Browsing by Author "Orce, Nico"
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Item Coulomb excitation of 66ge(University of Western Cape, 2021) Abrahams, Kenzo; Orce, NicoThe Coulomb excitation of 66Ge has been performed for the rst time using \safe" bombarding energies at the HIE-ISOLDE facility at CERN in July 2017. A particle- coincidence experiment using the MINIBALL array and double-sided silicon detectors has allowed the determination of transitional and diagonal matrix elements in 66Ge, yielding new measurements of the reduced transition probability connecting the ground state, 0+1 , and the rst excited state, 2+1 , or B(E2; 2+1 ! 0+1 ) value, and the spectroscopic quadrupole moment of the 2+1 state, QS (2+1 ). A relatively large B(E2) = 29:4(30) W.u. has been extracted using beam-gated data at forward angles { less sensitive to secondorder e ects { as compared with the adopted value of 16:9(7) W.u., but in closer agreement with modern large-scale shell-model calculations using a variety of e ective interactions and beyond-mean eld calculations presented in the current work.Item The design and simulation of a new experimental set up to measure nuclear level lifetimes(University of the Western Cape, 2016) Singh, Bhivek; Triambak, Smarajit; Orce, NicoMeasurements of nuclear level lifetimes are an important aspect of experimental nuclear physics. Such measurements determine transition matrix elements for nuclear structure research and also provide the widths of relevant excited states in nuclei that are of astrophysical interest. In the latter, the measured widths are used to obtain reaction rates in main sequence stars such as the Sun and in binary-star systems where the accretion of material from one star to another provides an opportunity to study extreme stellar environments such as novae and x-ray bursts. This thesis work describes the design and simulation of a new experimental set up at iThemba LABS that will allow for highprecision femtosecond-level lifetime measurements of nuclear states using the Doppler Shift Attenuation Method (DSAM). We use the Solid Edge computer-aided design (CAD) software to design a new scattering chamber with a cooled target ladder specifically for such measurements using inverse-kinematic transfer reactions with ion implanted targets. The light charged ejectiles from the reaction will be detected with a ΔE - E silicon telescope, while Doppler shifted rays will be registered using a high-purity and 100% efficient germanium (HPGe) detector. We also describe preliminary Monte Carlo simulation codes that are being developed in a relativistically invariant framework to optimize the experimental set up and to obtain predicted lineshapes of γ rays from several astrophysically relevant states in nuclei using this experimental set up.Item Design of a helium-6 production target for the iThemba LABS Radioactive-ion Beam Facility(University of the Western Cape, 2018) Davis, Lance Garth; Orce, Nico; Bark, RobertIt is well known, that there is a severe lack of information available pertaining to neutron rich nuclei, specifically of those nuclei with mass numbers ≥ 60. These neutron rich nuclei are not easy to access in current experimental facilities or be produced with sufficient yield to allow for it to be studied. In order to expand our understanding of nuclear physics by studying the properties and characteristics of these nuclei, the development of new facilities producing Radioactive-ion Beams (RIBs) is required. The applications for RIBs are wide, allowing for deeper investigations into the properties of nuclei, their interactions and the manner in which they were formed in the early universe. Additionally, there are various interdisciplinary fields such as medicine, biology and material science in which RIBs can be utilized as a driving mechanism for new research and technological innovation. The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS), South Africa, has proposed a new facility for the production and acceleration of radioactive-ion beams (RIBs). The RIB Project is to be developed in sequential phases and would produce a range of neutron-rich isotopes for low-energy materials science and nuclear physics research. Of specific interest, is the production of the Helium-6 isotope (6He), for its potential applications in various areas of nuclear physics research. The aim of this research work was to design, model and optimise a RIB production target capable of producing high intensity 6He beams, guided by the characteristics of the primary proton beam available for use at iThemba LABS. This research work/design study is however limited, due to the absence of experimentally measured and verified 6He cross section data for proton induced reactions on the proposed target materials (Graphite and Boron Carbide). However, best-estimate approaches were adopted through the use of validated computer codes. Additionally, all 6He yield results are presented as in-target yields, as this study did not cover the diffusion (isotope release) efficiency of the target systems in question. Three RIB production targets types were investigated using Graphite, Boron Carbide and Beryllium Oxide as potential target materials. Following numerous optimisation processes, a Boron Carbide RIB target was converged upon, proving to be suitable for the production of high intensity 