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[en] Complete text of publication follows. The ionization of the hydrogen atom in intense laser field is studied theoretically employing quantum mechanical and classical models. Classical calculations were performed within the framework of the classical trajectory Monte-Carlo (CTMC) method and the obtained results are considered to be the 'exact' ones. For quantum mechanical calculations the Volkov and the first order models were used. The main difference between them is that in the Volkov model the Coulomb interaction between the ionized electron and the residual ion is neglected. The comparison of the first order and Volkov results allow us to study in details the influence of the Coulomb interaction during the laser pulse. The angular distribution of the electrons ejected at given energies is studied in details. At low ejection energies (see Fig. 1.) big discrepancies are observed between the first order and Volkov distributions. The observed discrepancies diminishes and disappears at higher ejection energies, which indicates that during the ionization the electrons with high ejection energies are less influenced by the Coulomb interaction then the electrons with low ejection energies. This can be explained using a simple intuitive picture presented on Fig. 2. The electrons with high ejection energy have their momentum in the initial state in the same direction as the net momentum transfer leading to a trajectory, with a very small portion close to the core where the Coulomb potential has a significant influence. On the other hand the electrons with lower ejection energies have a momentum in the initial state, which leads in the opposite direction of the net impulse transfer. In this way the low energy electrons need to 'go around' the core, leading to a trajectory, which has a long portion close to the core, where they can be influenced significantly by the Coulomb interaction
[en] Complete text of publication follows. It is very difficult to quantify the uptake kinetics and bio-distribution of magneto pharmaceuticals in humans using MRI (Magnetic Resonance Imaging). The well-know PET (Positron Emission Tomography) technique, however, could give a solution to this problem in the case of those MRI contrast agents that are based on manganese as paramagnetic contrast enhancer. Luckily manganese has a proper radioisotope, namely the 51Mn (T1/2= 46.2 min, β+ = 97%), which can be easily employed (in the form of 51Mn-labelled contrast agents) for PET studies. Recently, for the production of this radioisotope proton and deuteron induced nuclear reactions were suggested using natural and enriched Cr targets, respectively. In this work we studied the natV(3He,x)51Mn nuclear processes in detail from their respective threshold energies up to 40 MeV. For natural vanadium, the 51V(3He,3n)51Mn reaction (natural isotopic composition of 51V: 99.75%) forms the majority of the required radioisotope. The cross-sections were measured by the conventional stacked-foil method. Two stacks containing 10 and 8 pieces of thin natural V foils were irradiated in external collimated 3He beams of the AVF-930 isochronous cyclotron of NIRS. Thin copper and titanium foils served as energy degraders. The activations lasted for 1 h with a beam current of 100 nA. The activity of the irradiated samples was measured without chemical separation by using the usual gamma-ray spectroscopy. Since the 51Mn has a very weak gamma-line at 749 keV (Iγ=0.265%) its activity was measured via decay curve analysis of the annihilation peaks. We also measured the excitation functions of those reactions which form the major radio-contaminants i.e. 52mMn (T1/2=21.1 min, Eγ=1434.068 keV(Iγ=98.3%)) and 52Mn (T1/2 = 5.591 d, Eγ=744.223 keV (Iγ=90%), Eγ=935.538 keV (Iγ=94.5%)). The excitation function curve of the natV(3He,x)51Mn nuclear process shows one maximum of about 40 mb at about 30 MeV. The maximum cross-sections of the natV(3He,x)52mMn and natV(3He,x)52Mn processes are about 55 (at about 14.5 MeV) and 85 mb (at about 14.5 MeV), respectively. As far as we know, the present measurements describe a first systematic study of the above processes up to 40 MeV. Based on the results of the present measurements, thick target yields were also calculated for the production of 51Mn, 52mMn and 52Mn up to 40 MeV. We also evaluated the optimum production circumstances both for the targets and the applicability circumstances of the product. According to the results, the natV(3He,x)51Mn route seems to be useful for practical production purposes in the energy range of 23 → 50 MeV
