iThemba LABS Research Articles

Abstract ID: 201 Code sharing of MC beam models for advanced radiotherapy.

The development and validation of a beam line MC model is an essential part of many projects, particularly those involving particle therapy. This is often a very time consuming and sometimes costly computational process. However, many experiments use the same beam lines and the medical physics community would benefit greatly from a structured code and data sharing site. A new project [1] , funded through the STFC Global Challenge Network+ in Advanced Radiotherapy [2] , aims to collate, host, validate experimentally, and disseminate common Geant4 beam line models. We will present an overview of the aims of the project, and details of how to gain access to these structured models; a structured model includes both code and data with related information on use and best practice. To illustrate this, three validation results will be shown; the iThemba LABS proton therapy centre in Cape Town (South Africa), the medical beam line of the University of Birmingham MC40 Cyclotron (United Kingdom), and ongoing work with multiple models of the Clatterbridge Cancer Centre proton eye therapy beam line (United Kingdom). We aim to develop these models in TOPAS/GATE to facilitate their use for non Geant4/C++ experts, for example, medical physicists working in hospitals. A status report of this final step will be provided.

International Workshop on Discovery Physics at the LHC

Foreword Proceedings Kruger2016The fourth biennial conference on “Discovery Physics at he LHC” was held December 4-9 2016 at the Kruger Gate Hotel in South Africa. In total there were about 100 participants coming from Canada, Croatia, France, Germany, Hungary, Italy, Korea, Poland, Russia, Spain, Sweden, Switzerland, UK and USA. The latest results from the Large Hadron Collider, the Relativistic Heavy Ion Collider as well the latest theoretical insights were presented. With the majestic Kruger national Park in the background this led to a very stimulating conference with many exchanges taking place. The proceedings reflect the high level of the conference.The financial contributions from iThemba LABS, the Department of Science and Technology, the SA-CERN programme, the UCT-CERN Research Centre, the Universities of Cape Town and of the Witwatersrand are gratefully acknowledged.

Radiation damage effects on the optical properties of plastic scintillators

We report on the radiation damage to the optical properties of plastic scintillators following irradiation using a 6MeV proton beam produced by the 6MV tandem accelerator of iThemba LABS, Gauteng. A comparative is drawn between polyvinyl toluene based commercial scintillators EJ200, EJ208, EJ260 and BC408 as well as polystyrene based scintillator UPS923A and scintillators manufactured for the Tile Calorimeter. Results on the proton induced damage indicate a reduction in the light output and transmission capability of the plastics. Scintillators containing a larger Stokes shift, i.e. EJ260 and EJ208 exhibit the most radiation hardness. The EJ208 is recommended as a candidate to be considered for the replacement of Gap scintillators in the Tile Calorimeter for the 2018 upgrade.

Characterization of the proposed 4−α cluster state candidate in O16

The $\mathrm{^{16}O}(\alpha, \alpha^{\prime})$ reaction was studied at $\theta_{lab} = 0^\circ$ at an incident energy of $\textrm{E}_{lab}$ = 200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton and $\alpha$-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 $\mathrm{^{16}O}$ located at $E_{x} = 15.097(5)$ MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the 4-$\alpha$ cluster state. The results of this work suggest the presence of a previously unidentified resonance at $E_{x}\approx15$ MeV that does not exhibit a $0^{+}$ character. This unresolved resonance may have contaminated previous observations of the $0_{6}^{+}$ state.

