2. NUCLEAR AND PARTICLE PHYSICS ________________________________________________________________________________________________

2.1. EXPERIMENTAL NUCLEAR PHYSICS

The experimental nuclear physics carried out at the Accelerator Laboratory is discussed separately in section 1.

Collaboration with Matti Leino's group at the gas-filled recoil separator facility (RITU) at the Accelerator Laboratory of the University of Jyväskylä has been continued. For a major part of the year RITU was run in connection with the JUROSPHERE array of large germanium detectors. This unique combination of a gas-filled mass separator and "crystal ball" detector setup allowed a very selective means to study prompt gamma ray emission tagged by subsequent recoil-alpha decay event pairs. The technique was very successfully applied to a wide range of highly neutron-deficient nuclides centered in the lead region. Our collaborative studies dealt with Po and Ra isotopes.

In addition well-established, stand-alone RITU experiments were continued. The work on the new isotope 206Ac and the accompanying isomer was completed. Analysis of seemingly unorthodox alpha decay data in the 195At region has been continued. New detailed information on alpha decay characteristicss of 190Po, 189mBi and 186Pb has been obtained.

High spin spectroscopy studies were carried out in co-operation with groups at the Accelerator Laboratory in Jyväskylä and Department of Physics at Åbo Academy and Royal Institute of Technology in Stockholm. The main interest has been focused on studies of shell structure and shape co-existence in nuclei in the lead region.

Kari Eskola and Björn Fant

ALPHA DECAY CHARACTERISTICS OF NEUTRON-DEFICIENT 190Po, 189mBi AND 186Pb ISOTOPES STUDIED IN THE 48Ti + 144Sm REACTION

A.N. Andreyev*, N. Bijnens*, J.F. Cocks**, T. Enqvist***, K. Eskola, K. Helariutta**, M. Huyse*, P. Kuusiniemi**, M. Leino**, W.H. Trzaska**, J. Uusitalo**** and P. Van Duppen*

As a continuation of our program to study light neutron-deficient nuclides of Pb-Po we performed an experiment with the aim to get more precise data for 190Po and 186Pb. The results and a detailed description of our first experiment (June '96), have been published elsewhere [1]. Here we present our improved data, based on the results of both measurements.

The experiments were performed in June '96 and January '97 by using a beam of 48Ti with an initial energy of ELab = 225 MeV. For the 144Sm target (450 *g/cm2 thickness, 88.6% enrichment) we performed the measurements at beam energies of ELab = 202, 208, and 221 MeV. To identify the 190Po isotope, a search for position and time correlated events has been carried out. Altogether about 100 recoil-alpha1 (190Po) correlation chains have been found. Moreover, about 25 of these recoil-alpha1 correlations have a second alpha decay with the measured energy and half-life value corresponding to the alpha-decay properties of 186Pb. Additionally, we observed a few recoil-alpha1-alpha2-alpha3 events having a third alpha-decay with an energy and half-life matching those of 182Hg. On the basis of these data we assigned the observed correlation chains to alpha-decay of 190Po. Its alphaparticle energy and half-life value are Ealpha = 7545(15) keV and T1/2 = 2.53(33) ms, respectively, in agreement with our data from [1] and with those of Batchelder et al. [3].

By comparing the numbers of recoil-alpha1(190Po) and recoil-alpha1(190Po)-alpha2(186Pb) correlations an improved branching ratio value for 186Pb equal to alphabr(186Pb) = 0.38(9) was deduced. From the time intervals between the implantation of EVR and its first alpha decay with Ealpha = 7300(15) keV by using the maximum likelihood method we determined a half-life value of T1/2 = 4.8(5) ms for 189mBi.

1. A. Andreyev et al., Z. Phys. A358 (1997) 63
2. M. Leino et al., Nucl. Instr. and Meth. in Phys. Res. B99 (1995) 653
3. J. C. Batchelder et al. Phys. Rev. C 55 (1997) 2142

* Instituut voor Kern- en Stralingsfysica, University of Leuven, Belgium
** Dept. of Physics, Univ. Jyväskylä, Jyväskylä
*** Present address: GSI, Darmstadt, Germany
**** Present Address: Argonne National Laboratory, Argonne, Illinois, USA

ALPHA DECAY OF THE NEW ISOTOPE 206Ac

K. Eskola, P. Kuusiniemi*, M. Leino*, J. F. C. Cocks*, T. Enqvist**, S. Hurskanen*, H. Kettunen*, W. H. Trzaska*, J. Uusitalo***, R. G. Allatt****, P. T. Greenlees**** and R. D. Page****

The new neutron-deficient isotope, 206Ac, was produced in the fusion evaporation reaction 36Ar+175Lu. Altogether six bombarding energies in the range 177-192 MeV were used in two irradia-tions with 36Ar8+ particles and average beam intensities of about 300 enA. The target thickness was 320 µg/cm2. Nickel degrader foils were used to reduce the bombarding energy starting from 5.5 MeV/nucleon as delivered by the K = 130 MeV cyclotron. The heavy ion beams were pulsed in both irradiations.

The daughter nuclide of 206Ac, 202Fr, has two isomeric states with similar decay properties [1]. A half-life T1/2 of (340±40) ms and an alpha particle energy Ealpha of (7237±8) keV have been determined for both the 3+ and the 10­ isomeric level. Proper identification of 206Ac on the basis of the correlation method requires an observation of the granddaughter decays. The 3+ level of 198At has the following reported decay properties: Ealpha = (6755±4) keV and T1/2 = (4.2±0.3) s [1]. For the 10­ level the corresponding data are (6856±4) keV and (1.0±0.2) s [1].

