Rotational and vibrational bands in 108Pd

Abstract

The 108Pd nucleus has been studied with the 100Mo(11B, p2ngamma) reaction at 43 MeV incident energy. gamma -gamma - t, gamma -gamma - charged particle coincidences and directional correlation ratios were measured using the gamma spectrometer formed by four Compton suppressed HPGe detectors and a 4pi charged-particle ancillary detector system. The struture of the bands was interpreted within the framework of the cranked shell model and total Routhian calculations.

Rotational and vibrational bands in ¹⁰⁸Pd

J.A. Alcántara-Núñez; J.R.B. Oliveira; E.W. Cybulska; N.H. Medina; M.N. Rao; R.V. Ribas; M.A. Rizzutto; W.A. Seale; F. Falla-Sotelo

Instituto de Física, Universidade de São Paulo, C.P. 66318, São Paulo, SP, 05315-970, Brazil

ABSTRACT

The ¹⁰⁸Pd nucleus has been studied with the ¹⁰⁰Mo(¹¹B, p2ng) reaction at 43 MeV incident energy. g – g – t, g – g – charged particle coincidences and directional correlation ratios were measured using the g spectrometer formed by four Compton suppressed HPGe detectors and a 4p charged-particle ancillary detector system. The struture of the bands was interpreted within the framework of the cranked shell model and total Routhian calculations.

The nuclei around Z = 44 and N = 60 are susceptible to dramatic changes in shape with the addition or subtraction of a small number of nucleons [1]. In the region of Pd the rotational alignment of proton (g_9/2) and neutron (h_11/2) intruder orbitals can give rise to a significant increase in aligned angular momentum, resulting in a variety of collective behaviors. Previous experiments on ^100-106Pd show that the majority of states populated by (HI,xn) reaction are members of decoupled collective bands built on quasineutron states [2-4].

On the other hand, the nuclei in the region of Pd and Ru are characterized by vibrational structures and modest deformation at low energies because they are near the Z = 50 shell closure. It can be commonly mentioned that the structure of their lowest-lying states is dominated by vibrational degrees of freedom as reflected in the typical vibrational 0⁺, 2⁺ and 4⁺ triplet of states at about twice the energy of the first 2⁺ state [5]. This statement is corroborated by empirical analysis E-GOS, proposed by P. Regan [6].

In this work, we present the results of an investigation of ¹⁰⁸Pd with the ¹⁰⁰Mo(¹¹B,p2ng) reaction at 43 MeV beam energy. The beam was provided by the Pelletron Tandem Accelerator of the University of São Paulo. The target used was sufficiently thick in order to stop the recoils. Gamma rays and charged particles have been detected using the SACI-PERERE array. SACI [7] (Sistema Ancilar de Cintiladores) is a 4p charged particle telescope system consisting of 11 plastic phoswich scintillators, disposed in the geometry of a dodecahedron, which enabled the selection of the evaporated charged particle fold in coincidence with the observed g-rays. PERERE [8] (Pequeno Espectrômetro de Radiação Eletromagnética com Rejeição de Espalhamento) is the g-ray spectrometer consisting of 4 HPGe detectors with BGO Compton-shields. A partial selection of specific channels can be made by detecting and identifying some or all of the evaporating particles in coincidence with g-rays. Events were collected when at least 2 Ge detectors fired in coincidence. A total of 85×10⁶ Compton-suppressed events was collected. The data have been sorted into symmetrized g-g and g-g-charged particle matrices. The g-ray transitions belonging to ¹⁰⁸Pd were identified by setting gates on charged particle fold 1p (see Fig. 1). The data were analysed using the VPAK [9] and RADWARE [10] spectrum analysis codes.

Previous to the present work, only three bands were known [11, 12]. Two new rotational structures at relatively high excitation energy were identified (labelled 3b and 4 in Fig. 2).

One of these new bands (3b) corresponds to the unfavored signature of n(h_11/2 Ä g_7/2) configuration. The favored signature band (3a) was identified in previous work [11]. TRS calculations (Fig. 3(a)) predict a nearly axial symetry with b₂» 0.19 and g » 4º as deformation parameters.

