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Classical beta spectra calculations are briefly described, highlighting the usual assumptions and limitations. To go beyond these usual assumptions, the numerical resolution of the Dirac equation for the atomic and beta electrons is necessary. This allowed us to determine the parameters λiinvolved in the theoretical shape factors exactly. A systematic comparison between theoretical and experimental shape factors led us to disqualify the usual λi= 1assumption for all forbidden transitions. The usual ξ-approximation was proved to be incorrect for numerous first forbidden non-unique transitions, and for all higherorder non-unique transitions. A more accurate screening correction was defined and the atomic exchange effect was taken into account, an effect which is always neglected in usual calculations. The beta spectra of 63Ni and 241Pu, recently measured down to very low energies, are well reproduced in our calculations. The exchange effect was demonstrated to have a great influence on the spectral shape within this energy range.

The atomic exchange effect has previously been demonstrated to have a great influence at low energy on the 241 Pu β- transition. The screening effect has been given as a possible explanation for a remaining discrepancy. Improved calculations have been made to consistently evaluate these two atomic effects, compared here to the recent high-precision measurements of 241 Pu and 63Ni β spectra. In this paper a screening correction has been defined to account for the spatial extension of the electron wave functions. Excellent overall agreement of about 1% from 500 eV to the end-point energy has been obtained for both β spectra, which demonstrates that a rather simple ß decay model for allowed transitions, including atomic effects within an independent-particle model, is sufficient to describe well the current most precise measurements. [ABSTRACT FROM AUTHOR]

27Ne has been investigated through the one neutron transfer reaction 26Ne(d,p)27Ne in inverse kinematics at 9.7 MeV/nucleon. The results support the existence of a low lying negative parity state in 27Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.

The exchange effect has been previously given as a possible explanation for a significant deviation from an allowed shape observed at low energy in the 241 Pu β spectrum. Calculations set out here confirm that this atomic effect explains a large part of this deviation. The equations needed to calculate the exchange effect are detailed, as well as the evaluation of the confluent hypergeometnc function for complex arguments of large magnitudes. After a review of the possible other effects that could explain the remaining discrepancy at low energy, the screening correction using effective nuclear charges seems to be the best explanation. For radionuclides with high Z, this work has demonstrated the necessity to take into account the spatial variation of the nuclear charge experienced by the ejected electron to accurately correct for the screening effect. [ABSTRACT FROM AUTHOR]