Fast Digital Filtering of Spectrometric Data for Pile-up Correction

International audienceThis paper considers a problem stemming from the analysis of spectrometric data. When performing experiments on highly radioactive matter, electrical pulses recorded by the spectrometer tend to overlap, thus yielding severe distortions when computing the histogram of the pulses' energies. In this paper, we propose a fast recursive algorithm which estimates efficiently this histogram from measurements of the duration and energies of overlapping pulses. Its good performances are shown both on simulations and real data. Furthermore, its lower algorithmic complexity makes it more fitting for real-time implementation

The Determination of the Half-Life of Si-32 and Time Varying Nuclear Decay

The aim of this work is to make an accurate determination of the half-life of $^{32}$Si as well as to investigate reported variations in nuclear decay rates. Primarily, we focus on a previous experiment at Brookhaven National Lab (BNL) \cite{BNL}, which measured the ratio of interwoven $^{32}$Si and $^{36}$Cl decays over a 4 yr period and reported unexplained annual periodicities. Utilizing the very same sources and shuffling apparatus, we have observed the decay of $^{32}$Si and $^{36}$Cl for more than 6,000 hours each, over the last $\sim 2$ yr, while recording $\sim 5\times 10^6$ individual environmental readings. The half-life of $^{32}$Si, which has been quoted between 101(18) yr and 330(40) yr, is redetermined by this data to be $159.4\textrm{ yr} \pm 1.9\textrm{ yr (statistical)} \pm 3.7\textrm{ yr (systematic) with } \chi^2_{DOF} = 1.01$. We have also observed a periodic signal in the $^{32}$Si/$^{36}$Cl ratio decay data, similar to that in the original experiment at BNL, which we analyze at length. Finally, a related topic concerning self-induced decay and its relationship to systematic dead-time corrections is presented

Spectroscopy of ionizing radiation using methods of digital signal processing

Nuclear spectroscopy is an interdisciplinary subject of physics and electronics, which adopts state-of-the-art digital electronic technology and computer technology to analyze the information in ionizing radiation. The use of FPGAs shortens the development cycles of the digital circuit design and reduces system noise with compact electronics size. As a result, digital spectrometers with FPGAs are gaining popularity in research and industrial markets. The motivation behind this work was to replace conventional analog electronics with modern digital technology to provide an excellent energy resolution for different kinds of nuclear detectors and experiments. In this thesis, a SiPM-based scintillation detector is first designed based on the basic principles of ionizing radiation. The readout circuit of the detector is given in detail. Subsequently, a real-time DPP module is designed using the FPGA of Lattice. The system noise of the DPP is measured, compared, and analyzed after the hardware verification and implementation of digital algorithms to assess the capability of the DPP module. Afterward, digital pulse processing algorithms are investigated in detail to improve the performance of the designed digital module. The design and implementation of multipass moving average and trapezoidal filter are presented. The PZC and BLR are designed and implemented according to the analysis of the trapezoidal filter’s weakness to have a better energy resolution of the digital system. Algorithms are designed and implemented on a Simulink platform. Experimental results and analyses are provided at the end of this thesis. The acquired data are analyzed in real-time or by offline software. Spectra and resolutions are demonstrated of different detectors to evaluate the performance of digital module and algorithms implementation. The resolution of the scintillation detector can be obtained to 4.2%, which is almost the optimal value based on their datasheet. The implementations of digital algorithms are verified. Other applications are provided, such as coincidence and cosmic muons measurements

Optimizing Compton camera performance

Amore realistic simulation approach is used to study the behavior of the Compton camera in this thesis than previous studies to date. The Compton camera differs from gamma cameras in that the collimator is replaced by a detector known as the ‘scatterer’. Gamma rays may be Compton scattered in the scatterer and subsequently detected by an ‘absorber’ which is the equivalent of the detector in a gamma camera. By measuring the energies and the positions of the points on the scatterer and the absorber where the incident and scattered gamma rays interacted with the detectors, an image of the source can be reconstructed. Because there is no collimator present, the potential sensitivity of the Compton camera is much higher than the gamma camera, resulting in reduced acquisition times. Most of the work described in this thesis was done with the GEANT4 Monte Carlo simulation software. GEANT4 has been proven to be very robust and efficient in modelling physics problems of radiation transport and interactions with matter in complex geometries. Four major studies are carried out to estimate and optimize the performance of this novel equipment. The first study takes a look at the scatterer’s imaging parameters with the aim of prescribing an optimal scatterer material and geometry. In the second study, the contribution of the absorber to the overall Compton camera performance is evaluated, considering detector material, interaction type and geometry. The third study explores the limitations imposed by the detector energy threshold and dead time on the Compton camera performance, using a simplified model of the general electronic architecture. An evaluation of Compton camera for scintimammography was performed in the fourth study. For this study, three dual-head Compton camera models (Si/CZT, Si/LaBr₃:Ce and Si/NaI(Tl) Compton cameras) were simulated, and the effect of scintillation photons’ interactions with the photomultipliers was implemented. The results show that silicon of about 1 cm thickness would be adequate as the Compton camera scatterer. Analyses suggest however, that the choice of silicon is not completely flawless. Doppler broadening for this detector material contributes as much as 7.3 mm and 2.4 mm to full-width-at-half-maximum (FWHM) image resolution at 140.5 keV and 511 keV respectively. On the other hand, detector spatial resolution which accounts for the least image degradation at 140.5 keV is found to be the dominant degrading factor at 511 keV, suggesting that the absorber parameters play major roles in image resolution at higher diagnostic energies. Findings further suggest that cadmium zinc telluride (CZT) would be themost suitable detector as the absorber since thematerial demonstrated the highest efficiency and least positioning error due to multiple interactions as well as good spatial resolution. The inclusion of the energy threshold and detector dead time at 140.5 keV, reduced the Compton camera detection efficiency by 48% and 17% respectively, but improved the image resolution from 10.7 mm to 9.5 mm at the source-to-scatterer distance of 5 cm. At 511 keV, the inclusion of these parameters reduced the efficiency by 6% and 13% respectively, but made no significant difference on the camera resolution. For a challenging detection case in scintimammography, 5 mm breast tumours of tumour/background uptakes of 10:1 and 6:1 at 511 keV were used. The best signal-to-noise ratio (SNR) was attained for the Si/CZT Compton camera model, with the SNR values of 12.2 and 5.3. It is therefore envisioned that with an optimal camera geometry, improved reconstruction technique and adequate filter algorithm, the combination of Si and CZT as the scatterer and the absorber of the Compton camera would make a very promising imaging system for nuclear medicine studies at higher gamma ray energies where the collimated SPECT systems perform very poorly due to increased septal penetration. It is equally evident from the studies that with improved technology, new detectors such as LaBr₃:Ce could replace the traditional NaI(Tl) detector as imaging detectors

