Properties of 3 H-particles emitted during the thermal neutron-induced fission of 239 Pu

The study of the physical characteristics of light particles emitted from nuclei at the moment of fission is useful in the evaluation of the nuclear state in the configuration of scission. The importance of this evaluation, especially with respect to the velocity of the fragments at the instant of their separation, has prompted much experimental work in the past ten years to determine the energetic and angular properties of these light particles (1). In a preceding article (2) we reported for the first time the angular distribution of the particle 6He. This distribution is taken with respect to the line of flight of the light fragment in the thermal fission of 239pu. This distribution, together with the energy-angle correlations obtained, has shown that the emission of the isotope 6He has properties analogous to those measured for long-range alpha-particles and thus provides a proof that the emission of these isotopes of helium is due to the same physical mechanism. In this Letter we report briefly on the experimental results concerning the angular distribution of the particle aH emitted in the thermal neutron-induced fission of ~39pu. The experimental apparatus described earlier (2) pcrmits for each event thc simultaneous measurement of the energy of one of the fragments and of the light particle, which is also identified by a selniconductor telescope. Four separate runs were made, for which the angle between the directions of the light particle and the observed flagment was 68 ~ 88 ~ 108 ~ and 129 ~ respectively. During each run the fission fragments were detected by five surface barrier detectors in parallel, all placed at the same angle with respect to the direction of flight of the light particle. The angular acceptance of both the AE detector of the telescope and of the fragment detectors with respect to the centre of the target was • 5 ~ Unfortunately, due to the thickness (19 izm) of the AE detector available, it was not possible to observe completely the spectrum of the 3H-particles. The analysis was limited to the energy interval of 3.5 to 7 McV.