The detector that converts X-ray intensity into a digitized electronic signal, processed into an image, often limits the performance of modern X-ray imaging systems, whether for medical, research, or industrial applications. Digital radiography uses the following two types of detectors: indirect detectors, where a scintillating film converts X-ray into an optical signal read by an optical detector, and direct detectors using semiconductors. Semiconductor X-ray detector technology typically uses a silicon wafer, an amorphous Se (aSe) film, or a relatively thin CdZnTe crystal bump bonded to a readout platform. Although the use of aSe and CdZnTe improves the attenuation coefficient beyond that of silicon, a further increase in performance is highly desirable for improved imaging off higher energy X-rays (greater than 20 keV). This paper presents results on thallium bromide (TlBr) films coupled to a read-out integrated circuit (ROIC). Thallium Bromide, an attractive material for hard X-ray imaging due to its high atomic number and high density, achieves an image resolution of 20 μm as a 1 cm2 × 100-μm thick columnar film deposited on CMOS ROIC.

Thallium bromide (TlBr) is a promising material for semiconductor gamma-ray detectors due to its high atomic number (Tl = 81, Br = 35), high density (7.56 g/cm3), and relatively low melting point (460 degrees C). These properties give TlBr several competitive features, including high detection efficiency, good energy resolution, and lower fabrication costs, making it suitable for applications requiring compact, highly sensitive gamma-ray detection systems. Recent developments of TlBr detectors have progressed toward thicker crystals, up to 10-20 mm, configured as pixelated or virtual Frisch-grid (VFG) devices with volumes >1 cm3. Despite these advances, challenges remain in practical implementation, including material and fabrication issues that affect energy resolution, device stability, performance, and longevity. These challenges are similar to those faced by HgI2 and lead-halide perovskite detectors. This study presents results from characterizing over 100 position-sensitive 5x5x12 mm3 virtual Frisch-grid detectors fabricated by Radiation Monitoring Devices Inc. For a prototype Radioisotope Identification Device (RIID). The work identified factors limiting the performance and longevity of TlBr detectors, including stochastic response variations that limit the energy resolution to 2-3 % (at 662 keV) for most devices, performance degradation associated with defect decoration, and contact delamination. Addressing these issues advances the use of TlBr and similar materials for reliable, high-resolution gamma-ray spectroscopy.

Room temperature semiconductor materials provide important performance advantages in photon-counting X-ray imaging applications. This paper presents preliminary results on columnar thallium bromide (TlBr) films directly deposited on TimePix readout chips. This paper presents the current-voltage data for $\sim 100 \mu \mathrm{m}$ thick, $1^{\prime \prime} \times 1^{\prime \prime}$ or larger area, TlBr films deposited on ITO glass. The TlBr films exhibited resistivities $\gt1 \times 10^{9} \Omega \cdot \mathrm{cm}$. The work also shows the effect of thin barrier layers, intended to protect the electrode from parasitic electrochemistry, between the TlBr film and the electrode on the IV curve and X-ray response. The IV curve from barrier layers deviates from linearity, however, the magnitude of the current indicates the transport of charge across the barrier. All devices respond to a 70 kVp X-ray pulse, where the bare TlBr film device generated the most signal current, and with the barrier was lower by a factor of 2 to 4. A TlBr film was directly deposited on a TimePix chip and photon-counting events from the irradiation by a ${ }^{241} \mathrm{Am}$ source were imaged.

Semiconductor X-ray detector technology currently relies on the use of a silicon wafer, or a relatively thin CdTe crystal, bump bonded to their respective readout platform. Although the use of CdTe extends the energy range of these detectors beyond that of silicon, a further increase in energy performance is highly desirable. In this paper, we present preliminary results on thallium bromide (TlBr) films coupled to two readout chips, or application-specific integrated circuits (ASICs). TlBr is an attractive material for hard X-ray imaging due to its high atomic number and high density. Additionally, TlBr has a relatively low melting point (460 °C), melts congruently, and has a simple cubic crystal structure.

