### Regular Series

#### Vol. 28 (1997), No. 1 – 2, pp. 9 – 456

The Nuclear Thomas–Fermi Model with Angular Momentum: Fission Barriers, Superdeformations, Moments of Inertia

Acta Phys. Pol. B 28, 9 (1997)

page 9 •

abstract

We added a rotational energy to our Thomas–Fermi nuclear model of W.D. Myers, W.J. Świątecki, Acta Phys. Pol. B27, 99 (1996) and W.D. Myers, W.J. Świątecki, Nucl. Phys. A601, 141 (1996) and present here a survey of fission barriers, deformation energies, $\gamma$-ray rotational cascades and moments of inertia. We explore a hypothesis according to which the moment of inertia of a deformed nucleus can be estimated by subtracting from the rigid-body value the moment of inertia of an inscribed sphere.

Properties and Synthesis of Heaviest Nuclei

Acta Phys. Pol. B 28, 21 (1997)

page 21 •

abstract

Theoretical studies on the properties of heavy and superheavy nuclei, performed in recent years, are shortly reviewed. Such properties as mass and half-lives are discussed. Even-even nuclei with proton number $Z$=82–120 and neutron number $N$=126–190 are considered. Prospects for synthesis of still heavier nuclei, than obtained up to now, are outlined.

On the Fission Half-Lives of Spherical Superheavy Nuclei

Acta Phys. Pol. B 28, 31 (1997)

page 31 •

abstract

Spontaneous fission of spherical superheavy nuclei is studied in a multi-dimensional deformation space. Potential energy, fission barrier and fission half-life are analyzed. The half-life is studied in a dynamical approach. Even-even isotopes of the element 114, with neutron number $N$=174–186, are considered. Alpha-decay half-lives are also given, for completeness.

full authors' list

M. Csatlós, A. Krasznahorkay, M. Hunyadi, J. Gulyás, Z. Máté, J. Molnár, J. Timár, J. Végh

Properties of $^{236}$U at the Third-Minimum Deformation

Acta Phys. Pol. B 28, 37 (1997)

page 37 •

abstract

The fission probability and the angular distribution of the fission fragments has been measured for the $^{235}$U$(d, pf)$ $^{236}$U reaction at an excitation energy slightly below the top of the fission barrier as a function of the excitation energy in $^{236}$U. A strong resonance was observed at $E^*$=5.45 MeV. The fine structure of the resonance was measured and interpreted as a hyperdeformed rotational band.

Condensed-Matter Nuclear Physics with Radioactive Beams Snowballs in Superfluid Helium

Acta Phys. Pol. B 28, 41 (1997)

page 41 •

abstract

Novel methods were applied successfully to trap radioactive ions and atoms and to freeze out their nuclear spin polarization inside aggregates of helium atoms, “snowballs” in superfluid helium. Polarized beta-ray emitting $^{12}$B ions ($T_{1/2}$=20.4 ms) were introduced into superfluid helium and snowballs were created around the impurity ions. Beta rays were detected to locate snowballs and the beta-ray asymmetry was measured to obtain the nuclear polarization of decaying $^{12}$B. The comparison with the initial value of $^{12}$B polarization establishes that no relaxation in polarization was observed throughout lifetime of $^{12}$B. Most of the impinged high-energy ions are neutralized. While they may constitute background in the above measurements, the neutrals are themselves interesting in their behaviour. It has been found that there are two components that stay in a position after stopping and that move quite swiftly in superfluid helium. The strange behaviour of neutrals is discussed.

Charged Particles in Coincidence with Superdeformed States: a Test of the Compound Nucleus Decay at Large Angular Momenta

Acta Phys. Pol. B 28, 47 (1997)

page 47 •

abstract

The reaction $^{37}$Cl+$^{120}$Sn at 187 MeV bombarding energy has been investigated in a coincidence experiment between charged particles and $\gamma$-rays at the GASP spectrometer. The population of the superdeformed (SD) band of $^{152}$Dy has been studied in the $p4n$ and $d3n$ channels. Alpha particle spectra in coincidence both with normal deformed (ND) and SD bands of $^{150}$Tb have been measured. A clear difference is observed between the two spectra, that in coincidence with the SD states being shifted to lower energies. This effect is understood in terms of different angular momentum regions from which the ND and SD states originate. The standard statistical model fails to describe the angular momentum dependence of the alpha particle energies. The use of statistical model in the region where the fission-evaporation competition is strongly affected by dynamical effects is discussed.

The Spectroscopy of Fission Fragments

Acta Phys. Pol. B 28, 59 (1997)

page 59 •

abstract

High-resolution measurements on $\gamma$ rays from fission fragments have given a wealth of information on neutron-rich nuclei and on the mechanism of the fission process. In recent years important data have been obtained on the yrast and near-yrast structures of neutron-rich nuclei with up to ten more neutrons than the nearest stable isotope. This paper discusses the scope of measurements which can be made on prompt gamma rays from spontaneous fission fragments, the techniques used in the experiments and some results recently obtained.

New Vistas of Exotic Heavy Nuclei

Acta Phys. Pol. B 28, 75 (1997)

page 75 •

abstract

We report studies of examples of reflection-asymmetric nuclei which are difficult to access using compound nucleus reactions. The most octupole deformed nuclei should be uranium isotopes with $N \approx$ 132; preliminary measurements of these very fissile nuclei suggest that they are within reach of current spectroscopic techniques. The octupole radium isotopes with N > 132 and radon isotopes are not accessible by reactions employing stable targets and beams; we have shown that multinucleon transfer reactions can populate these nuclei with sufficient yield for their structure to be determined. We report high spin studies in $^{218,220,222}$Rn and $^{222,224,226}$Ra which reveal upbending effects in this mass region for the first time and show that the electric dipole moment is constant with spin.

Shell-Model Studies of Nuclei at the Proton Drip-Line

Acta Phys. Pol. B 28, 87 (1997)

page 87 •

abstract

Binding energies for nuclei with $A \leq 70$ can be predicted by exploiting analog symmetry. This is accomplished by computing the Coulomb energy difference between mirror nuclei and adding the Coulomb shift to the experimental binding energy of the neutron-rich mirror. The location of the proton drip-line is investigated and candidates for the detection of the exotic decay mode known as di-proton emission are identified. The consequences of analog-symmetry breaking on the calibration of solar neutrino detectors are discussed.

