Browsing by Department "Department of Physics"
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- ItemOpen AccessA compact neutron spectrometer for detecting neutrons produced by cosmic rays(2023) Jarvie, Erin; Buffler, Andrew; Hutton TanyaThe high energy secondary neutron fields from cosmic radiation that are found in aviation and space environments have impacts on both biological and technological systems. A compact neutron spectrometer that is safe to operate in an aviation environment and suitable for detecting these secondary neutrons has been developed. The detector consisting of a (0.6×0.6×12.0 cm3 ) EJ-276 plastic scintillator coupled to a silicon photomultiplier (SiPM) has been designed, constructed and characterised with neutrons up to 62.5 MeV at the UCT n-lab, IRSN AMANDE and iThemba LABS fast neutron facilities. The compact detector displays good quality pulse shape discrimination for the separation of neutron and gamma ray events. Neutron energy spectra were obtained through time-of-flight (ToF) measurements and spectrum unfolding. The energy spectra obtained from ToF display energy resolution similar to that obtained by a BC-501A reference detector. A matrix of measured response functions for neutrons with energies between 11 MeV and 62 MeV at 3 MeV intervals selected by ToF has been constructed for the detector. The response functions have been used to successfully unfold neutron energy spectra across the entire range of energies.
- ItemOpen AccessA new approach to the optimal target selection problem(2007) Elson, E C; Bassett, B A; van der Heyden, K; Vilakazi, Z ZContext.This paper addresses a common problem in astronomy and cosmology: to optimally select a subset of targets from a larger catalog. A specific example is the selection of targets from an imaging survey for multi-object spectrographic follow-up.Aims.We present a new heuristic optimisation algorithm, HYBRID, for this purpose and undertake detailed studies of its performance.Methods.HYBRID combines elements of the simulated annealing, MCMC and particle-swarm methods and is particularly successful in cases where the survey landscape has multiple curvature or clustering scales.Results.HYBRID consistently outperforms the other methods, especially in high-dimensionality spaces with many extrema. This means many fewer simulations must be run to reach a given performance confidence level and implies very significant advantages in solving complex or computationally expensive optimisation problems.Conclusions.HYBRID outperforms both MCMC and SA in all cases including optimisation of high dimensional continuous surfaces indicating that HYBRID is useful far beyond the specific problem of optimal target selection. Future work will apply HYBRID to target selection for the new 10 m Southern African Large Telescope in South Africa.
- ItemOpen AccessA new compact neutron spectrometer(2016) Comrie, Angus; Buffler, Andrew; Smit, RickyA new compact neutron spectrometer has been designed, developed and characterized. The detector is based on EJ299-33 plastic scintillator coupled to silicon photomultipliers, and a digital implementation of pulse shape discrimination is used to separate events associated with neutrons from those associated with gamma-rays. The spectrometer is suitable over the neutron energy range 1 – 100 MeV, and the development illustrated with measurements made using an Am-Be radioisotopic source, a D-T sealed tube neutron generator and quasi-monoenergetic neutron beams produced using the iThemba LABS cyclotron. A segmented variation of the spectrometer is capable of providing directional information through the comparison of count rates between scintillator cells.
- ItemOpen AccessA New Digital Data Acquisition System for Neutron Metrology(2022) Sole, Chloe; Buffler, Andrew; Hutton,TanyaWithin the neutron metrology and spectrometry community digital pulse processing systems are being developed for measurements of Fast neutron fields in a wide variety of contexts, for example at accelerator and medical radiation facilities, around nuclear power plants, in airplanes in flight and space stations. These fields often vary widely with respect to both energy and intensity, which complicates measurements of energy dependent fluence. Investigations have been completed into the suitability of a CAEN DT5730 digitiser unit as a viable alternative to a traditional analogue system for data acquisition for fast neutron metrology. Experiments were completed at the fast neutron facilities of AMANDE (IRSN, Cadarache) using a BC501A scintillation detector and both the DT5730 digitiser and an analogue system based on NIM electronics and an MPA-3 multichannel analyser acquisition unit. Follow-up measurements were made at the n-lab facility at the University of Cape Town. The measurements covered an energy range from 0.5 MeV to 20 MeV, over a large range of intensities. The energy and intensity response for both systems and digital configurations were investigated based on the unfolding of measured light output spectra using an existing neutron response matrix for the detectors. Deadtime behaviour, rate dependent losses and linearity over the energy range were explored. The quality of the measured neutron spectra was compared through both uncertainty budgets and shape comparison analyses. Factors to consider when migrating from analogue signal processing to digital signal processing are discussed for the measured energy and intensity range, including advantages and disadvantages using digital pulse processing for metrology both in the laboratory and in the field. While there is more research required for a thorough bench-marking, the present results indicate that digital data acquisition technology has matured to the point where it can now be considered for use within neutron metrology.
