X-ray Detector Applications Research Articles

Lead-free halide perovskite Cs3Bi2Br9 single crystals for high-performance X-ray detection

All-inorganic lead-free halide perovskites have attracted interest owing to their high ambient and thermal stabilities, excellent optoelectronic properties, and environmental friendliness. Herein, the bismuth-based halide perovskite Cs3Bi2Br9 single crystals were successfully grown to a diameter of 12 mm and length of 40 mm using a modified Bridgman method for the first time. The resistivity and transmittance of transparent and crack-free Cs3Bi2Br9 single crystal are ~6.8×1011 Ω cm and ~80%, respectively. The carrier mobility of the (−120) plane is 0.17 cm2 V−1 s−1 along the [010] orientation (b axis), and the trap density is 9.7×1010 cm−3. Moreover, Cs3Bi2Br9 single crystals exhibit excellent potential for X-ray detection, including a high absorption coefficient, a superior X-ray sensitivity of ~230.4 µC Gyair−1 cm−2, and an ultra-low and no-drift dark current density of ~17.8 pA mm−2, which enables lower noise and is also beneficial to the ultra-low detection limit for X-ray detectors. Our study shows that Cs3Bi2Br9 is a promising candidate for X-ray detection applications.

Highly efficient copper halide scintillators for high-performance and dynamic X-ray imaging.

Progress towards high performance X-ray detection and dynamic imaging applications, including nondestructive inspection, homeland security, and medical diagnostics, requires scintillators with a high light yield, a reasonable decay time, low cost, and eco-friendliness. Recently, copper halide scintillators have drawn tremendous attention due to their outstanding radioluminescence performance. Here, we first employed β-Cs3Cu2Cl5 as a high-performance scintillator, with a photoluminescence quantum yield (PLQY) of 94.6%, a radioluminescence light yield of 34 000 ± 4000 photons per MeV, a low detection limit of 81.7 nGyair s-1, and good operational stability under a total X-ray dose of 174.6 Gyair in air. In addition, this scintillator presents a high spatial resolution of 9.6 lp mm-1 at the modulation transfer function of 0.2 and a superb performance at 60 frames per second in our X-ray imaging system. Overall, this highly efficient scintillator demonstrates outstanding comprehensive performance and shows great potential for broad applications in X-ray detection and dynamic imaging.

Sb2Se3 film with grain size over 10 µm toward X-ray detection.

Direct X-ray detectors are considered as competitive next-generation X-ray detectors because of their high spatial resolution, high sensitivity, and simple device configuration. However, their potential is largely limited by the imperfections of traditional materials, such as the low crystallization temperature of α-Se and the low atomic numbers of α-Si and α-Se. Here, we report the Sb2Se3 X-ray thin-film detector with a p-n junction structure, which exhibited a sensitivity of 106.3 µC/(Gyair·cm2) and response time of < 2.5 ms. This decent performance and the various advantages of Sb2Se3, such as the average atomic number of 40.8 and μτ product (μ is the mobility, and τ is the carrier lifetime) of 1.29 × 10-5 cm2/V, indicate its potential for application in X-ray detection.

Highly Sensitive and Stable X-ray Detector Based on a 0D Structural Cs4PbI6 Single Crystal.

Lead halide perovskite single crystals with a high X-ray stopping power and large μτ product have been successfully used for X-ray detection. However, poor air stability and ionic migration lead to the degradation of the devices for long-term operations. Here, we report a solution-processed lead 0D bulk Cs4PbI6 single crystal, which have I atoms in isolated [PbI6]4-. This 0D Cs4PbI6 single crystal has a μτ product of 9.7 × 10-4 cm2 V-1 and an activation energy of 321.28 meV. We have fabricated a photoconductor device with a symmetric Au/Cs4PbI6/Au sandwich structure, which exhibits a high sensitivity of 451.49 μC Gy-1 cm-2 under 30 keV X-ray irradiation at an applied voltage of 30 V. After storing the device in air at room temperature for three months, the device retains nearly the same sensitivity as the original. These results demonstrate that this 0D Cs4PbI6 perovskite single crystal has great potential for practical X-ray detection applications.

