James Tickner

Monte Carlo simulation of X-ray and gamma-ray photon transport on a graphics-processing unit

Graphics-processing units (GPUs) suitable for general-purpose numerical computation are now available with performances in excess of 1 Teraflops, faster by one to two orders of magnitude than conventional desktop CPUs. Monte Carlo particle transport algorithms are ideally suited to parallel processing architectures and so are good candidates for acceleration using a GPU. We have developed a general-purpose code that computes the transport of high energy (>1 keV) photons through arbitrary 3-dimensional geometry models, simulates their physical interactions and performs tallying and variance reduction. We describe a new algorithm, the particle-per-block technique, that provides a good match with the underlying GPU multiprocessor hardware design. Benchmarking against an existing CPU-based simulation running on a single-core of a commodity desktop CPU demonstrates that our code can accurately model X-ray transport, with an approximately 35-fold speed-up factor.

An on-belt elemental analyser for the cement industry

On-line control of raw mill feed composition is a key factor in the improved control of cement plants. A new and improved on-conveyor belt elemental analyser for cement raw mill feed based on neutron inelastic scatter and capture techniques has been developed and tested successfully in Adelaide Brightons Birkenhead cement plant on highly segregated material with a depth range of 100 to 180 mm. Dynamic tests in the plant have shown analyser RMS total errors of 0.49, 0.52, 0.38 and 0.23 wt% (on a loss free basis) for CaO, SiO2, Al2O3 and Fe2O3 respectively, when 10-minute counting periods are used.

Determination of the spatial response of neutron based analysers using a Monte Carlo based method

One of the principal advantages of using thermal neutron capture (TNC, also called prompt gamma neutron activation analysis or PGNAA) or neutron inelastic scattering (NIS) techniques for measuring elemental composition is the high penetrating power of both the incident neutrons and the resultant gamma-rays, which means that large sample volumes can be interrogated. Gauges based on these techniques are widely used in the mineral industry for on-line determination of the composition of bulk samples. However, attenuation of both neutrons and gamma-rays in the sample and geometric (source/detector distance) effects typically result in certain parts of the sample contributing more to the measured composition than others. In turn, this introduces errors in the determination of the composition of inhomogeneous samples. This paper discusses a combined Monte Carlo/analytical method for estimating the spatial response of a neutron gauge. Neutron propagation is handled using a Monte Carlo technique which allows an arbitrarily complex neutron source and gauge geometry to be specified. Gamma-ray production and detection is calculated analytically which leads to a dramatic increase in the efficiency of the method. As an example, the method is used to study ways of reducing the spatial sensitivity of on-belt composition measurements of cement raw meal.

Development and commercialization of a fast-neutron/x-ray Cargo Scanner

There is a pressing world-wide need for a technology that can rapidly scan air freight containers for a range of threats, including explosives, narcotics, CBRN materials and other contraband. Starting in 2002, CSIRO developed a cargo scanning concept that combines fast neutron and high-energy gamma-ray or x-ray radiography. The scans can be processed to produce high-resolution radiographic images that show both areal density and composition. In 2008, CSIRO and Nuctech Company Limited launched a joint venture for the commercialization and global deployment of the technology. In early 2009 a commercial prototype unit was demonstrated that combined neutron and dual-energy, binocular-vision x-ray technologies from the two partners to achieve significantly higher image quality and cargo throughputs. This paper reviews the scientific, technological and commercial developments that have been critical to the development of the Cargo Scanner.

Quantitative mineral phase analysis of dry powders using energy-dispersive X-ray diffraction.

Energy-dispersive X-ray diffraction (EDXRD) spectrometry is being investigated as a means to perform direct on-line quantitative mineral phase analysis on industrial mineral process streams. A laboratory prototype EDXRD analyser has been developed and a preliminary study has been made to determine its suitability for measuring mineral samples. This was carried out by collecting the diffraction spectra of 20 samples each containing six powdered minerals in dry form and using a linear regression analysis model to determine the mass of each mineral component contained in the samples. Using this method it was found that mineral components could be determined to an accuracy of better than 1 wt%.

X-ray beam hardening based material recognition in micro-imaging

We are developing a technique of X-ray imaging capable of identifying materials in the image. The presented technique is based on analysis of two or more images taken using different energy discrimination thresholds with a single photon counting imaging detector, Medipix2. The Medipix type detectors are devices suitable for material resolving imaging thanks to their energy sensitivity for individual photons, allowing transmission to be measured as a function of X-ray energy. The material recognition technique presented here uses the signal-to-thickness calibration method and can be applied to images collected with an X-ray tube without the use of filters.

Neutron generator burst timing measured using a pulse shape discrimination plastic scintillator with silicon photomultiplier readout

An EJ-299-34 plastic scintillator with silicon photomultiplier (SiPM) readout was used to measure the fast neutron output of a pulsed Thermo-Fisher A-325 Deuterium-Tritium sealed tube neutron generator (STNG). The SiPM signals were handled by a prototype digital pulse processing system, based on a free-running analogue to digital converter feeding a digital signal processor (DSP). Pulse shape discrimination was used to distinguish between detected fast-neutrons and gammas. Pulse detection, timing, energy and shape were all processed by the DSP in real-time. The time-dependency of the neutron output of the STNG was measured for various pulsing schemes. The switch-on characteristics of the tube strongly depended on the operating settings, with the delay between pulse turn-on and the production of neutrons ranging between 13 μs to 74 μs for the tested pulse rates and duty cycles. This work will facilitate the optimization and modeling of apparatus that use the neutron generators pulsing abilities.

Neutron-Gamma Pulse Shape Discrimination Using Organic Scintillators With Silicon Photomultiplier Readout

Neutron/Gamma pulse shape discrimination (PSD) was measured using stilbene and EJ-299-34 plastic scintillators with readout by silicon photomultipliers (SiPMs). The SiPM pulses were digitized and processed for energy and pulse shape information using a digital technique performing numerical weighted integrations on each pulse. A genetic algorithm (GA) was developed to optimize the weighting vectors used for the pulse shape discrimination. Efficient PSD was obtained down to an electron-equivalent energy of 127 keV with stilbene and 391 keV with the EJ-299-34 PSD plastic. Separation at lower energies was possible at reduced detection efficiency, down to 78 keV and 186 keV at 50% efficiency. The detectors were used to measure separated gamma and neutron spectra from an Americium-Beryllium neutron source and from a Na-22 gamma source. The GA-optimized weighted integration was compared with digital charge comparison (DCC). The GA exhibited slightly improved performance with a 400 MSps digitization rate and showed a significant advantage at sample rates below 100 MSps.

FAST NEUTRON AND GAMMA-RAY DETECTORS FOR THE CSIRO AIR CARGO SCANNER

A critical aspect of the CSIRO Air Cargo Scanner has been the development of fast neutron and gamma ray detector arrays and electronics with the following desirable characteristics: • High efficiency, particularly for fast neutrons as the brightness of conveniently available neutron sources is low; • Small detector size. The size of the individual elements of the detector array determines the spatial resolution of the transmission images. A small detector element is desirable to provide sharper images but at the expense of increasing cost and complexity. • The cost of detector and signal processing electronics should be less than about US

Fast-neutron/gamma-ray radiography scanner for the detection of contraband in air cargo containers

200 per detector channel.