Quantum Monte Carlo Diagonalization Method for Realistic Shell Model Calculations

Acta Phys. Pol. B 28, 91 (1997)

page 91 •

abstract

A stochastic method for performing large-scale shell model calculations is presented, which utilizes the auxiliary field Monte Carlo technique and diagonalization method. This method overcomes the limitation of the conventional shell model diagonalization and can extremely widen the feasibility of shell model calculations with realistic interactions for spectroscopic study of nuclear structure.

full authors' list

M. Würkners, J. De Boer, M. Loewe, H. Steffens, E. Maier, J. Srebrny, P.J. Napiorkowski, J. Iwanicki, A. Kordyasz, M. Kisielinski, M. Kowalczyk, J. Choinski, T. Czosnyka, A.I. Levon, J. Kvasil, C. Günther, G. Sletten

Gamma-Spectroscopy of $^{231}$Pa by Particle-Coincidences Using a New Pin-Diode Detector Array

Acta Phys. Pol. B 28, 97 (1997)

page 97 •

abstract

The nucleus $^{231}$Pa lies outside the calculated region of stable octupole deformation. Investigating the $K = 1/2^-$ g.s. band and transitions therein can serve to test calculations in the quasiparticle-plus-phonon model including small octupole correlations. To this end a $^{231}$Pa target was Coulomb-excited by 260 MeV $^{58}$Ni projectiles. For background reduction a new PIN- diode-mosaic particle detector was constructed to enable the readout of the 20 germanium detectors of NORDBALL in coincidence with the backscattered particles with good time resolution and position sensitivity. Doppler correction and $\gamma$-$\gamma$-coincidences help to clarify the structure of $^{231}$Pa.

full authors' list

M. Loewe, J. de Boer, H.J. Maier, M. Würkner, P. Olbratowski, J. Srebrny, J. Choinski, T. Czosnyka, J. Iwanicki, P.J. Napiorkowski, G. Hagemann, G. Sletten, S.A. Karamian, P. von Neumann-Cosel, A. Richter, C. Schlegel, H.J. Wollersheim

The Rotational Band on the 9$^-$ Isomer in $^{180}$Ta

Acta Phys. Pol. B 28, 101 (1997)

page 101 •

abstract

In-beam Coulomb excitation of the K$^\pi$ = 9$^-$ isomer in $^{180}$Ta was studied with $^{32}$S and $^{58}$Ni beams. Excited levels in $^{180}$Ta were identified by comparison of results with a natural (0.01%) and an enriched (5.6%) target. The rotational band built on the 9$^-$ isomer could be followed up to the 16$^-$ level and the cascade-to-crossover branching ratios are consistent with a spin-independent value for (g$_K$-g$_R$)/Q$_0$.

Two Recent Experiments at Gammasphere

Acta Phys. Pol. B 28, 105 (1997)

page 105 •

abstract

Two Gammasphere experiments of the past year are described to illustrate the new capabilities possible. One involves lifetime measurements on M1 bands in $^{198,199}$Pb to see if the predictions of the TAC model of Frauendorf for $B$(Ml) values of the “shears bands” hold experimentally. The other is a search for discrete linking transitions between the superdeformed and normal deformed bands in $^{194}$Pb in order to determine the spins, parity, and excitation energy of the superdeformed band.

Identical Bands: What Is New From Large Arrays?

Acta Phys. Pol. B 28, 121 (1997)

page 121 •

abstract

The phenomenon of identical bands is studied by analyzing the distributions of fractional changes in the dynamical moments of inertia for pairs of bands in superdeformed (SD) nuclei. These distributions show that there exists a significant excess of identical bands in SD nuclei compared to normally-deformed nuclei at low spins. This is attributed to the weaker pairing correlations and the stabilizing role of intruder orbitals on the structures of SD bands. To go into further details, precise level lifetimes have been measured for various superdeformed bands in $^{148,149}$Gd and $^{152}$Dy with the Doppler-shift attenuation method. From the derived quadrupole moments, $Q_0$, we find large differences in deformation between the yrast bands and some excited bands in the gadolinium isotopes. Moreover, two of the excited Gd bands and the $^{152}$Dy yrast band, which have identical moments of inertia, have similar $Q_0$ moments, supporting the picture that alignment and deformation effects cancel in identical bands. Calculations performed with the cranking-Hartree-Fock model show that the charge moments calculated with respect to the doubly magic SD core $^{152}$Dy can be written as independent contributions from the individual orbitals.

Tests of Nuclear Friction in Saddle-to-Scission Motion of Fissioning Nuclei

Acta Phys. Pol. B 28, 133 (1997)

page 133 •

abstract

A selected set of precise measurements of the mean kinetic energy of fission fragments has been used for testing nuclear dissipation. Calculations of the kinetic energy release on the descent from saddle to scission have been done with a dynamical model based on the classical Rayleigh– Lagrange equations of motion. Both alternative dissipation mechanisms, one-body dissipation and two-body viscosity have been tested. An increasing strength of the dissipation with increasing mass of a composite system has been observed. However this effect may partly result from a specific shape parametrization used in the dynamical model.

full authors' list

M. Popkiewicz, S. Osuch, Z. Szelfiński, A. Sztampke, M. Wolińska, R. Zagańczyk, Z. Wilhelmi, Z. Żelazny

Test of Modular Neutron Spectrometer in Measurement of Neutron Spectra from Fission of $^{252}$Cf

Acta Phys. Pol. B 28, 139 (1997)

page 139 •

abstract

An array of eight large BC-501A liquid scintillators for use in neutron time-of-flight experiments was build. Pulse-shape-discrimination method was applied to distinguish between neutrons and $\gamma$-rays. The array has been tested in the measurement of neutron spectra from spontaneous fission of $^{252}$Cf. Future experiments on heavy ion beam are in preparation.

