Table of contents

After an unintended time gap of five years, the series of regular Nordic meetings on nuclear physics was continued with the 6th Nordic Meeting, August 10–15, 1989. The site was Utgarden in the outskirts of Kopervik, the administration center for the Saga island of Karmøy on the west-coast of Norway. Utgarden, a "peoples high-school'' with a kitchen, housing facility and a neighboring modern gymnasium with fine lecture halls, proved to be an inexpensive and adequate site for the meeting.

From the time of the Vikings, the sound between Karmøyy and the mainland has been a vital part of the way to the north. Mobility and international orientation is still a signature of an area where today essential parts of Norway's oil- and metal industry are located. The conference program included a session on nuclear physics in industry and society, with contributed talks from a number of companies and technology/research institutions, which also sponsored the meeting. Lunch visits to Hydro's aluminium plant on Karmøy or alternatively to Statoil's gas terminal on the mainland, were included in the program.

The scientific program gives a cross section of nuclear physics activities in which researchers from the Nordic countries are involved nowadays. The spectrum is rich, and the emphasis has shifted to higher energies than was the case five years ago. We appreciate the possibility to present this overview in a separate volume of Physica Scripta. The present issue covers nearly all the talks given at the meeting. The order deviates, however, somewhat from that of the conference program.

The organizing committee tried to encourage in various ways the participation of young physicists; this effort was truely rewarded. The young participants put their imprint on the activities in the lecture halls and even more on the soccer arena.

The meeting was sponsored by The University of Bergen, The Nordic Accelerator Committee, NORDITA, The Norwegian Research Council for Science and the Humanities (NAVF), The Norwegian Physical Society, The Community of Karmøy, Hydro Aluminium Karmøy, Statoil, Laborel, Aanderaa Instruments and also by other local firms and institutions. The financial as well as practical support from all of these sources is gratefully acknowledged.

We will in particular express our appreciation for the indispensable help from cand. real. Konrad Bardsen and his colleagues at the gymnasium where the daily scientific activity, and also major parts of the social program, took place.

The meeting could not have been organized nor carried through without the professional help from Karen-Margrete Hovland, backed by her sister Alice and a team of hard-working graduate students from Bergen: Erling Andersen, Håvard Helstrup, Torbjørn Rogde and Espen Staubo.

We thank all of our speakers for a smooth cooperation.

* The logo of the meeting, Seidmennene på Skrattaskjaer (Old Norse: seidmann = "shaman", skrattasker = wizard or troll skerry) has resulted in colorful interpretations and comments. The organizing committee noticed, with satisfaction, that the logo had been correctly identified in one of the contributions. The still curious reader may read about this thousand year old historic event in the sagas of the NORSE KINGS by Snorri Sturluson, in the saga of Olav Trygvason.

In this paper, some achievements of an equations-of-motion approach to nuclear structure theory are discussed. As an introduction to the main subject, a brief survey of some early work is given. We then describe a formalism for treating the pairing-force problem and show, by numerical applications, that at the lowest order of approximation it provides an advantageous alternative to the BCS method. Finally, we discuss how to treat a general shell-model Hamiltonian within the framework of the seniority scheme. This makes it possible to further reduce seniority-truncated shell-model spaces, as is illustrated by examples.

We discuss spin excitations in 3 regions. The low excitation energy region dominated by giant resonances of low multipolarity. The spin response in the quasielastic region and finally the Δ-excitation. We are especially interested in correlations in the spin longitudinal channel.

This paper gives a brief review of some recent results on the structure of nuclei at the boundaries of the nuclear chart, that is near to or at the limit of particle stability. The extremely neutron-rich nuclei appear especially interesting. Their low neutron separation energy combined with the fact that the nuclear force is attractive and of short range leads to a spatially extended neutron wave function, the neutron "halo", which for the case of ¹¹Li is estimated to extend to about 8 fm, well beyond the 2.5 fm radius of ⁹Li. New manifestations of the halo in electromagnetic processes and beta decay are discussed. Ther first example of beta-delayed deuteron emission has been observed in the isotope ⁶He, and is expected to occur also in the decays of other drip-line nuclei.

