Investigation of low spin states inCr48with the MINIBALLγ-ray spectrometer

Abstract

Low spin states in the self-conjugate even-even nucleus 48 Cr were investigated using the Miniball γ-ray spectrometer. At the FN tandem accelerator in Cologne the 46 Ti( 3 He,n) reaction was used for the measurement of γγ coincidences for an excitation function from 7 to 12 MeV beam energy. 17 excited states were observed, nine for the first time by means of γ-ray spectroscopy, and new spin assignments were made. No excited states apart from the ground band were observed below 3.4 MeV. PACS number(s): 21.10. Hw, 25.70.Gh, 27.40.+z Much experimental and theoretical work has recently been devoted to the investigation of self-conjugate nuclei, which exhibit unique and interesting properties. For example, they are the only nuclei in which isospin zero states (T = 0), nearly degenerate isospin doublets, and large isovector M 1 transitions can be found. In particular the N = Z nucleus 48 Cr, placed amidst the doubly magic nuclei 40 Ca and 56 Ni, has been the subject of recent studies [1,2,3,4,5,6,7]. The ground state rotational band of 48 Cr has been successfully described both by the shell model [5] and the collective model [6]. The latter approach shows 48 Cr as a deformed rotor. On the other hand the shell model reproduces the states of the yrast structure impressively well and predicts many additional states above the pairing gap of about 3 MeV. So far these additional states had not been observed experimentally. The question of their existence puts the shell model to a crucial test. A recent 40 Ca+α+α cluster model approach [7] also describes level energies and B(E2) values of the ground band very well and also predicts many so far unobserved states.Early particle-spectroscopic works on the energy levels of 48 Cr following the (p,t) reaction had shown that about nine excited states are to be found at level energies between 3.4 and 6.1 MeV [8,9,10]. Recent measurements used heavy ion induced reactions to examine both yrast and non-yrast structures of 48 Cr [1,2,3]. The non-yrast states were assumed to belong to a K π = (4 − ) band, but the spin and parity assignments had only been tentative [3]. The aim of the present work was to extend the known level scheme and remove the spin ambiguity for the side band. Therefore we performed a complete γ spectroscopy experiment using the new Miniball spectrometer [11], which has been designed as an extremely efficient array for the detection of low multiplicity γ events.The population of non-yrast states in 48 Cr was carried out following the 46 Ti( 3 He,n) reaction at the FN tandem accelerator of the University of Cologne. The Miniball spectrometer was used to measure γγ coincidence events. It was designed to have a high full-energy peak efficiency and effective granularity, needed for the Doppler correction of γ-rays emitted by fast recoiling nuclei. Miniball was equipped with 18 6-fold segmented encapsulated Ge detectors, clustered in six Triple-Cluster cryostats with three detectors each. For this experiment we used only the total energy signal, which is read out from...