No abstract

Room-temperature semiconductor radiation detectors (RTSD) have broad applications in medical imaging, homeland security, astrophysics and others. RTSDs such as CdZnTe, CdTe are often pixelated, and characterization of these detectors at micron level can benefit 3-D event reconstruction at sub-pixel level. Material defects alongwith electron and hole charge transport properties need to be characterized which requires several experimental setups and is labor intensive. The current state-of-art approaches characterize each detector pixel, considering the detector in bulk. In this article, we propose a new microscopic learning-based physical models of RTSD based on limited data compared to what is dictated by the physical equations. Our learning models uses a physical charge transport considering trapping centers. Our models learn these material properties in an indirect manner from the measurable signals at the electrodes and/or free and/or trapped charges distributed in the RTSD for electron–hole charge pair injections in the material. Based on the amount of data used during training our physical model, our algorithm characterizes the detector for charge drifts, trapping, detrapping and recombination coefficients considering multiple trapping centers or as a single equivalent trapping center. The RTSD is segmented into voxels spatially, and in each voxel, the material properties are modeled as learnable parameters. Depending on the amount of data, our models can characterize the RTSD either completely or in an equivalent manner.

In this work we present high-flux experiment and simulation results of 3D-position-sensitive CdZnTe pixelated detectors operated in pulse mode. Charge transport properties used in our simulations were carefully calculated through direct comparison between measured and simulated charge induced signals using two different methods: irradiating with a-particles on the lateral side surface of the detector at normal bias and irradiating with Mo-Kc"X-rays (Molybdenum) on the cathode surface at reverse bias. Measured and simulated spectra as a function of increasing flux showed energies shifting towards lower energy bins followed by complete absence of spectral information, which was found to be caused by positive space charge build up distorting and completely breaking down the operating field as flux increased. More importantly, we developed a complete 3D framework that can be extended to other semiconductor detector technologies to study and predict their performance under high flux scenarios.

No abstract

OBJECTIVE: This project aims to provide a novel method for performing dosimetry measurements on TRT radionuclides using a custom-made SPECT/CT compatible phantom, common active and passive detectors, and Monte Carlo simulations. In this work we present a feasibility study using 99mTc for a novel approach to obtaining reproducible measurements of absorbed dose to water from radionuclide solutions using active and passive detectors in a custom phantom for the purpose of benchmarking Monte Carlo-based absorbed dose to water estimates. 
APPROACH: A cylindrical, acrylic SPECT/CT compatible phantom capable of housing an IBA EFD diode, SNC600c Farmer type ion chamber, and TLD-100 microcubes was designed and built for the purpose of assessing internal absorbed-dose-to-water at various points within a solution of 99mTc. The phantom is equipped with removable inserts that allow for numerous detector configurations and is designed to be used for verification of SPECT/CT-based absorbed-dose estimates with traceable detector measurements at multiple locations. Three experiments were conducted with exposure times ranging from 11 to 21 h with starting activities of approximately 10-16 GBq. Measurement data was compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2019. 
MAIN RESULTS: In general, the ionization chamber measurements agreed with the Monte Carlo simulations within k=1 uncertainty values (±4% and ±7%, respectively). Measurements from the TLDs yielded results within k=1 agreement of the MC prediction (±6% and ±5%, respectively). Agreement within k=1 uncertainty (±6% and ±7%, respectively) was obtained for the diode for one of three conducted experiments. 
SIGNIFICANCE: While relatively large uncertainties remain, the agreement between measured and simulated doses provides proof of principal that dosimetry of radionuclide solutions with active detectors may be performed using this type of phantom with potential modifications for beta emitting radionuclides to be introduced in future work.