[en] In the field of preclinical X-ray tomography, spectral tomography is actively explored. The aims of spectral tomography are the characterisation of tissues and contrast agents together with the quantification of the latter and the enhancement of contrast between soft tissues. This is achieved by the exploitation of spectral information (i.e. energy) and not only the detected quantities of X-ray photons. The interest in spectral tomography is enforced by the arrival of photon counting cameras like the hybrid pixel detector XPAD3, because of their ability to select photons according to their energy. The XPAD3 camera was built to be used in the micro-CT prototype PIXSCAN developed at CPPM. In this context, this thesis has two goals: first a contribution to the development of the PIXSCAN prototype and second a realisation with it of a proof of concept of spectral tomography. The first goal is achieved by developing the data acquisition system of PIXSCAN. To accomplish the second objective, we will perform spectral tomography by implementing component separation in order to isolate photoelectric, Compton and contrast agent contributions. This work begins with the characterisation of this method and ends by a proof of concept on real data acquired with the PIXSCAN prototype. (author)

[en] The simulation of nuclear reactors requires being able to accurately model several different interrelated physical processes including neutronics, heat production, fluid/thermal physics, structural mechanics, fuel behavior, chemistry, and balance-of-plant. In order to design and use new types of nuclear reactors, modern codes have been developed which couple together the behavior of these different physical models to accurately consider the feedback effects between different physical effects. The SHARP toolset, part of the Nuclear Energy Advanced Modeling and Simulation (NEAMS) project, is an example of an effort to combine models of neutronics, thermal-hydraulics, and structural mechanics in a coupled simulation to model any type of nuclear reactor as accurately as possible. The validation of multi-physics coupling techniques requires new experimental benchmark experiments specifically designed for that purpose. As a preliminary step towards design of these experiments, a series of measurements have been performed to demonstrate the temperature-dependent neutronic behavior of a well-characterized reactor assembly for comparison with the available codes. The facility adopted for these experiments is Walthousen Reactor Critical Facility (RCF) at Rensselaer Polytechnic Institute (RPI). The RCF is unique in several aspects and provides several advantages to this type of experiment. Built in 1954 by the American Locomotive Company (ALCO), the RCF has been used to design small compact reactors. RPI acquired the facility in 1963, and the highly-enriched uranium fuel was later swapped for low-enriched fuel pins that were excess from the SPERT facility experiments. Nowadays, The RCF is regarded as a teaching and research facility, permitting the training of licensed Senior Reactor Operators, and offering a critical experiments laboratory course to students. The reactor core standard configuration consists of 332 or 333 UO₂ fuel pins, containing a total of 35.2 grams of U- 235 each (4.81 % enriched). The pins are arranged in a regular lattice with a pitch of 1.6256 cm. The number and configuration of these pins are flexible, and the reactor's overall thermal power is limited to no more than 15 W. As the low thermal output of the reactor does not induce any measurable temperature changes, the effect of temperature in the reactor is observed by applying two 18- kW electric heaters to the water moderator, with mechanical agitators to maintain uniformity in moderator temperature. Before beginning experiments, each fuel pin inside the core was surveyed for mechanical defects (bending or warping) that may contribute to uncertainty in core behavior. Every fuel pin has been individually removed from the core and checked for any irregularities on its surface (caused by shocks or usage). About 12 % of the 333 pins were found to have a small degree of warping and were replaced with excess pins available in the facility. In doing this, we made sure all the pins are identical, thereby increasing the accuracy of future models. (authors)

[en] The Walthousen Reactor Critical Facility (RCF), located in Schenectady, New York, is currently owned and operated by Rensselaer Polytechnic Institute (RPI). The facility was originally constructed by the American Locomotive Company (ALCO) for the research and development of small, portable reactors. Originally designed for highly-enriched uranium fuel, the prototype reactor first came online in August 26, 1956, and was transferred to RPI ownership in 1963. Several changes to the reactor's design and operation have been made, the most notable of which was the conversion from high-enriched uranium to low-enriched uranium fuel. The RCF is an open-pool critical assembly that operates at a low power with a license limit of less than 100 watts. This low operating power permits a much greater flexibility in configuration and operation as compared to other research reactors. Because of this, experiments may be designed that would be unfeasible for research reactors that operate at much higher powers. The RCF's functional capabilities provide opportunities to perform several reactor experiments and measurements, such as: core criticality experiments, differential and integral rod worth measurements, moderator temperature coefficient of reactivity calculations, critical rod height and critical water height measurements, etc.. These experiments are regularly performed in conjunction with RPI course-work education and Senior Reactor Operator (SRO) training. The unique capabilities and experiments that may be performed in the RCF are particularly useful for validating certain aspects of computational analysis codes. A computational model of the RCF reactor has been previously generated and validated using the Monte Carlo N-Particle (MCNP) code. For the purpose of future experiments, this model is also being adapted for use with the SHARP Toolset, which includes the PROTEUS neutronics code, the Nek5000 fluid dynamics code, and the Diablo structural mechanics code. (authors)

