Browsing by Author "McNamara, AL"

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    The determination of the efficiency of a Compton suppressed high purity germanium detector using Monte Carlo simulations
    (South Pacific Radioactivity Association, 2010-09-01) McNamara, AL; Heijnis, H; Fierro, D; Reinhard, MI
    The low level radiochemistry laboratory at the Australian Nuclear Science and Technology Organisation (ANSTO) performs radioactivity measurements of various environmental samples for a broad range of low level radionuclides. The laboratory's low-level gamma-spectrometry facility contains two Compton suppressed high purity Germanium (HPGe) detector systems. A Compton suppressed HPGe detector is well suited to the analysis of small environmental samples, however the nature of these samples (range of different geometries, densities and compositions) can make it difficult to construct an efficiency curve for the instrument. Currently, efficiency calibrations are f, carried out using reference materials packed into a particular geometry, e.g. a petri-dish. This makes the analysis of samples with different geometries difficult and time-consuming. Monte Carlo simulations can be a powerful tool in estimating the efficiency of the detector, especially for complicated detector systems and unusual sample compositions and geometries, provided enough geometric information on the system is available. Monte Carlo radiation transport simulations can also be used to determine self absorption, random and coincidence summing corrections, cascade and background effects. We model the gamma-Compton suppressed system using the simulation toolkit Geant4 for the efficiency calibration and compare the calculated efficiencies with the measurement of standard radionuclides in the low energy region of ~ 40 to 1500 keV. The calculated I7 efficiencies have the same dependence on energy as the measured values and the discrepancies between the two values can be attributed to incomplete knowledge of the detector geometry.
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    The determination of the efficiency of a Compton suppressed HPGe detector using Monte Carlo simulations
    (Elseiver, 2012-04-01) McNamara, AL; Heijnis, H; Fierro, D; Reinhard, MI
    Compton suppressed high-purity germanium (HPGe) detector is well suited to the analysis of low levels of radioactivity in environmental samples. The difference in geometry, density and composition of environmental calibration standards (e.g. soil) can contribute to excessive experimental uncertainty to the measured efficiency curve. Furthermore multiple detectors, like those used in a Compton suppressed system, can add complexities to the calibration process. Monte Carlo simulations can be a powerful complement in calibrating these types of detector systems, provided enough physical information on the system is known. A full detector model using the Geant4 simulation toolkit is presented and the system is modelled in both the suppressed and unsuppressed mode of operation. The full energy peak efficiencies of radionuclides from a standard source sample is calculated and compared to experimental measurements. The experimental results agree relatively well with the simulated values (within similar to 5 - 20%). The simulations show that coincidence losses in the Compton suppression system can cause radionuclide specific effects on the detector efficiency, especially in the Compton suppressed mode of the detector. Additionally since low energy photons are more sensitive to small inaccuracies in the computational detector model than high energy photons, large discrepancies may occur at energies lower than similar to 100 keV. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.
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    Dose enhancement effects to the nucleus and mitochondria from gold nanoparticles in the cytosol
    (IOP Publishing, 2016-07-20) McNamara, AL; Kam, WWY; Scales, N; McMahon, SJ; Bennett, JW; Byrne, HL; Schuemann, J; Paganetti, H; Banati, RB; Kuncic, Z
    Gold nanoparticles (GNPs) have shown potential as dose enhancers for radiation therapy. Since damage to the genome affects the viability of a cell, it is generally assumed that GNPs have to localise within the cell nucleus. In practice, however, GNPs tend to localise in the cytoplasm yet still appear to have a dose enhancing effect on the cell. Whether this effect can be attributed to stress-induced biological mechanisms or to physical damage to extra-nuclear cellular targets is still unclear. There is however growing evidence to suggest that the cellular response to radiation can also be influenced by indirect processes induced when the nucleus is not directly targeted by radiation. The mitochondrion in particular may be an effective extra-nuclear radiation target given its many important functional roles in the cell. To more accurately predict the physical effect of radiation within different cell organelles, we measured the full chemical composition of a whole human lymphocytic JURKAT cell as well as two separate organelles; the cell nucleus and the mitochondrion. The experimental measurements found that all three biological materials had similar ionisation energies  ~70 eV, substantially lower than that of liquid water  ~78 eV. Monte Carlo simulations for 10–50 keV incident photons showed higher energy deposition and ionisation numbers in the cell and organelle materials compared to liquid water. Adding a 1% mass fraction of gold to each material increased the energy deposition by a factor of  ~1.8 when averaged over all incident photon energies. Simulations of a realistic compartmentalised cell show that the presence of gold in the cytosol increases the energy deposition in the mitochondrial volume more than within the nuclear volume. We find this is due to sub-micron delocalisation of energy by photoelectrons, making the mitochondria a potentially viable indirect radiation target for GNPs that localise to the cytosol. © 2016 Institute of Physics and Engineering in Medicine
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    Positron emission tomography coincidence detection with photon polarization correlation
    (SPIE, 2013-02-09) McNamara, AL; Wu, KW; Boardman, DA; Reinhard, MI; Kuncic, Z
    Two-photon annihilation quanta are emitted in a pure quantum state and when detected in coincidence, the photon pairs possess orthogonal polarizations. We propose that this polarization correlation can be exploited in Positron Emission Tomography (PET), which relies crucially on accurate coincidence detection of photon pairs. In this proof of concept study, we investigate how photon polarization information can be exploited in PET imaging by developing a method to discern true coincidences using the polarization correlation of annihilation pairs. We demonstrate that the unique identification of true photon pairs with their polarization correlation can dramatically enhance overall PET image quality, especially for high emission rates, when conventional, energy- based coincidence detection methods become increasingly unreliable. Our results suggest that polarization-based coincidence detection offers new prospects for in vivo molecular imaging with next-generation PET systems. © 2013 Society of Photo-Optical Instrumentation Engineers (SPIE).
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    Predicted ionisation in mitochondria and observed acute changes in the mitochondrial transcriptome after gamma irradiation: a Monte Carlo simulation and quantitative PCR study
    (Elsevier B.V., 2013-11-01) Kam, WWY; McNamara, AL; Lake, V; Banos, C; Davies, JB; Kuncic, Z; Banati, RB
    It is a widely accepted that the cell nucleus is the primary site of radiation damage while extra-nuclear radiation effects are not yet systematically included into models of radiation damage. We performed Monte Carlo simulations assuming a spherical cell (diameter 11.5 μm) modelled after JURKAT cells with the inclusion of realistic elemental composition data based on published literature. The cell model consists of cytoplasm (density 1 g/cm3), nucleus (diameter 8.5 μm; 40% of cell volume) as well as cylindrical mitochondria (diameter 1 μm; volume 0.5 μm3) of three different densities (1, 2 and 10 g/cm3) and total mitochondrial volume relative to the cell volume (10, 20, 30%). Our simulation predicts that if mitochondria take up more than 20% of a cell's volume, ionisation events will be the preferentially located in mitochondria rather than in the cell nucleus. Using quantitative polymerase chain reaction, we substantiate in JURKAT cells that human mitochondria respond to gamma radiation with early (within 30 min) differential changes in the expression levels of 18 mitochondrially encoded genes, whereby the number of regulated genes varies in a dose-dependent but non-linear pattern (10 Gy: 1 gene; 50 Gy: 5 genes; 100 Gy: 12 genes). The simulation data as well as the experimental observations suggest that current models of acute radiation effects, which largely focus on nuclear effects, might benefit from more systematic considerations of the early mitochondrial responses and how these may subsequently determine cell response to ionising radiation. © 2013 Elsevier B.V.