUTONIUM-240 AND PLUTONIUM-242

Previously published values of the half lives of Pu/sup 240/ and Pu/sup 242/ are not sufficiently accurate because only very small amounts of the isotopes were available. A series of measurements was started, using preparations with a small surface density and a high concentration of the isotope studied. A Xe-filled scintillation counter was used as the fission-fragment detector; its resolution was better than that of ionization chambers. Before starting the measurements, the counter was evacuated to 5 x 10/sup -7/ mm of Hg, then filled with Xe to a pressure of 2.5 to 3 atm. The stability of the system was tested by measuring the energy spectrum of U/sup 233/ fission fragments. The spontaneousfission half life of Pu was found to be (1.45 plus or minus 0.02) x 10/sup 11/ years, using a preparation with a surface density of 7 x l0 microgram/ cm and a Pu /sup 239//Pu/sup 240/ ratio of 7.04. A second series of tests with a preparation possessing a P /sup 240//P /sup 242/ == 1.24 ratio yielded a corresponding half life value of (7.45 plus or minus 0.17) x 10/sup 10/ years for Pu/sup 242/. The energy spectrum of the spontaneous fission fragments is alsomore » presented. (TTT)« less