Various approaches to calculate the coefficient of the transport equation describing the process of multinucleon transfer are analysed. It is shown that the expressions for transition probabilities containing the effects of the nuclear shell structure can be obtained if we do not use averaging of the matrix elements over many shell configurations. On this basis, in the framework of the model with the realistic single-particle scheme of levels the charge distributions of the reaction products in the collisions ⁵²Cr + ¹⁸¹Ta, ⁵¹V + ¹⁹⁷Au and ²⁰Ne + ¹⁹⁷Au are investigated. The role of the Coulomb interaction in a dinuclear system is discussed. It is shown that the Coulomb interaction increases the formation probability for very asymmetric configurations.

The Dynamic Deformation Model has been extended to the problem of fission in such a way that several thousand channels including particle-decay, α-decay, heavy-ion-emission, asymmetric fission, and symmetric fission can be taken into account. The model also includes a Kinetic Shell Correction which was ignored in previous predictions for Superheavy nuclei. This model is in better agreement with experimental life-times. A new location of the Superheavy peak is predicted at Z = 116 (eka-Polonium), A = 300, total half-life = 1079 years. New heavy-ion-fusion experiments and the means of identifying the Superheavy Elements are suggested.

The novel method for rapid on-line mass separation, the IGISOL, is described. The method has been shown applicable to mass separation of short-lived proton-induced fission products without any chemical dependence on radionuclide. Recent results on isomeric states with half-lives of the order of milliseconds produced directly in proton-induced fission of ²³⁸U and separated with the IGISOL are presented. The future plans for connection of the new K-130 cyclotron of the University of Jyväskylä with the IGISOL are reviewed.

The feasibility and limitations of a DWBA approach to the (³He, t) reaction formulated as a direct reaction in terms of an effective ³He-nucleon force is investigated. While the central V_τ and the tensor component V_Tτ can well be determined, the spin-orbit force V_LSτ cannot be used without a spin dependent normalization. Analysis of the analyzing power of the "free" process ³He(, p,)t indicates the need of an imaginary LSτ-component but does not solve the normalization problem. The evidence on energy dependence of the effective force component is discussed briefly.

The properties of Giant Dipole Resonances (GDR) built on excited nuclear states are sensitive to the structure of heated and rotating nuclei. Experiments measuring the spectral and strength distribution of the high energy gamma rays emitted in the decay of excited state GDR's can thus provide information on the deformations and fluctuations of the shapes of hot rotating nuclei, and on the characteristics and limits of collective motion at high temperatures.

Hot rotating ¹⁵⁸Yb nuclei show a transition from quasirotational structure at high spins and moderate temperatures to quasivibrational structure at low spins and temperatures. Thermal fluctuations in i and ω do not wash out this transition. The standard deviations Δi and Δω decrease at moderate temperatures.

This contribution summarizes results on nuclear structure of rare earth nuclei at high intrinsic excitation energy and low spin obtained with the (³He, ⁴He) and (³He, ³He') reactions. For E_x > 4.5 MeV we find that the γ-spectra from the two reactions are similar indicating that equilibrium is present prior to γ-emission. An unresolved γ-ray bump at 1 MeV is observed in the decay. This structure is ascribed the depopulation of the vibrational regime, defined around E_x = 1-2 MeV, to the ground state band. The technique of extracting first-generation γ-ray spectra is described and surprising low-energy γ-rays are found at high excitation energy. Preliminary results from the new CACTUS multidetector system are presented.

I survey some features of our knowledge of the hadronic interactions and their implications on the model building with applications to heavy ion reactions. Finally I present a model for Bose-Einstein correlations which at present is applied to the Fritiof model and at least in e⁺ e^--annihilation reactions imply "intermittency" in the sense of factorial moment growth in small rapidity intervals.

The proximity potential is discussed for the inelastic scattering of a spherical nucleus on a deformed nucleus or the mutual interaction of two deformed nuclei. It is shown that the proximity potential is, in general, geometrically more correct than the usual centerline prescription used in inelastic scattering analyses. For the cases where the proximity potential is inadequate a folding model approach is advocated. Techniques to facilitate the coupled channels analysis are presented.

