New proton radioactivity measurements from odd-odd drip-line nuclei

• Main Author: Livingston, Kenneth
• Published: University of Edinburgh 1993
• Subjects: 539.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.653971

A series of six experiments was carried out to search for proton radioactivity in odd-odd drip line nuclei in the region Z = 61 - 73. The candidate nuclei were produced with 1p3n fusion evaporation reactions and separated from beam particles and other reaction products using the Daresbury Recoil Mass Separator. Recoiling evaporation residues were implanted in a double-sided silicon strip detector which recorded the energy, position and time of implantation events and subsequent proton or alpha decays. The position resolution of the detector (300μm on both axes) provided a high correlation efficiency between decay events and parent nuclei, resulting in a system which was capable of measuring proton transitions with half-lives π 1μs and cross sections π 1μb. A new proton line has been observed in the nucleus <SUP>156</SUP>Ta at energy of 1103±12keV, and half-life of 320±80ms. The transition is assigned to a ˜3% decay branch from a high spin h<SUB>11/2</SUB> isomeric state on the basis of systematics. Two transitions have been also observed in the new isotope <SUP>146</SUP>Tm at energies of 1119±5keV and 1189±5keV. The measured half-lives of 235±27ms and 72±23ms are well reproduced by simple WKB calculations assuming proton emission from h<SUB>11/2</SUB> orbitals and the transitions are assigned to a 10<SUP>+</SUP> isomeric state and a low lying 5<SUP>-</SUP> or 6<SUP>-</SUP> level. The proton separation energies have been used to assess the particular quality of the Möller-Nix and Liran-Zeldes mass models far from stability, and the measured partial half-lives are compared with WKB calculations based on different nuclear potentials. The subsequent implications for level ordering beyond the proton drip-line are also discussed. No evidence was found for proton radioactivity in the nuclei <SUP>128</SUP>Pm, <SUP>132</SUP>Eu, <SUP>138</SUP>Tb and <SUP>142</SUP>Ho. On the basis of mass model systematics it was concluded that the odd proton in these nuclei is not sufficiently unbound for proton emission to compete successfully with β-decay.