Cherenkov Charge Induction (CCI) detectors have the potential to provide the characteristic energy and spatial resolution of semiconductor detectors together with a sub-nanosecond time resolution by detecting Cherenkov light. The first CCI detectors we have developed consist of thallium bromide (TlBr), which, additionally, is one of the semiconductor detector materials with highest detection efficiency for gamma-rays. In this work, we report on the working progress of a CCI TlBr detector with dimensions of 10 mm x 10 mm x 10 mm and a 2.5 mm pixel aimed at the detection of gamma-rays with energies above 1 MeV. We present spectroscopic data obtained with Co-60, with energy resolutions of 11% and 8% at 1.17 and 1.33 MeV, respectively. The 2D histogram of cathode-to-anode ratio vs anode ADC shows a significant fraction of events with greater energy read by the cathode, which suggests the impact of charge sharing on the dataset is significant. Next steps include expanding the energy calibration up to 2.6 MeV using a Th-228 source and an extensive charge sharing study for this detector at 1.33 MeV and 2.6 MeV.

Position sensitivity enables the correction of response non-uniformities in room-temperature semiconductor detectors caused by crystal defects and other factors. It can also be used to pinpoint the exact location of crystal defects responsible for the response variations. This work describes a technique for revealing and visualizing the detector regions affecting the charge collection efficiency in CdZnTe (CZT), TlBr, and CsPbBr3 detectors configured as position-sensitive virtual Frisch-grid (VFG) devices. The technique correlates the photopeak events in energy spectra with their spatial distributions inside the detectors using the position information. By selecting the events from narrow energy intervals within a photopeak, we can visualize the detector volumes with particular charge collection efficiencies, which, in turn, correlate with the locations of electrode and crystal defects. We demonstrate this technique in several examples. Columnar structures in the volume plots (position distribution maps) are consistent with signal losses near or at the anode in selected samples of CZT and TlBr. Structures exhibiting a distinct depth dependence are consistent with grain boundaries or other crystal defects.

Thallium bromide is a very promising semiconductor as an efficient, room temperature gamma ray detector material due to its high density, high atomic number, and wide band gap. Additionally, due to its relatively low melting point it has the potential to be lower cost than CZT. High electron mobility-lifetime products have enabled the fabrication of 10 mm thick TlBr arrays with energy resolution of about 2% FWHM at 662 keV without corrections for the interaction depth at room temperature. To reach optimum performance, TlBr detectors often require continuous bias for an initial "conditioning phase." In this paper, we report not only on the investigation of the initial conditioning phase with fresh arrays, but the second or subsequent conditioning phases. To probe the active volume during the conditioning phase, a collimated isotopic source will irradiate at various depths of 10 mm thick arrays.

CsPbBr 3 is a promising semiconductor material for X-and ℽ-ray detection at room temperature due to a wide bandgap of 2.3 eV, high stopping power, and relatively high mobility-lifetime products of electrons and holes, and the capability of growing large crystals. In this paper we report on fabrication and testing of a 3.8 mm thick, 3 × 3 array with 1.25 mm pitch. Raw 137 Cs spectra were acquired with applied bias of 400 V, 500 V, and 600 V. Energy resolution of between 2 and 3 % FWHM at 662 keV was obtained from most pixels without any correction at room temperature. After 29 days under continuous bias at 600 V, no polarization effects were observed.

Thallium bromide (TlBr) is a promising material for room temperature gamma radiation detection due to its high density, high atomic number, and wide bandgap. Additionally, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature. Advances in material purification, crystal growth and device processing have led to improved material quality including a significant increase in the mobility-lifetime product of electrons in TlBr. This has enabled single carrier collection devices with thicknesses of 1 cm and beyond. The arrays have been flip-chip bonded to carrier boards using a low temperature curing conductive polymer. In this paper we report on results from planar and pixelated devices. Planar TlBr devices with dimensions of 12 mm × 12 mm × 7 mm exhibit an energy resolution ranging from 3% to 5% FWHM at 662 keV when using a shaping time of 2 s. The energy resolution in planar devices improves with a reduction of the shaping timing consistent with the expected amelioration of the depth dependence. The 1-cm thick pixelated arrays, with a pitch of 1.72 mm, produce an energy resolution in the anode spectrum ranging from 1.8% to 4.4%, without applying depth corrections. This work presents spectra from a selected pixel for 133Ba and 57Co irradiation. Measurements of the room-temperature stability of the planar and pixelated detectors show that the position of the 662-keV photopeak is stable over a period of ~200 days, but the shape of the photopeak in the anode spectra exhibits small changes. These detectors show promise for applications in radio-isotope identification devices and for medical imaging.

