Browsing by Author "Aschman, David G"

Now showing 1 - 9 of 9
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    Development of a recoil detector and the study of exotic asymmetric shapes in nuclei
    (2011) Ntshangase, Sifiso Senzo; Aschman, David G
    A number of interesting physics phenomena have been predicted to occur in the actinide region. These can be studied by measuring fusion-evaporation residues produced in a heavy-ion fusion reaction. However reactions leading to this region are characterized by very low cross-sections and fission is a dominant channel...
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    Fast neutron scattering analysis
    (1998) Buffler, Andy; Brooks, F D; Aschman, David G
    The scattering of a beam of fast monoenergetic neutrons is used to determine elemental compositions of small (0.2-1 kg) samples of materials. Particular emphasis is placed on the measurement of concentrations of the elements H, C, N and O, which are the principal constituents of contraband materials, such as explosives and narcotics. Scattered neutrons are detected by liquid scintillators located at forward and at backward angles, and different elements are identified by their characteristic scattering signatures derived either from a combination of time-of-flight and pulse height measurements or from pulse height measurements alone. Atom fractions for H, C, N, O and other elements are derived from unfolding analyses based on these scattering signatures and used to identify materials. Effects of neutron interactions in surrounding materials, either in the neutron beam or between the scatterer and the detectors, can be detected and allowed for in such a way as not to interfere significantly in the identification of the scatterer. The Fast Neutron Scattering Analysis technique provides a non- intrusive method for detecting and identifying sub-kilogram quantities of contraband materials. Methods for locating the positions of small contraband items in packages of volume up to about 0.5m³ are described and a two-stage screening system for detecting contraband hidden in small packages is proposed.
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    Investigation of the ⁵¹V(n,d)⁵⁰Ti reaction
    (1992) Qangule, Lusanda Lungelwa; Aschman, David G
    The ⁵¹V(n,d) ⁵⁰Ti reaction has been studied with neutrons of energy of 21.6 MeV. The measurement of the angular distribution of the differential cross section was made with the aid of a spectrometer designed by Dr. McMurray and Prof K. Bharuth - Ram. The spectrometer consists of three multiwire proportional counters followed by a curved plastic scintillator. The proportional counters act as ΔE detectors and the scintillator acts as an energy detector. The ΔE - E method based on the Bethe - Bloch formula was used for particle identification. The spectrometer allows accumulation of data over an angle range of ≥ 70°, has angular resolution (FWHM) ≈ 5° and energy resolution (FWHM) ≈ 0.8 MeV. A review of the optical model and the reaction mechanism is presented. Theoretical DWBA cross sections are performed by using the Distorted Waves code DWUCK4. Spectroscopic factors for transitions to the 0⁺, 2⁺ and (4⁺ , 6⁺ ) states in ⁵⁰Ti have been determined from the comparison of experimental differential cross sections to theoretical DWBA differential cross sections. These have a mean value of 0.42, 0.30 and 0.38 respectively. The spectroscopic factors are compared with the results from (e,e' p) and (d,³He) measurements and shell model calculations.
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    Nucleon transfer from heavy-ion reactions using the AFRODITE gamma-ray spectrometer
    (2005) Benatar, Marco; Aschman, David G; Sharpey-Schafer, John F; Mullins, S M
    The 1- radiation following the interactions of 1271 on 197 Au and 194Pt at ELAB = 730 MeV has been studied. The beam energy is approximately 9.5% above the Coulomb barrier. The aim of the present work is to study multi-nucleon transfer to and from the target. At energies above the Coulomb barrier, stripping and pickup reactions occur, quasi-elastic and deep-inelastic events dominate, with the target-like and projectile-like fragments remaining in contact over a sufficient period of time for degree of mass and NIZ ratio equilibration to occur. Relative intensities of various target-like fragments as well as projectile-like fragments have been extracted using the RADWARE and GRAZING program respectively. The spectroscopy of the fragments has been investigated by 1-1 coincidence techniques using the AFRODITE Spectrometer from the iThemba Laboratories. Isotopes of Au and Pt have been observed as well as other nuclei having lost or gained one to two protons in the process. Q-values are also calculated and plotted versus the relative intensities. The results of these plots are compared with the predictions of the GRAZING program. The aim of the present work is to determine whether the unpaired proton from both the projectile and the target influences the transfer of nucleons and whether the transfer is done in purely statistical way or again if the unpaired proton does playa part in the transfer. It was found that for both 1271 on 197 Au and 194Pt at ELAB = 730 MeV, the maximum number of transferred nucleons was only 4- and that the predictions from the GRAZING program do not agree with the extracted relative intensities from RADWARE.
