PRECISE ATOMIC MASS-DIFFERENCES IN PB AND TL AND THE DECAY ENERGY FORPB-205

The Manitoba II high resolution mass spectrometer has been used to mea sure precise values for the spacings of four mass doublets in the mass spectra of Tl and Pb chlorides. Our new data are combined with other precise experimental data to derive an improved value for the Pb-205-T l-205 atomic mass difference, viz., 51.06 +/-0.51 keV. This value agre es with most previous data, identifies one inconsistent datum and subs tantially improves the precision of this mass difference. This improve ment in precision, in turn, reduces the uncertainty in one aspect of t he derivation of the cross section for the neutrino capture reaction, nu + Tl-205 --> Pb-205 +e-. This reaction has been proposed as the pos sible basis for a favourable solar neutrino detector.