Browsing by Author "Bignell, LJ"

Now showing 1 - 12 of 12
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    Activity measurements of H-3 using the TDCR method and observation of source stability
    (Elsevier, 2010-07) Mo, L; Bignell, LJ; Steele, T; Alexiev, D
    Activity measurements of 3H were performed using the triple to double coincidence ratio method for the 2009 international comparison organized by the Bureau International des Poids et Mesures (BIPM). The activity obtained by using the FPGA (field programmable gate array) acquisition system and software event analysis was compared to that using MAC3 (Module d’Acquisition de Coincidences triples). Agreement to better than 0.4% was obtained. The stability of sources prepared in Ultima Gold™, Ultima Gold™ LLT, Insta-Gel® Plus and Optiphase “Hisafe”-3 over 140 days is presented. © 2010, Elsevier Ltd.
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    Can the ZoMBieS method be used to characterise scintillator non-linearity?
    (Elsevier, 2014-05) Bignell, LJ
    Measurements of the detection efficiency as a function of deposited electron energy in a liquid scintillation cocktail between 4 keV and 49 keV are obtained using the ZoMBieS method. Comparison is made between the measured data and the Poisson–Birks detection efficiency model. Measurements of the Birks non-linearity parameter, kB, and the linearised scintillation response of each photomultiplier, ωi, were made using these data. However, the value of kB that best linearises the scintillator response is found to vary depending upon which photomultiplier is used in its determination, and the measured kB and ωi vary depending on the external source geometry. The cause of this behaviour is unknown. The triple-coincident detection efficiency appears to be unaffected by any systematic errors. © 2013 Elsevier Ltd.
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    Effect of multiple γ-ray interactions on ionisation quenching corrections in liquid scintillants
    (Elsevier, 2010-03-01) Bignell, LJ; Mo, L; Alexiev, D; Hashemi-Nezhad, SR
    The effect of multiple γ-ray interactions within a liquid scintillation detector caused by a single radionuclide decay event on ionisation quenching corrections has been determined. Ionisation quenching corrections to the energy deposition spectrum have been carried out over all electron-generating gamma interactions of the decay event. Comparison has been made with the approximate method typically used to correct for ionisation quench. Both calculations were carried out using the Geant4 simulation package. The two models are compared using the values of detection efficiencies of the logical sum of double coincidence obtained for 131I, 123I and 177Lu measured using the Triple-to-Double Coincidence Ratio method of absolute activity measurement. Finally, predictions are made as to the circumstances under which the two quench correction approaches will be most discrepant. © 2010, Elsevier Ltd.
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    Evaluation of lead shielding for a gamma-spectroscopy system
    (Elsevier, 2008-05-01) Smith, ML; Bignell, LJ; Alexiev, D; Mo, L; Harrison, JJ
    Three types of lead shielding assemblies were investigated to establish which provides the lowest background as preliminary work for determining the efficiency of a High Purity Germanium (HPGe) detector. The background continuum and peaks were examined using gamma spectroscopy with a coaxial and a planar HPGe detector. The Pb-210 concentration in each lead sample taken from the assemblies was quantitatively determined. A French lead sample was found to have the lowest background in this relative comparison with a Pb-210 concentration of 29.1 +/- 1.3 Bq kg(-1). © 2008, Elsevier Ltd.
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    FASEA: a FPGA acquisition system and software event analysis for liquid scintillation counting
    (Elsevier, 2009-10-11) Steele, T; Mo, L; Bignell, LJ; Smith, M; Alexiev, D
    The FASEA (FPGA based Acquisition and Software Event Analysis) system has been developed to replace the MAC3 for coincidence pulse processing. The system uses a National Instruments Virtex 5 FPGA card (PXI-7842R) for data acquisition and a purpose developed data analysis software for data analysis. Initial comparisons to the MAC3 unit are included based on measurements of 89Sr and 3H, confirming that the system is able to accurately emulate the behaviour of the MAC3 unit. © 2009, Elsevier Ltd.
