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On the context of the Cern n_TOF collaboration the 232Th(n,2n)231Th reaction cross section has been measured relative to the 56Fe(n,p)56Mn and 27Al(n,a)24Na reaction cross sections by the activation method for neutron energies up to 11 MeV. The neutrons were produced via the 2H(d,n) reaction using a deuterium filled gas-cell, at the 5.5MV TANDEM Accelerator of NCSR "Demokritos". In addition to the experimental work, theoretical Statistical model calculations have been performed using the computer code STAPRE/F. The code STAPRE is designed to calculate energy averaged cross sections for particle induced nuclear reactions with several emitted particles and gamma rays under the assumption of sequential evaporation. For the first evaporation step preequilibrium emission is taken into account while population of states resulting from the first equilibrium evaporation step is calculated using the Hauser-Feschbach theory. Fission process competition is also taken into account in the evaporation steps. Sensitive parameters for the calculation, like level density parameters, have been adopted after fitting experimental data for the competing (n,f) reaction. The results are being compared to the experimental data.

The neutron flux and energy distribution of the neutrons produced in a gas cell were measured using a silicon semiconductor detector. The energy spectra produced by the borbardment of the silicon detector with neutrons were simulated using Monte Carlo techniques and were compared with the measured spectra

The 7Li + 11B reaction has been investigated in the energy range 5.5 MeV < E lab < 19MeV, by detecting γ-ray resulting from the de-excitation of evaporation residues. Statistical compound-nucleus calculations have been performed in order to extract both the cross-sections of individual exit channels and the fusion cross-section of the system. The total angular momentum that the compoundn ucleus 18O can support has been deduced and is seen to exhibit saturation for a limiting value of 8.5ħ at the high-energy extreme. The results are discussed in terms of the entrance channel and statistical yrast line limitations.

The mean size of Cu clusters produced in a hollow-cathode, dc plasma-discharge source, increases significantly with decreasing flow rate and less pronouncedly with increasing Ar/He abundance ratio and increasing discharge current. The effect of pressure is mixed. Between 0.4 and 0.6 mbar the size is not affected. Above 0.6 mbar, the size increases significantly with increasing pressure. With the help of the above parameters, the mean cluster size can be shifted at will between 20 and 250 A. Implications concerning the mechanisms associated with clustering in plasma are discussed.

The size of the clusters produced in a plasma-discharge, hollow-cathode source depends sensitively on the distance separating the cathode from a grounded grid introduced into the source to intercept the plasma beam. On this evidence it was derived that a crucial parameter for cluster growth in that source is the time interval during which particles and plasma coexist. In fact, an approximately linear correlation between cluster size and plasma interaction time is indicated by the experimental data, implying that up to about 1 ms the rate of cluster growth is constant. Monte Carlo calculations for particle coagulation in low-temperature plasma, predict reasonably well the size evolution but not the ionization of the clusters.

Summarization: The competition between neutral, positive and negative metal clusters produced via hollow-cathode discharge, was found to depend on the speed of the pump used to evacuate the cluster source as well as on the distance separating the cathode from a grounded grid introduced into the source to intercept the plasma beam. On this evidence it is concluded that the competition between positive and negative cluster ionization depends on the time interval during which clusters and plasma coexist. For coexistence time intervals less than about 1 ms, the relative abundances remain constant and the ratio of negative to positive cluster intensities ranges between 1.6±0.3 for Cu and 2.6±0.3 for Fe. For longer times, the abundance of negative clusters increases and that of positive decreases to the point of complete extinction. In terms of the cluster ionization mechanisms involved, it is suggested that initially several ionizing mechanisms producing negative as well as positive clusters proceed with comparable rates. At later time intervals, however, electron attachment becomes the dominant, if not the exclusive, particle ionization process. Presented on: Vacuum

Deflection by an electric field has been tested as a means to deposit size selected clusters. A hollow-cathode, glow-discharge, source has been used for the production of Cu clusters in the size range between 4 and 38 nm, while a field emission scanning microscope has been used for HREM investigation of the size and morphology of clusters supported on Alfoil. The deflected negative clusters demonstrate size selection as a function of displacement while the positive do not follow the same displacement function. Size selection is also observed in the neutral clusters, with the heaviest clusters deposited at the center of the undeflected beam spot. Implications concerning the synthesis of cluster-composed, nanostructured materials are discussed.

A new method is presented for transmission measurements of total neutron cross sections. The detection system has low background with good monitoring of the incident neutron flux and can be used for measurements at En > 5.5 MeV. This method has been applied to total cross section measurements for the system 11B + n at En = 7.3–8.5 MeV. The results show the potential of this method for nuclear spectroscopy measurements of high accuracy.

The cross section of the11B(n, α)8Li reaction has been measured atEn=7.6 to 12.6 MeV. The neutron beam was produced via theD(d, n)3He reaction and aBF3 counter (with naturalB isotopic composition) served both as target for the11B nuclides and as detector for the observation of the delayedα-activity of8Li. The data match well with previous results obtained atEn=12.5 to 20.0 MeV. Using the principle of detailed balance the data were converted to the case of the8Li(α, n0)11B reaction. The associated astrophysicalS(E) factor is dominated by a resonance atER=0.58 MeV of widthΓR =200 keV, withS(ER)=8400 MeV b. ThisS (ER) value for the n0 channel alone is already three times higher than the constantS(E) factor assumed previously and, thus, strengthens the significance of inhomogeneous Big Bang nucleosynthesis.