α clustering in Si28 probed through the identification of high-lying 0+ states

Background: Aspects of the nuclear structure of light $\ensuremath{\alpha}$-conjugate nuclei have long been associated with nuclear clustering based on $\ensuremath{\alpha}$ particles and heavier $\ensuremath{\alpha}$-conjugate systems such as $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$. Such structures are associated with strong deformation corresponding to superdeformed or even hyperdeformed bands. Superdeformed bands have been identified in $^{40}\mathrm{Ca}$ and neighboring nuclei and find good description within shell model, mean-field, and $\ensuremath{\alpha}$-cluster models. The utility of the $\ensuremath{\alpha}$-cluster description may be probed further by extending such studies to more challenging cases comprising lighter $\ensuremath{\alpha}$-conjugate nuclei such as $^{24}\mathrm{Mg}, ^{28}\mathrm{Si}$, and $^{32}\mathrm{S}$.Purpose: The purpose of this study is to look for the number and energy of isoscalar ${0}^{+}$ states in $^{28}\mathrm{Si}$. These states are the potential bandheads for superdeformed bands in $^{28}\mathrm{Si}$ corresponding to the exotic structures of $^{28}\mathrm{Si}$. Of particular interest is locating the ${0}^{+}$ bandhead of the previously identified superdeformed band in $^{28}\mathrm{Si}$.Methods: $\ensuremath{\alpha}$-particle inelastic scattering from a $^{\mathrm{nat}}\mathrm{Si}$ target at very forward angles including ${0}^{\ensuremath{\circ}}$ has been performed at the iThemba Laboratory for Accelerator-Based Sciences in South Africa. Scattered particles corresponding to the excitation energy region of 6 to 14 MeV were momentum-analysed in the K600 magnetic spectrometer and detected at the focal plane using two multiwire drift chambers and two plastic scintillators.Results: Several ${0}^{+}$ states have been identified above 9 MeV in $^{28}\mathrm{Si}$. A newly identified 9.71 MeV ${0}^{+}$ state is a strong candidate for the bandhead of the previously discussed superdeformed band. The multichannel dynamical symmetry of the semimicroscopic algebraic model predicts the spectrum of the excited ${0}^{+}$ states. The theoretical prediction is in good agreement with the experimental finding, supporting the assignment of the 9.71-MeV state as the bandhead of a superdeformed band.Conclusion: Excited isoscalar ${0}^{+}$ states in $^{28}\mathrm{Si}$ have been identified. The number of states observed in the present experiment shows good agreement with the prediction of the multichannel dynamical symmetry.

CAKE: the coincidence array for K600 experiments

The combination of a magnetic spectrometer and ancillary detectors such as silicon detectors is a powerful tool for the study of nuclear reactions and nuclear structure. This paper discusses the recently commissioned silicon array called the "CAKE" which is designed for use with the K600 magnetic spectrometer at iThemba LABS.

Acceptance Tests for AMS Radiocarbon Measurements at iThemba LABS, Gauteng, South Africa

The accelerator mass spectrometer was commissioned recently at the iThemba LABS 6 MV tandem accelerator. Improvements in the vacuum system, requiring procurement of cryo-pumps and the reducing the tank pressure of the N2 + CO2 insulation gas mixture below the level used for IBA measurements, were necessary. This resulted in the reduction of the nitrogen background and improved the resolution of 14C from 14N background in the ionisation chamber. The nitrogen was leaking to the stripping canal because of inadequate sealing. The analysing magnet was scaled to detect C3+ ions, at 3 MV terminal potential. The first sensible spectra allowed for the pin-pointing of many persistent issues. This resulted in measurements with a precision better than 1 pMC, and current blank levels correspond to 12 half-lives of 14C or ∼68000 years. The radiocarbon sample preparation laboratory has reached production status. A brief outlook of the work towards the implementation of the measurement and chemical preparation protocols for radionuclides 10Be and 26Al is also summarised in the conclusion

Statistical gamma-ray decay studies at iThemba LABS

A program to study the γ-ray decay from the region of high-level density has been established at iThemba LABS, where a high-resolution gamma-ray detector array is used in conjunction with silicon particle-telescopes. Results from two recent projects are presented: 1) The 74 Ge(α,α′γ) reaction was used to investigate the Pygmy Dipole Resonance. The results were compared to (γ,γ′) data and indicate that the dipole states split into mixed isospin and relatively pure isovector excitations. 2) Data from the 95 Mo(d,p) reaction were used to develop a novel method for the determination of spins for low-lying discrete levels utilizing statistical γ-ray decay in the vicinity of the neutron separation energy. These results provide insight into the competition of (γ,n) and (γ,γ′) reactions and highlights the need to correct for angular momentum barrier effects.