Electromagnetic processes in peripheral nucleus-nucleus collisions are reviewed. First I discuss applications to radiative capture processes of astrophysical interest, the use of Coulomb-dissociation for the study of the structure of exotic secondary beams, and multiphoton excitations of multiphonon giant resonances and giant resonances coupled to low lying surface modes. In the second part, electromagnetic effects at relativistic heavy ions colliders are discussed, they are a serious source of loss for beam particles, which leads to a decrease of luminosity with time. Finally, the possibility to use ultra-relativistic heavy ion collision for fundamental studies in γ-γ physics is explored. The essential advantage over γ-γ-physics at e⁺ e-colliders is, due to the coherence effect, the Z⁴-factor (⩽ 10⁸) in the cross section.

Recent developments in models used to describe nuclear collisions and fragmentation are reviewed. A framework for the classification and relations of these models is discussed. Results on the mean free path of a nucleon in nuclear matter, relevant to dynamical model calculations, are quoted. A prescription for incorporating fluctuations in these models is described. Statistical multifragmentation theories are viewed as approximations to a transition-state treatment generalized from the standard theory of binary fission. A quasifragment in equilibrium with a nucleon vapor is considered. The interplay between dynamical and statistical aspects of nuclear collisions and fragmentation is briefly reviewed.

Nucleus-nucleus scattering and charge-exchange reactions are studied in a region where rainbow-like phenomena are seen. Optical potentials are found which give fits to the cross sections, but in spite of the rainbow-like phenomena and contrary to apparently accepted expectations, not determined unambiguously. Other possibilities of resolving the ambiguities are discussed.

The experimental results on the emission of intermediate mass fragments obtained by collaborations of our group at Michigan State University with groups from GANIL, GSI, Indiana University, Oak Ridge National Laboratory, and Washington University are discussed.

Results are presented from 60 and 200 A GeV p, ¹⁶O and ³²S projectiles with C, Cu, Ag and Au nuclei. Energy spectra at zero degrees and transverse energy distributions in the pseudorapidity interval 2.4 to 5.5 are shown. Transverse momentum distributions of inclusive photons and neutral pions at midrapidity measured with a lead glass array are also presented. For all target projectile combinations an increase in average p_T is observed for small entropy values, which is deduced from the central multiplicity density. Different from proton induced reactions, 200 A GeV ¹⁶O + Au data show a plateau like region at large values of entropy density characteristic for a mixed phase.

The yields and the transverse momentum distributions of pions, kaons, and antiprotons produced in the central region of pp collisions at √s = 1.8 TeV at the Fermilab Tevatron collider have been measured up to a charged particle pseudo-rapidity density of approximately 20. The average transverse momentum ⟨p_t⟩ as a function of ⟨dN_c/dη⟩ for all three types of particles is presented. We find that transverse flow does not explain the ⟨p_t⟩ separation. The most plausible interpretation is that the ⟨p_t⟩ vs. N_c spectra represent in isobar in the two phase region.

Various models of relativistic heavy ion collisions are compared and their predictions are compared to experimental data. Especially the model predictions for rapidity distributions for nucleons and mesons and to less degree transverse energy production and intermittency are investigated in those cases where experimental data exists. The two types of models analysed are hydrodynamical models and Fritiof. They are very different in the sense that the latter does to a large extent assume transparency of nuclear matter, whereas the hydrodynamical models does the opposite. Yet they are found to give similar results agreeing well with the data in several cases and the reasons for this are discussed.

It is discussed how self-consistent descriptions of undamped nuclear collective motion can be extended to transport theory. A previous formulation with linear response theory is modified to describe fluctuations consistently with average motion. A Hamiltonian for an extended space of collective plus intrinsic degrees of freedom is used and finally applied to a non-perturbative Nakajama-Zwanzig approach. The equations of motion contain non-Markovian features. Microscopically computed transport coefficients will be discussed from which inferences on applications to fission and heavy ion collisions can be drawn.