The 232 Th(n,2n) 231 Th reaction cross-section was measured relative to the 56 Fe(n,p) 56 Mn and 27 Al(n,a) 24 Na reaction cross-sections with the activation method for neutron energies up to 11 MeV. Neutrons were produced by the 2 H(d,n) reaction in the 5.5 MV Tandem accelerator of the NCSR ‘‘Demokritos’’. The characteristic g lines of the product nuclei 231 Th (25.646 keV) and 56 Mn (846.771 keV) or 24 Na (1368 keV) were measured with an 80 mm 2 Si(Li) and a 20% reverseelectrode coaxial germanium detector, respectively. The results indicate that our values are in agreement with the JEF2 and BROND2 database values up to 9 MeV and slightly higher than those from all the libraries for higher energies. r 2003 Elsevier Science B.V. All rights reserved.

A new technique for measurements of total neutron cross sections is presented. This method has been applied to measurements of the excitation function for the "B + n system at E = 7.2 - 8.5 MeV, in order to obtain the 8Li(n , 1 B thermonuclear reaction rate. 1. INTRODUCTION In this work, we present a system for transmission measurements of total neutronaoss sectionsusing two silicon semiconductor detectors. One detector is placed in front of the target and monitors the incident neutron flux. The second detector is placed behind the target and measures the attenuation of the incident neutrons as well as their energy.This system has been used to measure the excitation function of the "B(ri. x) reaction , where x is either a neutron or an alpha, and at incident neutron energies between 7.2 and 8.5 MeV.The parameters of the resonances observed in this reaction together with those observed in the previously measured "B(n, a)8 Li reaction2 were used to derive the stellar rate for the 8L i(a, ri)1' B

Cross sections and angular distributions for the 26 Mg(p, n) 26 Al and 23 Na(α, n) 26 Al reactions, populating the ground state and the first three excited states in 26 Al, have been measured between 5.3 and 7.0 MeV proton bombarding energy and between 3.9 and 5.9 MeV alpha bombarding energy, using the neutron time-of-flight technique. The measured total cross sections were used to derive the cross sections for the astrophysically interesting reactions 26 Al(n, p 0 ) 26 Mg and 26 Al(n, α 0 ) 23 Na. The 26 Mg(p, p′) 26 Mg and 26 Mg(p, α) 23 Na cross sections were also measured at E p = 5.3–6.1 MeV. A resonance analysis of the measured 26 Mg+p excitation functions is used to obtain information on cross sections for the 26 Al(n, p') 26 Mg reaction.

The 96Mo(p, n) and 96Mo(p, nγ) reactions have been studied for proton energies between 3.8 and 5.5 MeV. Energy levels in 96Tc up to 632 keV excitation energy have been determined. Possible spin and parity assignments are given for several levels based on the neutron enhancement and angular distributions observed on and off resonance of the 5 2 >+ isobaric analog state in 97Tc, as well as the observed γ-yields. The first excited state reported at 34 keV was found to be a close doublet only 0.8 keV apart. The observation of this doublet in the (p, n) reaction was used to determine the ground state Q-value Q = − 3.760±0.010 MeV.

A resonance analysis was performed of the26Al(n, p0)26Mg(g.s.) and Al(n, α0)23Na(g.s.) reactions atEn=(0÷150) keV. The extracted resonance parameters were used to make predictions about the destruction rate of26Al via the26Al(n, p)26Mg and26Al(n, α)23Na reactions, at stellar temperaturesT=(108÷109)K.

The deep inelastic reactions of 32S with 27A1 have been studied at incident energies of 152.6 and 175 MeV by measuring inclusive fragment distributions, fragment-fragment and fragment-light particle coincidences. The observed Q-value distributions of the primary deep inelastic process range down to large negative Q-values as predicted by time-dependent Hartree-Fock calculations in the presence of low partial waves. No distinct low-l window for deep inelastic reactions can, however, be inferred from the data.

Elastic scattering and evaporation residues have been measured for the system 32 S + 27 Al at E c.m. = 66.4, 73.2 MeV and 32 S + 48 Ti at E c.m. = 96.0 MeV. Reaction cross sections have been obtained by use of the optical theorem and are found to be about 60 % larger than the fusion cross sections.

Energy and angular distributions of neutrons, protons, and alpha particles were measured in coincidence with the projectilelike fragment in deep inelastic collisions of $^{16}\mathrm{O}$ on $^{40,44,48}\mathrm{Ca}$ at 102 and 142 MeV. Using an event-by-event analysis which takes into account particle emission from both fragments and the recoil correction due to light particle emission, we show that our data are consistent with the assumption that the light particles are evaporated in the rest frame of the fully accelerated projectilelike and targetlike fragments. This conclusion is based on the results for neutrons and alpha particles. But because of kinematic limitations which affect the alpha particle data, the conclusions based on the latter are less definitive.