abstract

The statistical Thomas–Fermi model is applied to a comprehensive survey of macroscopic nuclear properties. The model uses a Seyler–Blanchard effective nucleon–nucleon interaction, generalized by the addition of one momentum-dependent and one density-dependent term. The adjustable parameters of the interaction were fitted to shell-corrected masses of 1654 nuclei, to the diffuseness of the nuclear surface and to the measured depths of the optical model potential. With these parameters nuclear sizes are well reproduced, and only relatively minor deviations between measured and calculated fission barriers of 36 nuclei are found. The model determines the principal bulk and surface properties of nuclear matter and provides estimates for the more subtle, Droplet Model, properties. The predicted energy vs. density relation for neutron matter is in striking correspondence with the 1981 theoretical estimate of Friedman and Phandaripande [1]. Other extreme situations to which the model is applied are a study of Sn isotopes from \(^{82}\)Sn to \(^{170}\)Sn, and the rupture into a bubble configuration of a nucleus (constrained to spherical symmetry) which takes place when \(Z^2/A\) exceeds about 100.

abstract

The new gamma-ray detector array, Gammasphere, is described and compared with other new arrays. The physics from this detector system is discussed briefly and two topics are considered in greater detail. The first topic is “identical bands” where it is shown that the similarities between these bands axe not accidental, but also that it is a phenomenon which is not yet understood. The second topic discusses the study of nuclear structure in neutron-rich nuclei formed in deep-inelastic reactions.

abstract

The progress in \(\gamma \)-spectroscopy is discussed with special emphasis on the transition region in excitation energy, from the rather well known cold nuclei characterized by regular rotational motion built on specific single particle configurations with well defined quantum numbers, to the region above 800 keV in heat energy, where the rotational motion is damped, as a result of a spreading of the rotational transition strength, due to residual interactions acting between the close lying bands. A general analysis scheme is advocated, aiming at a more direct comparison between the experimental results and theoretical models to give a more complete picture of nuclear structure going from cold to warm nuclei. A simulation model for describing the rotational and statistical decay of high spin states has been developed based on microscopically calculated energies and the spreading of rotational transition strength due to residual interactions. The calculations predict that scars of regular rotational motion may be found at excitation energies as high as 2 MeV above yrast, far into the region characterized by damped rotational motion. Traces of this new phenomenon are identified and studied in the experimental spectra from high spin states in rare earth nuclei. A prominent example of isolated structures in the quasi-continuum region are bands built on high-K quantum numbers. Recent results from a fluctuation analysis of counts in low-K and high-K configurations in \(^{163}\)Er are discussed, and indicate that states in the quasi-continuum feeding high-K bands are different from states feeding the low-K bands.

abstract

Some of the single-nucleon configurations in the rotating nuclear potential appear to be almost insensitive to nuclear rotation. The contributions both to the angular momentum alignment and to the dynamical moment of inertia are almost negligible for these special orbits. It is suggested that the appearance and properties of the identical bands discovered in superdeformed regions depend crucially on the population of such special orbits. Examples of the resulting identity relations between various superdeformed bands in nuclei are discussed.

abstract

The importance of the ground-state shell structure for the formation and stability of excited exotic nuclear configurations is discussed in terms of the multicluster model based on dynamical symmetries of a three-dimensional harmonic oscillator. As a spectacular example, it is shown that the density distribution at the third hyperdeformed minimum in the actinide nuclei resembles a di-nucleus consisting of a nearly-spherical fragment around the doubly-magic \(^{132}\)Sn, and a well-deformed fragment from the neutron-rich \(A \sim 100\) region.

abstract

New features in superdeformed nuclei observed with the new generation of large gamma-ray detector arrays are presented. The first part gives for the first time a microscopic scenario for the decay of a superdeformed band in the mass 130 region. The second part presents experimental evidence for a new type of symmetry, namely C\(_4\) symmetry, in superdeformed nuclei. A phenomenological study of how the staggering occurs for a Y\(_{44}\) deformation is discussed.

abstract

Transitions linking the highly deformed band to low deformation states have been established in \(^{137}\)Nd, \(^{137}\)Sm, \(^{139}\)Sm and \(^{139}\)Gd with experiments performed at GASP. Whereas in \(^{137}\)Nd the linking transitions account for 25% of the band intensity, in the other three nuclei almost 100% of it is found. The data prove that the HD bands are built on the \(\nu i_{13/2}\)[660]1/2\(^+\) intruder orbital. In all cases, the sudden termination of the HD band is explained through the disappearance of the second minimum in the potential energy surface. A wide systematic of excitation energies and spin is now available for detailed comparison with calculations.

