Prompt neutron multiplicity in correlation with fragments from spontaneous fission of 252Cf

摘要

The spontaneous fission of 252Cf serves as an excellent benchmark of prompt emission in fission since experimental data can be obtained without the need of an incident beam. With the purpose of providing experimental data on the prompt fission neutron properties in correlation with fission fragment characteristics an experiment on 252Cf(SF) has been performed. In addition, the experiment serves as benchmark of setup and analysis procedures for measurements of fluctuations in the prompt neutron properties as a function of incident neutron energy in fission of the major actinides 235U and 239Pu. The experiment employs a twin Frisch grid ionization chamber as fission fragment detector while neutrons where counted using a liquid scintillator placed along the symmetry axis of the ionization chamber. Average neutron multiplicity has been obtained as a function of fission fragment mass and total kinetic energy (TKE). The average multiplicity as a function of mass agrees well with available literature data in the mass range from 80 to 170 amu. The existence of additional sawtooth structures in the far asymmetric mass region could not be confirmed, although the statistical accuracy of the present experiment is as good as the previous study where such structures have been reported [C. Budtz-Jorgensen and H.-H. Knitter, Nucl. Phys. A490, 307 (1988)]. The available literature data on the TKE dependence of the multiplicity show strong deviations. Therefore, effort was focused on investigating experimental factors in low efficiency neutron counting experiments that may lead to faulty determination of this dependence. Taking these factors into account a result that agree well with data from high efficiency neutron counting experiments is obtained. The experimental arrangement allows determination of the angle between the detected neutron and the fission axis, which permits the neutron properties to be transformed into the fission fragment rest frame. Fission neutron emission spectra in the fragment center of mass frame has thereby been obtained as a function of the fission fragment mass and TKE.