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 the Accelerator Laboratory of the University of Jyväskylä has been continued. The research effort has concentrated on studies of nuclides not far from lead. Two specific cases are the discoveries of two new alpha emitting nuclides: the mercury isotope 174Hg and the actinium isotope 206Ac. A tentative identification of an oblate 0+ state in 188Pb, showing up its existence through a weak alpha fine-structure line in the decay of 192Po, was possible through an ingenuous use of an array of silicon detectors. The use of rotating target wheels was introduced in our studies of neutron-deficient U and Pu isotopes.


High spin spectroscopy studies were carried out in co-operation with groups at the Physics Department at the Royal Institute of Technology and The Svedberg Laboratory in Sweden, and at the University of Jyväskylä. Level properties of nuclei in the lead region and in the mass 120 region were studied.


Kari Eskola and Björn Fant

ALPHA DECAY OF THE NEW ISOTOPE 174Hg

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

We have continued our program aimed at the region of known nuclides towards the proton drip line among the heavy elements. The new alpha decaying even-even isotope  174Hg was synthesized in the reaction 36Ar + 144Sm [1]. It was identified on the basis of correlated decay chains leading to 170Pt and 166Os. The measured alpha particle energy and half-life were 7069 keV and 2.1 ms, respectively. The region of very neutron-deficient Hg isotopes is interesting from the point of view of nuclear shape coexistence in the vicinity of the Z = 82 shell closure [2]. The measured alpha particle energy fits in well with alpha particle energy systematics and reveals no surprises. One should note that the 36Ar + 144Sm cross section for the production of 176Hg in the reaction measured in the present work, 1.3 ub, makes it possible to study the excited states of this nuclide using RITU in recoil decay tagging mode in connection with in-beam gamma-ray spectroscopy. Preparations for such a study are underway.

1. J. Uusitalo et al., submitted to Z. Phys.
2. N. Bijnens et al., Physica Scripta T56 (1995) 110 

* Dept. of Physics, Univ. Jyväskylä
** Dept. of Physics, Univ. Liverpool
*** GSI, Darmstadt

ALPHA DECAY FINE STRUCTURE OF  192Po

R.G. Allatt*, T. Enqvist**, K. Eskola, P.T. Greenlees*,***, M. Leino***, P. Kuusiniemi***, R.D. Page*, W.H. Trzaska*** and J. Uusitalo*** 

The alpha decay fine structure of 192Po has been investigated using RITU. Previous alpha-decay studies of heavier neutron-deficient polonium isotopes indicate the existence of excited 0+ states a few hundred keV above the ground state in their respective lead daughters. Systematics indicate a decrease in the excitation energy of these states with decreasing neutron number, the minimum of which is predicted to occur around 188Pb, where it should represent the lowest excited state. These 0+ states have been assigned to oblate configurations but calculations also predict prolate minima in the potential energy surfaces in neutron-deficient lead isotopes, implying the existence of excited 0+ states based on prolate configurations. In  188Pb the energy of the prolate and oblate states is expected to be comparable.


An experiment was performed at Jyväskylä using beams of  36Ar ions to bombard a 500 um/cm2 thick 160Dy target at centre of target beam energies in the range 172 - 184 MeV, in order to produce 192Po through the 4n evaporation channel. The recoil separator RITU was used to separate the reaction products, which were implanted into a silicon PAD detector in order to detect evaporation residues and their subsequent alpha-decays. An array of six 5 cm by 5 cm silicon detectors, subdivided into quadrants and mounted around the PAD detector, was used to detect conversion electrons emitted in the decay of the excited 0+ states. The signal for alpha decay to an excited 0+ state would then involve a coincidence between an alpha particle in the PAD detector and an electron in the surrounding detectors.


The alpha-decay of 192Po to the ground state of 188Pb yielded approximately 13000 counts in the PAD detector energy spectrum. Energy spectra of alpha particles detected in the PAD detector in coincidence with conversion electrons in the surrounding detector array revealed a clear peak containing approximately 100 counts at an energy 570 keV lower than the main 192Po decay line. The half-life of this decay line was measured and found to be consistent with the value of 33 ms determined for the ground state line. The difference in energy between this new decay line and that of the ground state line is in good agreement with extrapolations of the systemati c trends observed for the oblate states in heavier isotopes.


We therefore tentatively identify this new decay line as the alpha-decay of 192Po to the first excited (oblate) 0+ state in 188Pb. Analysis of the data searching for evidence of alpha-decays to the expected prolate 0+ state is currently in progress.

