Department of Physics

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Analyzing power and cross section distributions of the 12C (p,pα)8Be cluster knockout reaction at an incident energy of 100 MeV
(Stellenbosch : University of Stellenbosch, 2010-03) Mabiala, Justin; Cowley, A. A.; Fortsch, S. V.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The (p, pα) reaction on 12C was investigated experimentally using polarized incident protons of 100 MeV. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25◦ to 110◦, were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well. The observed agreement allows the extraction of distorted momentum distributions from experimental data. These distributions are very consistent over a wide range of angle pairs at which cross section energy-sharing distributions vary considerably. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of spin-orbit distortions were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-α cross section enters as a multiplicative factor in the three-body (p, pα) cross section expression. Spectroscopic factors derived from the data are fairly consistent with the trend of the theoretical predictions. Analyzing power data also follow the trend of free p-4He scattering data, and comparisons with DWIA predictions are in reasonable agreement. The theory reproduces also very well analyzing power angular distributions of the projectile-cluster two-body scattering at large angular momentum of the residual nucleus. This indicates that a quasifree knockout mechanism dominates the reaction. The two-body interaction response between the projectile and the α cluster was found to resemble the scattering of protons from a free α particle to a remarkable degree, the present results strongly imply the existence of preformed α clusters in 12C.
Cross sections and analysing power energy-sharing distributions of valence (p,2p)-knockout from 208Pb with a projectile of 200MeV
(Stellenbosch : Stellenbosch University, 2000-12) Bezuidenhout, Jacques; Cowley, A. A.; Arendse, G. J.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The aim of this work was to study the 208Pb(p,2p)207Tl quasi-free knockout process. The experimental data were measured at the National Accelerator Centre using incident polarised protons of 200 MeV. The two scattered particles, from the knockout reaction, were detected in coincidence and their energies were determined using a magnetic spectrometer and a solid state detector telescope. Cross section and analysing power energy distributions were extracted from the experimental measurements and these were compared with theoretical values for the Distorted Wave Impulse Approximation. The theoretical cross-section calculations predict the experimental cross-section distribution well for all combinations of distorting potentials and bound states that were investigated, both with regard to shape, as well as absolute magnitude. However the theoretical analysing power distributions did not agree with the experimental quantities. Therefore it is not clear whether the analysing power is a useful tool to extract information on the specifics of the quasi-free reaction mechanism. The spectroscopic factors were found to be consistent with the results obtained in previous studies, thereby inspiring confidence that the problem with the analysing power distribution is not ascribable to a possible deficiency in the experimental techniques exploited in this work.
Near-target and other heavy residues in the interaction of ¹²C and ¹⁶O with ¹⁰³Rh
(Stellenbosch : University of Stellenbosch, 2004-04) Buthelezi, Edith Zinhle; Cowley, A. A.; Steyn, G. F.; Van der Walt, T. N.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: This study forms part of a larger investigation which has as a primary objective the development of a comprehensive theoretical description of all the processes which contribute to the continuum in the interaction of 12C and 16O with nuclei. Previous investigations of 12C and 16O induced reactions on targets with mass close to A = 100 have shown that the experimental excitation functions and recoil range distributions of heavy residues can be reproduced satisfactorily by means of a theoretical model which takes relatively few dominant reaction mechanisms into account. These include the complete fusion of the projectile with the target, the incomplete fusion of break-up α-type fragments (i.e. single α particles, 8Be fragments and for the 16O induced reactions also 12C fragments) with the target and single-nucleon transfer at incident energies above about 15 MeV/nucleon. The mean-field interaction is mainly responsible for these interactions. The thermalization of the intermediate excited nuclei produced in this first stage of the reaction is described by an intranuclear interaction cascade, during which pre-equilibrium emission of particles and clusters may occur, followed by evaporation after statistical equilibrium has been attained. The model also included the probability that break-up α particles may escape with a large fraction of their initial energy after only a few interactions with individual target nucleons following their initial incomplete fusion. The theory also predicted an enhanced isobaric yield for residues with mass similar or near to that of the target. The subsequent analysis of the emission spectra of intermediate mass fragments in these reactions, however, indicated that two additional aspects need to be considered as well in order to reproduce the experimental data. The first is that the projectile may lose a substantial amount of energy in an initial-state interaction before breaking up, which can be described as a friction dissipative process. The second is that several other incomplete fusion channels of “non-α-cluster”- type fragments should also be included in a more complete description of these reactions as their contributions are not negligible. The present study has two main objectives. Firstly, to investigate the isobaric yield in the neartarget mass region by measuring production cross sections for 103Pd, 103mRh and 103Ru. Previous studies only provided data for 103Ag, which constitute only a few percent of the A = 103 isobaric yield. The new data constitute more than 80% of the A = 103 isobaric yield, which provide experimental confirmation of the enhanced isobaric yield in the near-target mass region. The second objective is to perform extensive new calculations of the excitation functions and recoil ranges in order to investigate the predictive power of the extended model in a priori calculations for the entire available data set.
