Program Listings

CZT crystal growth depends significantly on the crystal growth furnace, such as furnace structure, temperature profile, growth temperature and temperature gradient etc. It is very time consuming to optimize such parameters in crystal growth process due to the long cycle of CZT crystal growth, especially for a THM process, in which the growth rate is much lower than Bridgman method. Here we report the result from thermodynamic modeling of THM furnace with different scenarios and the results show that with right selection of refractory materials and design of furnace structure, the temperature profile and growth temperature gradient which are required by THM CZT growth process can be achieved. The modeling result has been further verified in THM furnace. CZT crystal was tested and results will be available soon.

CdTe material is of prime technological importance for making high-quality room-temperature semiconductor radiation detectors used in nuclear homeland security and medical imaging fields. To make such a high-performance x-ray or gamma-ray radiation detector, crystallographically perfect and high resistivity materials are needed. However, there have been several reports on CdTe crystals deviated from desired material properties. Thus, compensation doping by adding proper amounts of dopants (e.g. Cl) into CdTe was intensively investigated to make semi-insulating crystals. Meanwhile, Jayatirtha et al. reported that there was no indication of Te inclusions in their Cl-doped CdTe samples. However, exact mechanisms underlying these two kinds of observations (i.e. compensation doping, and annihilation of Te inclusions by Cl doping) are not fully understood yet. Also, there are few reports on the combined effects of Cl doping on the compensation doping and annihilation of Te inclusions. Thus, in this presentation, we report the effects of Cl doping on the material properties of CdTe single crystals. Histograms of the Te inclusion size analyzed from several IR microscope images indicate that the average size and surface coverage of Te inclusions decreased with increasing Cl doping concentrations. Photoluminescence (PL) measurement results show that Cl-doped samples have better crystalline quality from the optical perspective. Also, samples doped with different amounts of Cl exhibited an increase in resistivities over 5 orders of magnitude. Based on the measurement results (IR microscope, PL, SE, Resistivity, Fermi level positions), we can ensure that Cl doping is the effective and promising way to control both Te inclusions and resistivity of the CdTe samples at the same time for better-performance semiconductor radiation detectors.

Gamma radiation detection instrument based on coplanar grid CdZnTe detector (CPG CZT) with charge sensitive integration detection amplifier and a differential amplifier was developed in the Institute of Physics. Standard pulse shaping amplifier and pulse height analyzer is used to record the spectrum with the resolution of 2% of the Full Width at Half Maximum (FWHM) at Cs 137 (662 keV). The determination of internal electric field profile and the charge collection efficiency belongs to important tasks in any semiconductor radiation detectors characterization. Transient Current Technique (TCT) is used as a standard method for this purpose. Laser induced TCT (L–TCT) uses direct oscilloscope triggering taken from the driving generator so that it provides much higher signal to noise ratio in compare to non–synchronized sources thus very low level CPG detector L–TCT current waveform with high content of additive noise component can be recorded. In this work L–TCT electron waveform measurement on CPG CZT detector under various bias values between the cathode and the grid will be presented. The influence of the voltage applied between collecting and non-collecting grids on the current waveforms shape will also be discussed. Electron transient waveforms were measured with our setup on collecting grid of CPG CZT detector in wide bias range (400 V to 1900 V) and it enables time resolved charge collection process characterization and to evaluate internal electric field in CPG detector volume. We will analyze additional component of the electrical characteristics of the detection circuitry in the transient waveform shape. Parasitic inductive and capacitive components and the detector electrical replacement circuit will be discussed in view of the identification its influence on L–TCT waveform shape, which is significant for the correct interpretation markedly in CPG detectors as it is more complex electronic component in compare to standard planar detector.