6He beams at iThemba LABS, by meeting the material thermal and mechanical limiting criteria for operation. This target system was found to produce an in-target 6He yield rate of 2 ~ 3 x 1011 6He/s, considered sufficient for experimental application at iThemba LABS.Item Determining the Spectroscopic Quadrupole Moment Qs of the 2+1 state in 40Ar(University of the Western Cape, 2017) Mokgolobotho, Makabata Jeremia; Orce, NicoThe rst reorientation-e ect Coulomb-excitation experiment has been performed at iThemba LABS by bombarding 40Ar beams on a heavy 208Pb target at a \safe" energy of Elab =143.2 MeV. The goal was to determine, the spectroscopic quadrupole moment, QS , of the rst excita- tion 2+ 1 at 1.461 MeV in 40Ar at \safe" energies. The scattered particles were detected at backward angles using a double-sided CD-type S3 silicon detector, composed of 24 rings and 32 sectors for angular distribution and Doppler correction, respectively, in coincidence with de-excited rays collected by 8 clover detectors in the AFRODITE array. The collected coincidence data were analysed using a state-of-the-art sorting code spe- cially developed for this kind of measurements, which allowed, by setting up di erent energy and time conditions, a clean -ray spectrum for further analysis with the Coulomb-excitation coupled-channels code, GOSIA. A diagonal matrix element of h2+ 1 j jE^2 j j2+ 1 i = +0:047(22) eb has been determined, which yields QS (2+ 1 ) = +0:036(17) eb, after detailed GOSIA analysis and minimisation. This value is 8.5 times more precise than the only previous measurement of QS (2+ 1 ) = +0:01(4) eb by Nakai and col- laborators in 1970, and further supports the zig-zag of quadrupole shapes observed at the end of the sd shell. A possible explanation regards the in uence of proton holes driving the nuclear shape towards an oblate con- guration.Item Determining the spectroscopic quadrupole moment of the first 2+ state in 36Ar(University of the Western Cape, 2019) Montes, Elias Jose Martin; Orce, NicoA series of reorientation-effect Coulomb-excitation experiments were carried out at iThemba LABS from April to May 2016. Enriched ion beams of Ar, Ar and S were bombarded at safe energies, well below the Coulomb barrier, onto a heavyPt target with thickness 1mg/cm2. The three experiments were performed to determine the spectroscopic or static quadrupole moment 𝑄, of the first excitation 2 in these nuclei in order to shed light on the zip-zap of nuclear shapes found at the end of the sd shell. This thesis will be dealing with the particular case of Ar, accelerated at a safe bombarding energy of 134.2 MeV. For this purpose, a particle-gamma coincidence experiment has been carried out using the AFRODITE array composed of 8 high-purity germanium clover detectors to detect the de-exciting gamma energies, coupled to a double-sided CD-type S3 silicon detector at backward angles composed of 32 sectors and 24 rings to detect the scattering particles. A new optimised sorting code has been developed which included fast computing processing, non-Doppler (194Pt) and Doppler correction (36Ar), add-back, and energysharing, particle and time tagging conditions. The peaks of interests in the spectra were analysed using the Coulombexcitation code GOSIA. Using the normalization method, a more precise determination of 𝑄2 0.093 eb has been accomplished.Item Measuring transitional matrix elements using first-order perturbation theory in Coulomb excitation(University of the Western Cape, 2019) Masango, Senamile Khethekile Ntombizothando; Orce, NicoThe aim of nuclear structure physics is to study the interplay between singleparticle and collective degrees of freedom in nuclei and to explain how nuclei get excited and decay under di erent external conditions, such as strong electric and magnetic elds. If nuclei absorb a large amount of energy and angular momentum, like in a scattering reaction when you bombard a target that is in the ground state with a projectile at high bombarding energies, the energy from the projectile gets transfered to the target and vice versa, hence both projectile and target may get excited. During the de-excitation process nuclei may release the energy in a form of electromagnetic radiation (gamma rays) which carries angular momentum. The atomic nucleus is a many-body system, whose structure is de ned in terms of interactions between protons and neutrons. In nature there are only around 300 stable isotopes [1]. They are all in their ground states (although some are in a low-energy excited isomeric state with a long lifetime). To study excited states in these nuclei one needs to provide energy to the system. In addition, there are some 3000 unstable nuclei, most of which do not exist in nature. Many have been produced and studied in research laboratories, and there could be more than 3000 other unstable nuclei that can in principle exist in astrophysical environments, but have not yet been synthesized on Earth [1].Item New formulas for the (−2) moment of the photoabsorption cross section, σ−2(American Physical Society, 2015) Orce, NicoTwo new formulas for the (−2) moment of the photoabsorption cross section, σ−2, have been determined, respectively, from the 1988 photoneutron evaluation of Dietrich and Berman and a mass-dependent symmetry energy coefficient, asym(A). The data for A 50 follow, with a RMS deviation