MSX — A Monte Carlo Code for Neutron Efficiency Calculations for Large Volume Gd -Loaded Liquid Scintillation Detectors

Acta Phys. Pol. B 28, 145 (1997)

page 145 •

abstract

Some properties of the code newly developed to simulate the neutron detection process in a NMM are briefly described.

full authors' list

E. Gueorguieva, A. Minkova, Ch. Vieu, C. Schuck, F. Hannachi, Ts. Venkova, A. Lopez-Martens, A. Korichi, J.S. Dionisio, M. Kaci, I. Deloncle, B. Kharraja

Study of High-Spin Structures in $^{191}$Au

Acta Phys. Pol. B 28, 149 (1997)

page 149 •

abstract

The high-spin states in $^{191}$Au have been studied with two complementary experimental devices using the $^{186}$W($^{11}$B,6n) reaction. The $\gamma$-$\gamma$ coincidences and angular distributions of the $\gamma$-rays have been established using the EUROGAM II array; the electron conversion coefficients were obtained in an additional e$^-$-$\gamma$ experiment. The level scheme of $^{191}$Au was extended up to 9.6 MeV excitation energy and spin of 75/2 $\hbar$. The shape coexistence of different structures in the $^{191}$Au nucleus is discussed.

full authors' list

J. Iwanicki, J. Choiński, T. Czosnyka, J. Kownacki, M. Kisieliński, P. Napiorkowski, L. Zemło, J. Srebrny, J. deBoer, M. Loewe, M. Würkner

Coulomb Excitation at the Warsaw Cyclotron

Acta Phys. Pol. B 28, 153 (1997)

page 153 •

abstract

The results of first Coulomb Excitation experiments at the Heavy Ion Laboratory of Warsaw University are presented for two very different nuclei: $^{150}$Nd (a subject of an extensive Coulex project in the past few years) and $^{19}$F, not investigated by Coulex up to now. New detector system, dedicated to Coulex experiments is also presented.

full authors' list

O. Stezowski, F.A. Beck, Th. Byrski, D. Curien, C. de France, G. Duchêne, Ch. Finck, B. Gall, B. Haas, N. Khadiri, B. Kharraja, D. Prévost, C. Rigollet, H. Savajols, J.P. Vivien, K. Zuber

Superdeformed Nuclei Produced in $\alpha xn$ Channel in the $A \simeq 150$ Mass Region

Acta Phys. Pol. B 28, 157 (1997)

page 157 •

abstract

The aim of this work is to get more information about the population mechanism of superdeformed (SD) bands in the $A \simeq 150$ mass region. Usually, SD bands were populated in heavy-ion induced fusion reactions and most of them have been seen in xn exit channels. With the new generation of gamma-ray arrays, we are able to see SD structures populated in charged particle channels. Four experiments have been performed at the Vivitron accelerator in Strasbourg using the EUROGAM II spectrometer to study these phenomena (especially in $\alpha$-particle channels).

full authors' list

Ch. Finck, D. Appelbe, F.A. Beck, T. Byrski, D. Cullen, D. Curien, G. de France, G. Duchêne, S. Erturk, B. Haas, N. Khadiri, B. Kharraja, D. Prévost, C. Rigollet, O. Stezowski, P. Twin, J.P. Vivien, K. Zuber

Deexcitation of Superdeformed Bands in the Nucleus $^{151}$Tb

Acta Phys. Pol. B 28, 161 (1997)

page 161 •

abstract

The aim of this work is to get more informations about the decay-out of superdeformed bands. One of the best candidates in the mass $A \simeq 150$ region for that kind of research is the nucleus $^{151}$Tb. From previous works, it has been established that the first excited band goes lower in frequency than the band SD(1) [2], and it is the only known case in that mass region. One way to investigate these different behaviours, is to perform measurements of the average entry spin. The nucleus $^{151}$Tb has been populated via the fusion-evaporation reaction $^{130}$Te($^{27}$Al,6$n$)$^{151}$Tb at a beam energy of 155 MeV. This experiment has been perfornied at the Vivitron facility accelerator in Strashourg using the Eurogam II spectrometer, consisting of 54 Germa nium detectors. It came out of this study, that the band SD(2) and band SD(3) feed the normal deformed well 9$\hbar$ lower than the band SD(1). For the moment, it is not possible to explain the observed shell structure effects and to conclude about the statistical decay-out. The analysis is still under progress.

full authors' list

H.J. Jensen, W.F. Mueller, W. Nazarewicz, W. Reviol, L.L. Riedinger, J.-Y. Zhang

Systematics of Energy Signature Splitting for $\pi h_{9/2}$[541 1/2$^-$] Rotational Bands in odd-Z Rare-Earth Nuclei

Acta Phys. Pol. B 28, 169 (1997)

page 169 •

abstract

Systematics of the energy signature splitting, $\Delta e^\prime$, for rotational bands built on the $\pi h_{9/2}$[541 1/2$^-$] Nilsson intruder configuration in the $Z$ = 69–79 region are presented. Predictions from Cranked Shell Model calculations, based on frequency adiabatic configurations, are compared with the experimental data for isotopes of Tm, Lu and Ta.

full authors' list

A. Savelius, K. Helariutta, P.M. Jones, R. Julin, S. Juutinen, P. Jämsen, M. Muikku, M. Piiparinen, S. Törmänen, P. Simecek, D. Cutoiu

Coexisting Structures in $^{116}$Sn from Weak Fusion-Evaporation Reactions

Acta Phys. Pol. B 28, 173 (1997)

page 173 •

abstract

Excited states in $^{116}$Sn have been studied by using the DORIS Ge array combined with charged particle detectors. High-spin states were observed up to $I \approx 20\hbar$ region. Spherical as well as regular, deformed level structures were found. The spherical states are interpreted to arise from pure neutron configurations, while the deformed, intruder bands are most likely to involve the proton 2$p$ – 2$h$ excitations across the $Z$ = 50 shell gap.

full authors' list

B. Kharraja, U. Garg, S.S. Ghugre, M. Carpenter, B. Crowell, S. Fisher, R.V.F. Janssens, T.L. Khoo, T. Lauristen, D. Nissius, W. Mueller, W. Reviol, L.L. Riedinger, R. Kaczarowski, E. Ruchowska, I.M. Govil

