Neutron-deficient isotopes in the vicinity of the N = Z line have attracted considerable experimental and theoretical attention over the last years. Among the key points of interest are investigations in the framework of the astrophysical rp-process [1] and studies of the residual proton-neutron interaction [2]. Reaction network calculations of the rp-process [3] identified, along with other isotopes such as ⁹²Pd and ^96,97Cd, the very proton-rich nucleus ⁹³Pd (Z = 46, N = 47) as a probable waiting point in the reaction path in X-ray burst scenarios. Calculations of this kind require, amongst other nuclear properties, b-decay half-lives as input parameters.
The nucleus ⁹³Pd was produced in the fusion-evaporation reaction ⁵⁸Ni(⁴⁰Ca,an)⁹³Pd at a beam energy of 188 MeV. The reaction products were collected and ionised in a FEBIAD-E ion source. The mass-separated A = 93 beam collected from the GSI Online Mass Separator was implanted in a thin carbon foil in front of a DE-E silicon telescope which served as a detector for b-delayed protons. Figure 1 shows the energy spectrum containing 205 events, obtained within a measuring time of 14.3 hours. For part of the time, the beam was flipped between the collection position on the carbon foil and a beam dump, thus allowing for the recording of time-differential data. A comparison of the number of b-delayed protons registered during beam-on and beam-off periods yielded a half-life of T_1/2 = 0.7^+0.2_-0.1 s.

Fig. 1 Energy spectrum of b-delayed protons recorded at mass A = 93.

In addition to the measurement of b-delayed protons, b-delayed g-rays were investigated by using an array of 13 germanium detectors and a plastic scintillator, mounted at a separate beamline. Gamma transitions of 239.7 keV, 381.7 keV, 621.7 keV, and 864.1 keV were identified in measurements of mass-separated A = 93 sources and preliminarily assigned to the decay of ⁹³Pd. The time characteristics of these g-lines yield a half-life of T_1/2 = (1.0 ±0.3) s in good agreement with the value deduced from the proton data. Shell model calculations in the mass range A = 86-100 [4] predict a ground-state spin of either 7/2⁺ or 9/2⁺ for the isotope ⁹³Pd, while a low-lying 1/2^- isomer can not be excluded. The calculated disintegration mode of the ground-state is an allowed Gamow-Teller b-decay with a half-life of T_1/2 = 1.4 s. Due to the good agreement with the calculations, we attribute the observed activity of b-delayed protons and g-rays to the ground-state decay of ⁹³Pd. ⁹³Ag, on the other hand, is predicted [4,5] to be unbound with respect to direct proton emission from its ground state. The search for direct protons at mass A = 93 was unsuccessful.

¹ GSI, D-64291 Darmstadt, Germany
² Universita degli Studi di Milano, I-20133 Milano, Italy
³ N.C.S.R. Democritos, GR-15310 Aghia Paraskevi, Greece
⁴ Lund University, P.O. Box 118, S-22100 Lund, Sweden
⁵ University of Edinburgh, Edinburgh EH9 3JZ, United Kingdom

References

[1] R.K. Wallace and S.E. Woosley, Astrophys. J. Suppl. 45, 389 (1981)
[2] A.L. Goodman, Phys. Rev. C 60, 014311 (1999)
[3] H. Schatz et al., Physics Reports 294, 167 (1998)
[4] H. Herndl and B.A. Brown, Nucl. Phys. A 627, 35 (1997)
[5] P. Möller et al., At. Data and Nucl. Data Tables 59, 185 (1995)  IKH 06/25/01 © G. Rainovski