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Nuovo Cimento. A; v. 23(2); p. 200-208

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Uguzzoni, A.; Gaertner, K.; Lulli, G.; Andersen, J.U., E-mail: uguzzoni@bo.infn.it

AbstractAbstract

[en] A first estimate of the minimum yield was obtained from Lindhard's theory, with the assumption of a statistical equilibrium in the transverse phase-space of channeled particles guided by a continuum axial potential. However, computer simulations have shown that this estimate should be corrected by a fairly large factor, C (approximately equal to 2.5), called the Barrett factor. We have shown earlier that the concept of a statistical equilibrium can be applied to understand this result, with the introduction of a constraint in phase-space due to planar channeling of axially channeled particles. Here we present an extended test of these ideas on the basis of computer simulation of the trajectories of 2 MeV α particles in <1 0 0> Si. In particular, the gradual trend towards a full statistical equilibrium is studied. We also discuss the introduction of this modification of standard channeling theory into descriptions of the multiple scattering of channeled particles (dechanneling) by a master equation and show that the calculated minimum yields are in very good agreement with the results of a full computer simulation

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S0168583X99010186; Copyright (c) 2000 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)

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Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms; ISSN 0168-583X; ; CODEN NIMBEU; v. 164-165(4); p. 53-60

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[en] In diffusion models for ion propagation along a crystal axis the average over the accessible area in the transverse plane of a 'local diffusion coefficient' is required. The nuclear contribution to this quantity has been calculated by different procedures and approximations. A comparison is shown among the classical Lindhard's formula, the quantum treatment by Ohtsuki and a Monte Carlo simulation. The Monte Carlo results agree very well with the Lindhard formula at large distances from the string and the Ohtsuki one at small distances

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[en] In this paper some problems present in the extraction of nuclear lifetimes from blocking experiments are discussed with reference to experimental results relative to the 'zero reaction time' (p, α) resonance at 1183 KeV in

^{27}Al single crystals. The crystal preparation and the particle detection are described in detail. A simple method of reducing the collected data to a one-dimensional radial distribution is also presented. The accuracy of a computer simulation of the blocking effect of 2 MeV α-particles in the crystal is discussed. It is shown that by such a method an angular distribution of emerging particles from the crystal is obtained that agrees with the experimental one if reasonable 'multiple scattering' effects are taken into account by properly folding in a gaussian angular broadening. (author)Primary Subject

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Radiation Effects; v. 32(1-2); p. 109-112

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[en] We have used the crystal blocking technique to measure the lifetimes of

^{28}Si compound states excited by the^{27}Al (p,α) reaction at E/sub p/ < 937 and 885 keV. Axial blocking on the <110> axis of thick Al crystals and Kodak LR 115 plastic detectors are used. Measurements have been made at a 0.2 μm depth. A study is made of the yields of some (p,α) resonances on Al and of the depth dependence of blocking dips. The data are analyzed by comparison with a realistic Monte Carlo model. The resulting lifetime, tau_{937}< 17 as (as < 10^{-18}s) and tau_{885}< 38atsub -at9^{+10}as are compared with previous workPrimary Subject

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Physical Review. C, Nuclear Physics; ISSN 0556-2813; ; v. 19(5); p. 1606-1614

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[en] Two measurements are compared in view of the different methods of analysis. It is shown that the results are the same if thermal vibrations are included in the continuum model for blocking. The combined level width is Digamma equal to 17

^{+-4}_{3}eVPrimary Subject

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Lettere al Nuovo Cimento; ISSN 0024-1318; ; v. 25(3); p. 65-68

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[en] The note deals with the spectrum of Bremsstrahlung γ-rays associated with a low-energy nuclear reaction and proposes the introduction of the time development of the nuclear-wave packets in a previously developed quantum treatment of the problem. Calculations are reported in the case of a Lorentzian energy weight function of the wave packet

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Lett. Nuovo Cim; v. 16(9); p. 257-260

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[en] The width of the 12.291 MeV level of

^{28}Si was measured by the axial blocking of α particles from the^{27}Al( p,α_{0}) resonance at E/sub p/=731.4 keV. The result GAMMA=28^{+13}/sub -/6 eV compares very favorably with that obtained from yield measurements but not with a previous planar blocking experimentPrimary Subject

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Physical Review. C, Nuclear Physics; ISSN 0556-2813; ; v. 21(4); p. 1646-1647

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AbstractAbstract

[en] In this paper a new method is proposed for calculating, for any transverse energy, the diffusive nuclear contribution to the rate of increase with depth of the mean transverse energy of positively charged ions subjected to the stochastic effects of the thermal vibrations of the string atoms of a crystal. A thermally averaged single-string potential is used to calculate the average force and the initial transverse energy distribution. The corresponding diffusion equation is solved in the case of axial blocking of approx. 2 MeV α-particles in aluminium single crystal, already studied in a previous work. The results here reported are still in satisfactory agreement with Monte Carlo simulations. (author)

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Radiation Effects; ISSN 0033-7579; ; v. 69(1-2); p. 113-125

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[en] To study the charge exchange cooling of channeled ions, we have made simulations of a single collision with an atomic string for 1.5 MeV/u Al ions in (1 0 0) Si. The relative magnitude of the probabilities for capture and loss varies with the distance from the string, and the irreversibility leading to cooling is associated with an asymmetry in time of the ion charge state during the collision. Combining the simulations with simple analytical calculations, we investigate the transition from the perturbation limit with very few charge exchanges to the adiabatic limit with a local charge state equilibrium. In this limit, the symmetry in time is restored and the cooling is strongly suppressed. The simulations illustrate the interplay of charge exchange and multiple scattering due to thermal displacements of atoms, and cooling by charge exchange can be included in the diffusion equation for dechanneling. We use the results of the simulations to obtain an estimate of the flux enhancement in an axial direction from an initially isotropic angular distribution. The conclusion is that the observed magnitude of this enhancement can be explained by charge exchange cooling, with plausible assumption about the impact parameter dependencies of electron capture and loss

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S0168583X02007371; Copyright (c) 2002 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)

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Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms; ISSN 0168-583X; ; CODEN NIMBEU; v. 193(1-4); p. 118-127

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