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    High-purity CdMnTe radiation detectors: a high-resolution spectroscopic evaluation
    (Intistute of Electrical Engineers, 2013-02-07) Rafiei, R; Reinhard, MI; Kim, KH; Prokopovich, DA; Boardman, DA; Sarbutt, A; Watt, GC; Bolotnikov, AE; Bignell, LJ; James, RB
    The charge transport properties of a high-purity CdMnTe (CMT) crystal have been measured at room temperature down to a micron-scale resolution. The CMT crystal, doped with indium, was grown by the vertical Bridgman technique. To reduce the residual impurities in the Mn source material, the growth process incorporated a five-times purification process of MnTe by a zone-refining method with molten Te solvent. The resulting 2.6 mm thick crystal exhibited an electron mobility-lifetime product of μnτn=2.9 × 10-3 cm2V-1. The velocity of electron drift was calculated from the rise time distribution of the preamplifier's output pulses at each measured bias. The electron mobility was extracted from the electric field dependence of the drift velocity and at room temperature it has a value of μn=(950±90) cm2/Vs. High-resolution maps of the charge collection efficiency have been measured using a scanning microbeam of 5.5 MeV 4He2+ ions focused to a beam diameter <; 1 μm and display large-area spatial uniformity. The evolution of charge collection uniformity across the detector has been highlighted by acquiring measurements at applied biases ranging between 50 V and 1100 V. Charge transport inhomogeneity has been associated with the presence of bulk defects. It has been demonstrated that minimizing the content of impurities in the MnTe source material is highly effective in achieving major improvements in the CMT detector's performance as compared to previous data. © 2013, IEEE.
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    Monte Carlo simulation of a Au-198 thin foil: the response of a 4πβ-γ detector
    (Institute of Electrical and Electronics Engineers (IEEE), 2008-12) Bignell, LJ; Mo, L; Smith, ML; Alexiev, D; Hashemi-Nezhad, SR
    Monte Carlo simulations have been performed of the decay of a(198) Au solid source within a 4πβ-γ coincidence detector. Calculations of the proportional counter efficiency to both β and γ emissions are simulated. A comparison of results obtained using the MCNP-5 and Geant4 simulation packages indicates that Geant4 better evaluates the K correction to the coincidence equation than MCNP-5 predictions. This difference can be accounted for in terms of how the codes handle the physical interactions occurring in the foil and surrounds. © 2008, Institute of Electrical and Electronics Engineers (IEEE)
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    New modes of THz generation by low-temperature-grown GaAsSb
    (Elsevier, 2009-02) Hargreaves, S; Bignell, LJ; Lewis, RA; Sigmund, J; Hartnagel, HL
    The low-temperature growth of GaAs1−ySby with y = 0.4 and 0.85 has been reported recently along with characterization by X-ray diffraction, Hall, and current–voltage measurements. Here we extend the characterization by employing reflectance spectroscopy in the range 5–18 THz to confirm the compositions of the grown layers. In the course of this work we established for the first time that GaAs1−ySby may serve as an emitter of THz radiation under optical excitation by ultrashort pulses of near-infrared radiation in two distinct experimental arrangements: THz is generated when an electrical bias is applied through a simple electrode structure, attributed to a photoconductive effect; and THz is generated by the pristine layers themselves, attributed to a surface-field effect. In each case the THz emission is compared directly with that from low-temperature-grown GaAs. The results presented here are for as-grown material. Suitable annealing may improve the THz emission even further. © 2009, Elsevier Ltd.
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    Plasmonic light yield enhancement of a liquid scintillator
    (AIP Publishing LLC, 2013-05-28) Bignell, LJ; Mume, E; Jackson, TW; Lee, GP
    We demonstrate modifications to the light yield properties of an organic liquid scintillator due to the localization of the tertiary fluorophore component to the surface of Ag-core silica-shell nanoparticles. We attribute this enhancement to the near-field interaction of Ag nanoparticle plasmons with these fluor molecules. The scintillation light yield enhancement is shown to be equal to the fluorescence enhancement within measurement uncertainties. With a suitable choice of plasmon energy and scintillation fluor, this effect may be used to engineer scintillators with enhanced light yields for radiation detection applications. © 2013, AIP Publishing LLC.
