Browsing by Subject "Nuclear Physics"
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- ItemOpen AccessAlpha decay and alpha elastic scattering by heavy nuclei(1993) Tripe, Peter; Perez, Sandro MA simple three parameter cluster model has previously been developed by Buck, Merchant and Perez to successfully describe alpha decay half lives for more than 400 nuclei. An important feature of this model is that it envisages preformed (preformation factor, P=1.0) alpha particles in the parent nuclei to be moving in orbits with a large value of a global quantum number, G. The discontinuity in decay half-lives at the N=l26 neutron shell closure is then naturally explained by an increase of G as the alpha particle is forced into a higher orbit by the shell closure. We consider alternative approaches to this model and extend it to consider different values of P, and different changes in G at shell closures. We find the original approach of Buck et al., a radius fit with ΔG=2, is still the most successful, but that a potential fit with ΔG=4 turns out to be competitive, lending support to the suggestion that the proton shell closure is also felt. We have also analysed low energy (24.7MeV) scattering of alpha particles from a number of heavy nuclei in an attempt to find a common set of potential parameters that adequately reproduces both the decay and scattering data. Although not completely successful in this attempt, we find that the potential parameters obtained in the decay calculations provide a good first guess at the scattering potential parameters. The above analysis constrains P to the range 0.01≤P≤0.1, with the value of P=1.0 not ruled out.
- ItemOpen AccessHigh-energy approximations to nuclear scattering(1973) Schürmann, Bernd; Frahn, W EWe study high-energy approximations to nuclear scattering. These are based on expansions of the free-particle propagator. We distinguish the eikonal expansion and the Fresnel expansion and interpret their physical meaning in optical terms. A "Fresnel approximation" is defined as a partial sum of the eikonal expansion which describes Fresnel diffraction effects. In Fresnel approximation we derive, by means of a unitary transformation, a closed representation of the scattering amplitude which is formally similar to the corresponding expression irt Glauber's high-energy scattering theory. The corrections to the Glauber model of first order in the reciprocal wave number are given by the second-order eikbnal approximation. These are evaluated explicitly for high energy elastic scattering of protons from light nuclei (⁴He, ¹²C, ¹⁶O).
- ItemOpen AccessMonte Carlo simulations of the iThemba LABS neutron beam facility(2010) Adam, Buthaina Abdalla Suleiman; Nchodu, Rudolph; Buthelezi, ZinhleThe iThemba LABS neutron beam facility is currently being used for various applications of fast neutron studies, such as measurements of fission cross sections, the biological effectiveness of high-energy neutrons, calibration of detectors used for dose monitoring in space and aircrafts, and the development of neutron dose monitors. Neutron beams with energies up to 200 MeV are produced at iThemba LABS by irradiating thin targets of 7Li and 9Be with protons from the separated-sector cyclotron. The neutrons are collimated to produce a beam with a diameter of about 50 mm at a flight path of 7.7 m from the target. The collimator geometry is designed to maximize the central part of the beam resulting in a beam with a uniform intensity throughout its diameter and a small penumbra. Secondary neutrons produced from the interactions of the primary charged particles with structural parts e.g. beampipes, shielding wall, target holder, etc. have been observed in the measured neutron fluence spectra. The Monte Carlo radiation transport code FLUKA were used to study the effects of secondary neutrons on the neutron fluence spectra. Results obtained from the calculations were compared with those obtained experimentally.
- ItemOpen AccessNeutron induced alpha production from carbon between 18 and 22 MeV(1976) Stevens, Alun Peter; Brooks, F DCross sections for neutron induced alpha production in carbon were measured at seventeen energies between 18 and 22 MeV, using a deuterated anthracene crystal as both target and detector. Pulse shape discrimination was employed to separate the alphas and elastically scattered deuterons from the other reaction products. Published (n,d) elastic scattering data were used as a standard to obtain the alpha production cross sections. Comparison with available measurements shows good agreement
- ItemOpen AccessThe neutron-deutron breakup cross section between 8 and 22 MeV(1973) Pauletta, Giovanni; Brooks, F DThe n-d breakup cross section has been measured at eleven energies. between 8 and 22 MeV by integrating the energy distributions of breakup protons in a deuterated scintillator. The breakup protons were separated from the recoil deuterons by pulse shape discrimination. A comparison of these measurements with recent calculations favours those performed with local potentials.
- ItemOpen AccessPolarization in neutron-deuteron scattering from 8 to 22 MeV(1975) Steinbock, Mary; Brooks, F DThe polarization in n-d scattering has been measured at incident energies 7.9, 16.4, and 21.6 MeV using neutrons from the ⁹Be(α,n) ¹²C and ³H(d,n) ⁴He reactions. The polarization analyzer was a deuterated anthracene scintillation crystal. Recoil deuterons from n-d elastic scattering within the crystal were studied and pulse shape discrimination (PSD) was used to determine the left-right asymmetry of the recoils. The experimental method depends on the fact that the PSD response of the crystal is sensitive to the direction of the recoil deuteron relative to the crystal axes. The results obtained are compared with theoretical calculations from the literature and with other measurements. A comparison of the n-d and p-d polarization data reveals no evidence of breaking of the charge symmetry of nuclear forces.
- ItemOpen AccessPrompt neutrons from fission of 252Cf and 235U(1977) Pringle, John Stephen; Brooks, F DMeasurements have been made of the n-n angular correlation for neutrons emitted in spontaneous fission of ²⁵²Cf and thermal-neutron- induced fission of ²³⁵U. The dependence of the angular correlation on the neutron detection threshold energy was investigated. These measurements and other experimental data on the fission spectrum and the neutron angular distribution are compared with Monte Carlo calculations based on the evaporation model. It is apparent that a model assuming isotropic evaporation of all neutrons from the moving fragments cannot adequately explain the observed distributions. No evidence was found favouring the emission of a fraction of the neutrons isotropically at scission, or during the fragment acceleration period.