- ItemOpen AccessA Review of the Intrinsic Heavy Quark Content of the Nucleon(2015) Brodsky, S J; Kusina, A; Lyonnet, F; Schienbein, I; Spiesberger, H; Vogt, RWe present a review of the state of the art of our understanding of the intrinsic charm and bottom content of the nucleon. We discuss theoretical calculations, constraints from global analyses, and collider observables sensitive to the intrinsic heavy quark distributions. A particular emphasis is put on the potential of a high energy and high luminosity fixed target experiment using the LHC beams (AFTER@LHC) to search for intrinsic charm.
- ItemOpen AccessA search for an excited muon decaying to a muon and two jets in pp collisions at $sqrt{s};=;8;{rm{TeV}}$ with the ATLAS detector A search for an excited muon decaying to a muon and two jets in pp collisions at √s = 8 TeV with the ATLAS detector(2016) Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; Abolins, M; AbouZeid, O S; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S; Alconada Verzini, M JA new search signature for excited leptons is explored. Excited muons are sought in the channel pp → µµ∗ → µµ jet jet, assuming both the production and decay occur via a contact interaction. The analysis is based on 20.3 fb−1 of pp collision data at a centre-of-mass energy of √ s = 8 TeV taken with the ATLAS detector at the Large Hadron Collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass mµ ∗ . For mµ ∗ between 1.3 TeV and 3.0 TeV, the upper limit on σB(µ ∗ → µqq¯) is between 0.6 and 1 fb. Limits on σB are converted to lower bounds on the compositeness scale Λ. In the limiting case Λ = mµ ∗ , excited muons with a mass below 2.8 TeV are excluded. With the same model assumptions, these limits at larger µ ∗ masses improve upon previous limits from traditional searches based on the gauge-mediated decay µ ∗ → µγ
- ItemOpen AccessA search for tWZ production in the Full Run 2 ATLAS dataset using events with four leptons(2021) Reich, Jake; Keaveney, James; Yacoob, Sahal
- ItemOpen AccessA search for tWZ production in the trilepton channel using Run 2 data from the ATLAS experiment(2021) Warren, Benjamin; Keaveney, JamesThis dissertation describes an analysis of events containing three leptons from 136 fb−1 of proton-proton collision data, with a centre of mass energy of 13 TeV, recorded by the ATLAS detector between 2016 and 2018. The aim of the analysis was to search for evidence of a top quark produced in association with a W boson and a Z boson (tWZ). An event selection scheme was developed using simulation to broadly suppress background events and to select signal events. Events were separated into mutually-exclusive regions of phase space to increase the ratio of signal to backgrounds and to calibrate the modelling of the backgrounds. Background events were further suppressed through the use of the Gradient Boosted Decision Tree (GBDT) machine learning algorithm. First, a hadronically-decaying W boson candidate was identified using a GBDT; this was used to suppress WZ background events. Then, an event-level GBDT was used to suppress all background events. A maximum likelihood fit was used to estimate the signal strength µ of tWZ production, where nuisance parameters were assigned to theoretical and experimental systematic uncertainties. The work presented here forms the basis of an official ATLAS experiment analysis, thus, the signal region was blinded to avoid potential biases in the future development of the official ATLAS analysis. The best fit value of µ resulting from a fit to a modified Asimov dataset was µˆ = 2.08+1.48 −1.45. This corresponds to an expected significance of Z exp µ = 0.72 σ. An expected upper limit µ exp up = 2.77+2.39 −1.28 was also determined from the fit to the modified Asimov dataset. Thus, this analysis has the potential to put the strongest ever constraint on tWZ production, but does not have the potential to observe tWZ production as predicted in the Standard Model. These constraints are limited by statistical uncertainties.
