In-beam gamma ray spectroscopy of 58Co

Abstract

The odd-odd 58Co nucleus has been studied with the 51V(10B, p2n) reaction at 33-MeV incident energy and the gamma-spectrometer Saci-Pererê. Excited states up to 8.0 MeV and spin up to 11+ have been observed. The results are compared to Shell Model calculations using the GXPF1 effective interaction, developed for use in the fp shell. The pif7/2-1<FONT FACE=Symbol>Än</FONT>(p3/2²f5/2¹) configuration was assigned to the yrast levels.

NUCLEAR STRUCTURE

In-beam gamma ray spectroscopy of ⁵⁸Co

M. A. G. Silveira; N. H. Medina; J. R. B. Oliveira; J. A. Alcântara-Nuñez; E. W. Cybulska; H. Dias; M. N. Rao; R. V. Ribas; W. A. Seale; K. T. Wiedemann

Instituto de Física, Universidade de São Paulo, São Paulo, SP, Brazil

ABSTRACT

The odd-odd ⁵⁸Co nucleus has been studied with the ⁵¹V(¹⁰B, p2n) reaction at 33-MeV incident energy and the g-spectrometer Saci-Pererê. Excited states up to 8.0 MeV and spin up to 11⁺ have been observed. The results are compared to Shell Model calculations using the GXPF1 effective interaction, developed for use in the fp shell. The pf_7/2^-1Än(p_3/2²f_5/2¹) configuration was assigned to the yrast levels.

Nuclei close to doubly magic shell closures are the object of extensive experimental and theoretical investigations [1-3]. Spectroscopic data from these nuclei provide essential information for the parameter sets of spherical shell-model calculations. They apply severe constraints on the outcome of such calculations and, consequently, define the effective nuclear forces. During recent years both experimental and theoretical efforts have been used to understand the nuclei near the Z=N=28 shell closure. There is an N or Z=28 "magic" number inside the major shell with the oscillator quantum number N=3. The shell gap at N=Z=28 is due to the spin-orbit lowering of the f_7/2 orbital. This gap is relatively small so that the particle-hole excitation across the gap has relatively low energies. For shell model calculations around this magic number, ⁵⁶Ni has often been assumed as an inert core. However, it has been shown that this core is rather soft and only a very limited description is provided by the closed-shell model for the magic number 28 [4]. These structures were successfully described for N or Z = 28 nuclei only after considering the existence of significant core-excitations in low-lying non yrast states as well as in high-spin yrast states [3]. These calculations were performed for the ⁵³Mn, ⁵⁴Fe, ⁵⁵Co and ^56,57,58,59Ni nuclei.

In this study we present new results on excited states of ⁵⁸Co, thus enriching the systematics of the nuclear structure along the N=31 chain. This nucleus has three particles and one hole relative to the ⁵⁶Ni core and has been studied so far with proton and a particle induced reactions [6-8], therefore very little was known regarding its high-spin structure.