abstract

The features of the superdeformed band in \(^{144}\)Gd have been studied. An \(i_{13/2}\) proton crossing has been observed in \(^{144}\)Gd which is absent in the isotope \(^{146}\)Gd. This has been explained as a deformation effect. The absence of this crossing in the superdeformed bands of the neighbouring \(N=80\) isotones of \(^{144}\)Gd can be understood in terms of a blocking effect.

abstract

Comprehensive spectroscopic studies of even-mass Cd, Sn and Te nuclei have been carried out within the Jyväskylä–Uppsala collaboration. A special emphasis is put on the systematics of collective intruder states and the role of proton excitations across the \(Z=50\) gap in these states.

abstract

Systematics of bandcrossing frequencies, \(\hbar \omega _c\), are presented for the \(\pi h_{9/2}\)[541 1/2\(^-\)] Nilsson intruder configuration and contrasted with the (0,+) yrast configuration in the even–even rare-earth nuclei. Predictions from Cranked Shell Model calculations, based on frequency diabatic configurations, are compared with the experimental data.

abstract

New results have been obtained on the dipole bands in \(^{194}\)Pb using the EUROGAM-1 spectrometer array. The two dipole bands recently observed have been extended to higher spin and excitation energy. Band crossing is observed at the top of one band, which leads to a new configuration based on six quasi-particle excitation. The configuration assigned to the second band is confirmed at higher spin.

abstract

Lifetimes have been measured in the \(\pi h_{9/2}\) (yrast) and \(\pi i_{13/2}\) (excited) bands in the nuclei \(^{181}\)Ir and \(^{187}\)Au using the Recoil Distance Method (RDM). The results clearly indicate that the \(\pi i_{13/2}\) band exhibits an increased deformation over the \(\pi h_{9/2}\) band, in keeping with the premise that the delayed crossing in this band is due to enhanced quadrupole deformation.

abstract

Level schemes of \(^{146}\)Sm and \(^{144}\)Nd have been studied with GASP. New evidence of collective octupole excitations coupled to low lying shell-model configurations, similar to those of \(^{148}\)Gd, has been obtained.

abstract

The origin of the \({\mit \Delta }I=4\) staggering effect which has recently been observed in several superdeformed bands is discussed. The Hamamoto and Mottelson model, which is based on a phenomenological parametrization of the Hamiltonian as a quartic function of angular momentum, is analyzed. A stability of the description with respect to the \(C_4\)-symmetry breaking terms is studied.

abstract

A detector (RFD) has been developed that measures evaporation residues in coincidence with \(\gamma \)-rays detected in a Ge-array. Evaporation residues are distinguished by time of flight from any other reaction products with different velocity, like from fission and reactions with light target impurities, and the background from these processes is eliminated. Also the energy of the \(\gamma \)-rays can be Doppler corrected event by event with the measured velocity vector of the emitting nucleus improving the resolution. Recent results of experiments with OSIRIS at VICKSI on \(^{189}\)Pb and nuclei around doubly magic 56Ni, that show the performance of the detector, are presented and the perspectives of the RFD application with EUROBALL are discussed.

abstract

A gas-filled recoil separator for studies of heavy elements produced in heavy-ion-induced fusion reactions has been constructed. New neutron-deficient isotopes with \(Z=85 - 90\) have been identified through their alpha decay.

abstract

The reasons for our interest in nuclei with \(N=Z\) are outlined. The methods used to study the \(N \sim Z\) nuclei up to \(A=100\) and the state of our knowledge of their properties are reviewed. Future experiments, involving both existing facilities and radioactive nuclear beams, designed to improve our knowledge of nuclei near the \(N=Z\) line are described. Finally the Radioactive Ion Source Test (RIST) project, designed to pave the way for a full-scale radioactive beam facility at the ISIS spallation neutron source, is described and its progress reported.

abstract

The present status of experimental approach to \(^{100}\)Sn and its shell model structure is given. New developments in experimental techniques, such as low background isomer spectroscopy and charged particle detection in \(4\pi \) are surveyed. Based on recent experimental data shell model calculations are used to predict the structure of the single- and two-nucleon neighbours of \(^{100}\)Sn. The results are compared to the systematics of Coulomb energies and spin-orbit splitting and discussed with respect to future experiments.