Nine quadruple event chains of the type ER-alpha1-alpha2-alpha3 , where ER stands for an evaporation residue and alpha1 for an alpha decay event and two triple event chains (alpha2 missing) in which the last alpha particle had an energy of 6.75 MeV were found. Similarly, five quadruple events and one triple event (alpha2 missing) in which the last alpha particle had an energy of 6.86 MeV were found. We conclude that 206Ac has two isomeric states. For a full report, see Ref. 2.

1. M. Huyse et al., Phys. Rev. C 46 (1992) 1209
2. K. Eskola et al., Phys. Rev. C 57 (1998) 417

* Department of Physics, University of Jyväskylä, Jyväskylä, Finland
** Present address: GSI, Darmstadt, Germany
*** Present address: Argonne National Laboratory, Argonne, Illinois, USA
**** Department of Physics, University of Liverpool, Liverpool, UK

SHELL STRUCTURE AND SHAPE COEXISTENCE IN 195Pb

Björn Fant, B. Cederwall 1, J. Cederkäll 1, L.O. Norlin 1, R. Wyss 1, P. Fallon 2, C.W. Beausang 3, P.A. Butler 3, J.W. Roberts 3, A.M. Bruce 4, D.M. Cullen 5, S.M. Mullins 6, R.J. Poynter 7, R. Wadsworth 7, M.A. Riley 8, W. Korten 9 and M.J. Piiparinen 10

Experiments on 195Pb were performed using the Tandem accelerators at Daresbury Laboratory and the Niels Bohr Institute. The reaction 164Dy(36S, 5n)195Pb at 170 MeV bombarding energy was used at the Daresbury Laboratory and the reaction 164Dy(34S, 3n)195Pb at 160 MeV was used at NBI. In the longest runs 540.106 events of fold selected double and triple coincidences were recorded.

Prior to this investigation high spin states in 195Pb were observed [1] up to spin 33/2+. In this investigation we report two dipole gamma cascades which feed into the yrast 25/2+ state and thus bypassing the isomeric 33/2+ state. The dipole bands extend to J = 53/2. Similar bands have also been observed with better accuracy in an EUROGAM investiagtion [2]. The stronger dipole band can according to the performed Total Routhian surface calculations be associated with a rotational band built on a band head with the configuration pi (h9/2-i13/2)K=11­ x nu i13/2, K= 27/2­. This band head is probably observed in the present experiment. The band crossing is associated with the alignment of the first unblocked pair of i13/2 neutrons. The structure of the other observed dipole band could either be of the same proton structure but involving the f5/2 or p3/2 neutrons in addition to the i13/2 neutrons or of the pi(h9/2)2K=8+ x nu f5/2­(i13/2)2 configuration.The first assignment is suggested by Kaci et al [2]. The observed backbends are interpreted in terms of aligment of i13/2 neutrons. New cascades feeding into the yrast 27/2­ state are also observed.

1. M. Pautrat et al., Phys. Scripta 34 (1986) 378 2. M. Kaci et al., Z. Phys. A354 (1996) 267

1 Royal Institute of Thechnology, Physics Department, S-104 05 Stockholm, Sweden
2 Lawrence Berkeley Lab., CA 94720, USA
3 Oliver Lodge Laboratory, Univ. Liverpool, Liverpool L69 3BX, UK
4 Department of Mathematical Sciences, Univ. Brighton, Brighton BN2 46J, UK
5 Oak Ridge National Laboratory, Oak Ridge, TN 37831-6371, USA
6 Dept. of Physics and Astronomy, McMaster Univ., Hamilton, ON L8C 4M1, Canada
7 Department of Physics, Univ. York, Heslington, York Y01 5DD, UK
8 Department of Physics, Florida State Univ., Tallahassee, FL 32306, USA
9 Institut für Strahlen- und Kernphysik, Universität Bonn, W-5300 Bonn, Germany
10 Accelerator Laboratory, Univ. Jyväskylä, FIN-40351 Jyväskylä, Finland

2.2. THEORETICAL NUCLEAR AND HADRON PHYSICS

K. Dannbom, L. Glozman*, C. Helminen, D.O. Riska, A. Acus**, E. Norvaisas** and F. Coester***

The research efforts of the theoretical nuclear and hadron physics group during 1997 concentrated in the following topics: 1) Application of the chiral quark model for the baryons previously developed by the group to the baryon current observables and the role of meson exchange currents [1,2]. 2) Construction of a phenomenological Poincaré invariant constituent quark model, which fits the baryon spectra in all flavour sectors and which can be applied to the calculation of nucleon resonance form factors in point [3] or instant form kinematics [4]. The development of an ab initio quantized version of Skyrme's topological soliton model for the baryons [5] along with a constructive realization of Skyrme's conjecture of the of the nonzero pion mass as a self consistent quantal effect [6]. The prediction by the group of a strange pentaquark at 2860 MeV [7] appears to have been empirically confirmed [8].

1. K. Dannbom, L.Y. Glozman, C. Helminen and D.O. Riska, Nuclear Physics A616 (1997) 555
2. C. Helminen, hep-ph/9711252
3. F. Coester and D.O. Riska, hep-ph/9707388
4. K. Dannbom, F. Coester and D.O. Riska, hep-ph/9711458
5. A. Acus, E. Norvaisas and D.O. Riska, Nuclear Physics A614 (1997) 361
6. A. Acus, E. Norvaisas and D.O. Riska, nucl-ha/9712071
7. D.O. Riska and N.N. Scoccola, Phys. Lett. B299 (1993) 338
8. E.M. Aitala et al., FERMILAB -pub -97/118-E

* Power Engineering Inst., Almaty, Kazakstan
** Inst. for Theoretical Physics and Astronomy, Vilnius, Lithuania
*** Phys. Div., Argonne Natl. Lab., Argonne, USA