Nuclear Structure and Shell Model Description of the 37Ar Nucleus.
N. H. Medina 1*, M. A. Silveira 2, J. R. Oliveira 1, R. V. Ribas 1, W. A. Seale 1, S. Zilio 3, S. M. Lenzi 3, D. R. Napoli 4, N. Marginean 5, F. Della Vedova 3, E. Farnea 3, M. Ionescu-Bujor 5, A. Iordachescu 5
1 Instituto de Física da Universidade de São Paulo, São Paulo, Brazil.
2 Centro Universitário da FEI, São Bernardo do Campo, São Paulo, Brazil.
3 Dipartimento di Fisica dell'Università and INFN, Sezione di Padova, Padova, Italy.
4 INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy.
5 National Institute for Physics and Nuclear Engineering, Bucharest, Romania.
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 . The major problem in describing the nuclear structure is the large number of active and interacting nucleons which is a challenge for both the nuclear structure models and effective interaction description. Many experimental and theoretical efforts have been made to elucidate the role of particle-hole sd-fp cross-shell intruder configurations in the structure of the sd shell nuclei . In these nuclei the low spin structure can be well reproduced by Shell Model calculations limited to the sd shell taking into account the USD residual interaction . Nevertheless, to describe the high spin states it is necessary to consider also the excitations to the pf shell. The knowledge of the nuclear structure at high angular momentum is used to test the proposed efective interactions [4,5]. In this work we report on the nuclear structure of the sd-fp 37Ar nucleus. This nucleus was produced via the fusion-evaporation reaction 24Mg(16O,3n), E = 70 MeV . The beam was delivered by the XTU Tandem Accelerator of the Legnaro National Laboratory, Italy. Two experiments were performed, one devoted to spectroscopic investigation, with a 0.4 mg/cm2 self-supporting 24Mg foil and another one with a target consisting of about 0.75 mg/cm$^2$ 24Mg, backed with 15 mg/cm2 197Au, employed for lifetime measurements using the Doppler Shift Attenuation method (DSAM). Coincident gamma rays were detected with the GASP array comprising of 40 Compton-suppressed HPGe detectors and an 80 element BGO ball. In the thin target experiment we have used also the 4 charged-particle detector ISIS consisting of 40 Si telescopes in order to select the fusion-evaporation channel. The level scheme of 37Ar has been extended to an energy of about 13 MeV. Since the B(E2) reduced rates can give information about rotational collectivity and B(M1) reduced rates are indicators of single particle features, lifetimes of 11 new excited states in 37Ar have been determined with the DSAM method and some other known lifetimes were confirmed. To interpret the level scheme and the observed transition properties in the 37Ar nucleus, shell-model calculations have been performed in the basis of the Large Scale Shell Model (LSSM) using the Antoine code  and the SDPF residual interaction , developed for this mass region. In this calculation we have considered up to 9 particle excitation from 2s1/2 and 1d3/2 orbitals to the 1f7/2 and 2p3/2 orbitals. The LSSM model reproduces quite well the positive and the negative parity state energies up to 17/2 and 13/2, respectively. The measured B(E2) and B(M1) transition rates are very well reproduced and the calculated wave functions indicate very high configuration mixture. The calculations fail to reproduce the higher spin state energies.
 E. Caurier et al., Review of Modern Physics 77, 427, 2005.
 M. Ionescu-Bujor et al., Phys. Rev. C 80, 034314, 2009 and references therein.
 B.A. Brown and B.H. Wildenthal, Ann. Rev. Nucl. Part. Sci. 38, 29, 1988.
 Y. Utsuno et al., Phys. Rev. C 70 044307, 2004.
 E. Caurier et al., Phys. Lett. B 522, 240, 2001.
 M.A.G. Silveira et al., AIP Conference Proceedings, 947, 315, 2007.
 E. Caurier and F. Nowacki, Acta Physica Polonica 30, 705, 1999.