Spectroscopy of $^{96-98}$Ru and Neighboring Nuclei: Shell Model Calculations and Lifetime Measurements

Acta Phys. Pol. B 28, 177 (1997)

page 177 •

abstract

High Spin states in $^{94,95}$Mo, $^{94-96}$Tc, $^{96-98}$Ru and $^{97,98}$Rh were populated via the $^{65}$Cu($^{36}$S,$xpyn$) reactions at 142 MeV. Level schemes of these nuclei have been extended up to a spin of $J \approx 20 \hbar$ and an excitation energy of $E_x \approx$ 12–14 MeV. Information on the high spin structure for $^{96}$Tc and $^{98}$Rh has been obtained for the first time. Spherical shell model calculations have been performed and compared with the experimental excitation energies. The level structures of the $N$ = 51, 52 isotones exhibit single-particle nature even at the highest spins and excitation energies. A fragmentation of intensity into several branches after breaking of the $N$ = 50 core has been observed. There are indications for the onset of collectivity around neutron number $N$ = 53 in this mass region. A sequence of E2 transitions, reminiscent of vibrational degree of freedom, were observed in $^{98}$Ru at spins just above the observed $N$ = 50 core breaking. RDM lifetime measurements have been performed to ascertain the intrinsic structures of these level sequences.

full authors' list

T.S. Tveter, I. Huseby, L. Bergholt, M. Guttormsen, E. Melby, J. Rekstad, S. Siem, R.K. Sheline

Number of Degrees of Freedom in Primary $\gamma$-Decay after Neutron Capture

Acta Phys. Pol. B 28, 185 (1997)

page 185 •

abstract

The intensities of primary $\gamma$-ray transitions following thermal and average resonance neutron capture have been found to depend on the final-state $K$ quantum number. The apparent $K$-hindrance effect is significantly stronger in the thermal than in the ARC case. After thermal neutron capture, the shapes of the transition probability distributions indicate that the $K$-allowed transitions are associated with a higher number of degrees of freedom than the $K$-forbidden ones.

New Results in High-Energy Tray Studies from Heavy-Ion Reactions at Around 10 MeV/u

Acta Phys. Pol. B 28, 189 (1997)

page 189 •

abstract

Recently measured data for the $^{12}$C + $^{26}$Mg reaction at 6, 8.5 and 11 MeV/u and for $^{12}$C + $^{24}$Mg at 11 MeV/u have been analysed using a new version of the CASCADE code allowing for simultaneous fitting of $\gamma$-ray spectra and angular distributions. The GDR parameters and bremsstrahlung parameters have been extracted. Comparison with BUU calculations are also discussed.

Giant Dipole Resonance in Excited $^{120}$Sn and $^{208}$Pb Nuclei Populated by Inelastic Alpha Scattering

Acta Phys. Pol. B 28, 197 (1997)

page 197 •

abstract

The evolution of the giant dipole resonance in $^{120}$Sn and $^{208}$Pb nuclei at excitation energies in the range of 30–130 MeV was studied by comparing high energy $\gamma$ ray measurements from the decay of the resonance at low angular momenta with theoretical model calculations. Both, a model of the adiabatic coupling of the GDR to the nuclear shape as well as a collisional model describe the increase of the width with temperature. However, while the collisional model is independent of angular momentum, the adiabatic model can describe the increase of the GDR width at high angular momenta.

full authors' list

F. Camera, A. Bracco, A. Ataç, R. Bark, I.G. Bearden, J.J. Gaardhøje, B. Herskind, M. Kmiecik, S. Leoni, M. Mattiuzzi, A. Maj, G. Turri

GDR in Superdeformed Nuclei

Acta Phys. Pol. B 28, 207 (1997)

page 207 •

abstract

A search for the $\gamma$ decay of the Giant Dipole Resonance built on superdeformed nuclear configurations was made. The superdeformed states of the $^{143}$Eu nucleus were populated using the reaction $^{110}$Pd($^{37}$Cl,4$n$)$^{143}$Eu at a beam energy of 165 MeV. High energy $\gamma$-rays were detected in 8 large BaF$_2$ scintillators in coincidence with discrete transitions measured with part of the NORDBALL array (17 HPGe detectors and a 2$\pi$ multiplicity filter). Spectra of high-energy $\gamma$-rays gated by low-energy transitions from states fed by the superdeformed bands show an excess yield in the 7–10 MeV region with respect to those gated by transitions from states not populated by the superdeformed bands. Because the dipole oscillation along the superdeformed axis of the nucleus is expected to have a frequency corresponding to $\approx$ 8 MeV (low energy component of the GDR strength function), the present result gives the first experimental indication of $\gamma$-ray emission of the GDR built on a superdeformed states.

full authors' list

M. Kicińska–Habior, Z. Trznadel, M. Kisieliński, M. Augsburg, D. Chmielewska, J. Dworski, A. Kordyasz, M. Kowalczyk, J. Kownacki, A. Krzyczkowska, J. Kwieciński, A. Maj, T. Matulewicz, J. Romanowski, Z. Sujkowski, Z. Żelazny

Progress with the Statistical Giant Dipole Resonance Decay Experiment at the Warsaw Cyclotron

Acta Phys. Pol. B 28, 219 (1997)

page 219 •

abstract

The giant dipole resonance (GDR) in highly excited $^{70}$Se populated in a heavy-ion fusion reaction $^{12}$C + $^{58}$Ni at 47.5 MeV beam energy from the Warsaw Cyclotron has been studied. High-energy $\gamma$-ray spectra have been measured with the JANOSIK set-up consisting of a large NaI(Tl) spectrometer in coincidence with the 32 elements multiplicity filter. The first experimental results and further plans are discussed. A brief description of the JANOSIK set-up is given.