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    The (p, n) charge-exchange reaction on ⁹⁰Zr at intermediate energies
    (1992) Kabutz, Rudolf T; Aschman, David G
    Using the Time-of-Flight facility at the National Accelerator Centre at Faure, the (p, n) charge-exchange reaction has been studied at intermediate energies of 120, 160 and 200 MeV, and at angles of 0°, 2° and 4°. In this work the data collected for the ⁹⁰Zr target will be presented. The influence on the data from slow neutrons due to previous pulses is discussed and the best manner of removing them from the spectra is recommended. It is shown how the background cosmic rays can be utilised to measure the intrinsic resolution of the detectors and to obtain an estimate of the neutron energy threshold. The differential cross-sections for the states corresponding to Fermi and Gamow-Teller transitions were extracted from the time spectra. The sum of the strength of all the discrete Gamow-Teller states was determined and compared to the Ikeda Sum Rule. It was found that only 50% of the sum could be accounted for in the discrete states. An overview of the theory that has been developed to extract Gamow-Teller strengths from the (p, n) cross-sections is given. Some of the theoretical models that have been used to describe the ⁹⁰Zr(p, n)⁹⁰ Nb reaction and account for the missing Gamow-Teller strength are briefly discussed.
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    Radiative proton capture to second harmonic giant dipole states in ¹³N
    (1990) Steyn, Douw; Aschman, David G
    Angular distributions have been measured for the radiative proton capture reaction ¹²C(p,ϒ)¹³N at beam energies from 40 to 54 MeV. Measurements were made with the new, large anticoincidence-shielded spectrometer HA GAR at angles from 30° to 150° to the beam direction. Differential and total cross-sections have been calculated from these data, and show evidence for resonant behaviour for the transitions to the ground state, the first excited state, and the unresolved second and third excited states. The total cross-sections for these transitions peak at a proton energy of about 46 MeV which is twice the centroid energy of the Giant Dipole Resonance (GDR) based on the ground state of ¹³N.
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    Signature splitting and non-axiality in the nuclei ¹⁶⁴Ta and ¹⁶⁵Ta
    (2001) Roux, David Gerhardus; Aschman, David G; Babu, B R S; Gueorguieva-Lawrie, E; Lawrie, J J
    High-spin states in two deformed nuclei (¹⁶⁴Ta and ¹⁶⁵Ta) have been studied in two separate experiments with the AFRODITE escape suppressed spectrometer array at the National Accelerator Centre, near Cape Town. For both of these nuclei, definitive rotational band structures up to high spins have been established for the first time. Excited states in ¹⁶⁵Ta (Z = 73, N = 92) were populated in both the first experiment, ¹⁴²Nd(²⁷Al,4n) ¹⁶⁵Ta, and the second experiment, ¹⁴¹Pr(²⁸Si,4n) ¹⁶⁵Ta. The yrast rotational decay sequence, identified up to spin 53/2⁻, has been assigned the π[514]9 /2⁻ configuration. A comparison of the experimental results with predictions of Cranking Shell Model (CSM) calculations reveal an unexpectedly large signature splitting, for a high-Ω configuration, for the yrast band. Further discrepancies between theory and experiment are observed for the bandcrossing frequency and the signature splitting of the B(Ml)/ B(E2) ratios. The possibility that these discrepancies are associated with a substantial deviation from an axially symmetric shape is explored. In addition a weaker excited band, possibly associated with the π[402]5/2⁺ configuration, has also been identified. High-spin states in ¹⁶⁴Ta ( Z = 73, N = 91) were populated in the first experiment. The yrast band, identified up to spin 21⁻, has been assigned the configuration π[514]9/2⁻X v[660]1/2⁺. Anomalous signature splitting of the yrast band is observed at low spins. The splitting is discussed in relation to the systematic trends and its implications for the nuclear shape are considered. Previous attempts to explain the phenomenon are reviewed. The 11⁻ level was found to be isomeric, both in this nucleus and in its N = 91 isotone ¹⁶²Lu, a by-product of the first experiment. This is the first time that a nanosecond-range isomer has been found in the light tantalum or lutetium nuclei. A less intense excited band was also identified, and was tentatively associated with a four-quasiparticle configuration.
