Research Articles (Physics)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/5709

Browse

Now showing 1 - 20 of 133

Accurate Laboratory Wavelengths of the A1Π(υ' = 0-5)-X1Σ+(υ'' = 0) Vibronic Bands of 12C17O and 12C18O
(IOP Science, 2006) Du Plessis, Anton; Rohwer, Erich G.; Steenkamp, Christine M.
Twenty nine rovibronic lines of 12C17O for which laboratory wavelengths were previously unavailable were detected in laser-induced fluorescence excitation spectra of the six vibronic bands A1Π(υ' = 0-5)-X1Σ+(υ'' = 0). Rovibronic lines of 12C16O, 13 C16O, 12C17O, and 12C 18O were detected in each band, allowing accurate determination of the unknown wavelengths using neighboring 12C16O and 13C16O lines as reference. The new wavelength data yield consistent heliocentric velocity values when applied to vacuum ultraviolet observations of 12C17O and 12C 18O in the interstellar medium.
Accurate laboratory wavelengths of the e 3 Σ–(ν' = 5) – X 1 Σ+(ν'' = 0) band of 12C16O
(IOP Science, 2010) Dickenson, G. D.; Nortje, A. C.; Steenkamp, C. M.; Rohwer, E. G.; Du Plessis, A.
The forbidden singlet-triplet transitions of carbon monoxide (CO) are important in the interpretation of vacuum ultraviolet interstellar absorption spectra and in particular for the measurement of large CO column densities. Twenty rovibronic lines of the e 3Σ–(ν' = 5) – X 1Σ+(ν'' = 0) band of 12 C 16O for which laboratory wavelengths were previously unavailable were identified in laser-induced fluorescence excitation spectra. Wavelengths were assigned to five rovibronic transitions to an average accuracy of 0.0028 Å. A further 15 lines could not be fully resolved and average wavelengths were measured for these groups of closely spaced lines. A wavelength difference of 0.011 ± 0.0028 Å between the measured wavelengths and the calculated wavelengths in the atlas of Eidelsberg & Rostas demonstrates the need for more experimental data on CO.
Accurate laboratory wavelengths of the vacuum ultraviolet A(v'=3)-X(v''=0) band of 12C17O and 12C18O
(IOP Science, 2003) Steinmann, Christine M.; Rohwer, Erich G.; Stafast, Herbert
Six individual rotational lines (J'' ≤ 3) of 12C17O, as well as four of 12C18O, were detected in the A 1Π(v' = 3)-X 1Σ+(v'' = 0) vibronic band, and their wavelengths determined using neighboring 12C16O and 13C16O lines as reference. The measurements (fluorescence excitation spectra) were performed in a pulsed supersonic jet (Ar or Ne as carrier gas) employing a tunable pulsed vacuum ultraviolet radiation source with a narrow bandwidth (~5 GHz). The new spectral data on 12C17O and 12C18O are applied to the interpretation of recent vacuum ultraviolet observations of 12C17O and 12C18O in the interstellar medium.
Advancing X-ray micro computed tomography in Africa : going far, together
(Elsevier, 2019) Du Plessis, Anton; Le Roux, Stephan G.; Tshibalanganda, Muofhe
X-ray micro computed tomography (microCT) is a high resolution non-destructive materials imaging technology and is emerging as a powerful tool for industrial and scientific research applications. The aim of this review paper is to present the capabilities and potential of this technique within an African context. This is done using a representative sample: all work done at the Stellenbosch CT facility during 2018 is used as an overview of the type of work done at such a facility in Africa. Besides the plethora of academic research topics, the most important industrial applications are also discussed, which assisted to keep the Stellenbosch CT facility financially operational. This provides a wider understanding of the opportunities and capabilities of this technique and how it can benefit African researchers and local industries. The question “what is it used for?”, and more specifically “what is it used for in Africa?” is therefore answered. The availability of such X-ray tomography facilities helps to fast-track research by providing local expertise and support in Africa for advancing African science. This model is not only applicable to microCT but applies to any collaborative scientific endeavor in Africa, with success rates depending on the efficient sharing of resources, providing expert skills and advancing African science in Africa. There is an African proverb “if you want to go fast – go alone; if you want to go far – go together”. Clearly, African science will go far by working together in such facilities.
