THE INFLUENCES OF PAIRING FORCE ON THE PROPERTIES OF ACTINIDE NUCLEI

Abstract

By making use of the method for treating the pairing force, which conserves the number of particles, we have analysed the influences of pairing force on the various properties of actinide nuclei, including the intrinsic spectra of odd-A nuclei and even-even nuclei, β-decays and α-decays, etc. The general formula expressing the influences of pairing force on the various types of α-decay, are given.1. First, Nilsson's parameters, which, after the pairing force being taken into account, can account for the observed intrinsic spectra of odd-A nuclei, are determined. It is found that they may be chosen as follows: K～0.050, η～4.4-5.4 (δ～kη)～0.22-0.27) μ～ 0.70(N = 5), μ～ 0.62-0.66(N = 6), for protons, μ～0.43-0.45(N = 6), μ～0.41-0.43(N = 7), for neutrons. 2. Subsequently, with such realistic single-particle spectra, the intrinsic spectra and β-decays of neighbouring even-even nuclei (Th 228 ,Th 230 ,U 232 ,U 234 , Cm 244 ) are analysed in detail. Some observed levels (E≥1 MeV.) may be explained as the pair-broken states or the pair-excitation states. (See Table 14, and Figs. 15, 16.) According to calculation, there exist still many levels in the energy region, E～l-2 MeV, but they have not been observed yet. In the β-decay experiments, the majority of these levels can not be detected easily, but some of them, easily. The systematically occurring vibrational bands in the spectra of even-even nuclei could not be considered as the pair-broken states or the pair-excitation states. They areγ-vibrational states, K π =2 + , E～0.8-1.0 MeV;β-vibrational states, K π =0 + , E～0.7-0.9 MeV;octupole b-vibrational states, K π =0 - , E～0.5-0.7 MeV. 3. The influences of pairing force on the α-decay probabilities are treated both with the BCS method and with the present method. The results, especially for the favoured a-decay, are quite different in the two methods. For example, when about 10 levels are involved, the retardation factors for the favoured α-decay of even-even nuclei are:R = R Z R N ≈200 (BCS wave function), R = R Z R N ≈ 70 (exact solution). Compared with the exact solution, the occupation probability of the v-th levels, v v 2 , in the BCS method converges too slowly. In the BCS wave function, the portion, in which the number of particles is equal to the average number of particles, is about one third. After the influence of pairing force is taken into account, the nuclear radius constant derived from the a-decay is about r 0 ～1.35fm. The favoured a-decay of odd-A nuclei and the unfavoured a-decay are also discussed. The so-called F-factors are calculated and compared with experiments.