Nuclear medium effects on analyzing power investigated with a proton knockout reaction
(Stellenbosch : Stellenbosch University, 2001-12) Neveling, Retief; Cowley, A. A.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Please see full text for abstract
Pre-equilibrium excitation of nuclei in ([rho], [alpha]) and ([rho], 3He) reactions induced by 120, 160 and 200 MeV protons
(Stellenbosch : Stellenbosch University, 1994) Arendse, Gillian John; Cowley, A. A.; Stellenbosch University. Faculty of Science. Department of Physics.
ENGLISH ABSTRACT: The study of pre-equilibrium reactions induced by medium-energy light ions have become of increasing importance in recent years as they represent the bulk of the measured spectrum. On a nuclear time scale these reactions are known to take place after a one-step process of exciting discrete states in the residual nucleus but long before the process whereby the projectile distributes its kinetic energy equally over all target nucleons to form a compound nucleus which decays statistically. In this work the systematic behaviour of the continuum of complex ejectiles was investigated as a function of incident proton energy and target mass. The continuum energy spectra for the reactions 27AI(p, X), 59Co(p,X), and 197 Au(p,X), with X=a and 3He, were measured using a five element detector telescope. Data were acquired at scattering angles between 10-100° at incident proton energies of 129 and 200 MeV, and at scattering angles between 20 - 140° for an incident proton energy of 160 MeV. Experimental angle-integrated cross sections were compared with calculated angle-integrated cross sections using the geometry dependent hybrid (GDH) model. The comparison between the experimental data and the calculations were used to quantitatively estimate the contribution of the evaporation process to the pre-equilibrium spectrum. The experimental angular distributions from the reaction (p,a) and (p,3 He) on 27 Al, 59Co, and 197 Au were compared with calculated angular distributions using the phenomenological parametrization of Kalbach. In general good agreement between the experimental data and the calculations were obtained. Minor discrepancies were observed with regard to target mass and incident proton energy dependence. It is believed that the characteristics of these discrepancies could provide guidance to the future attempts to develop a proper quantum mechanical description of the reaction mechanism of complex-ejectile production induced by energetic protons.
Pre-equilibrium helion emission induced by protons
(Stellenbosch : University of Stellenbosch, 2006-12) Bezuidenhout, Jacques; Cowley, A. A.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
This thesis is devoted to a study of the 93Nb( p r ,3He) and 59Co( p r ,3He) reactions at incident energies of 100 MeV, 130 MeV and 160 MeV. Double differential cross sections and analysing power distributions were measured from a threshold of ~30 MeV up to the kinematic maximum and at scattering angles between 15º and 120º. The experimental data were compared with theoretical calculations done by combining a statistical multistep theory with a deuteron pickup mechanism in the final stage. The contribution of the first three steps towards the total double differential cross section and analysing power was assessed. The theory described the experimental double differential cross section and analysing power data reasonably well over all incident and scattered energies and for both target nuclei. As the incident energy was increased, the characteristics of the reaction mechanism also remained consistent. Thus the results supported the underlying multistep-pickup theory. The two target nuclei demonstrated similar responses, suggesting these two share the same basic reaction mechanism. The total double differential cross section for the reaction dropped with an increase in incident energy. At a fixed emission energy, near the maximum allowed value, the slope of the double differential cross section increased with an increase in incident energy, resulting in the dominance of forward peaked reactions at higher incident energies. The sensitivity of analysing power to the multistep part of the reaction was valuable to the study. Forward-peaked contributions that were associated with single step direct reactions resulted in large analysing power values at small scattering angles, and values dropped to around zero at large angles. The analysing power also appeared to decrease to values approximating zero as the incident energy was increased to 160 MeV. Furthermore, this study also confirmed the dominance of higher step mechanisms at high excitation energies, irrespective of incident energy. In general the results were in agreement with the predicted properties of the assumed reaction mechanism at all incident and emission energies, which inspires confidence that the theoretical interpretation is likely to be correct.
