Observation of a new high-spin isomer inPd94

Abstract

A second γ -decaying high-spin isomeric state, with a half-life of 197 (22)ns, has been identified in the N = Z + 2 nuclide 94 Pd as part of a stopped-beam Rare Isotope Spectroscopic INvestigation at GSI (RISING) experiment. Weisskopf estimates were used to establish a tentative spin/parity of 19 − , corresponding to the maximum possible spin of a negative parity state in the restricted (p 1/2 , g 9/2 ) model space of empirical shell model calculations. The reproduction of the E3 decay properties of the isomer required an extension of the model space to include the f 5/2 and p 3/2 orbitals using the CD-Bonn potential. This is the first time that such an extension has been required for a high-spin isomer in the vicinity of 100 Sn and reveals the importance of such orbits for understanding the decay properties of high-spin isomers in this region. However, despite the need for the extended model space for the E3 decay, the dominant configuration for the 19 − state remains (πp −1 1/2 g −3 9/2 ) 11 ⊗(νg −2 9/2 ) 8 . The half-life of the known, 14 + , isomer was remeasured and yielded a value of 499 (13) The N ≈ Z nuclei just below 100 Sn are of particular interest in contemporary nuclear structure studies. As well as lying on the pathway for rp-process nucleosynthesis in x-ray burster scenarios [1], these nuclei also provide significant information in relation to shell structure close to the last doubly magic N = Z nuclide [2] and constitute a key region for testing the reliability of the shell model (SM) and SM interactions [3]. The region is generally well described by the SM in a minimum space comprising p 1/2 and g 9/2 protons and neutrons [3][4][5]. Exploration of the limits of this approach with respect to inclusion of the p 3/2 and f 5/2 orbitals and excitations across the N = Z = 50 shell closure is a challenge both to the SM and experimental techniques.This region is also remarkable for an abundance of isomeric states [6]. Particularly interesting is the occurrence of high-spin isomers which can provide an ideal testing ground for the study of neutron-proton interactions near the N = Z line [6]. One of the most striking cases is the (21 + ) level in 94 Ag, with recent papers reporting evidence for a variety of particle decay channels: β [7,8], βp [9], p [10], and 2p [11]. The latter of these is the subject of much debate in the literature [12][13][14][15][16].The T z = 1 nuclide 94 Pd has been studied in a number of experiments [7,8,[17][18][19][20][21] and has a known 14 + isomer that decays through γ decay and internal conversion (IC). The half-life of this state has been measured previously, yielding values of 800 (200) [7,8]. The current work presents data from a Rare Isotope Spectroscopic INvestigation at GSI (RISING) experiment showing clear evidence of a second isomeric state that feeds the 14 + level through some of these transitions. The reproduction of the decay properties of this isomer in shellmodel calculations requires the inclusion of the negative-parity p 3/2 and f 5/2 orbitals. 94 Pd and ...

Properties of γ -decaying isomers and isomeric ratios in the Sn100 region

“…In all four cases, the half-lives determined from this experiment are smaller than those in Ref. [47], but consistent with other literature values [10,21,[48][49][50][51][52]. One publication [53], reports consistently longer half-lives for these isomers, possibly pointing towards some systematic effect in that analysis.…”

“…Enhanced effective charges account for nucleon excitations in the p 3/2 and f 5/2 orbits that cannot be calculated in the reduced pg model space, but the (15 + ) isomer is a pure state composed of g 9/2 holes and thus requires no extra compensation. This argument is supported by the negligible difference in the calculated B(E2) value in the extended f 5/2 ,p,g 9/2 model space [48], as reported in Ref. [20].…”

N=50core excited states studied in the4696Pd50nucleus

“…Calculations marked GF were performed in the standard (p 1/2 , g 9/2 ) proton (π ) space, assuming a − ) are systematically calculated too high. This deficiency is partly cured in the calculations marked as FPG, were the GF model space was extended by including π (f 5/2 , p 3/2 ) orbitals, with realistic interactions derived from the charge-dependent Bonn (CD-Bonn) nucleon-nucleon potential [9,10]. The low-spin states in the FPG calculations are shifted too much down, though, which indicates that the pairing contribution should be tuned.…”

“…The experimentally most likely E1 decays could not be calculated because E1 transitions are forbidden in this model space. Two different sets of effective charges were used for electric transitions which were found to be reasonable in such calculations [9,10] Tables II and III. The M2 strength is low because an M2 transition in the adopted model space can result only from the g 9/2 → f 5/2 transition and the f 5/2 orbital is deep in the shell so its content in the wave functions is marginal. If the 685 keV transition were of type M2, the experimental reduced transition strength would be almost four orders of magnitude larger than the shell model value.…”

“…It has, however, been recently demonstrated that the inclusion of the odd-parity p 3/2 and f 5/2 orbitals from below the Z = 38 gap, which opens up the entire fpg shell, is necessary for reproducing the decay properties of the observed spin-gap isomers in 94 Pd [9] and 96 Ag [10]. Moreover, one of the key points and questions in the region is the role of the excitations of the N = Z = 50 core.…”