Gamma-Ray Spectroscopy in 37 Ar M.A.G. Silveira 1,2 , N.H. Medina 2 , W.A. Seale 2 , R.V. Ribas 2 , J.R. B. de Oliveira 2 , S. Zilio 3 , S.M. Lenzi 3 , D.R. Napoli 4 , N. Marginean 4 , F. Della Vedova 4 , E. Farnea 3 , M. Ionescu-Bujor 5 , A. Iordachescu 5 . 1 Centro Universitário da FEI, São Bernardo do Campo, Brazil 2 Instituto de Física da Universidade de São Paulo, Brazil 3 Dipartimento di Fisica and INFN Padova, 4 INFN Laboratori Nazionali di Legnaro, 5 National Institute for Physics and Nuclear Enginnering, Bucharest, Romania. INTRODUCTION The microscopic description of collective motion of the many-nucleon system is a fundamental goal of nuclear structure physics. The correspondence between theoretical treatments and experimental results can lead to an understanding of collective motion in terms of the spherical shell model [1,2,3]. Nuclei of the A=40 mass region are subject to renewed interest. Recently, very complex structures of coexisting spherical, deformed and even superdeformed states have been observed in nuclei 36,38 Ar [3,4,5]. The same features are expected to be found in the odd nucleus 37 Ar since this nucleus is also in the neighborhood of the doubly magic nucleus 40 Ca. In this work we report on preliminary results of the gamma-ray spectroscopy studies in the 37 Ar nucleus. Excited states in 37 Ar have been investigated previously with proton, deuteron and α induced reactions [6,7,8]. This nucleus was studied thirty years ago via heavy-ion-induced reactions with 18 O and 14 N bombardment of 24,26 Mg in the range of 20-60 MeV, using a few Ge(Li) detectors [9]. For low and medium spin values, observed in those experiments, the excited states were well described with the shell model calculations considering the sd valence space and using the USD residual interaction developed by A. Brown and Wildenthal [10]. Nevertheless, to describe the high-spin features it is necessary to use the residual interaction in large-scale-shell-model calculations in the sdpf valence space [1]. EXPERIMENTAL SET-UP AND RESULTS The fusion-evaporation reaction 24 Mg( 16 O,2pn) at 70 MeV was employed to populate the high-spin states in 37 Ar. The incident beam was provided by the Tandem XTU accelerator of the Legnaro National Laboratories. Two independent experiments were performed. In the first experiment, devoted to spectroscopic investigation, the target was a self-supporting 24 Mg foil with a thickness of 400 μg/cm 2 . In the second experiment, a target consisting of 750 μg/cm 2 24 Mg, backed with 15 mg/cm 2 197 Au was employed. The γ-spectrometer GASP, comprising 40 Compton-suppressed HPGe and an 80-element BGO ball, which acts as a γ-ray multiplicity and sum filter [11], was used for obtaining γ-ray double and triple coincidence spectra. The charged-particle detector array (ISIS) [12] enabled the selection of evaporated charged particles in coincidence with the γ-rays. We have used also six neutron detectors placed at 20º with respect to the beam axis. Events were collected on tape when at least two HPGe detectors and one BGO scintillator fired in coincidence. Energy and efficiency calibrations were performed with standard sources of 56 Co and 152 Eu. FIG. 1:Partial level scheme for the 37 Ar nucleus obtained from the present work. The asterisks indicate the new levels. LNL Annual Report Experimental Nuclear Physics