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The report summarises the current state of research findings; the first part gives the results of fission cross section measurements, and the second determinations of the mean number of neutrons emitted per fission. Progress has been made in measuring fission sections of fissile material with high alpha activity, through intrinsic suppression of the alpha particles and formation of very short pulses in ionisation chambers where the fission fragments are stopped completely. Very small cross sections have been measured, by using ultrapure neutron beams and improved methods of preparing specimens. The neutrons are counted by an appropriate detector (of the same nature, use and potential as a large liquid scintillator). This has made it possible to establish that the increase in E(g) (gamma radiation energy) as a function of the excitation energy observed in the fission induced by fast neutrons is due to the increase in the mean energy of the individual quanta emitted, the number of these remaining virtually constant.

Additional information

Authors: DERUYTTER A J, JRC Geel Estab. (BE);FREHAUT J, CEA, Centre d'Études de Bruyères-le-Châtel (FR)
Bibliographic Reference: Paper presented: Le Cinquantenaire de la Découverte de la Fission, Paris (FR), June 21, 1989
Availability: Available from (1) as Paper FR 34882 ORA
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