Online program Overview Session: RTSD-10

Abstract

Spatial resolution in high-magnification X-ray imaging is limited by the detector's pixel aperture and the X-ray source focal spot size. The latter caps the achievable geometric magnification, and depends on photon energy, observation direction, and sample X-ray attenuation. This work presents an energy-resolved characterization of the effective focal spot size of the Freiburg Materials Research Center nanofocus X-ray imaging system, that combines the Hamamatsu L10711-03 Tungsten-anode nanofocus X-ray tube with a 12 × 1 Timepix3 spectroscopic photon-counting detector (PCD) array equipped with 1 mm thick Cadmium Telluride (CdTe) sensors. A single high-absorbing Steel slanted edge is used to implement the double-distance edge-response method, applied in horizontal and vertical orientations to measure the focal spot size in both principal directions. For energy bins from 15 keV to 50 keV, an in-contact edge image captures detector-generated blur, while a magnified edge image embeds both detector blur and source penumbra. After energy calibration, flat-field correction, and event declustering, edge-spread functions are oversampled, differentiated to obtain line-spread functions (LSF), and Fourier-transformed to obtain energy-resolved Optical Transfer Functions, OTF(E). The per-bin ratio of the mixed and contact OTF removes the detector  contribution to image blur and yields a source-plane OTF and LSF, from which the effective focal spot size is extracted. Preliminary analysis suggests measurable variation in the recovered source-plane profiles across energy windows and between horizontal and vertical orientations, indicating that the effective focal spot cannot be treated as energy-independent. The study relies on a practical method for source characterization in spectroscopic photon-counting X-ray systems, to achieve a thorough resolution characterization of the nanofocus imaging system.

Abstract

Over the course of the past decade, CsPbBr₃ has emerged as perhaps the most promising alterative room temperature semiconductor material for gamma-ray spectroscopy. CsPbBr₃ has been shown to be grown quickly, and relatively inexpensively, while being surprisingly defect tolerant. Fifteen 1-cm-thick CsPbBr₃ samples, attached to printed circuit boards with a low-temperature melting alloy, were evaluated over the course of this project. A custom evaluation system containing COTS preamplifiers and oscilloscopes was used to correct these samples for depth-of-interaction effects. While yields were initially fairly poor, the best sample achieved better than 1.5% FWHM energy resolution at 662 keV for the entire 0.25 cm³ volume. Eight of the nine pixels in the device achieved energy resolution better and 1.5% FWHM at 662 keV. The device exhibited predicable conditioning effects. The device has been operating for several months, albeit not at continuous bias. The failing samples tended to have attachment issues; either the attachment failed and the CsPbBr₃ disconnected from the carrier board or all pixels exhibited poor performance and instability which could also be due to issues with the device attachment to the substrate. At the time of the conference, the authors expect to have preliminary results of a CsPbBr₃ gamma-ray spectrometer with volume greater than 1 cm³ read out with pulse-waveform sampling preamplifiers integrated into an Application Specific Integrated Circuit (ASIC).

Abstract

In this work, we present a CdZnTe/CMOS spectroscopic linear-array detection system enabled by a wedge-bondable High Density Interconnect (HDI) interposer that allows bump-bonding on the detector side and wedge-bonding toward the readout electronics, significantly simplifying the interconnection constraints between the two elements [1],[2],[3]. The interposer layout is optimized to suppress inter-channel parasitics, yielding measured pixel-to-pixel and pixel-to-guard capacitances of 10.2 fF and 25.7 fF, respectively. Two custom 8-pixel CdZnTe detectors (500 μm pitch, 25 μm gap with 1 mm and 2 mm thickness respectively) were integrated with an 8-channel ultra-low-noise SIRIO charge-sensitive amplifier [4],[5] and characterized at room temperature. After integration, spectroscopic characterization was performed with 55Fe, 241Am, and 57Co radioactive calibration sources, demonstrating the capability of the system to cover a wide energy range (5–150 keV). Using a 50 ns peaking time on an external digital pulse processor, an average FWHM of 744 eV (Detector 1) and 850 eV (Detector 2) was measured on the pulser line, recording an optimum energy resolution of 1.4% at 122 keV, and paving the way for additional studies on larger detector arrays in high-flux spectroscopy applications.

Abstract

With greater photopeak efficiency and lower temperature growth processes, CsPbBr₃ is a promising alternative to CdZnTe (CZT) for room temperature semiconductor-based gamma-ray detectors. H3D, Inc. has characterized the stability of both single-pixel and pixelated 10mm thick CsPbBr₃ gamma-ray detectors. The best pixelated devices show a pattern of conditioning, whereby the device response stabilizes after 60 hours of bias. The best resolution obtained is 1% at 662 keV on a by-pixel basis, and 1.5% overall. The most stable detectors show clear photopeaks from ¹³⁷Cs during over 1200 hours of continuous operation. Though the stability is improved by contact engineering, surface polishing, and encapsulation, there is still significant drift in the peak over time. This drift causes the resolution to deteriorate over time and invalidates earlier calibrations. H3D has developed an algorithm to stabilize the response of these CsPbBr₃ detectors further by utilizing the known relationships between photopeaks and escape peaks. The algorithm operates on a per-measurement basis, requiring no knowledge of what isotope is being measured nor the past history of the device. This enables it to be applied in the field to arbitrary measurements, without requiring a convenient check source for pre-measurement calibration.

This material is based upon work supported by the Defense Threat Reduction Agency under Contract No. HDTRA225CE001. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Defense Threat Reduction Agency.

Abstract

Gallium oxide (Ga₂O₃) is an emerging material for next-generation nuclear sensors due to its ultra-wide bandgap, high thermal stability, and large critical breakdown field. However, its radiation tolerance and defect evolution mechanisms in reactor environments remain insufficiently understood, limiting predictive assessment of long-term device reliability and performance. In this work, we systematically investigate the effects of neutron irradiation with simultaneous thermal annealing on β-Ga₂O₃ using positron annihilation lifetime spectroscopy (PALS), Raman spectroscopy, and polarized optical microscopy to probe irradiation-induced changes across atomic, defect, and microstructural length scales, providing complementary insight into defect formation, evolution, and recovery mechanisms under neutron exposure.

Monoclinic β-Ga₂O₃ samples were irradiated in the North Carolina State University PULSTAR reactor at room temperature (RT) and 700 °C. Microstructural characterization and PALS confirmed vacancy-type defects in the RT irradiated samples, while their absence at high temperature suggests thermally activated point defect suppression through increased atomic mobility. Raman spectroscopy revealed irradiation-induced microtilting in both cases, whereas the absence of Raman modes associated with gallium-clusters and oxygen-vacancy indicates overall defect concentrations remained low though detectable by the highly sensitive PALS technique. Together, these results show β-Ga₂O₃ is a promising material for nuclear sensing while unveiling key insights into its behavior under irradiation.