Jason P. Hayward

The Effect of B

This paper investigates the effects of boron and calcium co-doping on the measured energy resolution of Gd3Ga3Al2O12:Ce (GGAG: Ce). For this study, three samples of GAGG were grown using the Czochralski method. The first sample was doped with Ce3+ and was used as a reference for comparison. The next two samples were additionally doped with either B3+ or Ca2+. The boron co-doped sample was found to have an overall improved performance when compared to the reference sample. The light output of the GGAG:Ce,B was measured to be 10% greater than the reference sample. In addition, the sample was found to have less charge trapping and a more linear relative light yield response. These factors led to an observed improvement in energy resolution from 9% in the reference sample to 7.8% in the B 3+ co-doped sample. Co-doping with Ca2+ led to an overall reduction in charge trapping; however, the sample suffered a 30% reduction in light output, and it was found to have a less linear relative light yield than the reference sample. Its energy resolution was measured to be 10.1%. The relationship between the measured energy resolution and the other measured properties in these samples is discussed.

^{3+}

D-T neutron generators have been used as an active interrogation source for associated particle imaging techniques. The D-T reaction yields a 14-MeV neutron and an alpha particle. The kinetics of the reaction allow the directionality and timing of the neutron to be determined utilizing position sensitive detectors for both the alpha and neutron. This information may be used for imaging applications. Since position and timing are required to form images, improved certainty in directional and timing will result in improved imaging performance. This requires maximum light transmission from its origin in the scintillator to conversion at the photosensor. This work is a study of the timing resolution of a first generation associated particle detector. An optical transport code, coupled with a timing model is also used to simulate the timing resolution. Good agreement is shown. Fundamental limits are presented with the aid of simulation and measurements. Based on these results, implications on the next-generation design are discussed.

and Ca

In order to develop a high spatial resolution (micron level) thermal neutron detector, a detector assembly composed of cerium doped lithium glass microfibers, each with a diameter of 1 μm, is proposed, where the neutron absorption location is reconstructed from the observed charged particle products that result from neutron absorption. To suppress the cross talk of the scintillation light, each scintillating fiber is surrounded by air-filled glass capillaries with the same diameter as the fiber. This pattern is repeated to form a bulk microfiber detector. On one end, the surface of the detector is painted with a thin optical reflector to increase the light collection efficiency at the other end. Then the scintillation light emitted by any neutron interaction is transmitted to one end, magnified, and recorded by an intensified CCD camera. A simulation based on the Geant4 toolkit was developed to model this detector. All the relevant physics processes including neutron interaction, scintillation, and optical boundary behaviors are simulated. This simulation was first validated through measurements of neutron response from lithium glass cylinders. With good expected light collection, an algorithm based upon the features inherent to alpha and triton particle tracks is proposed to reconstruct the neutron reaction position in the glass fiber array. Given a 1 μm fiber diameter and 0.1mm detector thickness, the neutron spatial resolution is expected to reach σ∼1 μm with a Gaussian fit in each lateral dimension. The detection efficiency was estimated to be 3.7% for a glass fiber assembly with thickness of 0.1mm. When the detector thickness increases from 0.1mm to 1mm, the position resolution is not expected to vary much, while the detection efficiency is expected to increase by about a factor of ten.

^{2+}

Cherenkov detectors are widely used for particle identification in high-energy physics and for track imaging in astrophysics. Glass Cherenkov detectors that are sensitive to beta emissions originating from neutron activation have been demonstrated recently as a potential replacement for activation foils. In this work, we evaluate Cherenkov glass detectors for sensitivity and specificity to MeV photons through simulations using Geant4. The model has been previously compared with measurements of isotopic gamma sources. It includes Cherenkov generation, light transport, light collection, photoelectron production and time response in photomultiplier tubes. The model incorporates measured, wavelength-dependent absorption and refractive index data. Simulations are conducted for glasses the size of fabricated samples and also for the same glasses in monolithic, square-meter-size. Implications for selective detection of MeV photons are discussed.

Co-Doping on Factors Which Affect the Energy Resolution of Gd

Cherenkov detectors are widely used for particle identification and threshold detectors in high-energy physics. Glass Cherenkov detectors that are sensitive to beta emissions originating from neutron activation have been demonstrated recently as a potential replacement for activation foils. In this work, we set the groundwork to evaluate large Cherenkov glass detectors for sensitivity to MeV photons through first understanding the measured response of small Cherenkov glass detectors to isotopic gamma-ray sources. Counting and pulse height measurements are acquired with reflected glass Cherenkov detectors read out with a photomultiplier tube. Simulation was used to inform our understanding of the measured results. This simulation included radioactive source decay, radiation interaction, Cherenkov light generation, optical ray tracing, and photoelectron production. Implications for the use of Cherenkov glass detectors to measure low energy gamma-ray response are discussed.