Recent results on fragmentation and multifragmentation in heavy ion reactions in the hundreds of MeV/nucleon energy region are discussed. Some perspectives are given on the possibilities to utilize storage rings in combination with gas-jet targets in order to achieve more selective fragmentation experiments.

Data on spectra and yields of pions and kaons from central Si + Au interactions of 14.6 GeV/c per nucleon are presented. The results are discussed in terms of a thermal one source model.

The widths of distributions obtained in central ¹⁶O + Au interactions at 200 A GeV are examined. It is shown that the width of the rapidity region is important when fluctuations in the distributions of transverse energy and charged particle multiplicity are studied. We investigate the origin of the fluctuations, and conclude that they originate predominantly from the underlying distribution of emitting sources.

We study the color confinement and hadronization in the thermodynamic limit of the flux tube model, which was recently developed to describe the energy deposition, particle production and the early expansion of plasma in the central region of ultrarelativistic heavy ion collisions. In this purpose, we introduce a concept of an intermediate "mixed state", which is formed when the original giant flux tube is fragmented into many smaller tubes which contain the plasma and the Abelian color-electric field and which coexist with a dilute hadron gas. This mixed state is an extension of the mixed phase used in the pure hydrodynamic description utilizing the bag equation of state. We study the thermodynamics of this mixed state making use of the bag model and the classical models of color confinement. It appears that in the thermodynamic limit, the mixed state can be considered as a dielectric medium in which the color dielectric constant takes the values between 0 and 1. We also study how the "equations of electrohydrodynamics", which were formulated in earlier works to describe the evolution of the giant flux tube and plasma, are to be modified due to the dielectric nature of the new transient mixed state. A numeric simulation of a one-dimensional scaling expansion demonstrates the dynamic transition of the system from plasma to hadron phase and the characteristic reheating effect due to this hadronization transition. It is also shown that in the limit of low temperature or large bag constant, this formalism is reduced to the hydrodynamics of hadron gas produced by the fragmentation of multiple strings.

We give a short overview on the most important possibilities of entropy production in the relativistic heavy ion collisions, which is connected to the shock phenomena. We consider as example the E802 experiment, where one can determine the specific entropy content from measured strange particle ratios. The received large entropy value (S/N_B ∼ 14) can be explained by assuming quark-gluon plasma formation. The possibility of overcooling of quark-gluon plasma and its deflagration are also investigated.

Multifragmentation is described as a quasi phase transition during the dynamical evolution of heavy ion collisions. A 3 fluid model of the dynamical evolution and geometrical break ups leading to multifragmentation are considered. The possibility of surface multifragmentation is mentioned as an instability growth phenomenon.

New results are presented from the HELIOS (High Energy Lepton and Ion Spectrometer) detector on the full η, -0.1 < η < 5.5, range of the calorimeter, the dimuon mass distribution and P_T spectra of negative pions.

We report on recent results obtained by the EMUO1 collaboration from studies of ¹⁶O + emulsion, at 14.6, 60 and 200 A GeV, and ³²S + emulsion and ³²S + Au interactions at 200 A GeV. Shower-particle production in oxygen-induced interactions are studied over the energy range 2.1-200 A GeV. Scaling and limiting fragmentation are observed. Multiplicity and angular distributions of low energy target-associated particles are presented and compared with distributions obtained using the Ranft and the Fritiof simulation codes.

Recent studies on target-fragmentation in heavy ion induced reactions at intermediate energies are reviewed. No unambiguous signature for a true multifragmentation process could be found. Intermediate mass fragment production in the energy regime up to approximately 100 MeV per nucleon incident energy seems to be dominated by conventional multi-sequential decays.

The relevance of lepton and photon pairs as well as inclusive direct photons for Quark-Gluon Plasma diagnostics is examined.

Signals of the Quark-Gluon Plasma, supposedly a state of matter as a result of central collisions of two nuclei at ultrarelativistic energies are investigated with special emphasis on photons, photon pairs and muon pair production. Relics of the events at the primordial epochs of microseconds after the Big Bang origin of the universe, the Quark-Gluon Plasma condensing to hadrons, are investigated. The ⁷Li production rate is highlighted as a suitable boundary condition for early universe models.