Elemental Analysis of Commonly Consumed Rice in Nigeria using PIXE Technique

This study investigated the essential element (P, Cl, K, Ca, Mn, Fe, Cu and Zn) concentra-tions of some local and imported rice commonly consumed in Nigeria. This is done in order to select the rice with high nutrient to combat malnutrition, especially in children. Proton-induced X-ray emission (PIXE) techniques available at iThemba Labs Someset West, South Africa was used for the analysis. The elemental concentration obtained for Ofada are \(8805.5\boldsymbol{\pm}221\), \(276.5\boldsymbol{\pm}32\), \(3704.5\boldsymbol{\pm}39\), \(212$\boldsymbol{\pm}8\), \(110.5\boldsymbol{\pm}8\), \(125.5\boldsymbol{\pm}7\), \(14.5\boldsymbol{\pm}1\) and \(44.5\boldsymbol{\pm}4\) ppm, while for the imported rice we got \(2862.5\boldsymbol{\pm}150\), \(399\boldsymbol{\pm}22\), \(1433\boldsymbol{\pm}28\), \(4.15\boldsymbol{\pm}2\), \(8.4\boldsymbol{\pm}0.8\), \(4.75\boldsymbol{\pm}1\) and \(9.5\boldsymbol{\pm}2\) ppm for P, Cl, K, Ca, Mn, Fe, Cu and Zn respectively. Phosporus (P) has the highest elemental concentration in all samples. Ofada, one of the local rice was found to have the highest elemental concentration while pure imported rice had the lowest concentrations of the elements. The study suggests that consumption of locally-cultivated (Ofada) rice should be encouraged in order to reduce malnutrition in children in Nigeria.

Octupole correlations in N=88Dy154 : Octupole vibration versus stable deformation

We report on low-spin states of $^{154}\mathrm{Dy}$ populated via the reaction $^{155}\mathrm{Gd}$ $(^{3}\mathrm{He},4n)$ with a beam energy of 37.5 MeV from the Separated Sector Cyclotron at iThemba Laboratory. The AFRODITE \ensuremath{\gamma}-ray spectrometer was used to establish new $E1$ transitions between bands of opposite parity. The measurements broaden the $N=88$ systematics on the relationship between the first excited positive-parity pairing isomer band and the lowest-lying negative-parity band as the nuclear quadrupole deformation decreases with increasing proton number. In a region of strong octupole correlations the data suggest that the spectroscopy of $N=88$ nuclei is driven by stable octupole deformations and not by vibrations.

An experimental demonstration of a new type of proton computed tomography using a novel silicon tracking detector.

Radiography and tomography using proton beams promise benefit to image guidance and treatment planning for proton therapy. A novel proton tracking detector is described and experimental demonstrations at a therapy facility are reported. A new type of proton CT reconstructing relative "scattering power" rather than "stopping power" is also demonstrated. Notably, this new type of imaging does not require the measurement of the residual energies of the protons. A large area, silicon microstrip tracker with high spatial and temporal resolution has been developed by the Proton Radiotherapy Verification and Dosimetry Applications consortium and commissioned using beams of protons at iThemba LABS, Medical Radiation Department, South Africa. The tracker comprises twelve planes of silicon developed using technology from high energy physics with each plane having an active area of ∼10 × 10 cm segmented into 2048 microstrips. The tracker is organized into four separate units each containing three detectors at 60° to one another creating an x-u-v coordinate system. Pairs of tracking units are used to reconstruct vertices for protons entering and exiting a phantom containing tissue equivalent inserts. By measuring the position and direction of each proton before and after the phantom, the nonlinear path for each proton through an object can be reconstructed. Experimental results are reported for tracking the path of protons with initial energies of 125 and 191 MeV. A spherical phantom of 75 mm diameter was imaged by positioning it between the entrance and exit detectors of the tracker. Positions and directions of individual protons were used to create angular distributions and 2D fluence maps of the beam. These results were acquired for 36 equally spaced projections spanning 180°, allowing, for the first time, an experimental CT image based upon the relative scattering power of protons to be reconstructed. Successful tracking of protons through a thick target (phantom) has demonstrated that the tracker discussed in this paper can provide the precise directional information needed to perform proton radiography and tomography. When synchronized with a range telescope, this could enable the reconstruction of proton CT images of stopping power. Furthermore, by measuring the deflection of many protons through a phantom, it was demonstrated that it is possible to reconstruct a new kind of CT image (scattering power) based upon this tracking information alone.