Angular correlation measurement with the iThemba LABS segmented clover detector
(Jagellonian University, 2019) Shirinda, O.; Lawriea, E. A.; Buchera, T. D.; Mthembua, S. H.; Dinoko, T. R. S.; Easton, J. L.; Mtshali, W. X.; Noncolela, S. P.; Duprez, D.; Lawrie, J. J.
Measurements of the angular correlation function W(θ) were performed with the iThemba LABS segmented clover detector. A set of measurements with radioactive sources and irradiated targets was carried out to check the performance of the detector. At short detector-to-source distance, the detector covers the whole range of angles needed for precise angular correlation measurements. Preliminary results show that our detector can measure precisely angular correlation functions, including for transitions with large multipole order such as E3, M4, E4, etc., and deliver mixing ratios δ of mixed magnetic and electric nature.
Anomalous scaling of dynamical large deviations
(American Physical Society, 2018) Nickelsen, Daniel; Touchette, Hugo
The typical values and fluctuations of time-integrated observables of nonequilibrium processes driven in steady states are known to be characterized by large deviation functions, generalizing the entropy and free energy to nonequilibrium systems. The definition of these functions involves a scaling limit, similar to the thermodynamic limit, in which the integration time τ appears linearly, unless the process considered has long-range correlations, in which case τ is generally replaced by τξ with ξ≠1. Here, we show that such an anomalous power-law scaling in time of large deviations can also arise without long-range correlations in Markovian processes as simple as the Langevin equation. We describe the mechanism underlying this scaling using path integrals and discuss its physical consequences for more general processes.
Atmospheric heavy metal deposition in North Macedonia from 2002 to 2010 studied by Moss biomonitoring technique
(MDPI, 2020-08-30) Barandovski, Lambe; Stafilov, Trajce; Sajn, Robert; Frontasyeva, Marina; Andonovska, Katerina Baceva
Moss biomonitoring technique was used for a heavy-metal pollution study in Macedonia in the framework of the International Cooperative Program on Effects of Air Pollution on Natural Vegetation and Crops (UNECE IPC Vegetation). Moss samples (n = 72) were collected during the summers of 2002, 2005, and 2010. The contents of 41 elements were determined by neutron activation analysis, atomic absorption spectrometry, and inductively coupled plasma atomic emission spectrometry. Using factor and cluster analyses, three geogenic factors were determined (Factor 1, including Al, As, Co, Cs, Fe, Hf, Na, Rb, Sc, Ta, Th, Ti, U, V, Zr, and rare-earth elements–RE; Factor 4 with Ba, K, and Sr; and Factor 5 with Br and I), one anthropogenic factor (Factor 2, including Cd, Pb, Sb, and Zn), and one geogenic-anthropogenic factor (Factor 3, including Cr and Ni). The highest anthropogenic impact of heavy metal to the air pollution in the country was from the ferronickel smelter near Kavadraci (Ni and Cr), the lead and zinc mines in the vicinity of Makedonska Kamenica, Probištip, and Kriva Palanka in the eastern part of the country (Cd, Pb, and Zn), and the former lead and zinc smelter plant in Veles. Beside the anthropogenic influences, the lithology and the composition of the soil also play an important role in the distribution of the elements.
Attractive electromagnetic Casimir stress on a spherical dielectric shell
(Elsevier, 2013) Graham, N.; Quandt, M.; Weigel, H.
Based on calculations involving an idealized boundary condition, it has long been assumed that the stress on a spherical conducting shell is repulsive. We use the more realistic case of a Drude dielectric to show that the stress is attractive, matching the generic behavior of Casimir forces in electromagnetism. We trace the discrepancy between these two cases to interactions between the electromagnetic quantum fluctuations and the dielectric material.