full authors' list

P. Aguer, G. Bogaert, A. Coc, J. Kiener, A. Lefebvre, V. Tatischeff, J.P. Thibaud

Radiative Reaction Rates: from the Observatory to the Laboratory

Acta Phys. Pol. B 28, 227 (1997)

page 227 •

abstract

The gamma ray telescopes launched in the recent years provide observations which in turn trigger a better thought of the nuclear physics involved in the gamma ray production in stars, novae and supernovae. Two examples are presented, with suggestions of experiments to run, either with stable or unstable beams.

full authors' list

C.T. Zhang, P. Bhattacharyya, P.J. Daly, R. Broda, Z.W. Grabowski, D. Nisius, I. Ahmad, T. Ishii, M.P. Carpenter, L.R. Morss, W.R. Phillips, J.L. Durell, M.J. Leddy, A.G. Smith, W. Urban, B.J. Varley, N. Schulz, E. Lubkiewicz, M. Bentaleb, J. Blomqvist

Shell-Model High-Spin Yrast States of Fission Product Nuclei above Doubly Magic $^{132}$Sn

Acta Phys. Pol. B 28, 233 (1997)

page 233 •

abstract

Prompt $\gamma$-ray cascades in neutron-rich nuclei around doubly-magic $^{132}$Sn have been studied at Eurogam 2 array using a $^{248}$Cm fission source. Yrast states to above 5.5 MeV in the two- and three-proton $N =$ 82 isotones $^{134}$Te and $^{135}$I are reported. They are interpreted in terms of valence proton and particle-hole core excitations with the help of shell model calculations employing empirical nucleon–nucleon interactions from both $^{132}$Sn and $^{208}$Pb regions. A serious inconsistency in the accepted masses of $N =$ 82 isotones near $^{132}$Sn is discovered but not resolved.

New Mechanism of Collapse and Revival in Wave Packet Dynamics Due to Spin–Orbit Interaction

Acta Phys. Pol. B 28, 243 (1997)

page 243 •

abstract

This article discusses the properties of time evolution of wave packets in a few systems. Dynamics of wave packet motion for Rydberg atoms with the hierarchy of collapses and revivals is briefly reviewed. The main part of the paper focuses on the new mechanism of quantum recurrences in wave packet dynamics. This mechanism can occur in any physical system with strong enough spin–orbit interaction. We discuss here the spin–orbit pendulum effect that consists in different motions of subpackets with different spin fields and results in oscillations of a fraction of average angular momentum between spin and ordinary subspaces. The evolution of localized wave packets into toroidal objects and backwards (for other class of initial conditions) is also subject to discussion.

Production of Neutron-Rich Nuclei Using Deep-Inelastic Reactions

Acta Phys. Pol. B 28, 257 (1997)

page 257 •

abstract

We have used the reactions $^{48}$Ca + $^{176}$Yb, $^{154}$Sm + $^{176}$Yb and $^{154}$Sm + $^{208}$Pb to study the feasibility of populating high spin states in neutron-rich nuclei by deep-inelastic reactions. Using Gammasphere, we observed gamma transitions from nuclei with several neutrons richer than the target. Yrast states with spin up to 20 were populated in these reactions. High spin states in $^{175,177,178}$Yb were observed for the first time. Back bending behavior in Yb nuclei was studied and compared with cranked shell model calculations. Yields of neutron rich nuclei and high spin states are higher in the Sm induced reactions, But larger Doppler broadening limits the observation of the high spin states.

full authors' list

R. Julin, R.G. Allatt, P.A. Butler, K.J. Cann, J.F.C. Cocks, T. Enqvist, P.T. Greenlees, K. Helariutta, G.D. Jones, P.M. Jones, S. Juutinen, P. Jämsen, H. Kankaanpää, P. Kuusiniemi, M. Leino, M. Muikku, R.D. Page, M. Piiparinen, A. Savelius, W.H. Trzaska, S. Törmänen, J. Uusitalo

Studies of Neutron-Rich and Heavy Nuclei with the New In-Beam Facilities at JYFL

Acta Phys. Pol. B 28, 269 (1997)

page 269 •

abstract

In-beam $\gamma$-ray spectroscopic methods combined with binary reactions have been used for studies of intruder structures in the “neutron-rich” $^{114}$Cd and $^{116}$Cd nuclei. The recoil decay tagging method with a gas-filled recoil separator was used for the first observation of excited states in neutron-deficient $^{192}$Po. A newly developed broad range electron spectrometer has been employed in a search for superdeformed structures in $^{222}$Th.

Recent Results on $^{208}$Pb

Acta Phys. Pol. B 28, 277 (1997)

page 277 •

abstract

Recent measurements of $\gamma$ transitions in $^{208}$Pb provide much new and reliable information. Excited states of $^{208}$Pb are well described by one particle–one hole shell model wave functions. These wave functions have been determined for about 50 levels strictly from the experimental data, mainly spectroscopic factors and $\gamma$ branching ratios. The matrix elements of the residual interaction are calculated from the wave functions by inverting the Schroedinger equation. The resulting residual interaction is presented.

Trends in the Research Programme of the GSI Fragment Separator

Acta Phys. Pol. B 28, 289 (1997)

page 289 •

abstract

A selection of experiments performed at the Projectile Fragment Separator (FRS) in GSI in the last two years is presented with the focus on the presently reached limits for production of nuclei very far from the $\beta$-stability line.

full authors' list

J. Kownacki, A. Atac, A. Algora, J. Cederkäll, B. Cederwall, Zs. Dombrádi, C. Fahlander, H. Grawe, E. Ideguchi, D. Jerrestam, A. Johnson, R. Julin, S. Juutinen, M. Karny, A. Kerek, W. Klamra, A. Likar, M. Lipoglavšek, L-O. Norlin, B.M. Nyakó, J. Nyberg, S. Mitarai, H. Mäkelä, M. Palacz, G.E. Perez, J. Persson, M. Piiparinen, D. Seweryniak, R. Schubart, G. Sletten, S. Törmänen, A. Virtanen, R. Wyss

