Table of contents

Nordic meetings on nuclear physics have been held regularly in the various Nordic countries. The 4th Nordic meeting took place in August 16th-20th, 1982, at Fuglsø, situated in the "Mols mountains" in Jylland. The scientific nuclear physics program includes a broad spectrum of topics of current interest to nuclear physicists. The present issue of Physica Scripta covers nearly all the talks presented at the conference. The order of invited talks and contributions follows the conference program closely. We are pleased to have the opportunity to collect some of the highlights of nuclear structure and heavy ion reactions in one volume.

The meeting was sponsored by The Nordic Accelerator Committee, NORDITA, The Danish Natural Science Research Council and The Niels Bohr Institute. The financial support from these sources is gratefully acknowledged.

The practical organization of the conference was most professionally handled by Bitten Brøndum and Agnethe Elsving. We are indebted to J. Ross Sørensen and his staff at "Fuglsø Centret" for the excellent and comfortable conference and housing facilities.

We thank all of our speakers for a smooth cooperation.

Last, but not least, we express our gratitude to Nils Robert Nilsson for undertaking the responsibility in editing these proceedings.

Nuclei generate high spins by two methods, alignment of single particle angular momentum and collective rotation. The competition of these two modes is discussed for the highest spins 40 ≲ I ≲ 65 ℏ. Evidence is presented that alignment of the h_9/2 and i_13/2 proton orbitals from the next higher major shell produces large affects at high spins in rotational nuclei in the A = 160-166 region. It is suggested that such major shell effects produce the still larger irregularities known to occur in the lighter nuclei of this region.

Some features and quantum-numbers of one-particle motion in rotating potentials are summarized for "high-j" orbits and "normal-parity" orbits, respectively. As an example of collective aspects of rotating nuclei, the disappearance of pair-correlation at high-spin states is discussed especially in connection with available experimental information.

Two topics will be addressed in this paper. First, we discuss the variation of shapes with spin and neutron number for nuclei in the N ≃ 88 transitional region. Second, we present comments on the feeding times of very high spin single-particle yrast states.

Recent developments in the interpretation of high-spin, discrete-line data in terms of independent-particle motion in a rotating deformed nucleus are discussed. Particular attention is given to the interpretation of the residual interactions derived from a comparison of routhians corresponding to multiple-quasineutron configurations and those constructed from "experimental" values of the constituent single-quasineutron configurations. Similarly, the correlation between band-crossing frequencies in odd-N nuclei corresponding to the alignment of an i_13/2 pair of neutrons, and the shape of the valence quasineutron orbit is established for a variety of configurations. The correspondence between the residual-interactions and band-crossing frequencies is discussed, and the extracted estimate of the pairing reduction associated with the "blocking" of a single quasineutron is compared with BCS estimates for constant level spacings. Finally, some suggestions are made concerning the physical basis of the shift in the h_9/2 proton alignment frequencies between v_n = 2, π = + and v_n = 4, π = − bands in ¹⁸⁰Os.

Experimental information on multi-quasiparticle isomers is compared, with special emphasis on the hafnium isotopes. New results for ¹⁷⁴Hf are presented. The co-existence of deformation-aligning and rotation-aligning influences is considered. Residual interactions are studied as a function of the number of quasiparticles, with large effects being evident for large numbers of quasiparticles.

The N = 90 transition region is studied by gauge-space systematics for Nd through Yb nuclei and by the trend of transition quadrupole moments for ground-state bands. Lifetime measurements on quasiparticle bands in ^160,161Yb indicate a systematic decrease in the collectivity at higher rotational frequencies, indicating triaxial shapes. Spectroscopy measurements on ¹⁵⁹Tm are described. A change in the order of the signatures of the h_11/2 band after compared to before the back-bend is used to extract qualitative information on shape triaxiality. The systematics of 2 and 3 quasiparticle bands in N = 90 and 91 nuclei is presented and used to deduce the occurrence of a large-interaction crossing in ¹⁶⁰Yb and ¹⁶²Hf due to the alignment of negative-parity neutrons (h_9/2 + f_7/2).

A novel method to study nuclear structure at low spin as a function of temperature is developed and used on ^146,148Sm, ^154,156Gd and ^160,162Dy, by means of the (³He, α) reaction. The nuclear level density for a wide energy range is also studied. The γ-multiplicities and the first generation γ-ray spectra indicate a structure change in deformed nuclei at about 6 MeV.