Relativistic plane wave description of spin transfer observables for proton knockout reactions
(Stellenbosch : Stellenbosch University, 2001-03) Wyngaardt, Shaun Metzler; Cowley, A. A.; Hillhouse, G. C.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: In this dissertation we set out to develop the first relativistic model for calculating complete sets of (p, 2p) spin transfer observables. In addition to this a new technique has been developed which allows us to evaluate the transition amplitude, which is used to calculate the scattering observables for the reaction directly. The influence of various modiuin-modificd parameters ()1I the scattered wave functions anr] NN interact ion lH-IVC' })('('11 invest igatcd DlIC t,C) (I,llIlJigllitic's surrounding the nNN coupling we have included both pseudosoalar and pseudovector coupling into the nucleon-nucleon interaction model. Furthermore we have included two different kinematic prescriptions to obtain the effective NN laboratory kinetic energy and center of mass scattering angle, which are used to obtain the NN scattering amplitudes. The aim of this study is to investigate the effects of the various model parameters on complete sets of scattering observables. Our investigation has shown that although the analyzing power is not very sensitive to nuclear medium effects, and the various other spin transfer observables such as Dnn should provide valuable insight. Further refinements of the model would be to include nuclear distortions as well as the IA2 model of the NN interaction.
Response of NaI(T1) detectors in geophysical applications
(Stellenbosch : Stellenbosch University, 2005-03) Titus, Nortin P.; Cowley, A. A.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Airborne gamma-ray spectrometry is used for exploration and environmental monitoring over large areas. Such datasets of 7-ray are acquired using large 16l NaI(T1) detector crystals mounted in an aircraft. The response of these detectors to naturally occurring radioactive isotopes is a function of detector volume, efficiency and resolution. This response is modelled using the Monte Carlo method. A photon-electron transport computer code, NIPET, is written to calculate the intrinsic efficiency and photofraction of these large prismatic crystals. The intrinsic efficiency indicates the number of incident photons that interact in some way with the detector resulting in a deposition of energy in the crystal. The photofraction, on the other hand, indicates how many of these photons that impinge on the detector are actually recorded with their full initial energy. This study presents explicity calculated photofraction values for 16l NaI(Tl) crystals. Comparison of these new values, calculated with this computer program, with that of published values for small 3" x 3" crystals shows good agreement. However, for large crystals, the calculated values differ from those in the literature by almost 15%. The study also finds that for the gamma-ray energy range between 0.2 - 3.0 MeV, these detectors have an intrinsic efficiency above 90%, whereas the photofraction values range from 80% for low energy photons to above 40% for high energy photons in the case of large crystals.
The Role of a direct knock-out mechanism in the inclusive (p, α) reaction
(Stellenbosch : University of Stellenbosch, 2008-03) Van Zyl, Johannes Jacobus; Hillhouse, G. C.; Cowley, A. A.; Van der Ventel, B. I. S.
Experimental double differential cross sections and analyzing powers for the inclusive 93 Nb(p,α) r and 93 Nb(p, 3He) r reactions have been measured at an incident energy of 160 MeV. The results of this experiment, for emission angles ranging from 15º to 60º as well as previously measured data for 93 Nb(p,α) r at 100 MeV and 59Co(p,α) r at 100 and 160 MeV, have been compared with theoretical calculations based on a direct quasi-free α- cluster knockout mechanism. A computer code has been employed which calculates triple differential cross section and analysing power values using the distorted wave impulse approximation (DWIA). Inclusive double differential cross sections and analysing powers were calculated by integrating over the solid angle of the unobserved proton. It was found that the DWIA is limited in its ability to reproduce the experimental data. Some slight agreements have been observed in the region of large emission energies and small angles. This is consistent with the findings of previous studies based on a direct multi-step analysis which indicated that the first step knock-out reaction dominates in this region.

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