This work evaluates data from position-sensitive capacitive Frisch-grid (PS-CFG) TlBr detectors for correcting the gamma-ray energy spectrum based on the location of the interaction in the crystal. This enables the correction of non-uniformity in the signal response. Signals from 5×5×12 mm 3 TlBr PS-CFG detectors generate anode amplitude versus cathode-to- anode-ratio histograms, which map the interaction depth for a 32×32 grid of XY pixels. The width of the 662-keV photopeak from 137 Cs irradiation contains contributions from statistics and non-uniformity. The dependence of the signal on the interaction depth produces an asymmetric "tail" in the photopeak in the anode spectra. This work also evaluates the dependence of the non-uniformity corrections on long-term stability, while under constant 1500 V bias at room temperature after 3 and 5 months of operation. The 3D corrections compensate for some non-uniformity; however, the interaction location determined by the approach represents the centroid of the charge distribution. Variations in the sampling of non-uniformities produced by the shape of the charge distribution may not be corrected. We suspect that the current energy resolution may be limited by uncorrected non-uniformities.

Arrays of 3D position-sensitive detectors (3DPSD), operating at room temperature and using cadmium zinc telluride (CZT) and thallium bromide (TIBr) sensors, are suitable for gamma-ray spectrometry in many applications. One detector configuration, the 3D position-sensitive Virtual Frisch-Grid detector (VFG), is particularly advantageous for integrating into large area arrays. The signals generated inside each detector of the array are captured with the anode, cathode and four pads that enable the reconstruction of the position and energy of the ionizing interaction by measurements of amplitude and timing of the signals. For these applications, a low-noise front-end ASIC has been developed, capable of processing bipolar signals (needed because of AC-coupling of certain electrodes). The ASIC can be coupled to an ADC in order to form a compound “waveform digitizer” capable of post-processing the analog signals and determining amplitude and timing information. This paper describes a 32-channel front-end ASIC that is suitable for reading out a 3 × 3 or 4 × 4 element matrix in the VFG configuration. Each channel is composed of a low-noise charge amplifier with an adaptive continuous reset feedback circuit suitable for both positive and negative charge, a first order shaper and a single-to-differential converter output stage. Voltage and current references are all internally generated by 10-bit DACs and the chip is fully controllable with the I 2 C communication protocol. The readout channel response has been verified using the implemented injection circuit. Linear behavior up to ∼75 ke ± with the gain of ∼80 mV/fC, and up to ∼200 ke ± with the gain of ∼30 mV/fC was demonstrated. In conclusion, the first test result waveforms using a 137 Cs radioactive source on a 5 × 5 × 12 mm 3 TIBr crystal are reported.

TlBr is a promising material for room-temperature semiconductor gamma-ray detectors currently under development by several groups around the world. TlBr has the optimal combination of properties: high atomic number, high density, high mu-tau product, low Fano factor, and lower fabrication cost compared to other materials. The presence of crystal defects and ionic drift-diffusion enchained by the electric field affects the performance of today’s TlBr detectors. As a bias is applied across a detector, a defect distribution inside starts changing due to ion migration. The changes appear to be most pronounced in the first weeks of applying a bias to newly-manufactured crystals during the “conditioning” period. The 3-D position-sensitive detectors provide an opportunity to investigate these processes and their effects on the device performance and on corrections applied to the spectrum. Here, we present results from analyzing response changes in TlBr crystals under applied biases using position-sensitive capacitive Frisch-grid detectors. This work has been supported by the U.S. Department of Homeland Security, Countering Weapons of Mass Destruction Office, under competitively awarded contract 70RDND18C00000024. This support does not constitute an express or implied endorsement on the part of the Government.