Calculations for nuclei in the vicinity of ¹⁴⁶Gd based on a version of the deformed independent particle model are discussed. Particular attention is paid to systematic trends in the calculated deformations, deformation energies and pairing energies.

Data from recent (p, n) and (³He, t) experiments at intermediate energies are presented. The reactions show a spectacular selectivity for excitation of isospin-spin modes in nuclei. From the zero degree (p, n) spectra Gamov-Teller strength can be deduced, and only half the sumrule strength is observed. The very general nature of the sumrule for charge exchange modes suggests that internal degrees of freedom of the nucleon could be involved. The (³He, t) reaction at 2 GeV shows a strong excitation of the Δ-resonance, indicating that this resonance could be important for the isospin-spin excitations in nuclei.

Recent progress in obtaining detailed fine structure distributions of magnetic giant resonances in nuclei using high resolution inelastic electron scattering at low energy is discussed. Specific examples chosen are the medium heavy nuclei ^40,42,44,48Ca in which M1 excitations are due to neutron spin-flip transitions and the N = 28 isotones ⁵⁰Ti, ⁵²Cr and ⁵⁴Fe where in addition also proton excitations contribute to the measured M1 strength. It is found that the M1 strength in the investigated energy region is very fragmented and considerably quenched in comparison to predictions of shell model calculations in a model space that includes up to 2p-2h excitations. The amount of strength which might have escaped experimental detection is estimated with the help of a fluctuation analysis technique and the comparison of experimentally deduced strength distributions with a statistical one. The remaining quenching still leaves ample room for theoretical speculations and it is shown for two specific examples of M1 transitions in ⁴⁸Ca and ⁸⁸Sr that the Δ(1232)-hole model accounts for part of this quenching. Based on this mechanism the virtual Δ-isobar admixture in 1⁺ spin-isospin excitations in complex nuclei is estimated. Furthermore, the results from inelastic electron scattering are compared to the results from (p, n) charge exchange reactions exciting Gamow-Teller states and to recent high resolution (p, p') results. The similarities and differences in the excitation of spin modes is pointed out by using ⁴²Ca, ⁴⁸Ca and ⁵¹V as examples. Finally, the old problem of M1 strength in ²⁰⁸Pb is revisited and the results of a form factor measurement of a recently discovered low lying J^π = 1⁺ state by nuclear resonance fluorescence are presented.

A critical discussion is given of several recent attempts to introduce dynamical content in the shell model by means of adding corrections to the Hartree-Fock approximation. Numerical results are given single-particle neutron energies in ²⁰⁸Pb.

The damping of single-particle and giant resonances is studied. Doorway states containing low-lying surface vibrations are found to play a central role in this process. The coupling to these states lead to damping widths consistent with the empirical systematics. It is however not possible to directly relate these two quantities, because of the central role played by the correlation between the particle and the hole in the vibration.

This talk contains a survey of the efforts that a group in Copenhagen has made in recent years in order to establish a firmer basis for the understanding of heavy ion reactions at low bombarding energies. The topics cover phenomena from the calculation of the absorptive potential and excitation of giant resonances to fusion and critical angular momenta for fission.

Experimental data and statistical-model calculations for xn and αxn products of the reaction ²⁰Ne + ¹⁴⁶Nd at 136 MeV are shown to be in generally good agreement, indicating that equilibrium processes are dominant. Preliminary results on the heavy-ion ejectiles from ¹⁹F + ¹⁵⁹Tb are presented.

The nucleon-exchange model for damped nuclear reactions is briefly reviewed in the framework of macroscopic transport theory. Recent developments are discussed and some future directions indicated.

High energy nucleus-nucleus collisions can provide a unique method of studying nuclear matter at densities and excitations far from the normal ground state. In these collisions a source can be identified with a velocity intermediate between the projectile and target velocities as well as sources that can be associated with decay of excited projectile and target nuclei. The study of light particle spectra (p, d, t and alpha) associated with this intermediate source can provide information on the equation of state of nuclear matter. Results are presented for O and Ne induced reactions on heavy targets ranging in energy from 9 to 800 MeV/nucleon. Analysis of these light particle data in terms of a single moving source model leads to a consistent description of a system in thermodynamic and chemical equilibrium. Extracted d/p and t/p ratios are almost independent of incident energy while the alpha/proton ratio increases dramatically at low bombarding energies.

