Isotopieverschiebungskonstanten von Th, U, Pu und Am

The isotope shifts of unperturbed electron configurations have been determined from isotope shift measurements in the spectra of Th, U, Pu, and Am. The screening of the 7s electron charge density at the nucleus by 6d, 7s, and 7p electrons is discussed. It turns out that the same screening factors as for the 6s electron in lighter elements can be used. The screening of the 7s electron charge density at the nucleus by one 5f electron amounts to about 25%: [δT(f n s)−δT(f n )]/[δT(f n }s)−δ(f n} )]=0.75. The charge density at the nucleus due to the filleds (andp1/2) shells is considerabely screened by anf electron. The isotope shiftδT(fn}-1d m +2)−δT(f n d m ) produced by this effect is of the same order of magnitude as the isotope shiftδT(f n d m s)-δT(f n d m ) due to ans electron. The experimental isotope shift constants are found to be:Β Cexp(Th230–Th232)=880±120;Β Cexp(U233–U235)=1000±180;Β Cexp(U234–U236)=1070±200;Β Cexp(U236–U238)=1080±180;Β Cexp(Pu238–Pu240)=1200±120;Β Cexp(Pu239–Pu241)=1060±100;Β Cexp(Pu240–Pu242)=900 ±90;Β Cexp(Am241–Am243)=890±50 [10−3cm−1]. The ratiosΒ Cexp/Cth are discussed.