_{{3}}

The position-sensitive alpha-particle detector used to provide the starting time and initial direction of D-T neutrons in a fast-neutron imaging system was simulated with a Geant4-based Monte Carlo program. The whole detector system, which consists of a YAP:Ce scintillator, a fiber-optic faceplate, a light guide, and a position-sensitive photo-multiplier tube (PSPMT), was modeled, starting with incident D-T alphas. The scintillation photons, whose starting time follows the distribution of a scintillation decay curve, were produced and emitted uniformly into a solid angle of 4π along the track segments of the alpha and its secondaries. Through tracking all photons and taking into account the quantum efficiency of the photocathode, the number of photoelectrons and their time and position distributions were obtained. Using a four-corner data reconstruction formula, the flood images of the alpha detector with and without optical grease between the YAP scintillator and the fiber-optic faceplate were obtained, which show agreement with the experimental results. The reconstructed position uncertainties of incident alpha particles for both cases are 1.198 mm and 0.998 mm respectively across the sensitive area of the detector. Simulation results also show that comparing with other faceplates composed of 500 μm, 300 μm, and 100 μm fibers, the 10-μm-fiber faceplate is the best choice to build the detector for better position performance. In addition, the study of the background originating inside the D-T generator suggests that for 500-μm-thick YAP:Ce coated with 1-μm-thick aluminum, and very good signal-to-noise ratio can be expected through application of a simple threshold.

Ga

A technique of laser-etching repeating micrometer-scale structures into the exit surface of a thin, YAP:Ce scintillator to increase light extraction has been performed. Different etching techniques were performed on a single, monolithic crystal using varying laser power to study how light extraction is influenced by these different treatments. For the best case, a 90.4% increase in light extraction was measured without optical coupling, which compares to photonic crystal techniques. This technique finds application in associated particle imaging (API) where no optical coupling can be used for the alpha detectors scintillator, inside the Deuterium-Tritium (D-T) neutron generator vacuum. The surface characteristics of each treatment are quantified, and changes in light collection relative to a normal, polished surface are presented. Measurement of the number of detected photons is given for the case where no optical coupling is used between the scintillator and photosensor. Additionally, implications of performance improvements in YAP:Ce based alpha transducers within D-T generators, due to the measured increases in light collection, are presented.

_{{3}}

In this paper, nine different samples of YAIO3:Ce have been collected and analyzed. The light yield non-proportionality of each sample was measured and used to classify each sample as proportional or non-proportional. A variety of scintillation and optical measurements were conducted on each sample, and the proportional samples were generally found to have a higher light output and better energy resolution. In addition, a strong linear correlation was found between scintillation decay time and the degree of non-proportionality. Based on absorption measurements as well as radioluminescence data, it was determined that the non-proportional samples all shared a range of increased absorption near the cerium 5d absorption edge between about 325 and 400 nm. The increased absorption has been reported in literature, and it is believed to be the result of a material defect introduced during growth. Thermoluminescence glow curves were measured for two representative YAIO3:Ce samples, one from each proportionality grouping, and it was determined that there was an observable change in defect structure, but there were no additional traps visible in the glow curves of either the proportional or non-proportional samples. However, the intensity of the 105 K thermoluminescence peak was found to be approximately a factor of two greater in the non-proportional samples. Since the lifetime of this peak is known to be between 25 and 81 ns, it was determined to be the likely cause of the slower decay in the non-proportional samples.

Al

In order to obtain a meaningful gamma ray measurement in mixed neutron and high-energy photon environments, a quantitative understanding of neutron-induced backgrounds is crucial. Neutrons that can produce Cherenkov photons indirectly through (n, γ) activation reactions are found in radiation backgrounds in both passive and active-source search environments. The objectives of this research are to establish the properties and fabrication techniques for materials suitable for the production of photon-sensitive Cherenkov detectors (including low neutron sensitivity), or, for certain samples, to understand how the response to neutrons can be electronically discriminated from the signal caused by the interaction of MeV photons. Over a hundred glass samples have been fabricated, and radiation characterization results from selected samples are reported. The thermal neutron responses of these glasses were determined using neutron activation followed by time dependent radiation measurement and gamma ray spectroscopy. The results showed that the probability of thermal neutron capture, the thermal neutron activation time, the half-life of the activation products, the abundance of the target materials, and the magnitude of the energy of gamma and beta radiations that result from activation play significant roles in understanding the observed count rate due to Cherenkov radiation. Implications for Cherenkov detectors to be used for MeV photon detection are discussed.

_{{2}}

An SPX4 4H-silicon carbide detector consisting of 4 × 4 pixels was developed and studied experimentally. Its pixel size is 400 × 400 μm2. A timing resolution of 117 ± 11 ps full width at half-maximum (FWHM) has been measured for the detection of alphas. With such good timing performance and high granularity, the SiC pixel detector holds great promise as an associated alpha-particle detector for fast neutron imaging.