abstract

Binary reaction products of \(^{92}\)Mo+\(^{60}\)Ni, \(^{106}\)Cd+\(^{54}\)Fe, \(^{124}\)Sn+\(^{80}\)Se, \(^{76}\)Ge, \(^{208}\)Pb+\(^{64}\)Ni and \(^{160}\)Gd+\(^{36}\)S, \(^{37}\)Cl heavy ion collisions 10–15% above the barrier have been studied in \(\gamma \)-ray thick target experiments. The product yield distributions have been obtained for some of the reactions from \(\gamma \)–\(\gamma \) coincidence intensities as well as from target radioactivity measurements. The mass transfer between colliding nuclei is discussed in terms of mass and charge equilibration processes. These deep inelastic processes were found to populate a large number of neutron-rich nuclei strongly enough for yrast spectroscopy studies. New spectroscopic results are presented for the \(sdf\) shell nuclei \(^{33}\)Si, \(^{34}\)P, \(^{39}\)Cl, for \(^{64,65,66,67}\)Ni products and for heavy tin isotopes \(^{119,121,122,123,124}\)Sn. Also yrast excitations in the \(^{68}\)Ni have been identified showing a substantial subshell closure at neutron number \(N=40\).

abstract

Nuclear reactions \(^{180}\)Ta\(^m(\gamma ,2n)\) \(^{178}\)Ta\(^{m,g}\), \(^{178}\)Hf\(^{m_2}\) \((n,\gamma )\) \(^{179}\)Hf\(^{m_2}\) as well as the Coulomb excitation of a \(^{178}\)Hf\(^{m_2}\) target were newly studied. The results are promising for the progress in nuclear structure and nuclear reaction understanding.

abstract

The energy-corrected intensities of the primary \(\gamma \)-transitions in \(^{168}\)Er, \(^{178}\)Hf, \(^{166}\)Ho, \(^{176}\)Lu, \(^{182}\)Ta and \(^{177}\)Lu following thermal and average resonance neutron capture are calculated from data available in the literature. The data reveal a significantly lower average transition rate for \(K\)-forbidden than for \(K\)-allowed primary transitions. The effect is more pronounced in the data from thermal neutron capture than in the data from 2 keV neutron capture.

abstract

The Eurogam-1 array has been used to study \(\gamma \) rays emitted following the spontaneous fission of a \(^{248}\)Cm source. The stopping of the fission fragments in the source material leads to Doppler-broadened \(\gamma \)-ray lineshapes for those states that have lifetimes comparable to the stopping time. From the analysis of these lineshapes the transition quadrupole moments for the Yrast states in the neutron-rich nuclei \(^{100,102}\)Zr and \(^{104,106}\)Mo in the spin range 6–12 \(\hbar \) have been deduced.

abstract

The transition of the level density parameter \(a_{\rm eff}\) from the low excitation energy value \(a_{\rm eff}=A/8\) MeV\(^{-1}\) to the Fermi Gas value \(a_{\rm FG} \sim \) A/15 MeV\(^{-1}\) was discovered a few years ago in studying particle spectra evaporated from hot compound systems of \(A \sim 160\). A number of experiments have been recently performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Difficulties have been evidenced in extracting the nuclear temperature from the slope of the particle spectra because of angular momentum induced effects at relatively low excitation energy and of the onset of cluster and IMF emission at excitation energies \(\epsilon \geq 3\) MeV/u. In the mass region \(A \sim 110\), the transition to the FG value has been found at an excitation energy per nucleon larger than that for \(A \sim 160\), as expected from theoretical predictions. For lighter mass nuclei, experiments do not show a departure from the \(a_{\rm eff}=A/8\) MeV\(^{-1}\) value. On the contrary the need of an excitation energy dependent level density parameter has been evidenced in the mass region \(A \sim 200\). The effect of this level density on the statistical model predicted pre-scission multiplicities is very important. Deformation induced effects on proton spectra have been also searched for. Experimental proton spectra do not show remarkable differences setting gates on bands characterized by different deformation (oblate,prolate,SD) in the \(^{152}\)Dy residual nucleus.

abstract

The recent progress made in the study of the Giant Dipole Resonance in hot rotating nuclei in the medium mass region with mass number \(A=110\) and 170 is discussed. Particular emphasis is given to experimental studies of the spectral and angular distributions at temperatures between 1 and 2 MeV. The mechanisms that play a role in the width of the giant dipole resonance state are the nuclear shape and orientation and their fluctuations and the collisional damping, the latter found to be basically independent of temperature and rotational frequency.

abstract

The GDR gamma decay of highly excited \(^{272}\)Hs and \(^{269}\)Ns nuclei and their evaporation daughters was studied in coincidence with fission fragments. A difference technique was used to isolate the pre-fission component. Strong dipole collectivity was observed. The lifetime of the hot superheavy nuclei is estimated.

