Gamma de-excitation of fission fragments: (I). Prompt radiation

Abstract

The gamma radiation emitted in fission of Cf 252 has been investigated in coincidence with the fission fragments. The mass ratio of the fragments was recorded, various properties of the radiation being studied as a function of the mass ratio. This paper deals with the prompt radiation, i.e. radiation with a half-life shorter than 1 nsec. The life-time of the gamma emitting states was measured by the time-of-flight technique. Using a collimator, the gamma radiation was detected after the fragments had travelled different distances. In this way, the decay curve was investigated and the half-life of the radiation was found to be about 1 · 10 −11 sec. Using this information, it was possible to find a collimator setting which permitted a complete separation of the radiation from the two fragments. The properties of the radiation could then be studied as a function of mass. Thi arrangement was used to record gamma spectra for a number of different masses. It was found that there is a pronounced variation in spectrum shape. The spectra from fragments close to magic numbers are shifted to higher energies. With the same arrangement the yield of gamma rays was determined as a function of mass. The yield curve has a saw-tooth shape. The significance of the experimental results is discussed. The characteristic feature of the de-excitation process seems to be that it involves high spin states. It is estimated that the average value of the initial spin is about 10. The results of this work indicate that the radiation mainly consists of vibrational transitions. This fact is explained as a consequence of the conditions at scission.