On-line test using multi-foil SiC target at iThemba LABS

In a collaboration between the INFN-SPES project and iThemba LABS, an on-line test of the power dissipation of a multi-slice target concept for radioactive-ion beam production has been performed. Using the 66MeV proton beam from the iThemba LABS Separated Sector Cyclotron, a total power of 4kW was deposited in the target complex, consisting of 13 Silicon Carbide (SiC) discs of about 1 millimetre thickness housed in a graphite cylinder. The results of the measurements fully validate thermal FEM simulations and confirm that the multi-foil target system is a reliable and affordable device for second-generation ISOL-RIB facilities like SPES.

Search for two-phonon octupole excitations in 146Gd

The low-spin structure of the nearly spherical nucleus 146Gd was studied using the 144Sm(4He, 2n) fusion-evaporation reaction. High-statistics \(\gamma - \gamma\) coincidence measurements were performed at iThemba LABS with 7× 109 \( \gamma\)-\( \gamma\) coincidence events recorded. Gated \( \gamma\)-ray energy spectra show evidence for the \( 6^{+}_{2} \rightarrow 3^{-}_{1} \rightarrow 0^{+}_{1}\) cascade of E3 transitions in agreement with recent findings by Caballero and co-workers, but with a smaller branching ratio of \( I_{\gamma} = 4.7(10)\) for the \(6^{+}_{2} \rightarrow 3^{-}_{1}\) 1905.1 keV \( \gamma\) ray. Although these findings may support octupole vibrations in spherical nuclei, sophisticated beyond mean-field calculations including angular-momentum projection are required to interpret in an appropriate way the available data due to the failure of the rotational model assumptions in this nucleus.

K-shell X-ray production cross sections in Ti by 0.3–1.0 MeV/u 12C and 28Si ions for heavy ion PIXE

An inhibiting factor in the implementation of heavy ion PIXE is that heavy ion X-ray production cross sections are not well known for many materials. This contribution reports on work carried out at iThemba LABS to measure X-ray production cross-sections in Ti thin films irradiated with 12C and 28Si ions within the 0.3–1.0MeV/u energy range. Experimental data are compared to predictions by the PWBA, SCA-UA and ECPSSR theories. For the C–Ti asymmetric collision, ECPSSR theory describes experiment fairly well, but fails for the nearly symmetric Si–Ti collision, even after correction for multiple ionisation effects. The SCA-UA calculations tend to agree with data only at low (<0.5MeV/u) energies for the lighter 12C incident ions, but like ECPSSR, fail in the case of 28Si projectiles. The PWBA predictions grossly overestimate experiment in both cases.

Image Information Measures for Predicting Image Registration Performance on iThemba LABS Image Registration System

Image Information Measures for Predicting Image Registration Performance on iThemba LABS Image Registration System

Liquid metal ion source assembly for external ion injection into an electron string ion source (ESIS).

An assembly for a commercial Ga(+) liquid metal ion source in combination with an ion transportation and focusing system, a pulse high-voltage quadrupole deflector, and a beam diagnostics system has been constructed in the framework of the iThemba LABS (Cape Town, South Africa)-JINR (Dubna, Russia) collaboration. First, results on Ga(+) ion beam commissioning will be presented. Outlook of further experiments for measurements of charge breeding efficiency in the electron string ion source with the use of external injection of Ga(+) and Au(+) ion beams will be reported as well.

Beam experiments with the Grenoble test electron cyclotron resonance ion source at iThemba LABS.

At iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) an electron cyclotron ion source was installed and commissioned. This source is a copy of the Grenoble Test Source (GTS) for the production of highly charged ions. The source is similar to the GTS-LHC at CERN and named GTS2. A collaboration between the Accelerators and Beam Physics Group of CERN and the Accelerator and Engineering Department of iThemba LABS was proposed in which the development of high intensity argon and xenon beams is envisaged. In this paper, we present beam experiments with the GTS2 at iThemba LABS, in which the results of continuous wave and afterglow operation of xenon ion beams with oxygen as supporting gases are presented.