One-fluid 3D calculations of heavy ion collisions at energies 14.5 and 60 GeV/A are done including a quark-gluon plasma (QGP) and a mixed phase. Pion and nucleon measurables are calculated. The main effects of QGP formation are increased longitudinal and reduced transverse flow.

In the last decade more and more interest has been focussed on the problem of how to deduce the nuclear matter equation of state from experimental data, especially from heavy ion collisions. One has looked not only at densities around saturation to determine the stiffness of the equation of state, but recently also at very high densities (energies) in order to find signals for a phase transition to the quark gluon plasma. One problem is, however, the uncertainty in the bulk properties and the minor significance of the compression constant at ground state on the high density behaviour of the nuclear equation of state. Therefore it is a problem to deduce the equation of state from the ground state properties or vice versa.

On the other hand the key mechanism for high compression and heating of nuclear matter in the laboratory was unambiguously established with the discovery of compression shocks in nuclear matter, which had been predicted long ago. Our approach is therefore to study the behaviour of highly excited and more or less compressed nuclear matter by using the successful hydrodynamical model and extend it in order to describe ultrarelativistic heavy ion collisions.

Results are presented for collisions of heavy ions at high energies up to 200 GeV in full space and time (3 + 1 dimension) and compared to experimental data.

We have measured directed transverse momentum and multi-fragment emission for the systems of 35 MeV/nucleon ⁴⁰Ar-⁵¹V, 50 MeV/nucleon ¹²C + ¹²C, and 50, 70, and 130 MeV/nucleon ¹³⁹La + ¹³⁹La. We observe that directed transverse momentum increases with decreasing mass of the system at energies around 50 MeV/nucleon. In addition the directed transverse momentum goes to 0 as the beam energy is decreased from 130 to 50 MeV/nucleon for La + La. We present results for multiparticle emission from the same systems in terms of global momentum space variables. We demonstrate that there are observable differences between sequential and simultaneous emission dynamics using global variables. Our results for 35 MeV/nucleon Ar + V fall closer to the sequential simulations than the simultaneous.

Results from the NA35 experiment at the CERN SPS are described in the context of possible formation and identification of a quark-gluon plasma (QGP). Evidence is presented that the initial energy density and temperature are sufficient for the QGP to be produced, and that hydrodynamic flow occurs in the expansion stage. Evidence for an unexpectedly large pion source size and for enhanced strangeness production is presented, and discussed in terms of QGP formation. Plans for experiments in 1990-91 with an expanded set up are presented, and prospects for a program with Pb beams at the SPS are discussed.

Canberra Semiconductor produces germanium and silicon solid state detectors for nuclear radiation. There exists an increasing demand from industrial and scientific customers to detect weaker gamma rays above the spectral background in a shorter measuring time. This has led germanium detector manufacturers to developments yielding lower detection limits and higher throughput rates. The new 2002 preamplifier has a shorter risetime yielding better timing resolutions and Gaussian timing peak shapes.

The CELSIUS ring at the Svedberg Laboratory, Uppsala, is now being commissioned. The Gustaf Werner cyclotron injects into the ring particles, ranging from protons to krypton ions, which are accelerated and made to interact with a variety of internal targets. Here, we will discuss some ring parameters, the internal target systems and the experimental programme, divided into intermediate-energy physics, elementary-particle physics and heavy-ion physics.

Thermal neutron absorption cross section of geological samples is determined with the steady state neutron source method. Cross section measurements of North Sea sediments demonstrate that also materials with high contents of clay minerals may be investigated with the steady state method.

The spectral structure of 245-445 keV electrons and positrons in positron - throium scattering was studied. The coincident positron and electron energy spectra were measured in a double beta spectrometer. A small peak was seen in addition to Bhabha scattering at an energy about 320 keV for both the electrons and the positrons.

A brief review of the state of the PIXE technique is given. Principles and recent advances in beam systems, instrumentation and sample handling are covered. A rather comprehensive list of references regarding different methodological aspects and biomedical applications is given. Some applications are discussed. In particular, preliminary results on a preconcentration technique developed by the authors for the determination of manganese in milk are presented.