The quasiparticle Routhians, calculated with the CSM, based on a deformed Woods-Saxon potential including pairing interaction, show that the n(h_11/2 Ä g_7/2) configuration has a low excitation energy (for two quasineutrons) with an aligned angular momentum i_x » 8 and signature splitting De' » 100 keV. This is in agreement with the experimental Routhians (see Fig. 4) for bands 3a and 3b.

TRS (Fig. 3(b)) and CSM calculation show that another configuration with positive parity and lower excitation energy is the excited (nh_11/2)² with a = 1 as a favored signature and an aligned angular momentum i_x » 7. In this configuration the first neutron crossing (at w » 0.3 MeV) is blocked. Experimentally, band 4 presents similar characteristics with the assumption of a I^p = (5⁺) state as a band head. For this reason we assign to band 4 the excited (nh_11/2)² configuration.

A low spin g-vibrational band, with some states previously observed by L. Svensson [5], was identified and extended up to I^p = 10⁺.

Acknowledgments

We thank the technical staff of the Pelletron Tandem Accelerator of the University of São Paulo. 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 3 November, 2003

• [1] Nuclear Structure of the Zirconium Region, edited by J. Eberth, R. Meyer, and K. Sistemich (Springer-Verlag, Berlin, 1988).
• [2] J.A. Grau, L.E. Samuelson, F.A. Rickey, P.C. Simms, and G.J. Smith, Phys. Rev. C14, 2297 (1976).
• [3] Dan Jerrestam, W. Klamra, B. Fogelberg, R. Bark, A. Gizon, J. Gizon, E. Iddeguchi, S. Mitarai, M. Piiarinen, and G. Sletten, Nucl. Phys. A603, 203 (1996).
• [4] G.E. Perez, et al., Nucl. Phys. A686, 41 (2001).
• [5] L.E. Svensson, C. Fahlander, L. Hasselgren, A. Bäcklin, L.Westerberg, D. Cline, T. Czosnyka, C.Y. Wu, R.M. Diamond, and H. Klue, Nucl. Phys. A584, 547 (1995).
• [6] P.H. Regan, C.W. Beausang, N.V. Zamfir, R.F. Casten, Jing-ye Zhang, A.D. Yamamoto, M.A. Caprio, G. Gürdal, A.A. Hecht, C. Hutter, R. Krücken, S.D. Langdown, D.A. Meyer, and J.J.Ressler, Phys. Rev. Lett. Vol90, 152502 (2003).
• [7] J. A. Alcántara-Núńez, J.R.B. Oliveira, E.W. Cybulska, N.H. Medina, M.N. Rao, R.V. Ribas, M.A. Rizzutto, W.A. Seale, F. Falla-Sotelo, F.R. Espinoza-Qińones, and C. Tenreiro, Nucl. Instrum. Methods Phys. Res. A497, 429 (2003).
• [8] R.V. Ribas, J.R.B. Oliveira, E.W. Cybulska, M.N. Rao, W.A. Seale, M.A. Rizzutto, and N.H. Medina, Ann. Rep. of the Nucl. Phys. Dept., Institute of Physics, University of Săo Paulo, p. 63 (1996).
• [9] W. M. Milner - Oak Ridge National Laboratory (1986).
• [10] D. Radford, Nucl. Instrum. Methods Phys. Res. A361, 297 (1995).
• [11] K.R Pohl, P.H. Regan, J.E. Bush, P.E. Raines, D.P. Balamuth, D. Ward, A. Galindo-Uribarri, V.P. Janzen, S.M. Mullins, and S. Pilitte, Phys. Rev. C53, 2682 (1996).
• [12] P.H. Regan, T.M. Menezes, C.J. Pearson, W. Galletly, C.S. Purry, P.M Walker, S. Juutinen, R. Julin, K. Helariutta, A. Savelius, P. Jones, P. Jämsen, M. Muikku, P. A. Butler, G. Jones, and P. Greenlees, Phys. Rev. C33, 2305 (1997).

Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Received
  03 Nov 2003