Crystal growth and scintillation properties of pure and Tl-doped Cs3Cu2I5

In this work the Bridgman crystal growth and scintillation properties of both undoped and Tl-doped Cs3Cu2I5 are presented. This material is very attractive for gamma and X-ray detection applications, with a density of 4.53 g/cm3 and effective atomic number of 51.9. Undoped Cs3Cu2I5 had a light yield of 41,500 photons/MeV, with an energy resolution of 4.4% at 662 keV. Thallium doping at 0.5 mol % resulted in a much-improved scintillation response, in which light yield increased to 98,200 photons/MeV and energy resolution reduced to 3.3% at 662 keV. The X-ray excited emission is centered at 442 nm for the undoped and 500 nm for the Tl-doped crystals. The undoped emission is broad, typical of excitonic emission, while thallium doping results in an even broader band with features of both the undoped and thallium defect-mediated emissions.

A CMOS Image Sensor with a Novel Passive Pixel Array and High Precision Current Amplifier for a Compact Digital X-ray Detector

An active pixel sensor (APS) in a digital X-ray detector is the dominant circuitry for a CMOS image sensor (CIS) despite its lower fill factor (FF) compared to that of a passive pixel sensor (PPS). Although the PPS provides higher FF, its overall signal-to-noise ratio (SNR) is lower than that of the APS. The required high resolution and small focal plane can be achieved by reducing the number of transistors and contacts per pixel. We proposed a novel passive pixel array and a high precision current amplifier to improve the dynamic range (DR) without minimizing the sensitivity for diagnostic compact digital X-ray detector applications. The PPS can be an alternative to improve the FF. However, size reduction of the feedback capacitor causes degradation of SNR performance. This paper proposes a novel PPS based on readout and amplification circuits with a high precision current amplifier to minimize performance degradation. The expected result was attained with a 0.35-μm CMOS process parameter with power supply voltage of 3.3 V. The proposed PPS has a saturation signal of 1.5 V, dynamic range of 63.5 dB, and total power consumption of 13.47 mW. Therefore, the proposed PPS readout circuit improves the dynamic range without sacrificing the sensitivity.

Efficient X-ray Attenuation Lead-Free AgBi2I7 Halide Rudorffite Alternative for Sensitive and Stable X-ray Detection.

The poor attenuation capability of high-energy X-ray photons hinders the application of X-ray detectors in medical and astrophysical areas. Halide-based perovskites are promising candidates for X-ray detection because of their improved sensitivity. However, their inferior attenuation coefficient is still unsatisfactory for broadband X-ray detection. Here, a new kind of X-ray detection material, AgBi2I7 rudorffite single crystal (SC), is prepared and applied in X-ray detection for the first time, and it shows a higher attenuation coefficient than halide-based perovskites, commercialized a-Se, and the currently outstanding Cd0.9Zn0.1Te (CZT). The AgBi2I7 rudorffite SCs possess outstanding electric properties and excellent stability. AgBi2I7-SC detectors demonstrate a limit of detection (LoD) of 72 nGyair s-1 and a sensitivity of 282.5 μC Gyair-1cm-2 to X-rays and show only a slight performance degradation after ontinuous X-ray irradiation with a total dose of 58 Gyair. This work opens up a new perspective and broad opportunities for halide rudorffite in X-ray detection.

Development of Ce- and Eu-doped TlSr2Cl5 scintillators

Luminescence and scintillation properties of Eu2+- and Ce3+-doped TlSr2Cl5 crystals are reported for large effective atomic number (Zeff = 63.7) that have potential applications in X-ray and gamma ray detection. We have previously developed TlSr2Cl5 crystalline scintillators. To improve the scintillation properties of TlSr2Cl5, Ce3+ and Eu2+ were doped into the crystals in this study. The scintillation emission bands of Eu2+-doped TlSr2Cl5 were observed at 420 nm and those for Ce3+-doped TlSr2Cl5 were observed at 380, 450, and 500 nm. Band peaks at 450 and 500 nm were similar to those of undoped TlSr2Cl5, which are characteristic of self-activated thallium-based chloride crystals. The light yields of Ce3+- and Eu2+-doped TlSr2Cl5 were estimated to be approximately 16,000 and 21,000 photons/MeV, respectively. The light yield of Eu2+-doped TlSr2Cl5 is higher than that of undoped TlSr2Cl5 crystals (19,000 photons/MeV). It was suggested that Eu2+ is more suitable dopant for TlSr2Cl5 than Ce3+. The higher light yield of Eu2+-doped TlSr2Cl5 is due to the emissions by Eu2+ ions, which makes it easier to distinguish peaks from noise and improve the energy resolution.