A Shape Evolution Observed for Nuclei Close to $^{100}$Sn

Acta Phys. Pol. B 28, 299 (1997)

page 299 •

abstract

The present contribution concentrates on the studies of shape evolution of very weakly deformed nuclei, in terms of a coexistence of spherical and deformed states near closed shells due to polarizing effects of the aligned particles on the originally spherical or slightly deformed core. Using the level schemes information, the different modes of nuclear behaviour are seen. The possibility of tracing the onset of collectivity when moving away from double magic structure is considered.

full authors' list

M. Górska, M. Lipoglavšek, H. Grawe, J. Nyberg, A. Axelsson, R. Bark, J. Blomqvist, J. Cederkäll, B. Cederwall, G. de Angelis, C. Fahlander, A. Johnson, S. Leoni, A. Likar, M. Matiuzzi, S. Mitarai, L-O. Norlin, M. Palacz, J. Persson, H.A. Roth, R. Schubart, D. Seweryniak, T. Shizuma, Ö. Skeppstedt, G. Sletten, W.B. Walters, M. Weiszflog

Spectroscopy of the T$_z$=1 Nuclei Close to $^{100}$Sn

Acta Phys. Pol. B 28, 303 (1997)

page 303 •

abstract

The two nuclei $^{98}$Cd and $^{102}$Sn, closest neighbours of $^{100}$Sn, have been studied with a recoil catcher setup, following the reactions: $^{58}$Ni($^{46}$Ti, $\alpha$2n)$^{98}$Cd and $^{58}$Ni($^{50}$Cr, $\alpha$2n)$^{102}$Sn. Long lived isomeric states were measured in $^{98}$Cd $I^{\pi }$ = (8$^+$), $t_{1/2}$ = 0.48(8) $\mu$s and in $^{102}$Sn $I^{\pi }$ = (6$^+$) with $t_{1/2}$ = 1.0(6) $\mu$s. The proposed experimental level schemes of the isomeric decay are presented and compared to the shell model predictions.

full authors' list

M. Palacz, J. Cederkäll, M. Lipoglavšek, J. Persson, A. Atac, J. Blomqvist, H. Grawe, C. Fahlander, J. Iwanicki, A. Johnson, A. Kerek, W. Klamra, J. Kownacki, A. Likar, L-O. Norlin, J. Nyberg, R. Schubart, D. Seweryniak, Z. Sujkowski, R. Wyss, G. de Angelis, P. Bednarczyk, Zs. Dombrádi, D. Foltescu, D. Jerrestam, S. Juutinen, E. Mäkelä, G. Perez, M. de Poli, H.A. Roth, T. Shizuma, Ö. Skeppstedt, G. Sletten, S. Törmänen, T. Vass

In Beam $\gamma$-Ray Spectroscopy of Very Neutron Deficient Odd-Cadmium Isotopes

Acta Phys. Pol. B 28, 309 (1997)

page 309 •

abstract

Excited states in the very neutron deficient odd cadmium isotopes $^{99}$Cd, $^{101}$Cd and $^{103}$Cd are discussed in terms of the nuclear shell model. Systematics of excited states in $^{99--109}$Cd is presented.

full authors' list

A. Heinz, J. Benlliure, C. Böckstiegel, H.-G. Clerc, A. Grewe, M. de Jong, A.R. Junghans, J. Müller, M. Pfützner, K.-H. Schmidt, S. Steinhäuser

Fission of Relativistic Secondary Beams

Acta Phys. Pol. B 28, 315 (1997)

page 315 •

abstract

A new method is described to study nuclear fission of short-lived radioactive isotopes induced by electromagnetic excitations. Secondary beams of fissile nuclei were produced by fragmentation of $^{238}$U projectiles at 1 $A$ GeV. After being separated and identified in the fragment separator of GSI they impinged on a secondary lead target. Nuclear-charge distributions and total kinetic energies of fission fragments produced in low-energy fission have been measured for a number of neutron-deficient uranium, protactinium, thorium, actinium and radium isotopes. First experimental results are presented.

Formation of Hot Nuclei in Collisions below 100 MeV/u and the Caloric Curve

Acta Phys. Pol. B 28, 319 (1997)

page 319 •

abstract

4$\pi$ charged products arrays allowed us to determine that, at incident energies 35–95 MeV/u, the fusion cross section is very small in nearly symmetric light and medium systems. Dissipative binary collisions accompanied by mid-rapidity (participant) emission was found to dominate at all impact parameters. The excitation energy of the reconstructed quasi-projectile was obtained via calorimetry and reaches high values. Their apparent temperatures were obtained from several double isotopic yield ratios and slopes of kinetic energy spectra. These data can be explained by a steady increase of the initial temperature with excitation energy without evidence for a liquid–gas phase transition.

full authors' list

S. Ternier, G. Neyens, K. Vyvey, N. Coulier, S. Michiels, R. Coussement, R. Kulkarni, D.L. Balabanski

Quadrupole Frequency Measurements in Cubic and Magnetic Host Materials

Acta Phys. Pol. B 28, 325 (1997)

page 325 •

abstract

The Level Mixing Spectroscopy (LEMS) method has proven to be a powerful method to determine the quadrupole moment of high spin isomers. There are some limitations on the use of the method which are due to the fact that a non cubic host providing an electric field gradient and a large superconducting magnet are needed. In this contribution we report on the use of cubic host materials as a first expansion of the applicability of the LEMS method towards a wider region on the nuclear chart. In two experiments we have shown that a defect-associated electric field gradient, induced by the nuclear reaction in a cubic host material, is reproducible and can be used to determine quadrupole moments. We also discuss how the use of magnetic host materials cancels the need for a superconducting magnet by taking advantage of the very strong, temperature dependent hyperfine fields near the Curie temperature.

full authors' list

K. Vyvey, G. Neyens, S. Ternier, N. Coulier, S. Michiels, R. Coussement, D.L. Balabanski, A. Lépine-Szily

Improvements of the LEMS Technique to Determine Quadrupole Moments of High-$K$ Isomers

Acta Phys. Pol. B 28, 329 (1997)

page 329 •

abstract

Recent studies have shown examples of severe violations of the $K$-selection rule, raising questions about the usual understanding of the $K$-mixing process and the structure of the $K$-isomeric states. The LEMS technique enables us to measure the spectroscopic quadrupole moment of nuclei in high-$K$ isomeric states, which allows to investigate the deformation of these isomers. As poor statistics is the main problem we face in these experiments, different improvements have been investigated to increase the sensitivity of the LEMS technique.