An Active Interrogation Technique Based on Photofission for Non-destructive Assay of Used Nuclear Fuel

High-energy delayed γ-rays from photofission were demonstrated to be signatures for detection and identification of special nuclear materials. Such γ-rays were measured in between linac pulses using independent data acquisition systems. A list-mode system was developed to measure low-energy delayed γ-rays after irradiation. Photofission product yields of ²³⁸U and ²³⁹Pu were determined based on the measured delayed γ-ray spectra. The differential yields of delayed γ-rays were also proven to be able to discriminate nuclear and non-nuclear materials. The measurement outcomes were compared with Monte Carlo simulation results. It was demonstrated that the current available code has capabilities and limitations in the simulation of photofission process. A two-fold approach was used to address the high-rate challenge in used nuclear fuel assay based on photofission technique. First, a standard HPGe preamplifier was modified to improve its capabilities in high-rate pulsed photofission environment. Second, advanced pulse processing algorithms including template-matching and de-randomization methods were shown to greatly improve throughput rate without large sacrifice in energy resolution at ultra-high input count rate

Machine Learning and Neutron Sensing in Mobile Nuclear Threat Detection

A proof of concept (PoC) neutron/gamma-ray mobile threat detection system was constructed at Oak Ridge National Laboratory. This device, the Dual Detection Localization and Identification (DDLI) system, was designed to detect threat sources at standoff distance using neutron and gamma ray coded aperture imaging. A major research goal of the project was to understand the benefit of neutron sensing in the mobile threat search scenario. To this end, a series of mobile measurements were conducted with the completed DDLI PoC. These measurements indicated that high detection rates would be possible using neutron counting alone in a fully instrumented system. For a 280,000 neutrons per second Cf-252 source placed 15.9 meters away, a 4σ [sigma] detection rate of 99.3% was expected at 5 m/s. These results support the conclusion that neutron sensing enhances the detection capabilities of systems like the DDLI when compared to gamma-only platforms. Advanced algorithms were also investigated to fuse neutron and gamma coded aperture images and suppress background. In a simulated 1-D coded aperture imaging study, machine learning algorithms using both neutron and gamma ray data outperformed gamma-only threshold methods for alarming on weapons grade plutonium. In a separate study, a Random Forest classifier was trained on a source injection dataset from the Large Area Imager, a mobile gamma ray coded aperture system. Geant4 simulations of weapons-grade plutonium (WGPu) were combined with background data measured by the Large Area Imager to create nearly 4000 coded aperture images. At 30 meter standoff and 10 m/s, the Random Forest classifier was able to detect WGPu with error rates as low as 0.65% without spectroscopic information. A background subtracting filter further reduced this error rate to 0.2%. Finally, a background subtraction method based on principal component analysis was shown to improve detection by over 150% in figure of merit

Coded Aperture Imaging Applied to Pixelated CdZnTe Detectors.

In the past decade, there has been a significant increase in demand for radiation detectors to detect, identify, and locate potentially threatening nuclear materials. The Polaris system was developed to be used for such applications. This portable, room-temperature operated detector system is composed of 18 thick CdZnTe detectors, and has the ability to detect gamma rays of energies between 30 keV and 3 MeV with an energy resolution <1% FWHM at 662 keV. Detection is extended to source directionality using Compton imaging to map out gamma-ray distributions in 4-pi space. This modality is most effective at imaging gamma-ray energies greater than 300 keV. Due to the low Compton-interaction probability in CdZnTe at lower energies, an alternate imaging technique, coded aperture imaging (CAI), was implemented to extend gamma-ray imaging to the energy range where photoelectric absorption is most probable. The purpose of this work is to describe the evolution of the CAI modality as applied to the Polaris system. During the course of this study, for the first time, CAI is applied to thick 3D position sensitive CdZnTe detectors to image lower-energy gamma rays. With the knowledge of 3D positions of gamma interactions, masks are applied to five of the six sides of a single CdZnTe crystal, extending the field-of-view (FOV) to near 4-pi through simulation and measurement. Material properties such as “pixel jumping” that are caused by non-uniform electric fields within the detector that result in degradation of image quality are also studied. Next, a single mask is applied to a 3 x 3 array of detectors showing improved image quality when combining images from multiple detectors. Finally, CAI is combined with Compton imaging and applied to the 18-detector Polaris system allowing for the extension of gamma-ray imaging capabilities across the entire dynamic range of the electronic readout system. This work was funded by the US Department of Homeland Security Domestic Nuclear Detection Office and National Science Foundation Academic Research Initiative.PHDNuclear Engineering and Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108755/1/sonalj_1.pd