* Dept. of Physics, Univ. Liverpool
**GSI, Darmstadt
*** Dept. of Physics, Univ. Jyväskylä
 

2.2. THEORETICAL NUCLEAR AND HADRON PHYSICS

D.O. Riska, Mikael Björnberg, Kaj Dannbom, Christina Helminen, L. Glozmana, G.A. Millerb, U. Van Kolckc, A. Acusd, E. Norvaisasd, A. Kobushkine

The research in theoretical nuclear and hadron physics concerned (1) the application of the chiral quark model to the structure of the baryons - the spectra of the light flavor baryons [1] as well as the heavy hyperons [2,3]. The exchange current contributions consistent with a complete dynamical version of the quark model, which can predict the spectra of the light flavor baryons and the strange hyperons satisfactorily, were derived [4]. Another line of research concerned (2) the development of a consistent method of quantizing Skyrme's topological soliton models for the baryons [5]. The topological soliton model was also employed to investigate the contributio n of the vibrational breathing mode N(1440) on the spin-orbit component of the nucleon-nucleon interaction [6]. Finally [3] a solution based on short range exchange mechanisms to the puzzle of the near complete cancellation to near threshold cross section f or 0 production in pp collisions that is implied by chiral perturbations theory was proposed [7,8].

References

1. L.Ya. Glozman and D.O. Riska, "The Spectrum of the Nucleons and the Strange Hyperons and Chiral Dynamics", Physics Reports 268 (1996) 263

2. L.Ya. Glozman and D.O. Riska, "The Charm and Bottom Hyperons in a Chiral Quark Model", Nucl. Phys. A603 (1996) 326

3. D.O. Riska, Physics of Charmed Baryons and their Magnetic Moments, Nuclear Instruments and Methods in Physics Research B119 (1996) 259


4. K. Dannbom, L.Ya. Glozman, C. Helminen and D.O. Riska, "Baryon Magnetic Moments and Axial Coupling Constants with Relativistic and Exchange Current Effects", hep-ph/9610384


5. A. Acus, E. Norvaisas and D.O. Riska, "The Quantum Skyrmion in Representations of General Dimension", hep-ph/9605435


6. A.P. Kobushkin and D.O. Riska, The Breathing Modes of the B=2 Skyrmion and the Spin-Orbit Interaction, nucl-th/9602008


7. D.O. Riska, "The Long Road to Short Range Exchange Currents", Nucl. Phys. A606 (1996) 25


8. U. Van Kolck, G.A. Miller and D.O. Riska, "Meson Exchange and Pion Rescattering Contributions to the Cross Section for pp -> pp pi0 , nucl-th/9607026

a Institute for Theoretical Physics, Univ. Graz

b Department of Physics, Univ. Washington 

c Inst. for Nuclear Theory, Univ. Washington


d Institute for Theoretical Physics and Astronomy, Vilnius, Lithuania

e Bogoliubov Institute for Theoretical Physics, National Academy of Science, Ukraine

2.3. THE LEP/DELPHI EXPERIMENT

Heimo Saarikko*

The year 1996 was the first year of data taking at LEP200. In October 1996 an operational e+e­ collision energy of 172 GeV was achieved. This means that the collision energy is well above the W-pair production threshold, for the first time in accelerator produced e+e­ collisions. In the coming years, with increasing LEP energy, accurate W-boson mass measurements are foreseen at LEP200. The LEP Upgrade phase will continue until about the year 2000 so that a possibly existing light Higgs with mass below 100 GeV would be discovered.


Being the traditional responsibility of the Finnish DELPHI group the Hadron Calorimeter detector activity has included the maintenance of the calorimeter as well as tuning, testing and repairs during the shutdowns of the experiment. As a part of the LEP200 Upgrade, the final version of the anode read-out electronics, anode trigger card, event timer card, control card and anode trigger system have been developed.


A method to identify KL0 with the aid of the Hadron Calorimeter cathode read-out has been developed.


The cluster reconstruction and pattern recognition program for the Vertex Detector barrel part of 1996 Delphi Silicon Tracker was also developed.


The group has also participated in doing the centrally manned shifts for Slow Control and Data Quality checking as well as expert shifts of the Hadron Calorimeter and Vertex Detector.


The data set of about 3.5 million Z0 -> hadrons achieved in 1989-1996 is being analyzed, the Finnish group concentrating in fields closely related to the use of the detectors to which the group is presently contributing. Preparation for the LEP200 physics program has been initiated, the main area of interest being the search for the Higgs bosons in specific final state decay modes.

* The Research Institute for High Energy Physics (SEFT) was the official Finnish collaboration institute with CERN. Doc. Heimo Saarikko from the Department of Physics has been nominated the chief coordinator of the Finnish participation in the DELPHI project.