Metals, the plasma of the poor man?

The electron screening effect in the d(d,p)t reaction has been studied for deuterated metals, insulators, and semiconductors, i.e. 58 samples in total. As compared to measurements performed with a gaseous D2 target, a large effect has been observed in most metals, while a small (gaseous) effect is found e.g. for the insulators, semiconductors, and lanthanides. The periodic table provides the ordering of the observed small and large effects in the samples. An explanation of the large effects in metals is possibly provided by the classical plasma screening of Debye applied to the quasi-free metallic electrons. The data also provide information on the solubility of hydrogen in the samples.

Multipole Response of Ni and Sn Isotopes and its Momentum Dependence

We have used a self-consistent Skyrme-HF plus Continuum RPA model to study the low-multipole response of stable and neutron/proton-rich Ni and Sn isotopes using suitable Skyrme parametrizations. Experimental studies of these nuclei are being planned at RIKEN. We focus on the momentum dependence of the strength distribution, as it may provide information on the structure of excited nuclear states.

Parameter-Independent Symmetries in Nuclear Structure

The E(5) symmetry describes nuclei related to the U(5)--SO(6) phase transition, while the X(5) symmetry is related to the U(5)--SU(3) phase transition. A chain of potentials interpolating between the U(5) symmetry of the 5-dimensional harmonic oscillator and the E(5) symmetry is considered. Parameter independent predictions for the spectra and B(E2) values of nuclei with $R_4= E(4)/E(2)$ ratios 2.093, 2.135, and 2.157 (compared to the ratio 2.00 of the U(5) case and the ratio 2.20 of the E(5) case) are derived numerically and compared to existing experimental data, suggesting several new experiments. Furthermore, an exactly soluble model with $R_4 = 2.646$ is constructed and a chain of potentials interpolating between this new model and the X(5) symmetry is considered. Parameter independent predictions for the spectra and B(E2) values of nuclei with $R_4$ ratios 2.769, 2.824 and 2.852 (compared to the ratio 2.904 of the X(5) case) are derived numerically and compared to existing experimental data, suggesting several new experiments.

Influence of Model Assumptions on Radial Flow Investigations in Heavy-Ion Collisions

An effective way for the investigation of nuclear matter under the extreme conditions of high density and temperature is the study of nuclear fragmentation, realized in intermediate-energy heavy-ion collisions. The potential energy accumulated in the compression zone of the collision is converted into collective flow of matter in the later stages of the reaction. During the expansion of the system, fragments are formed in a clusterization process. In the final stage of the reaction, the fragments are moving along Coulomb trajectories and may deexcite by particle emission or secondary break-up.In order to verify the influence of the collective energy on the experimental energy spectra and extract quantitative information on the thermal and collective components, a model simulation was developed. In this procedure, the evolution of the disintegrating system in a multifragmentation process was described after the freeze-out stage with a Monte-Carlo approach. A reconstruction procedure for the fragment kinetic energies has shown the possibility to obtain the thermal and collective components from measured fragment spectra. In the present work, we scrutinize the model simulation for the evolution of the disintegrating system by testing the sensitivity of the collective expansion energy extraction on the model assumptions concerning the freeze-out characteristics, such as the size and shape of the freeze-out volume. Furthermore, thermal aspects of the early stage of the expansion are tested with an examination of the relationship between the assumption of the degree of thermalization of the emitting source and the extracted value of the collective energy.

Medical Applications of Nuclear physics at the School of Medicine of the University of Ioannina (in greek)

N/A

Development of a Monte-Carlo Evaporation Code for Multiple-Fragment Emission

An extended Hauser-Feshbach approach has been employed in a multi-step Monte-Carlo evaporation code designed to study the de-excitation of highly excited compound nuclei. The code is intended to account for emission of light particles ($\gamma$, n, $^{1,2,3}$H, $^{3-6}$He) and intermediate mass fragments in their ground and excited states (particle-bound or unbound). As a study case, we consider the decay of the compound nucleus $^{120}$Te$^*$ at excitation energy 100, 200 and 300 MeV. First chance decay widths are compared with treatments based on the Weisskopf and the s-wave approximation. Preliminary calculations are compared with experimental isotopic yields of intermediate mass fragments emitted in E/A = 50 MeV $^{4}$He + $^{116,124}$Sn $\rightarrow$ $^{120,128}$Te$^*$ reactions.

Correlation of Tritium Concentration in Rainwater with Cosmic Rays: Preliminary Results

The Radiochronology Center has been established in 1999 as a member of the Horizontal Laboratories network of the University of Ioannina. Recently it has added to its activities the development of a tritium measurements laboratory as part of the Radiochronology Center. The laboratory is equipped with a super low level background liquid scintillation counter, which is capable to measure very low concentrations of tritium for applications in radioprotection, dating and hydrology.For the aims of the present continuing study rainwater samples are collected monthly. Each sample is distilled and then it is passed through an electrolysis process to increase the tritium concentration. Five mL of the enriched sample are mixed with 15 mL of a scintillation cocktail, specially designed for tritium measurements and its beta activity is measured for 200 min. The LLD of tritium in the samples is estimated 3 Bq/L or 27 TU.Our preliminary results show that, during the measuring period tritium concentration increased with time. The tritium values are correlated with the cosmic ray neutron flux data at ground level, available for Greece in the same period. The measured tritium concentrations in rainwater, which range from 36 ± 8 to 64 ± 8 TU, may be used for local hydrology studies.

Measurements of Energy Loss of Charded Particles Travelling in Crystalline Materials along the Channeling Directon

A method for determining the energy loss and mean dechanneling distance of light charged particles traveling along a low index axis of a crystal in the backscattering geometry, is presented. The method is based on nuclear resonance reactions which act as a marker for the range in the backscattering spectra. Computer simulations based on the assumption of an exponential rate of dechanneling of the incoming particles in the crystalline material, are in good agreement with measured spectra. The results of applying this method to protons and alphas in crystals of Si, SiO2, SiC and MgO are discussed and possible improvements are indicated.

Some Issues Related to the Direct Detection of Dark Matter

In this paper we review the theoretical issues involved in the direct detectionof supersymmetric (SUSY) dark matter. After a brief discussion of the allowedSYSY parameter space we focus on the determination of the traditional neutralinodetection rates, in experiments which measure the energy of the recoiling nucleus,such as the coherent and spin induced rates and the dependence of the rate onthe motion of the Earth (modulation effect). Then we examine the novel featuresappearing in directional experiments, which detect the recoiling nucleus in a givendirection. Next we estimate the branching ratios for transitions to accessibleexcited nuclear levels. Finally we estimate the event rates leading to the atomionization and subsequent detection of the outgoing electrons.

The $^{135}$Cs(n,$\gamma$) cross section at 30 and 500 keV

The neutron capture cross section of the unstable isotope $^{135}$Cs was measured relative to that of gold by means of the activation method. The sample was produced by ion implantation in a high resolution mass separator and irradiated with quasi-monoenergetic neutrons at 30 keV and 500 keV, using the $^{7}$Li(p,n)$^{7}$Be reaction. After the irradiations at the above energies, one more irradiation with thermal neutrons was used for defining the sample mass and for measuring the half-life of $^{136}$Cs. The neutron capture cross section was determined as 164 $\pm$ 10 mbarn and 34.8 $\pm$ 3.0 mbarn at 30 keV and 500 keV, respectively, and were used to normalize the theoretically derived cross section shape.