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    Reflectance studies of candidate THz emitters
    (Springer, 2009-01) Bignell, LJ; Lewis, RA
    Semiconductors are efficient emitters of terahertz (THz, 1012 Hz) radiation. Non-contact means of accurately measuring the physical parameters of these materials are of great value. The reflectance of polar crystals yields important information. A dramatic change in reflectance occurs in the frequency range between the transverse-optical (TO) and the longitudinal-optical (LO) phonons. For many materials these frequencies are of the order of a few THz. Analysis of the reflectance in and near this region yields (a) the TO phonon frequency ω T , (b) the LO phonon frequency ω L , (c) the low-frequency or DC reflectance R(0), and thence the DC refractive index, n(0), and dielectric constant, ɛ(0); (d) the high-frequency or optical reflectance R(∞), and thence n(∞) and ɛ(∞) and (e) the phonon damping factor Γ. These constants depend on the lattice itself and may be described within the Lorentz model. If, in addition, the crystal possesses free carriers, reflectance measurements further yield (f) the plasma frequency ω P , and thence the carrier concentration n e/h and (g) the plasma damping factor γ which may be understood in terms of the Drude model. Samples in the form of a parallel plate give rise to interference fringes that yield (h) the sample thickness t. We have examined many polar crystals with a view to understanding THz emission from them with the overall goal of improving the emission efficiency. Measurements have been made in the region 1.5–21 THz (50–700 cm−1) of single and multilayer samples. We use the sum rule to check the internal consistency of the experimental measurements. We have re-examined the relationship between the phonon frequencies and the reduced ion mass. We find the effective spring constant is very similar in all I–VII materials studied and likewise within the II–VI and III–V classes. We use shell theory to account for these results. © 2009, Springer.
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    Sensitivity and uncertainty analysis of the simulation of 123I and 54Mn decay in liquid scintillation vials
    (Elsevier, 2009-09-10) Bignell, LJ; Mo, L; Smith, ML; Steele, T; Alexiev, D; Hashemi-Nezhad, SR
    General purpose, widely distributed Monte Carlo simulations for radiation transport have found many applications in the fields of radionuclide metrology, detector research and nuclear medicine. A full evaluation of the uncertainties due to the simulation of physical quantities and correction factors is required for model validation, uncertainty analysis and quality assurance, yet this evaluation is rarely explicitly carried out. The decay of the most important gamma and x-ray emissions of 123I and 54Mn in the ANSTO Triple-to-Double Coincidence Ratio (TDCR) liquid scintillation detector have been simulated using the radiation transport code Geant4. The energy deposition spectrum in the scintillant and the interaction probability for each emission has been determined. Both of these calculated quantities are required for an absolute activity measurement of these nuclides using the TDCR method. A sensitivity and uncertainty analysis of the simulation model with respect to these output parameters has been carried out and is presented here. The uncertainty in the simulation results due to the input parameter uncertainties was found to be several times larger than the statistical uncertainty component for a typical number of simulated decay events. The scintillant volume was the most sensitive input parameter to produce changes in the output quantities studied, indicating that the minimization of the uncertainty of this parameter would be most beneficial for reducing the uncertainty of the simulation outputs. The scintillant density and composition were also important, which is significant as the uncertainties associated with these input parameters are generally not well known. Estimates of the relative uncertainty associated with the simulation outputs due to the combined stochastic and input uncertainties are provided.
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    Sipping test: checking for failure of fuel elements at the OPAL Reactor
    (Korean Nuclear Society, 2010-02) Smith, ML; Bignell, LJ; Alexiev, D; Mo, L
    Sipping measurements were implemented at the Open Pool Australian Light water reactor (OPAL) to test for failure in reactor fuel elements. Fission product released by the fuel element into the pool water was measured using both High Purity Germanium (HPGe) detection via samples and a NaI(Tl) detection in-situ with the sipping device. Results from two fuel elements are presented. © 2010, Korean Nuclear Society