The ⁵⁸Co nuclei were produced with the fusion-evaporation reaction ⁵¹V(¹⁰B,p2n) at 33 MeV bombarding energy, with the 8MV Pelletron accelerator of the University of São Paulo (USP). The target consisted of a stack of 3 self-supporting natural ⁵¹V foils of 200 µg/cm². Gamma-gamma-charged particle coincidences were measured with the Saci-Pererê g-ray spectrometer. Saci [9] (Sistema Ancilar de Cintiladores) is a 4 p- charged particle system consisting of 11 plastic phoswich scintillator DE-E telescopes. Pererê [10] (Pequeno Espectrômetro de Radiação Eletromagnética com Rejeição de Espalhamento) is a g-array spectrometer composed of 4 GeHP detectors with BGO Compton shields (two detectors were Ortec GMX of about 20% efficiency and the other two were Canberra REGe of 60% efficiency). Two of these detectors were placed at 37º and the other two at 101º with respect to the beam direction. Events were collected when at least two HPGe detectors fired in coincidence. The charged particle detectors were screened against the scattered heavy ions with three Al foils of 3.0 mg/cm². A total of 48×10⁶ Compton suppressed g-g events was collected and registered on the hard disk of a PC. g-ray energy and efficiency calibrations were made with ⁵⁶Co,¹³³Ba, and ¹⁵²Eu sources. The data have been Doppler corrected and sorted into symmetrized g-g, a- gated, and proton-gated g-g matrices with 9.4×10⁷, 2.5×10⁶ and 10.5×10⁶ counts, respectively. Background-subtracted spectra generated from those matrices were used to construct the level scheme of ⁵⁸Co. Those matrices were analysed using the UPAK [11], GASPware [12] and Radware [13] spectrum analysis codes. The g-ray transitions belonging to ⁵⁸Co were identified by setting gates on charged particle fold 1p. In Fig. 1 the g-ray spectrum gated on the 321 keV low-lying transition of the ⁵⁸Co nucleus, and on protons detected by the SACI array is shown. g-rays from ⁵⁷Co (corresponding to the p3n channel), which is the main contaminant channel in the 1p-gated spectra, were identified from previous work [14]. The assignment of the spins to the ⁵⁸Co levels was based on the DCO (directional correlation from oriented states) ratios. A g-g matrix was constructed by sorting the data from the 2 detectors positioned at 37º and 101º. Gates were set on both axes on several strong dipole transitions and the intensity of other transitions observed in the two spectra has been extracted. We have assumed only positive parity states since the shell model does not predict negative parity states in ⁵⁸Co within the excitation energy and spin limits of this work.

A level scheme extending up to an excitation energy of about 8.0 MeV and spin I^p=11⁺ has been proposed, based on the coincidence relationships, intensity balances on each level and energy sums from different paths using the 1p-gated matrix (see Fig. 2). Several cascades at high excitation energy suggest a complex level structure. We have found 46 new g-transitions de-populating 37 new excited states. The level energies are referred to the low-lying 2⁺ state previously known [15]. The placement of the transitions in the level scheme is firmly established by coincidence relationships. The width of the arrows is proportional to the intensities of transitions as seen in the reaction studied here. The relative intensities of weaker transitions, with respect to the strong ones, have been deduced from coincidence spectra.

In order to understand the observed structure, spherical shell model calculations have been performed with the code MSHELL [16]. We have used the model space and the residual interaction named GXPF1, developed recently for the description of fp-nuclei [3, 5]. The calculation with the GXPF1 interaction was performed in the full fp shell with up to 8 particle excitations from the 1f_7/2 orbital to the 1p_3/2, 1f_5/2 and 1p_1/2 orbitals.

To make a correspondence between a predicted level and a detected level means that both the level energies and the decay patterns should be in fair agreement. For the seven lowest yrast levels, including the g.s., the largest disagreement is for the 3⁺ level at 112 keV instead of the 256 keV prediction. The theoretical calculations for the branching ratios give an idea of which transitions should be considered most important. In Fig. 3 a comparison of the experimental level energies with the calculations for the yrast excited states of the odd-odd nucleus ⁵⁸Co are shown. These calculations compare reasonably well up to the excited yrast 8⁺state. The yrast states were interpreted as .

In conclusion, the level scheme of the odd-odd⁵⁸Co nucleus populated with a heavy ion fusion-evaporation reaction was measured for the first time. The shell model calculations reproduce well the experimental yrast states. Additional level assignments are being performed and should allow a more detailed comparison with the theoretical results. More experimental information on nuclei in the region of ⁵⁶Ni and, in particular, lifetime analysis for the excited states in ⁵⁸Co, which are in progress, should improve the knowledge of the high spin state features.

The authors would like to thank Prof. B. A. Brown for fruitfull discussions and Prof. T. Mizusaki for providing the MSHELL calculation results. This work was partially supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil.

Received on 12 November, 2005

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Publication Dates

• Publication in this collection
  07 Nov 2005
• Date of issue
  Sept 2005

History

• Received
  12 Nov 2005