[en] Highlights: • Temperature feedback effects in the RPI Reactor Critical Facility are studied. • New experiments are designed to validate reactor coupled physics simulations. • A water loop with controlled temperature and flow rate is used in experiments. • Extensive reactivity insertion scenarios are studied under various conditions. • Experiment results are valuable for modern coupled-physics codes validation. - Abstract: Modern reactor simulation tools provide advanced prediction capabilities by coupling multiphysics models to simulate reactor behaviors, involving thermal, neutronic, and mechanical interactions. To assure high-fidelity predictions by these tools, experimental data are needed to validate coupled-physics models deployed by these tools. In order to provide data to benchmark the feedback between thermal-hydraulic and neutronic simulations, coupled-physics critical experiments are designed and performed at a Reactor Critical Facility (RCF). The facility houses a low power and open-pool type light water reactor operated at the atmospheric pressure. The reactor allows flexible reconfigurations for many unique critical experiments. Recently, a water loop system has been designed and installed in the facility with the heated water circulating through the center of the reactor core, which broadens the range of validation experiments available for neutronics/thermal-hydraulics couplings. Direct effects of the loop water thermal dynamic change on reactor power and derived reactivity are demonstrated through a series of different experiments, including reactivity change over different loop water temperatures and reactor power evolution under influence of flow transient conditions in the water loop. Changes as low as 1% in the reactor power/neutron flux caused by small water temperature perturbations are observable experimentally.

[en] We propose a novel prompt-gamma (PG) imaging modality for real-time monitoring in proton therapy: PG time imaging (PGTI). By measuring the time-of-flight (TOF) between a beam monitor and a PG detector, our goal is to reconstruct the PG vertex distribution in 3D. In this paper, a dedicated, non-iterative reconstruction strategy is proposed (PGTI reconstruction). Here, it was resolved under a 1D approximation to measure a proton range shift along the beam direction. In order to show the potential of PGTI in the transverse plane, a second method, based on the calculation of the centre of gravity (COG) of the TIARA pixel detectors’ counts was also explored. The feasibility of PGTI was evaluated in two different scenarios. Under the assumption of a 100 ps (rms) time resolution (achievable in single proton regime), MC simulations showed that a millimetric proton range shift is detectable at 2σ with 10⁸ incident protons in simplified simulation settings. With the same proton statistics, a potential 2 mm sensitivity (at 2σ with 10⁸ incident protons) to beam displacements in the transverse plane was found using the COG method. This level of precision would allow to act in real-time if the treatment does not conform to the treatment plan. A worst case scenario of a 1 ns (rms) TOF resolution was also considered to demonstrate that a degraded timing information can be compensated by increasing the acquisition statistics: in this case, a 2 mm range shift would be detectable at 2σ with 10⁹ incident protons. By showing the feasibility of a time-based algorithm for the reconstruction of the PG vertex distribution for a simplified anatomy, this work poses a theoretical basis for the future development of a PG imaging detector based on the measurement of particle TOF. (paper)

[en] Built on top of the Geant4 toolkit, GATE is collaboratively developed for more than 15 years to design Monte Carlo simulations of nuclear-based imaging systems. It is, in particular, used by researchers and industrials to design, optimize, understand and create innovative emission tomography systems. In this paper, we reviewed the recent developments that have been proposed to simulate modern detectors and provide a comprehensive report on imaging systems that have been simulated and evaluated in GATE. Additionally, some methodological developments that are not specific for imaging but that can improve detector modeling and provide computation time gains, such as Variance Reduction Techniques and Artificial Intelligence integration, are described and discussed. (topical review)