[en] Complete text of publication follows. LaBr3:Ce scintillators are attracting particular interest for γ-spectroscopy due to their excellent time and energy resolutions. Motivated by Giant Resonance studies and neutron-skin thickness measurements, in this work we tested the energy resolution and efficiency of 1'x1', 1.5'x1.5' and 2'x2' crystals for γ-rays up to 17.6 MeV. At low energies the resolution and efficiency were measured using standard 60Co and 152Eu γ-ray emitters, which were placed at 15 cm from the face of the detectors. The measurements for the high energy region were performed with proton beams from the 5 MV Van de Graaf accelerator of ATOMKI using different (p,γ) reactions on 7Li, 11B, 27Al, 23Na, and 39K targets in the similar way as we did previously in calibrating a Clover detector. The detectors were set at 15 cm from the target and at 55 deg with respect to the beam direction, in order to minimize the effect of the different angular correlations. All targets were made by evaporation onto 0.1 mm thick Ta backings in vacuum. The thicknesses of the targets were varied between 10 - 75 μg/cm2. Beam currents varied between 1 and 2 μA and the charge collection was continued till the statistical error for the high energy γ-rays becomes less than 5 %. A typical gamma-spectrum for e.g. 2'x2' detector is shown in Fig. 1. Although the photopeak efficiency decreases at higher energy, this detector can still be used. Around 10 MeV it is comparable to the efficiency of e.g. an Euroball type Clover detector (with more than four times larger active volume ) in adding back mode. For energies above 15 MeV a larger LaBr3:Ce crystal would be required. The relative energy resolution (FWHM) is shown as a function of the γ-ray energy in Fig. 2. It goes well below to 1 % at 10 MeV and reaches about 0.5 % at 17.6 MeV, which is the best for scintillation type detectors
[en] Complete text of publication follows. The 3He(α,γ)7Be reaction plays a crucial role in two distinct fields of nuclear astrophysics: in the hydrogen-burning of the Sun (and similar main sequence stars) and in the big-bang nucleosynthesis. In the pp-chain of solar hydrogen burning the 3He(α,γ)7Be reaction is the starting point of the 2nd and 3rd branches of the chain from where the high energy 7Be and 8B neutrinos are originated. On the other hand, the rate of the 3He(α,γ)7Be reaction determines the primordial 7Li abundance which provides a stringent test of the big-bang nucleosynthesis models. Therefore, the precise knowledge of the astrophysically relevant low energy cross section of 3He(α, γ)7Be is of high importance. The LUNA collaboration has carried out a research program aiming to measure the 3He(α,γ)7Be cross section with high precision at low energies. In the first phase of the experiments, the cross section has been determined with the activation technique measuring the decay of the produced 7Be nuclei. The measurements have been carried out at energies between Ec.m. = 93 and 170 keV [1,2], lower than ever reached before. The research program has been completed by the on-line measurement where the prompt γ-radiation from the reaction has been measured. Similarly to the activation, the online experiments have been carried out at the LUNA-2 400 kV accelerator at the Gran Sasso National Laboratory in Italy. An intense (300 μA) 4He beam bombarded a differentially pumped windowless 3He gas target. The prompt γ-radiation has been measured with an ultra low background HPGe detector. The cross section has been determined at three energies: Ec.m. = 93, 106 and 170 keV. In order to investigate the possible difference between the cross sections measured with the on-line and activation methods, simultaneously with the on-line experiment the activation measurement has been repeated. Perfect agreement has been observed between the two methods . With our new high precision results the uncertainty of the 8B and 7Be solar neutrino fluxes from the 3He(α,γ)7Be reaction rate is reduced form about 8% to 3% and the solution of the 7Li problem of primordial nucleosynthesis based on the limited knowledge of this reaction rate is excluded