Recent theoretical attempts to study high energy heavy ion reactions are reviewed. Special emphasis is given to the description of the dynamical evolution and to the production of pions and light fragments in the collision. Evidence for a collective quasi-hydrodynamic flow behaviour of compressed nuclear matter is presented. We show that the pion yield is sensitive to the nuclear equation of state and to the viscosity in hot dense nuclear matter. A 4π exclusive flow analysis of intermediate energy nuclear collisions can be used to reveal the compressibility coefficient of nuclear matter.

Nuclear reactions induced by intermediate energy heavy ions are discussed. The observation of large nuclear surface transparency and of nuclear rainbow scattering in elastic scattering of ¹²C ions is presented. The data obtained for projectile-like fragments are compared with corresponding results from collisions involving relativistic heavy ions. Finally, a model for the production of light particle spectra emitted in heavy ion reactions is suggested and compared with the experimental data.

Some selected aspects of nuclear reactions at beam energies below a couple of GeV/nucl. are reviewed. The different dynamical regimes from the micro- to the macro-dynamics and the late expansion stage are discussed and compared with respect to their implications on specific observable like inclusive data or global event observations.

A review of a hadrochemical and quarkochemical model of relativistic heavy ion reactions is presented. It is pointed out that the minimal bombarding energy necessary to create baryonic gluon plasma is about 6 GeV/nucleon. The signature of the quark matter is also investigated in the strange meson production.

The negative pion multiplicity is measured for inelastic and central collisions of ⁴⁰Ar with KCl at eight energies from 0.36 to 1.8 GeV/nucleon and for ⁴He and ⁴⁰Ar on BaI₂ at 977 and 772 MeV/nucleon, respectively. A systematic discrepancy in central collision data with a cascade model calculation which fits proton- and pion-nucleus cross sections but omits potential energy effects is used to derive the energy going into bulk compression of the system. A value of the compressibility constant of K = 240 MeV is extracted in a parabolic form of the nuclear matter equation of state.

The aim of nuclear fusion research is to make fusion energy available as a new energy source. Fusion processes occur naturally in the sun, where hydrogen nuclei release energy by combining to form helium. A fusion reactor on earth will require even higher temperatures than in the interior of the sun, and it will be based on deuterium and tritium reactions.

JET (Joint European Torus) is a major fusion experiment now under construction near Abingdon in the UK It is aimed at producing conditions approximating those necessary in a fusion reactor. The results expected from JET should permit a realistic evaluation of the prospects for fusion power and serve as a basis for the design of the next major fusion experiment.

The programme of LEAR, the new facility for low energy antiprotons at CERN, is described. Sixteen initial experiments will explore: states in the proton-antiproton system, annihilation, search for glueballs, structure of the proton and interaction of antiproton with nuclei.

A talk held at the 4th Nordic Meeting on Nuclear Physics.

The two-phonon 0⁺ strength in ⁶⁴Zn has been searched for by obtaining experimental results for the E0 and E2 transitions depopulating the excited 0₂⁺ and 0₃⁺ states. The previous identification of the 0₂⁺ state in ⁶⁴Zn at 1910.3 keV as a two-phonon state is completely washed out through the present experimental results: B(E2;0₂⁺ → 2₁⁺) = 0.058(3) W.u., B(E2;0₂⁺ → 2₂⁺) = 62(8) W.u. and ρ²(E0;0₂⁺ → 0₁⁺) = 3.9(4) × 10^-3.

Branching ratios of E0 and E2 transitions depopulating excited 0₂⁺ states in ^70,74Ge and ^74,76Se have been measured using conversion-electron and gamma-ray spectroscopy, and new half-life measurement techniques. The E0/E2 branching ratios and the life-times of the 0₂⁺ states are used to extract the B(E2; 0₂⁺ → 2₁⁺) and ρ²(0₂⁺ → 0₁⁺) values.

A multi-step shell-model method is applied to calculate up to four neutron holes in the core ¹³²Sn. It is found that most low-lying states in the three- and four-hole nuclei are members of multiplets arising from the coupling of one- and two-hole states.

