2. NUCLEAR AND PARTICLE PHYSICS

2.1. EXPERIMENTAL NUCLEAR PHYSICS


Participation in the extensive program to study the production and structure of exotic, highly neutron deficient nuclei in the lead region has continued at the gas filled recoil separator facility (RITU) at the Accelerator Laboratory of the University of Jyväskylä. Specific problems of high interest have been studies of alpha fine structure in 191Po and 192Po. Through these studies shape co-existence in the daughter nucleus 188Pb and identification of proton p(2p-2h) based states in 187Pb have been established. A number of Rn and Ra nuclides have been studied by means of the new method of recoil decay tagging (RDT). A highlight of the year was participation in the in-beam study of 254No carried out by a large international team of experimenters at the RITU facility.

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
 

PROBING TRANS-FERMIUM NUCLEI: EXCITED STATES IN 254No
M. Leino*, R.-D. Herzberg**, F.P. Hessberger***, Y. Le Coz+, F. Becker+, P.A. Butler**, A.J. Chewter**, J.F.C. Cocks*, O. Dorvaux*, Kari Eskola, J. Gerl***, P.T. Greenlees**, K. Helariutta*, M. Houry+, G.D. Jones**, P.M. Jones*, R. Julin*, S. Juutinen*, H. Kankaanpää*, H. Kettunen*, T.L. Khoo++, A. Kleinböhl***, W. Korten+, P. Kuusiniemi*, R. Lucas+, M. Muikku*, P. Nieminen*, R.D. Page**, P. Rahkila*, P. Reiter++, A. Savelius*, Ch. Schlegel***, Ch. Theisen+, W.H. Trzaska* and H.-J. Wollersheim***

In beam studies of 254No opened a new and exciting window to structure of nuclei beyond Z = 100 and attracted in 1998 two large international collaborarations to engage in such studies one at Argonne and one at Jyväskylä. In Jyväskylä the SARI gamma detector array and the RITU gas-filled separator were used in the study of excited states in 254No, produced in the reaction 48Ca + 208Pb. Stationary 208Pb targets of 250-700 µg/cm2 thickness were used. The beam current was 10 pnA. The maximum cross section was found to be about 2 µb at an energy of 216 MeV, corresponding to 21 MeV excitation of the compound system. A total of 12000 254No alpha particles with full decay energy were observed at the focal plane of RITU.

Using the RDT (Recoil Decay Tagging) method six successive gamma rays assigned to the ground state band downwards from the 16+ level. The energies of the six observed transitions (158.9, 214.1, 267.2, 318.2, 366.5, and 414 keV) are well reproduced using the rotational model with moment of inertia parameters J0 = 68.2 h2/MeV and J1 = 162.4 h2/MeV3. An excitation energy of 44 keV can be extracted for the 2+ state.

The two experiments performed at Argonne and JYFL confirm the predicted quadrupole deformation of the trans-fermium nuclide 254No and show that it survives against fission up to at least spin 16h. Further studies on the structure of odd-mass isotopes in this region are planned to learn about the single-particle energies.

* University of Jyväskylä
** University of Liverpool
*** GSI Darmstadt,
+ DAPNIA/SPhN CEA-Saclay
++ ANL Argonne

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

STUDIES OF EXCITED STATES IN LIGHT Ra ISOTOPES
J.F.C. Cocks*, O. Dorvaux*, Kari Eskola, K. Helariutta*, P. Jones*, R. Julin*, S. Juutinen*, H. Kankaanpää*, H. Kettunen*, P. Kuusiniemi*, Y.Le Coz**, M. Leino, T. Lönnroth***, M. Muikku, P. Nieminen*, P. Rahkila*, A. Savelius* and W.H. Trzaska*

Excited states have been studied for the first time in three neutron deficient isotopes 206,208,210Ra using the recoil tagging technique in conjunction with RITU.  Preliminary data on 206Ra and 208Ra were presented in the Annual  Report of last year (1). Since then excited states of 210Ra were studied using the reaction 174Yb(40Ar,4n)210Ra at a bombarding energy of 183 Mev.  Fusion products were detected using the RITU focal plane Si detector and four unsuppressed TESSA-type germanium detectors positioned adjacent to the Si detector. A half life of 2.24(1) µs was deduced for the 8+ state.

In the accompanying figure the systematic behaviour of of the levels in even-even Ra isotopes with N=118-126 is presented.  The data for 212Ra and 214Ra have been taken from refs. (2) and (3), respectively. The excitation energies of the 6+ and 8+ states interpreted to originate from the ¼h69/2 multiplet increase in a regular fashion down to N=120, but for N=118 (206Ra)  the 8+ state appears to be crossed by another 8+  state, probably of neutron-hole origin.

* Department of Physics, University of Jyväskylä, Jyväskylä, Finland
** DAPNIA/SPhN, CEA Saclay, France
*** Åbo Akademi, Turku, Finland

1. J.F.C. Cocks et al., JYFL Annual Report 1997, 40
2. T. Kohno et al.Phys. Rev. C33 (1986) 392
3. A.E. Stuchbery et al., Nucl. Phys. A548 (1992) 159
 
 

2.2. THEORETICAL NUCLEAR AND HADRON PHYSICS


The research effort in 1998 followed 4 main lines: (a) Quantitative, mainly numerical, study of the electromagnetic structure of the bound trinucleons with realistic nucleon-nucleon interactions including the three-nucleon interaction and D33 configurations and full account of the associated many-nucleon interaction currents. (b) Development of a quantitative covariant quark model for the baryons in all flavor sectors along with the evaluation of higher loop contributions to the hyperfine interaction between constituent quarks. (c) Application of the extended Skyrme model to the h-nucleus interaction with the rational map ansatz. A constructive realization was made of Skyrme's conjecture that the pion mass may appear in the topological soliton model as a self-consistent quantal effect. (d) Phenomenological determination of the Lorentz-structure of the effective linear confining interaction between quarks and antiquarks by quantitative analysis of the M1 decay rates of heavy quarkonia with a realistic Hamiltonian with interaction currents. The last project was carried out as an innovative research + teaching project involving highly motivated undergraduate students. The effort involved collaboration with scientists in 4 other countries.

Dan Olof Riska