Online program Overview Session: RTSD-01

Abstract

Three-dimensional position-sensitive single-crystal CdZnTe detectors (3D-CZT), with energy resolution of ~0.3% FWHM for single-pixel events and ~0.5% FWHM for all events at 662 keV, had been demonstrated previously on detectors with dimensions up to 2x2x1.5 cm³ (total 6 cm³). Larger volume single-crystal CdZnTe semiconductor gamma-ray detectors are needed to achieve higher intrinsic detection efficiency at higher gamma-ray energies. 4x4x3 cm³ (total 48 cm³) 3D-CZT detectors manufactured by Redlen Technologies have been explored for the first time. Energy spectra of Cs-137 were obtained on all three 4x4x3 cm³ CZT detectors. An energy resolution of 1.05% FWHM at 662 keV was obtained for single-pixel events on one 4x4x3 cm³ CZT detector #12R2. The feasibility of achieving close to 1.0% FWHM energy resolution on CdZnTe gamma-ray detectors with single-crystal detection volume of 48 cm³ has been demonstrated.

Abstract

Carrier Dynamics by Pockels Imaging (CDPI) is a novel all-optical, quantitative, and non-invasive method for direct spatiotemporal visualization of charge carrier clouds in CdTe radiation detectors. CDPI achieves micrometer-scale spatial resolution and nanosecond temporal resolution, with sensitivity to electric field differences down to ~2 V/cm – enabling tracking of carrier densities just above the intrinsic level. We directly observe electron cloud formation and evolution, including Gaussian shape preservation, diffusion-driven broadening, and field-dependent reshaping. Under high optical excitation, we resolve space-charge perturbed regimes with cloud expansion due to Coulomb repulsion (width up to 400 µm) and pronounced asymmetric tails due to detrapping processes, possibly correlated with a defective layer near the cathode surface. Crucially, CDPI correlates internal cloud dynamics with external circuit currents, linking microscopic physics to macroscopic detector performance. Although demonstrated in one dimension, CDPI is intrinsically two-dimensional and readily extendable to investigate charge sharing in segmented electrodes or pixel arrays. The technique also supports studies of ultra-high photogenerated charge densities relevant to synchrotron and free-electron laser applications and enables direct visualization of carrier injection from the contacts. CDPI offers a powerful new diagnostic tool for understanding and improving CdTe and other compound semiconductor radiation detectors.

Abstract

Thallium bromide (TlBr) is a promising compound semiconductor for the fabrication of gamma-ray detectors because of its high atomic numbers (Tl: 81, Br: 35), high density (7.56 g/cm³), and wide bandgap energy (2.68 eV). The Compton imaging capability of 1-cm-thick pixelated TlBr detectors was characterized in this study. Pixelated TlBr detectors were fabricated from a crystal grown by the traveling molten zone method using zone-purified material. The crystal size for the detector was 12 mm x 12 mm x 10 mm. The electrodes were formed on the two 12 mm x 12 mm surfaces by vacuum deposition of Tl, followed by Ni and Au. The anode consisted of nine pixel electrodes (2 mm x 2 mm each) surrounded by guard electrodes. The cathode on the opposite side was a planar electrode. For single-pixel events, the TlBr detector exhibited energy resolutions of approximately 2% full width at half maximum or better at 662 keV for all nine pixels with depth correction and event selection based on electron drift time. Compton images of a ¹³⁷Cs gamma-ray source were obtained using non-adjacent two-pixel events via the list-mode maximum likelihood expectation maximization (LM-MLEM) algorithm. Clear images of the source were reconstructed at positions consistent with the actual source locations. These results demonstrate that the TlBr detector fabricated in this study possesses high energy resolution and Compton imaging capability.

Abstract

This work presents a study of the influence of X-ray irradiation on the electric field profile in biased CdTe sensors with blocking contacts. The investigation focuses on the modification and potential stabilization of the internal electric field through controlled irradiation of the blocking anode under varying conditions. To emulate realistic detector operation, a silicon wafer was positioned in front of the anode to represent a readout chip, while the cathode was probed by laser-induced transient current technique to determine the electric field. A synchronized pulsed bias scheme enabled time-resolved monitoring of polarization dynamics. Transient current waveforms were recorded in a shielded setup with controlled X-ray exposure, and their temporal evolution was analyzed to assess changes in the electric field profile. The results show that, under dark conditions, the sensor exhibits pronounced space charge formation due to blocking anode. In contrast, appropriately tuned X-ray irradiation of the anode leads to a significant stabilization of the field within the sensor bulk. Under these conditions, the electric field remains nearly uniform over time. The observed behavior reflects the interplay between space-charge formation and X-ray–induced carrier generation, which can partially compensate accumulated charge, although the process remains sensitive to irradiation conditions and contact configuration. Furthermore, low-energy X-ray irradiation transmitted through a silicon layer produces effects comparable to optical excitation at the anode, indicating a similar mechanism of near-contact carrier generation. The presented results provide insight into the control of polarization effects and contribute to improving the operational stability of CdTe-based radiation detectors.

