β- and γ-spectroscopy study of 119Pd and 119Ag

Abstract

Neutron-rich 119Pd nuclei were produced in fission of natural uranium, induced by 25-MeV protons. Fission fragments swiftly extracted with the Ion Guide Isotope Separation On-Line method were mass separated using a dipole magnet and a Penning trap, providing mono-isotopic samples of 119Pd. Their β− decay was measured with γγ- and βγ-spectroscopy methods using low-energy germanium detectors and a thin plastic scintillator. Two distinct nuclear-level structures were observed in 119Ag, based on the 1/2− and 7/2+ isomers reported previously. The β−-decay work was complemented by a prompt-γ study of levels in 119Ag populated in spontaneous fission of 252Cf, performed using the Gammasphere array of germanium detectors. Contrary to previous suggestions, our data show that the 1/2− isomer is located below the 7/2+ isomer and is proposed as a new ground state of 119Ag with the 7/2+ isomer excitation energy determined to be 33.4 keV. Our data indicate that there are two β unstable isomers in 119Pd, a proposed ground state of 119Pd with tentative spin 1/2+ or 3/2+ and a half-life of 0.88 s and the other one about 350 keV above, having spin (11/2−) and a half-life of 0.85 s. The higher-energy isomer probably decays to the 1/2+ or 3/2+ ground state via a γ cascade comprising 18.7-219.8-X-keV transitions. The unobserved isomeric transition with energy X≈100 keV probably has an E3 multipolarity. Its hindrance factor is significantly lower than for analogous E3 isomeric transitions in lighter Pd isotopes, suggesting an oblate deformation of levels in 119Pd. Oblate configurations in 119Ag are discussed also.