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    The study of Gamow-Teller strength in the 51V(p,n)51 Cr reaction at intermediate energies
    (1992) Tshivhase, Victor Makondelele; Aschman, David G
    The ⁵¹V(p,n)⁵¹Cr reaction was studied at four different proton beam energies namely 90, 120, 160, and 200 MeV. The experiment was performed in the angular range between 0 and 4°, using the beam swinger facility at National Accelerator Centre(NAC) at Faure, near Cape Town. The cross sections of the ground state, Isobaric Analog State, and the giant Gamow-Teller resonance were calculated at 0° for each beam energy. The ratio of the cross section of the giant Gamow-Teller resonance to the cross section of the Isobaric Analog State were studied at beam energies 90, 120, 160 and 200 Me V, and beam angles 0, 2 and 4 °. The Gamow-Teller strength was estimated for energies 120, 160 and 200 MeV. The fractions of Fermi and Gamow-Teller strength in the Isobaric Analog State were also estimated.
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    A study of Gamow-teller strength using the 208Pb(p,n) and 181Ta(p,n) reactions at intermediate energy, Ep=122 MeV
    (1997) Tshivhase, Victor Makondelele; Aschman, David G
    The Gamow-Teller strength was measured in ²⁰⁸Pb(p,n)²⁰⁸Bi and ¹⁸¹Ta(p,n)¹⁸¹W reactions at an incident proton energy of 122 MeV. The experiment was performed using the beam-swinger facility at the National Accelerator Centre, Faure, South Africa. The time-of-flight(TOF) technique was employed with a flight-path of 100 m. Detectors comprised three stacks of four rectangular bars of NE 102A plastic scintillators. The outgoing neutrons were observed at laboratory angles, ΘLab = 0°, 2°, 4° and 10° . A multipole decomposition analysis of spectra associated with ²⁰⁸Pb(p,n)²⁰⁸Bi and ¹⁸¹Ta(p,n)¹⁸¹W reactions was performed using the results of distorted-wave impulse approximation (DWIA) calculations. The spectra associated with the two reactions were decomposed into L=0,1 and 2 transfer components. The major contribution to the spectra were found to be L=O, with a very small 1=2 multipole transfer contribution. The spectra were modelled with a combination of symmetrical Gaussians and a polynomial background. The GT strength was extracted by normalizing the cross section of the GT states to the cross section of the isobaric analogue state. The ΣB(GT) strength associated with the ²⁰⁸Pb(p,n)²⁰⁸Bi and ¹⁸¹Ta(p,n)¹⁸¹W reactions was found by summing the individual GT strength in the 0 - 23 MeV and 0 - 20 MeV excitation regions respectively. The ΣB(GT) strength associated with the ²⁰⁸Pb(p,n)²⁰⁸Bi and ¹⁸¹Ta(p,n)¹⁸¹W reactions were found to be less than the values predicted by the 3(N - Z) sum rule, being 81 ± 10% and 78 ± 3% of the sum rule strength respectively.