[en] Complete text of publication follows. The stability of the shell closures are marked by the size of the shell gaps measured as the energy difference between the single particle states belonging to different major shells. Going from 30Si to 24O, the N=20 shell gap is expected to gradually decrease, and N=14,16 shell gaps develop instead. Therefore, our aim was to determine the location of the excited states in 23O, which directly gives the single particle energies suitable to deduce the size of the N=16 and N=20 shell closures, via invariant mass spectroscopy combined with the (d,p) neutron transfer reaction. The experiment was carried out at RIKEN where a 94 A x MeV energy primary beam of 40Ar with 60 pnA intensity hit a 9Be production target of 3 mm thickness. The total intensity was approximately 1500 cps having an average 22O intensity of 600 cps. The separation of 22O particles was complete. The secondary beam was transmitted to a CD2 target of 30 mg/cm2. The reaction occurred at an energy of 34 A x MeV. The scattered particles were detected and identified by a 2 x 2 matrix silicon telescope placed 96 cm downstream of the target. The first two layers were made of strip detectors (with 5 mm width of each strip) to measure the x and y positions of the fragments. The protons emitted backward in the reaction were detected by 156 CsI(Tl) scintillator crystals read out by photodiodes. The neutrons coming from the decay of the produced 23O nuclei excited above the neutron separation energy were detected by a neutron wall. The energy of the neutrons was deduced from the TOF while the hit position was determined by identifying the rod that fired (in vertical direction) and by the time difference between the two photomultipliers attached to the ends of the rods (in horizontal direction). The excitation energy spectrum of 23O shown in Figure 1 was reconstructed from the momentum of the neutron and the heavy ion 22O by calculating the invariant mass and using the known neutron separation energy (2.74 MeV). Two peaks are clearly visible at 4.00(2) MeV and 5.30(4) MeV in the spectrum. The (d,p) reaction populates the single particle states of 23O. From a comparison with shell model calculations, the first one is the neutron d3/2 state, the energy of which gives the N=16 shell closure to be 4 MeV. This is large enough to explain why 24O is the last bound oxygen isotope. The second excited state observed in the present experiment does not have any counterpart in the sd model space, and corresponds to a state from the fp shell. Its energy relative to the d3/2 state determines the strength of the N=20 shell closure to be 1.3 MeV and provides a direct evidence for the disappearance of the N=20 shell closure at Z=8
[en] Complete text of publication follows. The Coulomb-Volkov approximation (CVA) has been widely used to describe the ionization of atoms by short laser pulses in the last decade. The CVA is a time-dependent distorted-wave theory that allows us to include the effect of the remaining core into the final state at the same approximation level as the external field. In this way, the collision dynamics due to the effects of the core potential on the detached electron can be directly probed. Several studies have been performed so far to determine the accuracy of the CVA. On the other hand, in the last two decades there was a great revival of the classical trajectory Monte Carlo (CTMC) calculations applied to atomic collisions involving three or more particles. These approximations gain importance in those cases when higher order perturbations should be applied or many particles take part in the processes. The CTMC method has been quite successful also in dealing with the ionization process in laser-atom collisions, when, instead of the charged particles, electromagnetic fields are used for excitation of the target. In the present work we study the efficiency of the strong field approximation (SFA), which is a variant of the CVA. The electron emission spectra of a hydrogen atom excited by ultra-short pulses are calculated within the framework of CVA and a classical trajectory Monte Carlo (CTMC) method (see Fig. 1.). We analytically prove that in the limit of zero pulse duration and finite momentum transfer, CVA reproduces the exact quantum mechanical electron yields