States in ¹³⁰Ce have been studied using ¹⁶O induced reactions on ¹¹⁷Sn and ¹¹⁸Sn targets. Mean lifetimes have been measured for the yrast states up to J^π = 22⁺ using both the Doppler shift attenuation and recoil distance methods. Magnetic dipole transition rates have been deduced from branching ratio measurements within a band based on an isomeric state at 2454 keV. Electric quadrupole transition rates have been determined from mean lifetime measurements for states in the yrast band up to J^π = 22⁺. Both sets of results show an unexpected behaviour.

High-K rotational bands may correspond to classical wobbling, which is a more complex motion than rotation around an intrinsic axis and therefore can in general not be described by the cranking model. Two microscopic models of wobbling are developed and found to give compatible results. One diagonalizes the response of a valence shell to a wobbling rotational core. The other is RPA with the field F ∼ [H_N,J₊], which turns out to be a major improvement over F ∼ r²Y₂₁ employed in previous calculations. General features seem to be < _rigid or under many circumstances a fragmentation of the collective mode. The highest-K data available, from a recent experiment on ¹⁷⁹W, are reproduced without parameter fitting. Recent quasicontinuum data on ¹⁵⁸Yb are discussed.

Gamma cascades through high-spin configurations with high seniority are found to feed the t_1/2 = 710 ns isomer in ¹⁷⁹W. The abnormally short half life for the isomeric state itself, which decays into the K = 7/2 ground-state rotational 31/2 member, violating the K-selection rule twelve times, is explained as a result of an accidental degeneracy in energy between the K^π = 35/2^- isomeric state and the 35/2^- groundband rotational state. Above the isomer, the yrast structure is dominated by deformation aligned states.

The very high spin behaviour of nuclei with ∼ 10 particles outside the ¹⁴⁶Gd core are studied. Our method of calculation, which is based on the cranked Nilsson-Strutinsky formalism, makes possible to follow individual configurations as functions of spin. We concentrate on ¹⁵⁸Er and ¹⁵⁴Dy which nuclei become oblate with rotation around the symmetry axes for I = 40-50. It is found that most such oblate states can be described as band terminations, i.e., they correspond to the maximum spin which can be formed if the distribution of nucleons over the j-shells is kept fixed. A strong argument which supports this conclusion is the M1-bump which has recently been observed for ¹⁵⁸Yb. Such an M1-bump seems to be a natural consequence of the terminating bands. A further consequence is that these bands appear to prohibit the formation of yrast isomers for spins much larger than I = 40.

Theoretical arguments, based on the pairing-selfconsistent independent quasi-particle model, are presented to support the idea of a collective transition from the superfluid to the normal phase (Mottelson-Valatin effect) in selected actinide nuclei. Detailed calculation results are in good agreement with existing experimental data. The strong relationship between the nucleonic angular momentum alignment and the disappearance of pairing correlations in nuclei is also discussed. It is suggested that a Mottelson-Valatin type mechanism dominates in the excitation pattern of ^232-238U, while in the neighbouring thorium and plutonium nuclei the influence of nucleonic alignment and of a band crossing on the high spin properties seems more pronounced.

The decay of the entry states in ¹⁵⁸Yb populated in the reactions of 149 MeV ²⁰Ne with ¹⁴⁴Nd and ¹⁴⁶Nd has been investigated with a 4π multidetector system gated by a Ge counter. The average excitation energy, the γ-ray spectra and the angular distributions as a function of multiplicity show several changes in the γ-ray decay. These changes suggest a transition from prolate to particle aligned oblate configuration at low spin. At I ∼ 38-48 collective transitions with dipole and quadrupole component possibly built on high K single particle states are observed. Furthermore, above I ∼ 48 the dipole component disappears suggesting a further change toward more triaxial shape.

The average properties of the gamma ray entry region and the decay from it are studied systematically, for 49 nuclear systems, in the spin spectrometer. Preliminary results are given for the mass the neutron number dependence of the gamma ray fold distribution and of unresolved γ spectra. The possibility of gating simultaneously on narrow regions of fold and excitation energy is exploited.

