The Heat of Fission of Uranium

Abstract

The heat produced during slow neutron bombardment in a 13.36-g sample of metallic uranium was measured on a resistance thermometer. Simultaneously the number of fissions produced in a 54-microgram layer of uranium was counted by an ionization chamber and pulse amplifier. The two uranium samples were within one cm of each other and inside the same 1.5-cm lead shield and surrounded by 5 cm of paraffin. The source of neutrons was the Princeton cyclotron, using the Be-H reaction at 6.5 Mev. The 54-microgram layer gave 40.1±0.2 alpha-particles per min. into $2\pi$ solid angle and under slow neutron bombardment gave about 340 fissions per min. At the same time the heat output from the 13-gram sample was 40 microwatts. A method of calculation was developed that allows for the transient effects of fluctuations in the neutron output of the cyclotron. The weighted average of 13 runs, after various corrections that together amount to 3.8 percent, is 177 Mev per fission. The average deviation from this figure was 5 Mev and the result is believed to have a probable error of about 1 percent. The actual kinetic energy of recoil carried by the fission fragments is about 12 Mev less than this, ±50 percent, depending upon more precise knowledge of the fission mechanism.