[en] Complete text of publication follows. In a recent experiment, we performed at GSI aiming at studying the neutron-skin thickness of 124Sn, six large volume (3.5'x8') LaBr3 scintillation detectors were used for detecting the γ-decay of the giant dipole resonance. These scintillators have very good energy and time resolution. The aim of the present work was to study the absolute full-energy peak efficiency of such LaBr3 detectors, in such conditions, which was used during the experiment (5 mm Pb, 3 mm Cu and 10 mm thick Al absorbers in front of the detectors). The experiments were performed at the Institute of Nuclear Research of the Hungarian Academy of Sciences (ATOMKI). At low energies the efficiency was measured using 60Co and 66Ga gamma-ray emitters, which were placed at 25 cm from the front face of LaBr3 detector. At higher energies a version of the pointpair or two-line method combining low energy radioactive lines with proton resonance capture lines was applied in the efficiency calibrations. This procedure requires two gamma transitions in a cascade. The efficiency for the high-energy region was obtained by normalizing in to the low energy part in the overlapping rage around 1.4 MeV (internal radiation). The target - detector distance was also 25 cm, and we got statistical errors less than 1%. The values of the following sources and reactions are used: 60Co, 66Ga, 23Na(p,γ)24Mg, 27Al(p,γ)28Si, 39K(p,γ)40Ca and 11B(p,γ)12C, 7Li(p,γ)8Be. All targets were made by evaporation onto thick tantalum backings in vacuum (0.1 mm). The absolute normalization of the efficiency was carried out by the 60Co source (calibrated by the National Office of Measures of Hungary), which had an activity of 47.52 kBq during the measurement. The full absorption efficiency of the detectors were divided by their solid angle, to get the internal efficiency. The measured efficiencies were compared with the calculated ones (shown by the red curve in Fig. 1) obtained using a GEANT4 code. The simulated efficiency was normalized to the measured values, by the factor of 1.15. The results are in a good agreement with the experimental data. The efficiency decreases with the γ-ray energy from 19.7(6) % for 1173.2 keV to 4.5(5) % for 17.6 MeV. Without absorbers the above efficiencies were 32 % for 1,17 MeV and 18 % for 17.6 MeV, respectively. Acknowledgements The work has been supported by the Hungarian OTKA Foundation No. K 72566.
[en] Complete text of publication follows. Understanding the capture process during atomic collisions is fundamental both the experimental and theoretical point of view. Theoretically, one of the main difficulties in the accurate determination of the charge exchange cross sections is that the many-body interactions among collision partners have to be taken into account. This behavior is significant for positron impact where the projectile trajectory is spread in three-dimensional space and cannot be approximated by a straight-line trajectory, as is done for heavy projectile impact. Therefore, the success of different approaches strongly depends on their ability to describe the many-body character of the collision. In this work the collisions between positrons and argon atoms are studied theoretically at impact energies between 10 and 60 eV. We treat the collision problem in the framework of a classical trajectory Monte Carlo (CTMC) model. It was shown that the CTMC method can be applied to light projectile impact. It is a non-perturbative method. All interactions between the colliding partners can be taken into account exactly during the collision. 106 of classical trajectories were computed to calculate total and state selective capture cross sections. Large numbers of trials were required because the total cross sections are composed of the cross sections for the many partial levels. The statistical error of the calculated total cross section data were below 2%. In the recent studies the maximum impact energy is 60 eV therefore electrons from n=3, i.e. from 3p and 3s shells of Ar atom contribute to the total positronium formation cross sections. The major contribution originates from 3p shell of the argon atom. The contribution of the 3s shell of the Ar to the total positronium formation cross section is about 1-5 %. We found that, in most of the cases, ground state positronium is formed. We found excellent agreement with the experimental data above 40 eV incident energies. We also presented an accurate prediction for excited positronium formation cross sections (see Fig. 1.). We hope that our present results will encourage experimentalists to measure these cross sections in the near future. Acknowledgements. This work was supported by the Hungarian Scientific Research Fund OTKA No. K72172. The author would like to express his personal thanks to Andrew Bergman for the critical reading of the manuscript.