This work presents results from analyzing position-sensitive capacitive Frisch-grid (PS-CFG) TlBr gamma-ray detectors. As a room-temperature semiconductor detector, TlBr exhibits a high atomic number, high density, and low Fano factor compared to other material. The use of the 3D position sensing technique provides information on the crystal uniformity. This technique presents the spatial variation in the histograms of the anode amplitude versus the cathode-to-anode ratio, necessary for depth correction, for detectors fabricated with 5×5×12 mm 3 TlBr crystals at room temperature operating at continuous bias (1.5 kV) over the course of nine months. This work also presents the temperature dependence of the leakage current for a TlBr PS-CFG detector over the temperature range of -20 to +60 °C, which varies from <0.2 to 40 nA, respectively, along with changes in the spectrum from the anode signal measured with 137 Cs irradiation. This work shows the utility of the 3D technique to evaluate the performance and uniformity of PS-CFG detectors for applications that require high-energy resolution gamma-ray spectroscopy, such as radionuclide identification.

Assessing the position of the Bragg peak (BP) in hadron radiotherapy utilizing prompt-gamma imaging (PGI) presents many challenges in terms of detector physics. Gamma detectors with the capability of extracting the best energy, timing, and spatial information from each gamma interaction, as well as with high detection efficiency and count rate performance, are needed for this application. In this work we present the characterization of a pixel Čerenkov charge induction (CCI) thallium bromide (TlBr) detector in terms of energy and and electron drift time for its potential use in PGI. The CCI TlBr detector had dimensions of 4 × 4 × 5 mm 3 and one of its electrodes was segmented in pixels with 1.7 mm pitch. A silicon photomultiplier (SiPM) was optically coupled to one of the faces of the TlBr slab to read out the Čerenkov light promptly emitted after the interaction of a gamma ray. The detector was operated stand-alone and the 1.275 prompt gammas from a 22 Na radioactive source were used for the study. The electron drift time was obtained by combining the Čerenkov and charge induction signals and then used as a measure of the depth of interaction. The electron mobility in TlBr was estimated as ∼27 cm 2 V −1 s −1 . Energy resolutions between 3.4% and 4.0% at 1.275 MeV were obtained after depth-correction. These values improved to 3.0%–3.3% when events with drift times of 3–6 μ s were selected. These results show the potential of pixel CCI TlBr detectors to resolve gamma interactions in the detector with mm-like accuracy in 3D and with excellent energy resolution. Previous studies with CCI TlBr devices have shown a timing resolution of <400 ps full width at half maximum when detecting 511 keV gamma rays, therefore, the timing accuracy is expected to improve with the increased energy of the gamma rays in PGI. While other important detector characteristics such as count rate capability remain to be studied, results from this work combined with other preliminary data show pixel CCI detectors can simultaneously provide excellent energy, timing, and spatial resolution performance and are a very promising option for PGI in hadron therapy.

Results from testing of position-sensitive capacitive Frisch-grid (PSCFG) TIBr gamma-ray detectors are presented. Due to its high atomic number, high density, and low Fano factor, TIBr offers excellent energy resolution and high detection efficiency over a wide energy range, thus providing significant advantages over other detector materials commonly used in hand-held instruments. Using high-fidelity 3-D position sensing enables the response non-uniformity caused by defects in the TIBr crystal to be corrected, thereby offering an approach to overcome one of the technical barriers limiting the use of this promising semiconductor material. By utilizing the 3-D position information, temporal and spatial variations of the charge collection efficiency are presented, which provide microscopic characterization of the PSCFG devices. Using the 3-D response correction technique, the best energy resolution measured is <1.6% (FWHM) at 662 keV; it is expected that this resolution will improve further with the introduction of the ASIC-based front-end electronics discussed in this paper. The results presented demonstrate the great potential of PSCFG TIBr detectors and the need for better understanding and control of ionic migration processes in order to achieve good long-term performance.