abstract

High-energy photon production in heavy-ion reactions at projectile energies around 10 MeV/u is discussed in the context of bremsstrahlung studies at \(E_p/A \geq 20\) MeV/u. Recent results on the \(^{12}\)C + \(^{26}\)Mg reaction at 6, 8.5 and 11 MeV/u and the \(^{12}\)C + \(^{24}\)Mg reaction at 11 MeV/u studied by the Warsaw-Seattle-Cracow collaboration are summarized.

abstract

The level density of low-spin states (0–10 \(\hbar \)) in \(^{162}\)Dy has been determined from the ground state up to approximately 6 MeV of excitation energy. Levels in the excitation region up to 8 MeV were populated by means of the \(^{163}\)Dy(\(^3\)He,\(\alpha \)) reaction, and the first-generation \(\gamma \)-rays in the decay of these states have been isolated. The energy distribution of the first-generation \(\gamma \)-rays provides a new source of information about the nuclear level density over a wide energy region. A broad peak is observed in the first-generation spectra, and we suggest an interpretation in terms of enhanced M1 transitions between different high-\(j\) Nilsson orbitals.

abstract

A difference method has been applied to determine the first chance GDR photon emission accompanying a deexcitation of the hot compound systems of \(^{162}\)Yb and \(^{158}\)Dy produced in heavy ion fusion reactions with 3 various projectile-target combinations. The GDR difference yield has turned out to be surprisingly small for the heaviest projectiles (Ti) being in dramatic conflict with the statistical model (code CASCADE) prediction, while for the lightest beam (O) an agreement with the model has been observed. For the intermediate (Si) projectiles a strong dependence of the difference yield on the transferred angular momentum has been noticed. With increasing I the difference yield decreases and reaches zero for the highest transferred \(I\) (50 \(\hbar \)). Recently additional checks with better definition of the fusion events applying the PPAC and CS HPGe detectors have been performed.

abstract

A recent exclusive measurement of the \(\gamma \) decay of the Giant Dipole Resonance in hot \(^{176}\)W nuclei is here presented. For the first time the two observables, strength function and angular anisotropy coefficient \(a_2(E_{\gamma }\)), were measured at a temperature of \(T=1.5\) MeV at different values of the angular momentum of the compound nucleus, spanning the interval from 35 up to 55 \(\hbar \). At this temperature shell effects are almost completely washed out and this nucleus shows deformations that are smaller than the ones at zero temperature. The \(a_2(E_{\gamma }\)) and GDR width data are very well reproduced in all the selected spin windows, by predictions based on thermal shape and orientation fluctuations and using the collisional damping width at zero temperature.

abstract

The Giant Dipole Resonance (GDR) from the decay of excited \(^{156}\)Er nuclei has been studied with the GASP spectrometer. The correlations between the emission of energetic gamma rays, the fold \(k\) distribution and low energy discrete gamma lines have shown that standard statistical calculations can be used to describe not contaminated GDR gamma spectra.

abstract

The “Adding Mode” technique has been applied to measure the response of a HPGe (82%) detector with a BGO anti-Compton shield for the 15.1 MeV \(\gamma \)-rays produced by the reaction D(\(^{11}\)B,n\(\gamma \))\(^{12}\)C. The measured energy resolution for the 15.1 MeV \(\gamma \)-rays emitted by \(^{12}\)C is 209 keV and the efficiency relative to a measurement in a “Suppressed Mode” increased of an order of magnitude. Due to the recoil velocity (\(\beta \simeq 0.05\)) of \(^{12}\)C*, the Doppler broadening contribution to the measured FWHM is 140 keV. The experimental results has been compared to simulated spectra calculated with the GEANT libraries. The effects that a segmentation of an HPGe crystal would have on energy resolution is also discussed.

abstract

A brief history is given of silicon detectors leading up to the development of ion-implanted strip detectors. Two examples of their use in low energy nuclear physics are discussed; the search for exotic alpha-chain states in \(^{24}\)Mg and studies of anomalous positron-electron pairs produced in collisions of very heavy ions.

abstract

Recent experimental investigations on low-energy heavy-ion reaction dynamics performed at Legnaro are reviewed. A short description is given of the setup which enables the study of elastic scattering and of quasi-elastic transfer reactions, as well as of fusion reactions. After a brief hint on the perspectives in the field of multinucleon transfer, the main part of the lecture is dedicated to the fusion reactions for which recent developments, like the studies of barrier distributions and the theoretical approach using the Interacting Boson Model, have led to a renewed interest. Some results obtained by our group are presented for the systems \(^{32}\)S + \(^{58,64}\)Ni, \(^{16}\)O + \(^{194}\)Pt (sensitivity to the target deformation) and \(^{58}\)Ni + \(^{60}\)Ni (evidence for multiphonon excitation in subbarrier fusion).