- ItemOpen Access
- 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 AccessAlpha decay of excited states of carbon-12(1970) Shackleton, David; Brooks, F DA crystal of the carbon-rich phosphor, anthracene, has been bombarded by monoenergetic fast neutrons. Alpha-particle decays of carbon nuclei have been separated from other events by a specialised application of the pulse shape discrimination technique. A numerical analysis of the data has yielded alpha-particle energy spectra over a wide energy range. A counter has been assembled to detect neutrons scattered by carbon nuclei in the crystal, and the measurements repeated in coincidence with these neutrons. The energy range of particle resolving power has been extended. Structure in the resulting alpha-particle and proton energy spectra is attributed to particle decays of specific levels in carbon-12. It is shown how the experiment may be modified and extended to measure decay parameters in absolute terms.
- ItemOpen AccessAlpha-radioactive isotopes in the marine environment.(1975) Higgo, Jennifer Joan Wynne; Cherry, R DVarious marine organisms collected from southern hemisphere waters around Cape Town were analysed by radiochemistry and alpha-spectrometry for Pu-239, 240. Many of these organisms were also analysed for Pu-238, Po-210 and - those isotopes of thorium and uranium that are alpha-emitters. In some samples an estimate of the Ra-226 content was also made. The Pu-239 concentrations are compared with the values reported in the literature for similar organisms found in the northern-hemisphere and an attempt is made to relate the differences in concentration to differences in the quantity of fall-out Pu-239 delivered to the sea-surface in the two hemispheres. Within the limits of experimental error it appears that the Pu-239 content of recent fall-out is reflected in the Pu-239 concentration in many marine organisms.
- ItemOpen AccessAn investigation of the validity criteria for local thermodynamic equilibrium in a nitrogen plasma(1973) Rash, Jonathan Paul Stuart[Abstract missing]
- ItemOpen AccessAn uncertainty budget for the precursor Watt balance for South Africa(2019) Mametja, Thapelo Given; Buffler, Andrew; Karsten, AlettaThe 26th General Conference on Weights and Measures (CGPM) held on the 16th November 2018 has adopted the revision of the International system of units (SI) to be based on the fundamental physical constants.
- ItemOpen AccessAnalysis of a deep neural network for missing transverse momentum reconstruction in ATLAS(2020) Leigh, Matthew; Yacoob, Sahal; Young, ChristopherThe ATLAS detector is a multipurpose particle detector built to record almost all possible decay products of the high energy proton-proton collisions provided by the Large Hadron Collider. The presence and combined kinematics of unobserved particles can be inferred by the observed momentum imbalance in the transverse plane. In this work, a deep neural network was trained using supervised learning to measure this imbalance. The performance of this network was evaluated in MC simulation and in 43 fb⁻¹ of data recorded at ATLAS. The network offered superior resolution and significantly better pileup resistance than all other pre-existing algorithms in every tested topology. The network also provided the best discriminator between events that did and did not contain neutrinos. The potential gain insensitivity to new physics was demonstrated by using this network in a search for the electroweak production of supersymmetric particles. The expected sensitivity to observe the production of said particles was increased by up to 26%.
- ItemOpen AccessAnalysis of combined isotropic and anisotropic line-width data with a view to improved rotational CARS thermometry(2002) Horner, Mark John Newlyn Horner; Robertson, GThe development of a new line-width model and recently measured rotational (anisotropic) line-widths has allowed for a critical analysis of the various basis rates models. These basis rates will be analysed using both vibrational (isotropic) and rotational line-width data. The data sets are analysed individually and in combination. For all fits, parameter correlations are determined. In addition, the sensitivity of the goodness of fit to variations in the effective interaction length for collisions, le, is investigated. To complete the investigation full rotational spectra are calculated and compared with experimental spectra.