[en] Complete text of publication follows. In hydrogen and fluorine containing plasmas, mutual neutralization in collisions of H+ and F- ions can be an important source of charge removal. The mutual neutralization reaction among the two ions has as far as we know never been studied theoretically nor experimentally. We have previously studied dissociative recombination and ion-pair formation in electron recombination with HF+ ions. In the present work, the cross section for mutual neutralization in collisions between H+ and F- ions at low energies (E < 10 eV) is calculated using a molecular close-coupling approach with a quasidiabatic representation of the potentials and couplings determined from adiabatic potentials. The nuclear dynamics for the coupled electronic states was described quantum mechanically including very high angular momentum for the colliding fragments. The effect of autoionization was considered in the so-called local (boomerang) model by letting the potential energy curves become complex valued. The coupled Schrodinger equation for the nuclear motion is solved using a numerical integration of the corresponding matrix Riccati equation and the cross section for mutual neutralization is computed from the asymptotic value of the logarithmic derivative of the radial wave function. The cross section has a sharp threshold and is dominated by the formation of the H(n 2)+ F(2P) fragments. The magnitude of the cross section is small compared to a system such as H++H-, which in contrary to the H++F- colliding system, has curve crossings between ionic and covalent states occurring at large internuclear distances. For H++F-, sharp oscillations are observed in the cross section. The structures are broadening or smeared out and the magnitude of the cross section is decreased when autoionization is added to the model. In order to understand and identify these resonant structures, angular momentum quantum numbers are assigned to them. Some resonant structures originate almost exclusively from a single partial wave, while for other structures, several partial waves contribute. Surprisingly little has been done on mutual neutralization reactions both experimentally and theoretically. Currently a new electrostatic storage device DESIREE is being built at AlbaNova, SU, in order to fill this gap. The colliding system studied here, can be a good candidate to be studied using this device. Acknowledgements. A.L. and N.E. acknowledge support from the Swedish Research Council. J.Zs.M. acknowledges support from the Wenner-Gren Foundation.
[en] Complete text of publication follows. Interpretation of the cross sections in multielectron ion-atom collisions is a challenging task for theories. The main difficulty is caused by the many-body feature of the collision, involving the projectile, projectile electron(s), target nucleus, and target electron(s). In the last decade large numbers of non-perturbative studies have been performed to explain experimental total-, single- and double-ionization and charge exchange cross sections. The success of various approaches depends strongly on how far a given theory is capable to describe the many-body character of the collision. Along this line the classical trajectory Monte Carlo (CTMC) method has been quite successful in dealing with the atomic processes in ion-atom collisions. The CTMC method treats the atomic collisions in a non-perturbative manner. Classical equations of motions are solved numerically with randomly selected initial conditions. One of the main advantages of the CTMC method is that the three-body interaction is exactly taken into account during the collision at a classical level. The Integrated Tokamak Modeling Task Force (ITM-TF) was set up in Europe, under EFDA (the European Fusion Development Agreement), in 2004. The main target is to coordinate the European modeling effort and provide a complete European integrated modeling structure, with the highest degree of flexibility, for prediction of actual experiments and -in the long term - of the International Thermonuclear Experimental Reactor (ITER). For the ITM-TF a large numbers of atomic data is necessary. We have experience in theoretical interpretations of processes in ion-atom and ion-solid collisions based on classical descriptions. We developed a classical code for the determination of the cross sections for fundamental scattering processes. To demonstrate the validity and the efficiency of the present code we performed test calculations to determine the state selective charge exchange cross sections for collision system of N7+ + H(1s) at a 50 keV/amu projectile energy (see Fig. 1). The present results are in excellent agreement with the previous results. We also performed systematic calculation to determine charge exchange cross sections with other charge states of N ion. Further works are in progress and will be published soon. Acknowledgements. This work, supported by the European Communities under the contract of Association between EURATOM-HAS, was carried out within the framework of the Task Force on Integrated Tokamak Modelling of the European Fusion Development Agreement.