abstract

Fusion cross sections and mean angular momenta measured for the five systems \(^{16}\)O+\(^{112}\)Cd, \(^{28}\)Si+\(^{90,100}\)Mo and \(^{58,64}\)Ni+\(^{64}\)Ni can be explained in the framework of the coupled channels (CC) approach. In particular the influence of 2n-transfer channels with positive \(Q\)-values shows up. The direct relation between excitation function and angular momenta has been proved by a simple analytic relation. On this basis a new way for extracting fusion barrier distributions \(D(B)\) out of complete angular momentum distributions is presented.

abstract

The antiproton beam from the Low Energy Antiproton Ring at CERN was recently used in a series of nuclear physics experiments performed by Berlin–Caen–Moscow–Munich–Orsay–Rossendorf–Warsaw collaboration. This paper presents some of the obtained results. In particular a new method of the nuclear periphery study using the annihilation of stopped antiprotons is presented. Also, simple as well as highly exclusive experiments studying the heating of nuclei by antiprotons are discussed.

abstract

Recently, the study of pion production in few-nucleon systems has advanced considerably, mainly due to experiments using internal targets in stored, cooled beams. Experimental progress has inspired theoretical insights: from measurements close to threshold we have learned that an enhancement of the axial charge caused by heavy-meson exchange significantly contributes to pion production. The study of the production of pions, as well as heavier mesons, with stored polarized beams and polarized internal targets is likely to have important consequences for our understanding of the nuclear force.

abstract

GEM is a detector with high spatial and energy resolution making it ideally suited for studies involving meson production close to threshold. The investigations proposed include study of the reaction mechanism of light mesons and the their interaction with nuclei. In a first experiment the \(p+p \to d+ \pi ^+\) reaction close to threshold was measured employing the magnetic spectrometer Big Karl together with dedicated detectors. A (preliminary) result is an anisotropy similar to the one in the time reversed reaction.

abstract

We study \(\gamma \), \(\pi \)- \(\eta \)- and vector-meson production in heavy-ion collisions. The dynamical evolution of the nucleus–nucleus collision is described by a transport equation of the Boltzmann–Uehling–Uhlenbeck type evolving phase-space distribution functions for nucleons, \(\Delta \)’s, \(N\)(1440)’s, \(N\)(1535)’s, \(\pi \)’s and \(\eta \)’s with their isospin degrees of freedom. Besides the dominant production of hard-photons in first chance n-p collisions a significant production of hard-photons in a later stage of heavy-ion collisions is predicted by BUU theory. The calculation predicts that the production of these photons strongly depend on the compressibility of the nuclear matter. Experimental indications for their production are found in the hard-photon energy spectra and the photon–photon correlation function measured at GANIL at energies 30–60.0 MeV/u. Furthermore, we present results for meson production in heavy-ion collisions at SIS energies. We show that the observed \(\pi \)’s and \(\eta \)’s are emitted dominantly at low density, on the other hand, vector-mesons provide a signal from the dense region of the reaction. They also contribute significantly to the dilepton invariant mass spectrum.

abstract

Effective meson Lagrangians are the appropriate theoretical tool for the non-perturbative regime of QCD. We introduce the concept of effective Lagrangians and apply it to meson–meson and meson–nucleon scattering. It is shown that with these information one is able to calculate also form factors of mesons and baryons. The same model can be applied to the calculation of quark distributions in baryons and mesons. The recently observed \(\bar u\)–\(\bar d\) asymmetry is explained within the meson–cloud model in a natural way.

abstract

Position measurements of high accuracy are required for ray tracing of relativistic heavy ions in various nuclear structure experiments planned at GSI. Position resolutions down to 0.1mm are required to allow for a transverse momentum measurement from which differential cross-sections for peripheral heavy ion collisions can be determined. We have explored the capability of two types of detectors, silicon-microstrip and scintillation-fiber detectors, prototypes of which were tested using an \(^{238}\)U beam of 600 A\(\cdot \)MeV and an \(^{40}\)Ar beam of 200 A\(\cdot \)MeV energy.

abstract

The GEM-detector system was developed to investigate meson production and meson–nucleus interaction with the external proton beam of the COSY accelerator at Jülich. GEM is a hybrid system consisting of the GErmanium Wall and the modified Magnetic spectrometer BIG KARL. The Germanium Wall is a stack of up to four annular position-sensitive semiconductor detectors made from high purity germanium. Its design makes GEM a 4\(\pi \)-detector system for recoiling nuclei from reactions close to the production threshold [1]. In this contribution the design and the actual status of the Germanium Wall is described.