Production of flexible nanocomposite membranes for x-ray detectors

Flexible membranes of poly(vinyl alcohol) (PVA) polymer and CuO nanoparticles for x-ray detection applications are reported in this work. PVA represents a polymer matrix for nanoparticles, and its flexibility and electrical conductivity are enhanced by addition of glycerol (GL) plasticizer. Nanoparticles of an average size of 6.3∓2.4nm are produced by a solvothermal method and added to the PVA + GL solution with different concentrations. The flexible membranes are fabricated by solution casting on glass substrates. The effect of blending of PVA + GL with nanoparticles on different characteristics of the membranes including the flexibility as well as the melting, glass transition, and degradation temperatures are tested by differential scanning calorimetry, thermal gravimetric analysis, as well as both Raman and Fourier-transform infrared spectroscopy. Electrical impedance tests reveal that both dc resistance and activation energy decrease with increasing temperature as well as nanoparticle concentration. The produced membranes reveal electrical response to x-ray due to the presence of CuO nanoparticles, and this response rises with x-ray generator voltage. The results presented in this study specify that the produced membranes are easy to produce with low cost, thus, they represent potential candidates for practical applications including x-ray detection.

TlSr2Cl5: New intrinsically activated crystalline scintillator

We report the luminescence and scintillation properties of TlSr2Cl5 crystals, which have a large effective atomic number (Zeff = 63.7), for application in X-ray and gamma ray detection. TlSr2Cl5 crystals were prepared under vacuum using the self-seeding solidification method. The scintillation emission bands were observed at 430 and 490 nm, which are close to the emission wavelengths of TlMgCl3 and TlCdCl3. The peak wavelengths were characteristic of self-activated thallium-based chloride crystals. The scintillation decay time constants were estimated to be 7.5 ns (61%), 99 ns (7%), and 1375 ns (32%). These decay time constants are close to those of TlMgCl3 and TlCdCl3 and are also characteristic of self-activated thallium-based chloride crystals. The light yield of TlSr2Cl5 was estimated to be approximately 19,000 photons/MeV, which is higher than that of Gd2SiO5(Ce) (GSO) (10,000 photons/MeV) and close to that of Lu2SiO5(Ce) (LSO) (27,000 photons/MeV).

Development of an x-ray detector by polycrystalline diamond and its application in Z-pinch x-ray detection.

Large-grain-sized polycrystalline diamond films are fabricated by electron assisted chemical vapor deposition. A pure SP3 carbon bond in the cubic lattice structure is confirmed by Raman spectrum analysis. The grain size is on the order of several hundreds of μm or larger. Interlaced-finger electrodes are imprinted onto a 6 × 6 mm2 × 500 µm film to fabricate the x-ray detector. The width of every finger is 25 µm, and the distance between nearby fingers is 25 μm. Strong x ray irradiates when a pulsed current flows through a double-layer nested tungsten wire array in the Z-pinch. This diamond detector now works as one of the main x-ray detectors for the Z-pinch device. The diamond detector is calibrated using a plastic scintillator. The comparison between the signal measured by these two methods confirms that the large-grain-sized polycrystalline diamond is a good candidate for the detection of nanosecond pulsed x ray.

Dark current–voltage characteristics of vacuum deposited multilayer amorphous selenium-alloy detectors and the effect of x-ray irradiation

Doped and stabilized amorphous selenium (a-Se) alloys in a multilayer form are currently used as a photoconductor in direct conversion flat panel x-ray imagers in mammography and tomosynthesis. While much progress has been made on the physics of such detectors, there are still unresolved questions on such issues as the principles of operation of the so-called p-i-n detector structure in extinguishing the dark current. The present paper examines dark current transients after the application of a voltage in seven types of a-Se alloy based devices: i-layer, i-n, n-i, i-p, p-i, p-i-n, and n-i-p structures. The substrate was ITO coated glass, the top (radiation receiving) electrode was chromium, and the films were fabricated by vacuum deposition. The nominal device thickness was 200 μm, similar to commercial mammographic detectors. It is shown that n-i, i-n, n-i-p, and p-i-n devices have dark currents less than 1 pA mm−2 at an applied field of 10 V/μm. The dark current in the p-i-n device is the lowest at approximately 0.01 pA mm−2 at an applied field of 10 V μm−1. Experiments have been carried out by subjecting the detector to a staircase voltage-time profile during the voltage application (turn-on) and a staircase voltage ramp-down during the turning-off of the bias voltage. Step-voltage ramp-up and step-voltage ramp-down current transient are typical of expected dark current transient behavior in a semiconductor with traps in which carriers are captured and released from various trap centers in the bandgap. The dark current transients are qualitatively similar to those expected from a capacitor in parallel with a large resistor and both in series with a much smaller resistor. Current transients during charging and discharging experiments were integrated to find how much of the injected charge is released during discharge experiments. It is shown that the majority of this trapped charge is stored in the n-type and p-type blocking layers, near the contacts. The trapped carriers in the i-layer represent a very small portion. The evolution of the dark current upon single and repeated x-ray exposure has also been examined in n-i and p-i-n devices. Right after the cessation of irradiation, there is an excess or residual dark current whose magnitude is roughly ∼20 times higher in p-i-n and about ∼2 times higher in the case of n-i under an exposure of 3.36 R and a mean photon energy of 34.2 keV. The absorbed dose is 1.53 Gy. The excess dark current has a fast decay component with a time constant ∼10 s and a slow component with a time constant ∼100 s. It is shown that the decay in the irradiation induced excess dark current is very similar to the initial dark current. There is no permanent change in the dark current, and within a few hundred seconds, the dark current reaches the same level as that in the unexposed detector. The experimental results in this work highlight the distinct advantages of p-i-n and n-i-type a-Se multilayer structures in x-ray detection applications.