- ItemOpen AccessAnalytical application of proton-induced prompt photon specrometry(1982) Gihwala, Dherendra; Peisach, MaxPrompt gamma-rays emitted tinder bombardment with 3.5 to 6.0MeV protons from the 77 stable non-gaseous elements were evaluated for analytical application. The compilation included the yields of about 2200 gamma-rays and their detection limit for analysis. Because the data were measured under identical experimental conditions, the relative values may be generally applicable. For each element, the gamma-ray spectrum and the numerical data are presented in tables listing identified gamma-rays with yields and detection limits for Ep = 4.5 MeV, the gamma yields as a function of proton energy for the most intense gamma-prays, and, the polynomial coefficients of the function relating the detection limit to the bombarding energy. An Atlas of Spectra and a Catalogue of gamma-rays summarised the data.
- ItemOpen AccessAnomalous evolution of the Near-Side Jet Peak Shape in Pb-Pb Collisions at √sNN = 2.76 TeV(2017) Adam, J; Adamová, D; Aggarwal, M M; Aglieri Rinella, G; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Aiola, S; Akindinov, A; Alam, S N; Albuquerque, D S D; Aleksandrov, D; Alessandro, B; Alexandre, D; Alfaro Molina, R; Alici, A; Alkin, A; Alme, J; Alt, T; Altinpinar, S; Altsybeev, I; Alves Garcia Prado, C; An, M; Andrei, C; Andrews, H A; Andronic, A; Anguelov, V; Anson, C; Antičić, T; Antinori, FThe measurement of two-particle angular correlations is a powerful tool to study jet quenching in a p T region inaccessible by direct jet identification. Here, the differences in the pseudorapidity (Δη) and azimuthal (Δφ) measurements are used to extract the shape of the near-side peak formed by particles associated with a higher pT trigger particle (1 < p T,trig < 8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at √ sNN=2.76 TeV. A significant broadening of the peak in the Δη direction at low pT is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1 < p T,assoc < 2 GeV/c, 1 < p T,trig < 3 GeV/c a novel feature is observed: a depletion develops around the center of the peak. Our results are compared to pp collisions at the same center of mass energy and ampt model simulations. The comparison to the investigated models suggests that the broadening and the development of themore » depletion is connected to the strength of radial and longitudinal flow.« less
- ItemOpen AccessThe anomalous magnetic moment of baryons in cavity QCD(1992) Lindebaum, R J; Viollier, Raoul DUsing a generalised form of the Gell-Mann and Low theorem, all the diagrams in cavity QCD to order as that contribute to the magnetic moment are calculated. The calculations are performed for massive quarks so a mass renormalisation scheme has been developed to cope with the new divergences this brings into the self-energy insert diagrams. The results of this work show that no improvement on the simple SU(3) model is made by including these corrections. These calculations point to a smaller value of αs than that which is usually used.
- ItemOpen AccessThe anomalous magnetic moment of the nucleon in cavity QCD(1991) O'Connor, M SPerturbative quantum chromodynamics is developed in a spherical cavity using a symmetric form of the Gell-Mann and Low theorem. This formalism allows one to generate any desired term in the perturbation series, in a manner which is similar to the familiar Feynman rules in free space. All corrections to order eg² in the electromagnetic and strong coupling constants which contribute to the magnetic moment of a baryon are generated using this formalism. The O(eg²) radiative corrections to the magnetic moment of the nucleon are calculated here in an arbitrary covariant gauge. The gauge-dependent parts are found to vanish identically, and the divergences arising from the loop diagrams cancel amongst each other, making renormalization unnecessary. However, it is shown here that one can, if it is necessary, remove the divergences from the cavity diagrams by subtracting from them a singular factor which is found using dimensional regularization in the analogous free-space diagrams.