γ-transitions preceding the fission decay of the shape isomers in ²³⁸U and ²⁴⁰Pu have been measured with the crystal ball. Lines at 616 keV and 808 keV are observed in ²³⁸U and ²⁴⁰Pu, respectively, which are tentatively interpreted as deexcitations of the β-vibrational band in the second minimum.

The relevant pieces of the collective transport theory is briefly sketched. Then numerical results for the fission and pairing degrees of freedom of the nucleus ²³⁸U are presented. Quadrupole and hexadecapole vibrations around the equilibrium deformation are extracted. The β- and giant quadrupole vibrations are identified. One hexadecapole and two new quadrupole vibrations are predicted. Pairing vibrations are obtained and the number of broken pairs during descent from saddle to scission is estimated.

The isospin symmetry breaking noted in pion inelastic scattering on ¹³C has been shown previously to be consistent with helium-ion scattering and charge exchange data. Here, proton scattering and charge exchange data at a beam energy of 26.1 MeV are shown to exhibit rather different features. Since the great isospin asymmetries are found for stretched-spin states very near the proper nucleon decay thresholds, it is suggested that external mixing is responsible for the consistent observations for strongly absorbed projectiles, but that nucleon beams are sensitive to different radial locations and so the proton-induced data may not be related simply to more surface dominated data.

Radioactive decay of T_z = - 3/2 nuclides, ²⁷P and ³¹Cl, predicted to be weak β-delayed proton precursors, have been looked for. Two proton groups having energies of 989 ± 15 and 1528 ± 20 keV and a half-life of 150 ± 25 ms were associated with the decay of ³¹Cl. The search for ²⁷P is still under way, but the first qualitative results are briefly discussed.

We derive a sum rule for the Gamow-Teller transition strength allowing for internal excitations of nucleons. The effect of the Δ(1236) isobar in the final state is to reduce the total strength to 36% of the purely nucleonic value. However, separately the strengths for opposite charge changes are both enhanced. We also discuss the effect of Δ admixtures in the initial state and find the change in the sum rule to be very small.

The emission of charged particles in coincidence with γ-rays from the ¹²C + ¹¹⁸Sn system at E(¹²C) = 118 MeV has been studied. Energy spectra and relative cross sections for He, Li, Be and B ejectiles have been measured. The results are compared to the sum-rule model of Wilczynski et al.

Excitation functions for elastic scattering and reactions of ²⁸Si + ²⁸Si show evidence for the existence of narrow (Γ = 100-200 keV) high-spin (J ≃ 40 ℏ) resonances. The interpretation of these resonances as high-spin fissioning shape isomers is discussed with reference to data for the ²⁸Si + ³⁰Si and ³⁰Si + ³⁰Si systems.

Experimental data on pion production in heavy ion reactions at energies below 100 A MeV is presented. The inclusive spectra of π⁺ and π^- from reactions at 50-100 A MeV are in general well reproduced by a first-nucleon-nucleon scattering mechanism when diffused internal momentum distributions, Pauli blocking, and reabsorption is considered.

Heavy target nuclei split into many fragments when bombarded by protons or heavy ions at very high energies (E_p > 10 GeV). A statistical model is presented, which calculates the relative phase-space available for the various multifragment decay channels. It is shown that the Coulomb-interaction between the fragments is crucial. Including it by a self-consistent mean field method leads to a significant enhancement of the survival of the most heavy fragments and a strong suppression of the probability for splitting into many medium fragments. At a temperature of ∼ 5 MeV an U-shaped mass-yield curve results from the model calculations for most of the heavy targets, whereas they show a clear fission peak for U-targets. Over the whole fragment-mass region the experimental mass-yield distributions for limiting fragmentation can thus be reproduced. Further investigations and refinements of the model are needed before a final judgement about the importance of the multi-fragment phase-space relative to dynamical effects is possible.

Some simple reaction mechanisms for fragmentation of nuclei at relativistic energies are discussed. The inclusive spectra of fragments in longitudinal and transverse momenta give valuable information about the momentum distribution of a nucleon inside the fragmenting nuclei as well as effects of the nuclear mean field. We also report on preliminary results from the experiment R418 at the CERN ISR, where pα and αα reactions were studied at CM energies: √s ≈ 88 and 125 GeV respectively. By examining two-particle correlations we can study the process of simple fragmentation and the so called hard-scattering process.