Investigation of flexible polymer-Tl2O3 nanocomposites for x-ray detector applications

We report on the synthesis and characterization of composite semiconducting polymer – nanoparticle membranes for X-ray detection applications. The membranes are made of poly(vinyl alcohol) (PVA) organic polymer that is doped with ionic liquid (IL) to control its electrical conductivity. Tl2O3 nanoparticles with different weight concentrations are added to the polymer, and membranes are produced using solution casting method. The nanoparticles are synthesized using a microwave assisted technique under controlled temperature and pressure, and their average size is 13.0 ∓ 1.9 nm. Structural and compositional tests confirm the fabrication of homogeneous polymer-nanoparticle films. Electrical impedance tests are conducted as a function of nanoparticle concentration and temperature for the membranes, and they reveal that the dc electrical resistance of the membranes decreases with increasing both nanoparticle concentration and temperature. The activation energy was found to increase with increasing nanoparticle concentration. The membranes are utilized as conductometric X-ray sensors, and their response increases with increasing X-ray generator voltage. The fabricated composite membranes exhibit semiconducting properties, easy to fabricate, low cost, and qualify practical device utilization in the X-ray dosimetry sector.

Time-resolved, nonequilibrium carrier and coherent acoustic phonon dynamics in (Cd, Mg)Te single crystals for radiation detectors

We present time-resolved nonequilibrium carrier and coherent acoustic phonon dynamics in (Cd, Mg)Te single crystals intended for detection of high-energy x-ray photons. Our (Cd, Mg)Te crystals were grown using a vertical Bridgman method and during the process were doped either with In (a shallow dopant) or Ge (a deep impurity). Next they were cut into platelets and, subsequently, tested as as-grown specimens, or, before testing, long-term annealed in a temperature range of 750 °C–800 °C to improve their crystalline properties. All samples exhibited high resistivities of 109–1010 Ω cm, as required for x-ray detection applications. We have performed femtosecond optical spectroscopy, pump-probe measurements on all samples in both one-color (high-intensity 800 nm pump and low-intensity 800 nm probe) and two-color (high-intensity 400 nm pump and low-intensity 800 nm probe) configurations, and analyzed the results using our carrier dynamics rate-equation model. The model allowed us to perform a comprehensive, time-resolved analysis of relaxation dynamics of photoexcited carriers and investigate the role of carrier traps present in all tested samples. Two-color measurements also allowed us to observe and analyze the propagation of acoustic coherent phonons excited by the high-energy pump pulse. We observed that the dependence of phonon mode frequency, after including a correction of the experimentally measured crystal index of refraction dependence on the probe wavelength, was dispersionless and led to the value for the sound velocity of 3367 m s−1, which is in excellent agreement with the literature data. The intrinsic lifetime of coherent phonons in (Cd, Mg)Te was estimated to be of the order of 10 ns. Finally, we conclude that among our tested samples, the In-doped, annealed (Cd, Mg)Te crystal exhibits the best characteristics for radiation detection applications.