Calculation of a complete set of spin observables for proton elastic scattering from stable and unstable nuclei
(American Physical Society, 2018) Yahya, W. A.; Van Der Ventel, B. I. S.; Kaya, B. C. Kimene; Bark, R. A.
A microscopic study of proton elastic scattering from unstable nuclei at intermediate energies using a relativistic formalism is presented. We have employed both the original relativistic impulse approximation (IA1) and the generalized impulse approximation (IA2) formalisms to calculate the relativistic optical potentials, with target densities derived from relativistic mean field (RMF) theory using the NL3 and FSUGold parameter sets. Comparisons between the optical potentials computed using both IA1 and IA2 formalisms and the different RMF Lagrangians are presented for both stable and unstable targets. The comparisons are required to study the effect of using IA1 versus IA2 optical potentials, with different RMF parameter sets, on elastic scattering observables for unstable targets at intermediate energies. We also study the effect of full-folding form versus the factorized form of the optical potentials on elastic scattering observables. As with the case for stable nuclei, we found that the use of the full-folding optical potential improves the scattering observables (especially spin observables) at low intermediate energy (e.g., 200MeV). No discernible difference is found at a projectile incident energy of 500MeV. To check the validity of using localized optical potential, we calculate the scattering observables using nonlocal potentials by solving the momentum space Dirac equation. The Dirac equation is transformed to two coupled Lippmann-Schwinger equations, which are then numerically solved to obtain elastic scattering observables. The results are discussed and compared to calculations involving local coordinate-space optical potentials.
Characterization of the proposed 4−α cluster state candidate in ¹⁶O
(American Physical Society, 2017) Li, K. C. W.; Neveling, R.; Adsley, P.; Papka, P.; Smit, F. D.; Brummer, J. W.; Diget, C. Aa.; Freer, M.; Harakeh, M. N.; Kokalova, Tz.; Nemulodi, F.; Pellegri, L.; Rebeiro, B.; Swartz, J. A.; Triambak, S.; Van Zyl, J. J.; Wheldon, C.
The 0¹⁶O(α,α′) reaction was studied at θlab=0∘ at an incident energy of Elab=200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton decay and α decay from the natural parity states were observed in a large-acceptance silicon strip detector array at backward angles. The coincident charged-particle measurements were used to characterize the decay channels of the 0⁺₆ state in ¹⁶O located at Ex=15.097(5) MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the 4−α cluster state. The results of this work suggest the presence of a previously unidentified resonance at Ex≈15 MeV that does not exhibit a 0⁺ character. This unresolved resonance may have contaminated previous observations of the 0⁺₆ state.
Chiral bands in 193Tl
(Jagiellonian University, 2017-03) Ndayishimye, J.; Lawrie, E. A.; Shirinda, O.; Easton, J. L.; Wyngaardt, S. M.; Bark, R. A.; Bvumbi, S. P.; Dinoko, T. R. S.; Jones, P.; Kheswa, N. Y.; Lawrie, J. J.; Majola, S. N. T.; Masiteng, P. L.; Negi, D.; Orce, J. N.; Papka, P.; Sharpey-Schafer, J. F.; Stankiewicz, M.; Wiedeking, M.
Since the introduction of chirality in nuclei, the search for chiral candidates in different mass regions has been a hot topic for about two decades. At iThemba LABS, a new chiral symmetry region, the thallium isotopes with mass A ≥ 90 was found. Candidate chiral bands were suggested in 198Tl and then in 194Tl. Most importantly, a comparison of the properties in the chiral partner bands in 194Tl showed that this nucleus is one of the best chiral candidates to date. A search for chiral symmetry in the neighboring thallium isotopes was undertaken. The results from a y-ray spectroscopy study in 193Tl performed at iThemba LABS, South Africa, are discussed.
Chiral geometry and rotational structure for ¹³⁰Cs in the projected shell model
(Elsevier, 2018) Chen, F. Q.; Meng, Jie; Zhang, S. Q.
