Mesure de faibles sections efficaces de fission à l'aide de détecteurs à barrière de surface

Abstract

The use of surface barrier detectors for the detection of fission fragments is difficult when the number of fragments is small relative to a large number of other charged particles, which may produce other coincidence events (random or true). It is especially difficult when the fragments kinetic energies do not differ very much from those of the other charged particles. From the measured kinetic energies of the coincident particles and the kinematics of fission in the laboratory system, we can calculate (1) the time-of-flight from the target to the detector for each particle, and (2) the difference in time-of-flight for each coincident event. If this event is due to fission and the fragment energies are correctly measured, this calculated difference must be equal to the measured time-of-flight difference. We applied this selection method to the fission of medium targets induced by 128 MeV 14N ions, and it proved to be very effective and easy to handle. In addition, this method eliminates most of the random coincidence events. The remaining part of random events is lower by an order of magnitude than it would have been with the usual fast coincidence method; moreover, this part is easily measured.