Prompt emission calculations for ²⁵²Cf(SF) focused on post-neutron fragment distributions and different correlations between pre- and post-neutron fragment quantities

Abstract

The present investigation answers to the international request of updating fission yields data for applications, by revisiting the calculations for ²⁵²Cf(SF) performed by the deterministic prompt emission model code DSE (Deterministic Sequential Emission) with focus on different distributions of post-neutron fragments, different correlations between pre- and post-neutron fragment quantities and distributions related to pre-neutron fragments in correlation with the sequential prompt neutron emission. The influence of both the energy partition in fission and the pre-neutron fragment distribution Y(A,TKE) on different distributions of post-neutron fragments (independent FPY, isotonic yield Y(N_p) and kinetic energy distributions of post-neutron fragments KE_p(Z), KE_p(A_p)) and on different correlations between pre- and post-neutron fragment quantities is investigated by using: a) two methods of TXE partition, one based on the modeling at scission implemented in the DSE model code and another based on a parameterization of the temperature ratio of complementary fully accelerated fragments R_T(A_H) and b) three reliable experimental Y(A,TKE) data. The DSE results of independent FPY (Y(Z,A_p), Y(A_p)) are obtained in good agreement with the existing experimental data of ²⁵²Cf(SF). Both the energy partition in fission and the Y(A,TKE) distribution influence the Y(A_p) structure. Again the most pronounced peaks in the Y(A_p) structure are due to even-Z fragments while the odd Z fragments contribute to pronounced dips. But the role of the even–odd effect is less important than in the previous studied case of ²³⁵U(n_th,f) because the global even–odd effect in Y(Z) is almost 10 times lower in the case of ²⁵²Cf(SF) compared to ²³⁵U(n_th,f). The correlation between the excitation energy of pre-neutron fragments E* and KE_p, consisting of a well delineated sawtooth shape (looking as a reflection in mirror of the well known sawtooth shape of ν(A)) is maintained in the case of ²⁵²Cf(SF), too. The large number of (A,Z,TKE) configurations taken into account in the DSE treatment allows the investigation of neutron excesses of pre- and post-neutron fragments. This reveals an interesting behaviour consisting of oscillations with a periodicity of about 5 mass units of both neutron excesses of pre- and post-neutron fragments as a function of A (in the asymmetric fission region). The oscillation amplitudes of neutron excesses are significantly higher and clearly visible in the case of ²³⁵U(n_th,f) compared to ²⁵²Cf(SF) for which they are less visible. This behaviour is a consequence of the charge polarization ΔZ(A) and rms(A) of the isobaric charge distribution (entering the deterministic construction of the fragmentation range of DSE model) which always exhibit oscillations with a periodicity of about 5 mass units, due to the periodicity of nuclear properties. The magnitude of the global even–odd effect in fragment charge is reflected in the magnitude of oscillation amplitudes in ΔZ(A) and rms(A) and, as consequence, in the magnitude of oscillation amplitudes of neutron excesses of pre- and post-neutron fragments. The almost linear decrease of neutron excess of post-neutron fragments as a function of TKE as well as the behaviour of excitation energy distributions of pre-neutron fragments leading to each number “n” of emission sequences are explained, too.