X-ray detection based on complementary metal-oxide-semiconductor sensors

Complementary metal-oxide-semiconductor (CMOS) sensors can convert X-rays into detectable signals; therefore, they are powerful tools in X-ray detection applications. Herein, we explore the physics behind X-ray detection performed using CMOS sensors. X-ray measurements were obtained using a simulated positioner based on a CMOS sensor, while the X-ray energy was modified by changing the voltage, current, and radiation time. A monitoring control unit collected video data of the detected X-rays. The video images were framed and filtered to detect the effective pixel points (radiation spots). The histograms of the images prove there is a linear relationship between the pixel points and X-ray energy. The relationships between the image pixel points, voltage, and current were quantified, and the resultant correlations were observed to obey some physical laws.

X-ray detector based on Mn doped MgAl2O4 and Si photodiode.

Luminescence properties of Mn doped MgAl2O4 crystals connected with the Si PIN photodiode (PD) have been evaluated for X-ray detector applications. Mn doped MgAl2O4 crystals emit green colored luminescence peaking at around λ = 520 nm with CuKα irradiation. The intensity of the output signal from Si PD increases linearly with the X-ray intensity of CuKα generated with the X-ray tube at a current from 2 mA to 40 mA. The intensity of the X-ray excited luminescence is strongest in the specimen with 3.0 mol. % Mn. Afterglow luminescence is weak in the specimen with 2.0 mol. % Mn. Mn doped MgAl2O4 crystals connected with the Si PD is considered to be a useful X-ray detector.

Crystallization control toward colorless cerium-doped scintillating glass.

The construction of cerium-doped materials is of great technological importance for various applications, including smart lighting, biological shielding and high-energy ray and particle detection. A major challenge is the efficient prevention of the undesired colorization after cerium doping. Here we present the nanocrystallization method for constructing colorless cerium-doped glass with extremely high cerium concentration (15 mol%). The structure and optical characterizations confirm that the notable color change of glass is associated with the precipitation of CeF3 crystalline phase during heat-treatment. The chemical state investigation shows that most of cerium ions exist in the form of Ce3+ in both the glass and glass-ceramic samples. The chemical environment study indicates a dramatic change in the local structure unit from -Ce-O- to -Ce-F-, which is believed to dominate the decoloring phenomenon in cerium doped glass. As a result, a significant improvement in the ultraviolet excited luminescence (~35 times enhancement in intensity) and scintillating performance can be achieved, pointing to potential applications in X-ray detection.

Indirect X-ray Detectors Based on Inkjet-Printed Photodetectors with a Screen-Printed Scintillator Layer.

Organic photodetectors (PDs) based on printing technologies will allow to expand the current field of PD applications toward large-area and flexible applications in areas such as medical imaging, security, and quality control, among others. Inkjet printing is a powerful digital tool for the deposition of smart and functional materials on various substrates, allowing the development of electronic devices such as PDs on various substrates. In this work, inkjet-printed PD arrays, based on the organic thin-film transistor architecture, have been developed and applied for the indirect detection of X-ray radiation using a scintillator ink as an X-ray absorber. The >90% increase of the photocurrent of the PDs under X-ray radiation, from about 53 nA without the scintillator film to about 102 nA with the scintillator located on top of the PD, proves the suitability of the developed printed device for X-ray detection applications.

Laser-induced surface recrystallization of polycrystalline PbI2 thick films for X-ray detector application

In this work, laser-induced surface recrystallization process was developed to improve the surface properties and device performance of the polycrystalline PbI2 thick films prepared by using close space vapor deposition method. A continuous polycrystalline PbI2 recrystallized layer with a better mechanical strength and reflectivity improved from 2% to 4%–6% was obtained by this recrystallization process for the films with mechanical pretreatment. Other polytypes is absent in the recrystallized layer with the 2H-polytype remaining before and after treatment and obtaining improved electrical and X-ray photoelectric response performance. The pretreatment such as mechanical cutting/polishing and hydrogenation is necessary to lower the non-wetting crystallization behavior during the recrystallization process due to the rough surface state and oxygen contamination.

Recent technological developments on LGAD and iLGAD detectors for tracking and timing applications

This paper reports the latest technological development on the Low Gain Avalanche Detector (LGAD) and introduces a new architecture of these detectors called inverse-LGAD (iLGAD). Both approaches are based on the standard Avalanche Photo Diodes (APD) concept, commonly used in optical and X-ray detection applications, including an internal multiplication of the charge generated by radiation. The multiplication is inherent to the basic n++–p+–p structure, where the doping profile of the p+ layer is optimized to achieve high field and high impact ionization at the junction.The LGAD structures are optimized for applications such as tracking or timing detectors for high energy physics experiments or medical applications where time resolution lower than 30ps is required. Detailed TCAD device simulations together with the electrical and charge collection measurements are presented through this work.