of 6%, the power law σ−2 = 2.4A5/3 μb/MeV, which is in agreement with Migdal’s calculation of σ−2 = 2.25A5/3 μb/MeV based on the hydrodynamic model and the σ−2 sum rule. The additional inclusion of asym(A) provides a deeper insight into the nuclear polarization of A 10 nuclei.Item Polarizability effects due to low-energy enhancement of the gamma-strength function(University of the Western Cape, 2018) Ngwetsheni, Cebo; Orce, NicoPhysics is the study of natural phenomena. Nuclear physicists have since the discovery of the nucleus been working on understanding its dynamics. The nuclear chart, analogous to the periodic table of elements, is illustrated in Fig. 1.1 and color coded according to decay modes. Several theoretical models, based on various hypothesis, have been developed during the years in order to understand nuclear phenomena such as nucleon-nucleon (n-n) interactions, binding energies, radii, excited states, etc. Unfortunately, no-unique model is actually able to grasp all nuclear phenomena at the desired level of accuracy. Among the di erent models, we notice that two distinct hypotheses can be used to describe nuclear properties. Firstly, the independent particle shell model (IPSM) + the n-n residual interaction, which assumes that a nucleon moves independently in a potential generated by other nucleons. Secondly, the macroscopic models, where a nucleus is considered as a whole, i.e. neutrons and protons behave cooperatively and are mutually coupled to each other; highlighting the short-ranged character of the nuclear force. The liquid-drop model is an example of such macroscopic models. Re nement of these models is dependent on experimental observations that are better detailed for nuclei along the line of - stability, making up a small fraction of the known isotopes, as shown in Fig. 1.1. In practice, various techniques for studying exotic nuclei up to neutron and proton drip-lines have been devised, including the use of radioactive ion beams. However, the main challenges are the synthesization and short lived periods of these exotic nuclei resulting in insu cient data collection from which the characteristics and structural information are extracted. In general, nuclei have unique structures represented by a particular con guration as given by the shell model (SM). These structures impact a number of physical quantities, e.g. transition probabilities, cross sections and photon-strength functions. Experimental methods such as Coulomb excitation or electromagnetic radiation are used to probe these structures without invoking the nuclear force.Item Synthesis of the metallocenes for the production of exotic high energy ion beams(University of the Western Cape, 2019) Kheswa, Ntombizonke Yvonne; Orce, NicoThe Subatomic Physics Department of iThemba Laboratory for Accelerated Based Sciences (iThemba LABS) conducts experiments that require a variety of particle beams in order to study nuclear properties (reaction, structure, etc.) of various nuclides. These particle beams are accelerated using the K-200 Separated Sector Cyclotron (SSC) and delivered to different physics experimental vaults. Prior to acceleration, the particle beam is first ionised using an Electron Resonance Ion Source (ECRIS). The main goal of this study is the production of exotic metallic beams of 60Ni8+ and 62Ni8+ using ECRIS4, which are required for the Coulomb excitation experiments approved by the Programme Advisory Committee (PAC) at iThemba LABS. In order to provide the metallic beams of nickel, a development study of organometallic materials containing 60Ni and 62Ni isotopes in a form of metallocene complexes was undertaken. The nickelocene (NiCp2) complex, a member of the organometallic family, was synthesised at the Physics Target Laboratory of iThemba LABS for the first time. Method development involved the use of natural nickel during the multi-step synthesis before the use of enriched nickel-60 (60Ni) and nickel-62 (62Ni). Nine samples of NiCp2 were synthesised; two were isotopically enriched nickelocene (60NiCp2 and 62NiCp2). The percentage yields of the synthesised nickelocene samples ranged between 16 to 50 %, and samples were characterised by investigating their crystal structure and bonding arrangements in the complexes by X-ray diffraction (XRD) , Fourier Transform Infrared (FT-IR) spectroscopy, and Proton Nuclear Magnetic Resonance (1H NMR). The synthesised nickelocene were further used with ECRIS4 for the production of Ni beams on the Q-line of the cyclotron. The Metal Ions from Volatile Compounds (MIVOC) technique was used for the conversion of 60Ni and 62Ni to ion species. The method used the organometallic compounds which are volatile at specific pressures at ambient temperatures. Metallic ion beams of nickel were successfully produced after a carefully pre-sample conditioning in the MIVOC container before connecting the MIVOC set-up to the new injection system of the ECRIS4. Measured beam intensities during the experiment for both 60Ni+ and 62Ni+ were approximately 30 μA, optimum for physics measurements. The development of the MIVOC technique opens up new beam-target combinations with the use of new exotic stable beams for new science cases at iThemba LABS. Reactions in inverse kinematics, multi-step Coulomb-excitation and other types of reactions will immensely benefit from these developments.