2. NUCLEAR AND PARTICLE PHYSICS

 

2.1. EXPERIMENTAL NUCLEAR PHYSICS

 

Studies of the production and structure of exotic, highly neutron deficient nuclei in and above the lead region have been continued at the gas filled recoil separator facility (RITU) at the Accelerator Laboratory of the University of Jyväskylä. Investigations have concentrated in both decay and in-beam studies of neutron deficient isotopes of the elements in the range of Bi to Ra. The recoil decay tagging (RDT) method has been successfully extended from the pioneering studies of 254No to 252No in a large international collaborative experiment at the RITU facility. The first series of experiments to probe the structure of 255Lr by the RDT method were also carried out.

High spin spectroscopy studies were carried out in co-operation mainly with groups at the Accelerator Laboratory in Jyväskylä and the Royal Institute of Technology in Stockholm and also with other European groups. The main interest has been focused on studies of shell structure and shape co-existence in nuclei close to the double magic nucleus 100Sn.

Kari Eskola

 

SPECTROSCOPY OF TRANSFERMIUM NUCLEI: 252No

R.-D. Herzberg1, N. Amzal1, F. Becker2, P. A. Butler1, A. J. Chewter1, J. F. Cocks3, O. Dorvaux3, Kari Eskola, J. Gerl4, P. T. Greenlees3, N. J. Hammond1, K. Hauschild2, K. Helariutta3, F. P. Hessberger4, M. Houry2, G. D. Jones1, P. Jones3, R. Julin3, S. Juutinen3, H. Kankaanpää3, H. Kettunen3, T. L. Khoo5, W. Korten2, M. Leino3, Y. Le Coz2, C. J. Lister5, R. Lucas2, M. Muikku3, P. Nieminen3, R. D. Page1, P. Rahkila3, P. Reiter6, C. Schlegel4, C. Scholey1, O. Stezowski1, Ch. Theisen2, J. Uusitalo3 and H. J. Wollersheim4

The spectroscopy of transfermium nuclei has been proven to be possible using powerful gamma spectrometers in conjunction with recoil separators [1,2]. Encouraged by the results obtained on 254No, an experiment was performed to study the excited states in the nobelium nucleus 252No.

The ground state rotational band of 252No was populated in the 206Pb(48Ca,2n)252No reaction at a beam energy of 215.5 MeV in the center of the 500 m g/cm2 target. Gamma rays were detected by JUROSPHERE II in coincidence with nobelium recoils in the focal plane of RITU. The reaction cross section was determined to be s ª 300 nb. With a beam current of 20 pnA, a total of 2800 252No full energy alpha decays were observed. The channel selection was aided greatly by the introduction of a MWPC detector just in front of the focal plane which acted as a veto detector to distinguish alpha decays from recoil events and allowed a channel selection on the basis of the energy loss in the MWPC.

Transitions in the ground state band of 252No were assigned via the recoil decay tagging technique. Based on a Harris fit and the comparison of the transition energies to the neighboring 254No, the level energies and spins of 252No were assigned. The preliminary data analysis suggests the beginning of an upbend at a frequency around 200 keV.

Further experiments at even lower production cross sections appear feasible and are planned for the future.

1) Univ. Liverpool, Liverpool L69 7ZE, UK

2) DAPNIA/SPhN, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France

3) University of Jyväskylä, Jyväskylä, Finland

4) Gesellschaft für Schwerionenforschung, D-64220 Darmstadt, Germany

5) Argonne National Lab., Argonne, IL USA

6) Ludwig-Maximilians-Universität, D-85748 Garching, Germany

1. P. Reiter et al., Phys. Rev. Lett. 82 (1999) 509

2. M. Leino et al., Eur. Phys. J. A 6 (1999) 63

 

ALPHA DECAY OF THE NEW ISOTOPE 195Rn

H. Kettunen*, Kari Eskola, P. T. Greenlees*, K. Helariutta*, P. Jones*, P. Kuusiniemi*, M. Leino*, M. Muikku* and J. Uusitalo*

The new neutron-deficient isotope, 195Rn, was produced in the fusion evaporation reaction 56Fe(142Nd,3n)195Rn. The evaporation residues were separated from unreacted beam using the gas-filled recoil separator RITU. The separated ions were implanted into a position sensitive silicon strip detector where their subsequent alpha decays were measured. In front of the Si detector a gas counter was used to distinguish the alpha particles from the low energy recoils. The recoil-alpha-alpha-(alpha) correlation technique was used for identification of potential new alpha particle decays.

Recently the daughter nuclide of 195Rn, 191Po, was studied thoroughly [1]. Two a -decaying states were observed with Ea = 7334(10) keV and T1/2 = 22(1) ms for 191gPo and Ea = 7378(10) keV and T 1/2 = 98(8) ms for 191mPo [1]. The observation of hindered (D l=0) alpha decay together with the evidence for fine structure in both decays led to the identification of proton p (2p-2h) based states in 187Pb and for evidence for shape staggering in 191Po. The identification of fine structure in the alpha decays was based on the observation of prompt a -g -ray and a -x-ray coincidences.

In the present work two alpha decaying states could be tentatively identified also for 195Rn. In the observed recoil-alpha-alpha chains one of the reported [1] fine structure alpha decays was identified. In addition, improved half-life and energy values of the alpha decay of even-even 196Rn [2] were measured. The data analysis is under progress and the alpha decay properties of the very neutron deficient isotopes 195Rn and 196Rn will be reported at a later date [3].

* Department of Physics, University of Jyväskylä, Finland

1. A. N. Andreyev et al., Phys. Rev. Lett. 82 (1999) 1819

2. Y. H. Pu et al., Z. Phys. A 357 (1997) 1

3. H. Kettunen et al., to be published

 

 

2.2. THEORETICAL NUCLEAR AND HADRON PHYSICS

 

The research effort of the group dealt with theoretical issues in nuclear and hadron structure and reactions during the year 1999. The role of intermediate nucleon resonances in low energy pion production reactions in proton-proton collisions was investigated. Calculations of higher order loop contributions to the hyperfine interaction between constituent quarks were carried out, with the goal of understanding the multiplet structure and small spin-orbit splittings in the baryon spectrum. Meson exchange and loop contributions to the charge radii of the nucleons were calculated, and shown to be desirably small in the case of the neutron charge form factor. The role of the interaction current implied by the linear scalar confining interaction between heavy quarks and antiquarks was investigated, and shown to be of crucial significance for understanding the magnetic transitions in heavy quarkonia. The strangeness magnetic moments of the nucleons measured in the SAMPLE experiment were calculated with the chiral quark model.

Dan-Olof Riska