Selection of Reaction Mechanisms and of Exotic Events in Heavy Ion Reactions

Acta Phys. Pol. B 28, 335 (1997)

page 335 •

abstract

Problem of signatures of particular reaction mechanisms is discussed for the symmetric $^{40}$Ca+$^{40}$Ca reaction at 35 MeV/nucleon. Special reaction filters are proposed. Some exotic events are presented.

Probing Phase Transitions in Nuclei via Calorimetry

Acta Phys. Pol. B 28, 355 (1997)

page 355 •

abstract

Heavy-ion reactions at relativistic energies offer the unique possibility for studying phase transitions in finite, hadronic systems. A general overview of this subject is given emphasizing the most recent results on the liquid–gas transition obtained via nuclear calorimetry.

full authors' list

J. Cub, J. Holeczek, S. Wan, T. Aumann, T. Baumann, K. Boretzky, W. Dostal, B. Eberlein, H. Emling, Ch. Ender, Th.W. Elze, H. Geissel, J. Gerl, A. Grünschloss, D. Habs, T. Happ, R. Holzmann, N. Iwasa, M. Kaspar, A. Kleinböhl, F. Köck, O. Koschorrek, J.V. Kratz, G. Kraus, R. Kulessa, Y. Leifels, A. Leistenschneider, G. Münzenberg, F. Nickel, I. Peter, P. Reiter, H. Schaffner, C. Scheidenberger, G. Schrieder, D. Schwalm, R. Schubart, R. Schubert, W. Schwab, H. Simon, G. Stengel, J. Stroth, K. Sümmerer, A. Surowiec, P. Thirolf, E. Wajda, H.J. Wollersheim, A. Zentek

Invariant-Mass and $\gamma$-Ray Spectroscopy Using Secondary, Radioactive Ion Beams

Acta Phys. Pol. B 28, 367 (1997)

page 367 •

abstract

Coulomb excitation of secondary beams (5 $\leq Z \leq$ 20) at energies around 250 $A \cdot$MeV was explored at GSI. For low-lying states, $\gamma$-ray spectroscopy was utilized, while high-lying excitations were investigated by means of invariant-mass spectroscopy.

full authors' list

T. Aumann, K. Boretzky, J. Stroth, E. Wajda, Th. Blaich, J. Cub, W. Dostal, B. Eberlein, Th.W. Elze, H. Emling, A. Grünschloss, W. Henning, J. Holeczek, R. Holzmann, H. Klingler, C. Kozhuharov, R. Kulessa, J.V. Kratz, D. Lambrecht, Y. Leifels, A. Leistenschneider, E. Lubkiewicz, T. Ohtsuki, P. Reiter, K. Stelzer, K. Sümmerer, A. Surowiec, W. Walus, M. Zinser, E. Zude

Coulomb Fragmentation and Coulomb Fission of Relativistic Heavy-Ions and Related Nuclear Structure Aspects

Acta Phys. Pol. B 28, 375 (1997)

page 375 •

abstract

The Coulomb excitation of $^{208}$Pb projectiles has been studied at an energy of 640 $A$ MeV. Cross sections for the excitation of the two-phonon giant dipole resonance were measured for different targets, and show clear evidence for a two-step electromagnetic excitation mechanism. The experimental cross sections exceed those calculated in the harmonic oscillator approximation by a factor of 1.33 $\pm$ 0.16. The deduced 2$\gamma$-decay probability is consistent with the expectation in the harmonic limit. Finally, the excitation of the two-phonon giant dipole resonance in the deformed and fissile nucleus $^{238}$U is discussed.

Light Particle Evaporation from Dynamical Systems

Acta Phys. Pol. B 28, 387 (1997)

page 387 •

abstract

For the statistical particle-evaporation model to be applicable to particle emission from dynamical time-evolving systems, the system should closely follow the quasistatic path, which represents a sequence of conditional equilibrium shapes. We show that quasifission paths predicted by the one-body dissipation dynamics satisfy this requirement all the way from the contact point to the scission point, excluding short time intervals spent near the contact point (when neck fills in) and during separation (when waist develops).

Measurements of D-States of Light Nuclei by Transfer Reactions

Acta Phys. Pol. B 28, 393 (1997)

page 393 •

abstract

This is a review paper on the investigations of the D-state component in few-body systems.

Preequilibrium Model Calculations of $\gamma$-Spectrum from $^3$He + $^{25}$Mg Reaction at $E_{{\rm LAB}}$ = 29.9 MeV

Acta Phys. Pol. B 28, 397 (1997)

page 397 •

abstract

High energy $\gamma$-ray cross section obtained from $^3$He +$^{25}$Mg reaction at $E_{{\rm lab}}$ 29.9 MeV exhibits an enhancement as compared to statistical model predictions including giant resonance strength functions. Within the preequilibrium exciton model approach which takes into account level density parameters and pairing for individual nuclei and uses the giant dipole resonance parameters found at lower excitation energy a satisfactory description of the $\gamma$-ray energy spectrum is obtained.

