Measurement and Theoretical Calculation of the ²⁵²Cf Spontaneous-Fission Neutron Spectrum

Abstract

Concerning the ²⁵²Cf(sf) neutron spectrum remarkable progress in experiment and theory have been made during the last three years. Experimental techniques and analysis procedures have been improved. The precise measurement of the standard neutron spectrum from spontaneous fission of ²⁵²Cf requires the optimum experimental arrangement corresponding to the energy range to be measured. Several types of data corrections have to be considered with care. The most important requirements to be met in a ²⁵²Cf(sf) neutron spectrum measurement are summarized briefly. We consider the high-energy range especially.

Theoretical models for the calculation of fission neutron spectra are based on the predominant emission mechanism, i.e. the evaporation from fully accelerated fragments. It is emphasized that an exact evaporation theory of fission neutron spectra should take into account the fragment distribution in nucleonic numbers, excitation energy, kinetic energy, and nuclear spin as well as the cascade neutron emission from highly excited, neutron-enriched fragments in competition to gamma-ray emission. However, practical applications require several approximations. Some approaches which were studied in the framework of both the Weisskopf formalism and the Hauser-Feshbach theory are discussed. We point out some of the deviations in spectrum calculation if neglecting or approximating typical characteristics of fission neutron emission.

The results of new Cf spectrum calculations are compared with recent experimental data which confirm a Maxwellian spectrum at energies below ∼ 1 MeV for T = 1.42 MeV. Between 1.5 and 4 MeV, measured data tend to exceed this Maxwellian by about 3%. Significant deviations from a Maxwellian with T = 1.42 MeV appear between 6 and 20 MeV where a fit of experimental data yields a value of T close to 1.37 MeV.

Recent theoretical calculations of the Cf neutron spectrum agree very good with measured data. Especially, the complex cascade evaporation model permits a conformable description of recent experimental data in the whole energy range (1 keV–20 MeV) if introducing the CMS anisotropy of emission.