Abstract

Lead halide perovskite single crystals are promising for photon-counting radiation detection due to their high atomic number and favourable charge-transport properties. However, most perovskite detectors operate in hole-transport configurations, while currently existing readout electronics are optimised for electron collection. Bridging this mismatch requires a clear understanding of carrier-polarity-dependent transport and the underlying device physics governing perovskite detectors. We conducted polarity-resolved α-particle (²⁴¹Am, 5.48 MeV) measurements on FAPbBr₃ planar detectors (Au/FAPbBr₃/Bi). Polarity-resolved α-particle measurements reveal strongly asymmetric degradation, with irreversible degradation localised beneath the Bi contact, whereas the Au interface develops a defect-rich recombination region that can be mitigated by ultrathin Ti or Cr interlayers, enabling stable bipolar transport operation. Detectors remain stable after ~100 h of continuous biasing at 5,000 V cm⁻¹ and ~3 months of shelf-life storage. The combination of high-quality crystals and interfacial protection layers enables the detectors to achieve 1.3% energy resolution for 662 keV (¹³⁷Cs γ-ray source) and 6.1% for 5.48 MeV α-particles in an electron-transport configuration. This work establishes interfacial degradation as the dominant performance-limiting factor in perovskite radiation detectors and identifies interfacial engineering as the key pathway toward stable and high-resolution electron-transport spectroscopy.

Abstract

The newly upgraded Swiss Light Source (SLS) 2.0, a fourth-generation synchrotron, has begun operation. Photons with energies up to 80 keV will be available at increased flux compared to the third-generation source it replaces. In particular, the brilliance is expected to increase by two orders of magnitude. In this energy range, silicon is no longer an efficient detection material. High-Z semiconductors are therefore required, with GaAs:Cr, CdTe, and CdZnTe being promising candidates. Single sensors (2 × 2) cm² made up of 256 × 256 pixels with a 75 μm pitch have been bump-bonded to the JUNGFRAU readout ASIC. Developed at PSI, JUNGFRAU is a charge-integrating readout chip with three gain levels and dynamic gain switching. In this talk, we will discuss the ongoing characterisation of high-Z sensors using JUNGFRAU, with a focus on charge transport properties of CdZnTe produced by Redlen Technologies. Furthermore, recent efforts to improve the linear dynamic range of JUNGFRAU in electron collection mode will be discussed. We will also look towards the future with a discussion of full JUNGFRAU modules using GaAs from Dectris or recently acquired (4 × 4) cm² CdZnTe sensors from Redlen. Additionally, the new MATTERHORN photon-counting ASIC has been designed for both silicon and high-Z sensors, with assemblies using GaAs undergoing characterisation.

Abstract

We characterized a Timepix4 application-specific integrated circuit bonded to a 1-mm-thick cadmium telluride sensor for photon-counting X-ray imaging in the diagnostic energy range. A per-pixel energy calibration was performed using X-ray fluorescence and gamma-ray sources, followed by a cluster-wise linear correction to compensate for spectral distortions associated with multi-pixel charge sharing and front irradiation. Flat-field, photon-counting, and spectral imaging tests were performed using a tungsten-anode microfocus X-ray tube operated at 80 kV. The detector achieved an energy resolution of 3.3 keV at 59.5 keV for the Am-241 source. Spectral images were reconstructed using 1 keV energy bins after offline clustering and fluorescence recovery correction. The detector demonstrated high-quality radiographic imaging capability after flat-field correction, despite spatial inhomogeneities typical of fine-pitch CdTe hybrid pixel detectors. A limiting spatial resolution of 8 lp/mm at 10% modulation transfer function was measured for the detector biased at -450 V, at a counting rate approaching 10⁷ cps per chip. The results demonstrate the potential of the Timepix4 CdTe assembly for spectral imaging applications.

Abstract

Metal halide perovskites emerge as promising materials for direct X-ray detection due to their high absorption efficiency, favorable charge-transport properties, and compatibility with scalable fabrication methods[1, 2]. While thin polycrystalline perovskite films integrated with pixelated electronics have already demonstrated high-resolution imaging[3], extending these approaches toward thicker highly crystalline absorbers remains essential for practical low-dose applications. Here, we investigate several perovskite multi-pixel detector architectures coupled to charge-integrating readout electronics for low-dose X-ray imaging. The developed devices exhibit stable operation, uniform photoresponse, and spatial resolution approaching pixel pitch limit. Highly crystalline film CsPbBr₃ detectors demonstrate spatial resolution up to 20 lp mm^-1, keeping high X-ray absorption and efficient charge collection together with scalable fabrication potential, while bulk single-crystalline integrated absorbers provide further improvements in charge collection and uniform low-dose response. Thick 3-4 mm single crystal sensors achieve modulation transfer function values of 35% at the Nyquist frequency under X-ray doses as low as 4 µGy_air with a detective quantum efficiency of 48%. In addition, we demonstrate high-contrast radiographic imaging of packaged integrated circuit and computed tomography of small multi-material objects, highlighting the potential of perovskite detectors for next-generation high-resolution imaging systems.