The projected shell model with configuration mixing for nuclear chirality is developed and applied to the observed rotational bands in the chiral nucleus 130Cs. For the chiral bands, the energy spectra and electromagnetic transition probabilities are well reproduced. The chiral geometry illustrated in the Kplotand the azimuthalplotis confirmed to be robust against the configuration mixing. The other rotational bands are also described in the same framework.
A class of spatio-temporal and causal stochastic processes, with application to multiscaling and multifractality
(Academy of Science for South Africa, 2005) Schmiegel, Schmiegel; Barndorff-Nielsen, Ole E.; Eggers, Hans C.
We present a general class of spatio-temporal stochastic processes describing the causal evolution of a positive-valued field in space and time. The field construction is based on independently scattered random measures of Lévy type whose weighted amplitudes are integrated within a causality cone. General n-point correlations are derived in closed form. As a special case of the general framework, we consider a causal multiscaling process in space and time in more detail. The latter is derived from, and completely specifed by, power-law two-point correlations, and gives rise to scaling behaviour of both purely temporal and spatial higher-order correlations. We further establish the connection to classical multifractality and prove the multifractal nature of the coarse-grained field amplitude.
Classical and quantum filaments in the ground state of trapped dipolar Bose gases
(American Physical Society, 2017) Cinti, Fabio; Boninsegni, Massimo
We study, by quantum Monte Carlo simulations, the ground state of a harmonically confined dipolar Bose gas with aligned dipole moments and with the inclusion of a repulsive two-body potential of varying range. Two different limits can clearly be identified, namely, a classical one in which the attractive part of the dipolar interaction dominates and the system forms an ordered array of parallel filaments and a quantum-mechanical one, wherein filaments are destabilized by zero-point motion, and eventually the ground state becomes a uniform cloud. The physical character of the system smoothly evolves from classical to quantum mechanical as the range of the repulsive two-body potential increases. An intermediate regime is observed in which ordered filaments are still present, albeit forming different structures from the ones predicted classically; quantum-mechanical exchanges of indistinguishable particles across different filaments allow phase coherence to be established, underlying a global superfluid response.
Classical dynamics on three-dimensional fuzzy space
(American Physical Society, 2018) Scholtz, F. G.
We derive the path integral action for a particle moving in three-dimensional fuzzy space. From this we extract the classical equations of motion. These equations have rather surprising and unconventional features: they predict a cutoff in energy, a generally spatial-dependent limiting speed, orbital precession remarkably similar to the general-relativistic result, flat velocity curves below a length scale determined by the limiting velocity and included mass, displaced planar motion, and the existence of two dynamical branches of which only one reduces to Newtonian dynamics in the commutative limit. These features may provide a stringent observational test for this scenario of noncommutativity.
Comparison of double-folding effective interactions within the cluster model
(American Physical Society, 2018) Kaya, B. D. C. Kimene; Wyngaardt, S. M.; Ibrahim, T. T.; Yahya, W. A.
Cluster-core hybrid potentials with Woods-Saxon plus cubic terms have been constructed to account for both the decay properties and positive-parity ground-state bands in 212Po, 218Rn, 222Ra, and 228Th. The hybrid potential parameters have been extracted from the real part of the double-folding interaction using the realistic Michigan- 3-Yukawa (M3Y) and a complex Gaussian effective interactions. We find that both the effective interactions exhibit similar behavior in the internal region, and the agreement between our estimated results with the existing experimental data is satisfactory.
Competition of rotation around the intermediate and long axes in 193Tl
(American Physical Society, 2019) Ndayishimye, J.; Lawrie, E. A.; Shirinda, O.; Easton, J. L.; Lawrie, J. J.; Wyngaardt, S. M.; Bark, R. A.; Bucher, T. D.; Bvumbi, S. P.; Dinoko, T. R. S.; Jones, P.; Kheswa, N. Y.; Majola, S. N. T.; Masiteng, P. L.; Negi, D.; Orce, J. N.; Sharpey-Schafer, J. F.; Wiedeking, M.