Nuclear Temperature Measurements with Isotopic Thermometer

Acta Phys. Pol. B 28, 401 (1997)

page 401 •

abstract

We study the corelation between “apparent” nuclear temperatures which can be measured in an experiment and “real” ones within the sequential statistical model. We extract temperature from slopes of kinetic energy spectra of light charged particles and from isotopic double ratios. We find that due to the chain of subsequent emissions the “apparent” temperature significantly differs from initial (average) temperature. We also find that the model of simple statistical evaporation works very well even for very high excitation energies.

full authors' list

N. Coulier, G. Neyens, S. Ternier, K. Vyvey, S. Michiels, R. Coussement, D.L. Balabanski, J.M. Casandjian, M. Chartier, M.D. Cortina-Gil, M. Lewitowicz, W. Mittig, A.N. Ostrowski, P. Roussel-Chomaz, N. Alamanos, A. Lépine-Szily

Direct Determination of the Alignment of Projectile Fragments from a $\beta$-Anisotropy Level Mixing Resonance (LMR) Measurement

Acta Phys. Pol. B 28, 407 (1997)

page 407 •

abstract

Using the Level Mixing Resonance (LMR) interaction, which converts alignment into polarization, the nuclear spin alignment of $^{12}$B produced in two different fragmentation reactions at intermediate energy (respectively $^{13}$C onto a $^{12}$C target and $^{22}$Ne onto a $^9$Be target) was determined by measuring the $\beta$-anisotropy. The nuclear spin alignment of $^{18}$N fragments, produced in the $^{22}$Ne reaction was measured for different selections in the longitudinal fragment momentum distribution. In the same measurement the ratio of the quadrupole frequency to the magnetic moment $\frac {\nu _Q}{\mu }$ of neutron rich $^{18}$N fragments implanted in Mg was found to be an order of magnitude larger compared to simple shell model calculations.

Light Particles Emission from Hot, Rotating, Compound Nuclei

Acta Phys. Pol. B 28, 413 (1997)

page 413 •

abstract

The competition between fission and $n, p$ and $\alpha$-particle emission is studied. The fission process is described by a Langevin equation coupled to the Masters equation for particles evaporation. The significant influence of the nuclear deformation, isospin dependence of the nuclear charge radius and of the initial spin distribution on the prescission particles multiplicities is found.

full authors' list

S. Siem, T.S. Tveter, L. Bergholt, M. Guttormsen, E. Melby, J. Rekstad

Level Densities in Rare Earth Nuclei

Acta Phys. Pol. B 28, 419 (1997)

page 419 •

abstract

An iterative procedure for simultaneous extraction of fine structure in the level density and the $\gamma$-ray strength function from a set of primary $\gamma$-ray spectra has been developed. Data from the reactions $^{163}$Dy($^3$He,$\alpha \gamma )^{162}$Dy and $^{173}$Yb($^3$He,$\alpha \gamma )^{172}$Yb reveals step like enhancements in the level density in the region below 5 MeV and peaks in the $\gamma$-ray strength function at low $\gamma$-energy ($E_{\gamma } \approx$ 2–3.5 MeV). Tentative physical interpretations are presented.

Nuclear Structure Experiments with the New Euroball-3 Detectors

Acta Phys. Pol. B 28, 423 (1997)

page 423 •

abstract

In a series of Coulomb excitation and transfer reaction experiments the newly developed CLUSTER detectors have been combined with the Darmstadt–Heidelberg Crystal Ball to a very powerful $\gamma$-spectrometer. An overview of four experiments performed at the UNILAC will be described. This includes spectroscopic studies of harmonic multi-phonon vibrations and investigations of the nuclear reaction mechanism in very heavy ion systems which are also used for the synthesis of superheavy elements.

full authors' list

P.H. Regan, C. Chandler, C.J. Pearson, B. Blank, R. Grzywacz, M. Lewitowicz, A.M. Bruce, W.N. Catford, N. Curtis, S. Czajkowski, P. Dessagne, A. Fleury, W. Gelletly, J. Giovinazzo, Z. Janas, C. Longour, C. Marchand, C. Miehe, N.A. Orr, R.D. Page, M.S. Pravikoff, A.T. Reed, M.G. Saint-Laurent, S.M. Vincent, R. Wadsworth, D.D. Warner, J.S. Winfield

Studies of Isomeric States and Limits of Particle Stability around N$\sim$Z$\sim$40 Using Fragmentation Reactions

Acta Phys. Pol. B 28, 431 (1997)

page 431 •

abstract

Fragmentation products from a $^{92}$Mo beam on a natural nickel target have been used to study structural properties of the very neutron deficient nuclei around N$\sim$Z$\sim$40. We present the first observation of isomeric decays in the $T_z=1$ systems $^{74}_{36}$Kr, $^{80}_{39}$Y and $^{84}_{41}$Nb. The isomer in $^{74}$Kr is interpreted as the hindered decay from an excited 0$^+$ state, confirming the prediction of prolate/oblate shape coexistence in this nucleus. Transitions from states below an isomer in the N=Z nucleus $^{86}_{43}$Tc have also been tentatively identified, making this the heaviest N=Z system for which decays from excited states have been observed. In addition, we have obtained the first conclusive evidence for the existence of the $T_z=-\frac {1}{2}$ isotopes $^{77}_{39}$Y, $^{79}_{40}$Zr and $^{83}_{42}$Mo. The data for $^{77}_{39}$Y is of particular interest in light of the reported instability in the lighter odd-Z, $T_z=-\frac {1}{2}$ systems $^{69}$Br and $^{73}$Rb.

full authors' list

W.H. Trzaska, V.A. Rubchenya, A.A. Alexandrov, I.D. Alkhazov, J. Äystö, J. von Kalben, S.V. Khlebnikov, A.V. Kuznetsov, V.G. Lyapin, V.E. Makarenko, I. Moltchanov, M. Mutterer, O.I. Ossetrov, H.-G. Ortlepp, G. Otroshtchenko, Yu.E. Penionzhkevich, Yu.V. Pyatkov, G.P. Tiourine, D.N. Vakhtin

Research on Fission Dynamics at JYFL Using HENDES — High Efficiency Neutron Detection System

Acta Phys. Pol. B 28, 443 (1997)

page 443 •

abstract

In an effort to extend the scope of basic research centred around the new K-130 heavy-ion cyclotron at the Physics Department of the University of Jyväskylä a new experimental device called HENDES has been constructed. It is designed for correlation measurements of fission fragments, neutrons, and light charged particles produced in heavy ion induced reactions in the energy range of about 5 to 10 MeV/A. Collective dynamical parameters of nuclear matter are extracted from the data. New results were obtained from $^{40}$Ar + $^{180}$Hf reaction at $E_{{\rm Ar}}$ = 190–250 MeV. Work is in progress to study the influence of neutron excess on the fusion–fission dynamics in different combinations of Ni beams on Sn targets.

Concluding Remarks

Acta Phys. Pol. B 28, 451 (1997)

page 451

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