New light on how nuclear fission fragments cool down
In close collaboration with the Swedish University of Chalmers, JRC scientists investigated prompt fission gamma-ray spectra from plutonium isotopes to reduce statistical uncertainties on the model parameters.
The investigation of prompt fission gamma-ray emission in the spontaneous fission of even-mass plutonium isotopes sheds new light on how fission fragments cool down. Whereas neutron emission depends on the compound nuclear temperature and occurs at a very early stage, g-ray emission strongly reflects fragment properties at later moments during the fission process. The systematic investigation of prompt fission gamma-ray emission is an important step to accurately assess prompt g-heating in the core of fast nuclear reactors. Those reactors are operated with a non-moderated, thus, fast-neutron spectrum. In order to arrive at meaningful predictions, the database, from which average photon multiplicity and total energy for fast-neutron induced fission are taken, must be sound. The study of spontaneous fission allows enlarging the database on prompt fission g-ray spectrum (PFGS) characteristics without relying on accelerator- or reactor-based neutron beams.
However, spontaneous fission rates are in most cases relatively low. To overcome this, a team of scientists from the JRC and the Swedish University of Chalmers has developed a so-called transfer-function method to deduce characteristics from spectra containing only a small number of events. The team was able to compare PFGS characteristics from fissioning nuclei produced in spontaneous as well as in thermal-neutron induced fission. The researchers conclude that PFGS characteristics are strongly connected with fission fragment properties, e.g. temperature and spin.