High-spin states in ¹⁹³Tl were studied and the level scheme was revised and extended including the observation of new rotational bands. A surprising competition between bands built on the same πh9/2⊗νi13/2⁻² nucleon configurations is observed. It is suggested that it is generated by two different rotational modes of this triaxial nucleus: (i) a rotation around the intermediate nuclear axis, producing a pair of chiral symmetry bands, and (ii) a rotation around the long nuclear axis, producing a third band with lower alignment.
Consistency between SU(3) and SU(2) covariant baryon chiral perturbation theory for the nucleon mass
(Elsevier, 2017) Ren, Xiu-Lei; Alvarez-Ruso, L.; Geng, Li-Sheng; Ledwig, Tim; Meng, Jie; Vacas, M. J. Vicente
Treating the strange quark mass as a heavy scale compared to the light quark mass, we perform a matching of the nucleon mass in the SU(3) sector to the two-flavor case in covariant baryon chiral perturbation theory. The validity of the 19 low-energy constants appearing in the octet baryon masses up to next-to-next-to-next-to-leading order [1] is supported by comparing the effective parameters (the combinations of the 19 couplings) with the corresponding low-energy constants in the SU(2) sector [2]. In addition, it is shown that the dependence of the effective parameters and the pion-nucleon sigma term on the strange quark mass is relatively weak around its physical value, thus providing support to the assumption made in Ref. [2] that the SU(2) baryon chiral perturbation theory can be applied to study nf = 2+1 lattice QCD simulations as long as the strange quark mass is close to its physical value.
The CT scanner facility at Stellenbosch University : an open access X-ray computed tomography laboratory
(Elsevier, 2016-10) Du Plessis, Anton; Le Roux, Stephan Gerhard; Guelpa, Anina
The Stellenbosch University CT Scanner Facility is an open access laboratory providing non-destructive Xray computed tomography (CT) and a high performance image analysis services as part of the Central Analytical Facilities (CAF) of the university. Based in Stellenbosch, South Africa, this facility offers open access to the general user community, including local researchers, companies and also remote users (both local and international, via sample shipment and data transfer). The laboratory hosts two CT instruments, i.e. a micro-CT system, as well as a nano-CT system. A workstation-based Image Analysis Centre is equipped with numerous computers with data analysis software packages, which are to the disposal of the facility users, along with expert supervision, if required. All research disciplines are accommodated at the X-ray CT laboratory, provided that non-destructive analysis will be beneficial. During its first four years, the facility has accommodated more than 400 unique users (33 in 2012; 86 in 2013; 154 in 2014; 140 in 2015; 75 in first half of 2016), with diverse industrial and research applications using X-ray CT as means. This paper summarises the existence of the laboratory’s first four years by way of selected examples, both from published and unpublished projects. In the process a detailed description of the capabilities and facilities available to users is presented.
Defect-induced supersolidity with soft-core bosons
(Nature Publishing Group, 2014) Cinti, F.; Macri, T.; Lechner, W.; Pupillo, G.; Pohl, T.
More than 40 years ago, Andreev, Lifshitz and Chester suggested the possible existence of a peculiar solid phase of matter, the microscopic constituents of which can flow superfluidly without resistance due to the formation of zero-point defects in the ground state of self-assembled crystals. Yet, a physical system where this mechanism is unambiguously established remains to be found, both experimentally and theoretically. Here we investigate the zero-temperature phase diagram of two-dimensional bosons with finite-range soft-core interactions. For low particle densities, the system is shown to feature a solid phase in which zero-point vacancies emerge spontaneously and give rise to superfluid flow of particles through the crystal. This provides the first example of defect-induced, continuous-space supersolidity consistent with the Andreev–Lifshitz–Chester scenario.

Browse

Browsing Research Articles (Physics) by browse.metadata.type "Article"

Results Per Page

Sort Options