Jonathan Dumazert

Understanding the behaviour of different metals in loaded scintillators: discrepancy between gadolinium and bismuth

Organometallic chemistry has recently gained a lot of attention in the domain of plastic scintillators. Homogenously dispersed metal complexes in a polymer matrix can afford plastic scintillators with unseen abilities. Heavy atom loading is very attractive as it gives access to plastics with increased sensitivity towards elusive radiations such as gamma and neutron. But this comes with a drawback, as heavy atoms tend to quench fluorescence, hence decreasing the scintillation yield. We present here a comprehensive study of this phenomenon with bismuth and gadolinium complexes. We investigate the influence of the ligand nature by varying organometallic and fluorophore concentration to probe their interaction. We also propose an explanation of the difference in behavior between these two metals. These results were applied to the fabrication of large volume loaded plastic scintillators (>100 cm3). Bismuth loaded scintillators displayed characteristics equivalent to lead loaded commercial materials, and gadolinium samples proved to be able to capture thermal neutrons and release gamma rays.

Estimation of Nuclear Counting by a Nonlinear Filter Based on a Hypothesis Test and a Double Exponential Smoothing

Online nuclear counting represents a challenge due to the stochastic nature of radioactivity. The counting data have to be filtered in order to provide a precise and accurate estimation of the count rate, while ensuring a response time compatible with the application in view. An innovative filter is presented in this paper to address this issue. The filter is nonlinear and based on a Centered Significance Test (CST) providing a local maximum likelihood estimation of the signal. This nonlinear approach allows enables to smooth the counting signal while maintaining a fast response when brutal change in activity occurs. The filter is then improved by the implementation of a Browns double Exponential Smoothing (BES). The filter has been validated and compared to other state-of-the-art smoothing filters. The CST* filter shows a significant improvement compared to all tested smoothing filters.

Gadolinium-loaded Plastic Scintillators for Thermal Neutron Detection using Compensation

Plastic scintillator loading with gadolinium-rich organometallic complexes shows a high potential for the deployment of efficient and cost-effective neutron detectors. Due to the low-energy photon and electron signature of thermal neutron capture by Gd-155 and Gd-157, alternative treatment to pulse-shape discrimination has to be proposed in order to display a count rate. This paper discloses the principle of a compensation method applied to a two-scintillator system: a detection scintillator interacts with photon and fast neutron radiation and is loaded with gadolinium organometallic compound to become a thermal neutron absorber, while a not-gadolinium loaded compensation scintillator solely interacts with the fast neutron and photon part of incident radiation. After the nonlinear smoothing of the counting signals, a hypothesis test determines whether the resulting count rate post-background response compensation falls into statistical fluctuations or provides a robust indication of neutron activity. Laboratory samples are tested under both photon and neutron irradiations, allowing the authors to investigate the performance of the overall detection system in terms of sensitivity and detection limits, especially with regards to a similar-active volume He-3 based commercial counter. The study reveals satisfactory figures of merit in terms of sensitivity and directs future investigation toward promising paths.

Compensated gadolinium-loaded plastic scintillators for thermal neutron detection and counting

Plastic, scintillator loading with gadolinium-rich organometallic complexes shows a high potential for the deployment of efficient and cost-effective neutron detectors. Due to the low-energy photon and electron signature of thermal neutron capture by gadolinium-155 and gadolinium-157, alternative treatment to Pulse Shape Discrimination has to be proposed in order to display a trustable count rate. This paper discloses the principle of a compensation method applied to a two-scintillator system: a detection scintillator interacts with photon radiation and is loaded with gadolinium organometallic compound to become a thermal neutron absorber, while a non-gadolinium loaded compensation scintillator solely interacts with the photon part of the incident radiation. Posterior to the nonlinear smoothing of the counting signals, a hypothesis test determines whether the resulting count rate after photon response compensation falls into statistical fluctuations or provides a robust image of neutron activity. A laboratory prototype is tested under both photon and neutron irradiations, allowing us to investigate the performance of the overall compensation system in terms of neutron detection, especially with regards to a commercial helium-3 counter. The study reveals satisfactory results in terms of sensitivity and orientates future investigation toward promising axes.

Nuclear counting filter based on a Centered Skellam Test and a double Exponential Smoothing

Online nuclear counting represents a challenge due to the stochastic nature of radioactivity. The count data have to be filtered in order to provide a precise and accurate estimation of the count rate, this with a response time compatible with the application in view. An innovative filter is presented in this paper addressing this issue. It is a nonlinear filter based on a Centered Skellam Test (CST) giving a local maximum likelihood estimation of the signal based on a Poisson distribution assumption. This nonlinear approach allows to smooth the counting signal while maintaining a fast response when brutal change activity occur. The filter has been improved by the implementation of a Browns double Exponential Smoothing (BES). The filter has been validated and compared to other state of the art smoothing filters. The CST-BES filter shows a significant improvement compared to all tested smoothing filters.

Shadow-Shielding Compensation for Moving Sources Detection

To monitor radioactivity emitted by a vehicle such as a pedestrian, a car, a train, or a truck, radiation portal monitors (RPMs) are commonly employed. It has been noted that these RPM suffer from a shadow-shielding effect when the source is transported by a dense and large vehicle such as a truck or a train. While methods using databases are listed as state of the art, we have developed an approach based on a state model, ensuring a gain in performance and flexibility. This new study presents a technique applied to RPM, efficiently compensating the shadow-shielding effect. A simulation study has been performed to highlight the efficiency and reliability of the method.

Plastic scintillators modifications for a selective radiation detection

Recent developments of plastic scintillators are reviewed, from January 2000 to June 2015. All examples are distributed into the main application, i.e. how the plastic scintillator was modified to enhance the detection towards a given radiation particle. The main characteristics of these newly created scintillators and their detection properties are given.

Nuclear signal simulation applied to gas ionizing chambers

Particle transport codes used in detector simulation allow the calculation of the energy deposited by charged particles produced following an interaction. The pulses temporal shaping is more and more used in nuclear measurement into pulse shape analysis techniques. A model is proposed in this paper to simulate the pulse temporal shaping and the associated noise level thanks to the output track file PTRAC provides by Monte-Carlo particle transport codes. The model has been dedicated to ion chambers and more especially for High Pressure Xenon chambers HPXe where the pulse shape analysis can resolve some issues regarding with this technology as the ballistic deficit phenomenon. The model is fully described and an example is presented as a validation of such full detector simulation.

Hypothesis tests for the detection of constant speed radiation moving sources

As a complement to single and multichannel detection algorithms, inefficient under too low signal-to-noise ratios, temporal correlation algorithms have been introduced to detect radiological material in motion. Test hypothesis methods based on the mean and variance of the signals delivered by the different channels have shown significant gain in terms of a tradeoff between detection sensitivity and false alarm probability. This paper discloses the concept of a new hypothesis test for temporal product detection methods, taking advantage of the Poisson nature of the registered counting signals, and establishes a benchmark between this test and its empirical counterpart. The simulation study validates that in the two relevant configurations of a pedestrian source carrier under respectively high and low count rate radioactive backgrounds, the newly introduced hypothesis test ensures a significantly improved compromise between sensitivity and false alarm, while guaranteeing the stability of its optimization parameter regardless of signal-to-noise ratio variations between 2 to 0.8.