Quantitative mapping of elemental distribution in leaves of the metallophytes Helichrysum candolleanum, Blepharis aspera, and Blepharis diversispina from Selkirk Cu–Ni mine, Botswana

Multi-element profiling is essential in understanding the metal-tolerant behavior of metallophytes. Although previous reports using multi-elemental analyses show that the metallophytes Blepharis aspera, Blepharis diversispina (Acanthaceae) and Helichrysum candolleanum (Asteraceae) take up metals, no information exists on elemental spatial distribution and concentrations in specific tissues of these plants. The aim of this study therefore was to assess the spatial distribution and concentration of copper, nickel and other elements in leaf tissues of these plants using micro-PIXE. Whole plants were collected with soil in pots from an operational copper and nickel mine (i.e., a copper and nickel mineralized area), Selkirk, about 40km north-east of Francistown, Botswana. On the same day the samples were transported by air to iThemba LABS in South Africa. Leaf specimens were cryofixed in liquid propane cooled by LN2. Parallel samples were embedded in resin for anatomical studies to facilitate interpretation of elemental maps. The distribution and concentration of copper, nickel, and other elements in leaf tissues were determined using micro-PIXE and proton backscattering spectrometry. Data evaluation was performed using GeoPIXE II software. Micro-PIXE showed that H. candolleanum had the highest whole leaf content of copper (70±3μgg−1 DW) and nickel (168±5μgg−1 DW), followed by B. aspera (Cu: 25±1μgg−1 DW; Ni: 166±4μgg−1 DW) and B. diversispina (Cu: 3±1μgg−1 DW; Ni, below detection limit). For specific leaf tissues, the highest levels of copper were found in the vascular bundle for H. candolleanum (167±7μgg−1 DW) and the lower epidermis for B. aspera (70±9μgg−1 DW). The highest levels of nickel were present in the vascular bundle of B. aspera (479±10μgg−1 DW) and H. candolleanum (90±5μgg−1 DW). Elemental maps showed a similar distribution pattern for copper and nickel in B. aspera and B diversispina, with these elements concentrated in the upper and lower epidermal regions. However, for H. candolleanum, both copper and nickel were concentrated in the mesophyll. The difference in concentration and accumulation patterns between B. aspera, B. diversispina and H. candolleanum suggests that the metal-tolerant adaptation of these metallophytes may differ.

Radiation hardness of plastic scintillators for the Tile Calorimeter of the ATLAS detector

The radiation damage in polyvinyl toluene based plastic scintillator EJ200 obtained from ELJEN technology was investigated. This forms part of a comparative study conducted to aid in the upgrade of the Tile Calorimeter of the ATLAS detector during which the Gap scintillators will be replaced. Samples subjected to 6 MeV proton irradiation using the tandem accelerator of iThemba LABS, were irradiated with doses of approximately 0.8 MGy, 8 MGy, 25 MGy and 80 MGy. The optical properties were investigated using transmission spectroscopy and light yield analysis whilst structural damage was assessed using Raman spectroscopy. Findings indicate that for the dose of 0.8 MGy, no structural damage occurs and light loss can be attributed to a breakdown in the light transfer between base and fluor dopants. For doses of 8 MGy to 80 MGy, structural damage leads to possible hydrogen loss in the benzene ring of the PVT base which forms free radicals. This results in an additional absorptive component causing increased transmission loss and light yield loss with increasing dose.

A comparative study of the radiation hardness of plastic scintillators for the upgrade of the Tile Calorimeter of the ATLAS detector

The influence of radiation on the light transmittance of plastic scintillators was studied experimentally. The high optical transmittance property of plastic scintillators makes them essential in the effective functioning of the Tile calorimeter of the ATLAS detector at CERN. This significant role played by the scintillators makes this research imperative in the movement towards the upgrade of the tile calorimeter. The radiation damage of polyvinyl toluene (PVT) based plastic scintillators was studied, namely, EJ-200, EJ-208 and EJ-260, all manufactured and provided to us by ELJEN technology. In addition, in order to compare to scintillator brands actually in use at the ATLAS detector currently, two polystyrene (PS) based scintillators and an additional PVT based scintillator were also scrutinized in this study, namely, Dubna, Protvino and Bicron, respectively. All the samples were irradiated using a 6 MeV proton beam at different doses at iThemba LABS Gauteng. The radiation process was planned and mimicked by doing simulations using a SRIM program. In addition, transmission spectra for the irradiated and unirradiated samples of each grade were obtained, observed and analyzed.