Lithium indium selenide (LiInSe2) is being developed for use as a room temperature semiconductor detector for thermal neutrons. The material has been studied for a number of applications including non-linear optics such as parametric oscillators, as anode material for lithium ion batteries, piezoelectrics, as a scintillation detector material, and as a semiconductor detector material. The recent advances of the crystal growth, material processing, and detector fabrication have led to semiconductor neutron detectors with up to 100 mm(2) active area. The theoretical thermal neutron detection sensitivity and gamma rejection ratio (GRR) are comparable to 10 atm, He-3 tubes of similar size. Detector fabrication and characterization are described and the results are discussed.

Thallium bromide (TlBr) is a compound semiconductor with a band gap of 2.68 eV making it ideal for room temperature radiation detection. The high atomic numbers, 81 and 35, and the high density of 7.56 g/cm3 give it excellent gamma-ray stopping power. TlBr is a cubic material that melts congruently at a relatively low temperature (~480°C). These properties make it relatively easy to grow good quality crystals with high yield. As a result of improvements in the purification of TlBr mobility-lifetime product of electrons, μτe, is now on the order of 10−2 cm2/V, which is similar to that of CZT. High μτe enables the fabrication of thicker detectors with good charge collection and energy resolution. The properties of TlBr make it ideal for use in room temperature gamma radiation operation [1]. Single carrier devices such as small pixel arrays [2] and Frisch collar devices [3] which were developed for CZT can also been applied to TlBr. For example, better than 1% FWHM at 662 keV has been obtained for single pixel events with small (e.g., 3 × 3 pixels, 1-mm pitch, 5-mm thick) arrays.

Thallium bromide (TlBr) is a semiconductor material and, simultaneously, a good Cerenkov radiator. The performance of a TlBr detector that integrates two different readouts, the charge induction readout and the detection of Cerenkov light, was evaluated. A TlBr detector with dimensions of 4  ×  4  ×  5 mm3, with a monolithic cathode and an anode segmented into strips, was manufactured. One of the bare and polished 4  ×  4 mm2 faces of the detector was coupled to a silicon photomultiplier (SiPM) to read out the Cerenkov light. Simultaneous timing and energy resolutions of  <400 ps full width at half maximum (FWHM) and ~8.5% at 511 keV were measured using the Cerenkov detection and charge induction readouts, respectively. A coincidence time resolution of 330 ps was obtained when selecting Cerenkov events with amplitudes above 70 mV. The combination of both readouts showed the potential to resolve the depth-of-interaction (DOI) positioning, based on the improvement of energy resolution when selecting events with similar electron drift times. This manuscript sets the stage for a new family of semiconductor detectors that combine charge induction readout with the Cerenkov light detection. Such detectors can provide, simultaneously, outstanding timing, energy, and spatial resolution, and will be an excellent fit for applications that require the detection of high-energy gamma photons with high timing accuracy, such as time-of-flight positron emission tomography (TOF-PET) and prompt gamma imaging (PGI) to assess the particle range in hadron therapy.

MD simulation of dislocation migration under an electrical field.

Room-temperature operable semiconductor gamma-ray detectors offer the potential of superior energy resolution compared to scintillators, which can be valuable in the design of radionuclide identification equipment. To achieve the best energy resolution and simultaneously provide good sensitivity, multiple detection elements must be arrayed to operate in parallel. This paper describes efforts to construct such an array from multiple elements of thallium bromide (TlBr), a material that inherently offers a high interaction probability and photopeak efficiency for gamma rays. The base design of the array comprises capacitive Frisch grid (CFG) elements. A key objective is optimizing the energy resolution of each element, for which the single-carrier characteristics of the CFG design were chosen. Gains are realized through signal processing that accounts for 1-D or 3-D material inhomogeneity and charge collection variations. Fully corrected energy resolution reaches values of 1.6% FWHM at 662 keV.