The performance of CdTe and CdZnTe bulk radiation detectors is critically affected by deep levels (DLs), which capture photo-carriers and induce detector polarization. The deep level analysis thus constitutes an indispensable step at the material characterization and the optimization of the detector manufacture. Various experimental methods were developed to determine properties of DLs in semi-insulating semiconductors, each of them having advantages and disadvantages. The principal problem of the most of techniques is their inability to distinguish the charge of captured carriers, i.e. the identification of electron and hole traps. The obstacle may be solved by the measurement of the charge transport in thermal gradient or in magnetic field. While the DLs characterization by thermoelectric effect spectroscopy (TEES) belongs to routine methods used in laboratories, the measurement of galvanomagnetic properties of photo-excited samples remained out of focus of the researchers for decades. In this presentation we fill in the gap in magnetic-field-based procedures at the research of DLs and report on the investigation of semi-insulating CdTe and CdZnTe by the Photo-Hall effect spectroscopy (PHES). The illumination in the wavelength range 1800 - 700 nm and dc electrical measurements are used. We show on a set of samples, both n- and p-types, the typical features of spectra and deduce properties of DLs responsible for observed effects. The extensive collection of experimental data allow us to define both positive and negative sides of PHES. Due to special character of the Hall voltage prioritizing more mobile electrons on holes the PHES appears particularly useful in p-types, where the character of principal DLs could be proven. In case of n-types the PHES does not enable us the unique interpretation and complementary investigation is necessary for explaining of observed transitions.

CdTe is a well-known semiconductor for the fabrication of room-temperature radiation detectors and electro-optical devices. Its unique physical properties can be improved by the post-growth high temperature annealing. As-grown CdTe crystals usually have a high density of dislocations and tellurium inclusions as a result of additional stress in the solidified material. These obstacles can be eliminated by the additional thermal treatment of as-grown CdTe single crystals. Herein we show a systematic study of dislocations in CdTe and CdZnTe single crystals, comparing to the migration of second-phase inclusions by selective etching. We report the influence of high-temperature (~1000-1100 K) annealing on the behavior of dislocations in CdZnTe single crystals, comparing to the thermomigration of inclusions. The defect structure of CdTe single crystals was studied by the selective etching method. Trapping role of dislocations was discovered as additional tension places in crystals for the localization of native point defects or inclusions. It was confirmed that the migration of native defects and dopants occurs by dislocation mechanism. Inclusions, which are localized near dislocations, can slowly dissolve during post-growth thermal treatment at high temperature defect equilibrium conditions. However, diffusion of excess atoms from the vapor phase is localized on the dislocation loop places. Moreover we found a new effect of tracks formation near the star-shaped inclusions which are localized in the center of dislocation loops. This phenomenon is caused by inclusion migration during post-growth annealing. Such investigations in comparison with another methods, such as EDX-analysis near dislocations loops can provide valuable local information about the nature of inclusions, stoichiometric composition and deeper understanding of the mechanism of inclusions migration that is required for the improvement of the electrophysical characteristics of CdTe- and CdZnTe-based devices.

Chemical etching of (111) CdTe crystals using hydrogen bromide (HBr)-based etching solution was studied, which consists of a mixture of HBr, bromine and water. Properties of the etched CdTe surface was investigated with x-ray photoelectron spectroscopy (XPS) and surface leakage current measurements, and the results were compared with that of a conventional Br-methnol (BM) etched CdTe surface. All crystals studied were cut from a larger CdTe wafer, and we further maintained the process consistency to ensure that the obtained results do not get affected due to the process related factors. XPS results revealed that BM etched surface had small but distinct TeO2 peaks, however, no such peaks were not observed in the HBr-etched CdTe surfaces. Moreover, the component of the surface leakage current was lower in the HBr-processed detectors, when compared to BM etched detectors. Details results on surface properties of the HBr-etched CdTe surfaces will be presented.

The design of Voxtel’s second generation, large format, dual-threshold x-ray photon counting (DT-XPC) readout integrated circuit (ROIC) is described. In the large format (192 x 192) DT-XPC sensor, each 100-µm x 100-µm pixel consists of a thick, fully depleted silicon photodetector, low-noise analog front end, dual-threshold pulse discriminator, and multi-mode photon-counting (30-bit) circuit. The DT-XPC ROIC design is optimized to detect hard X-rays in the range of 2 – 20 keV. The ROIC design supports two operating bandwidths, as well as control of the analog shaping time around the nominal operating bandwidths, allowing for noise optimization at the required pulse-pair resolution of the experiment. Simulations of the ROIC design have shown noise performance of 275 eV FWHM and non-uniformity corrected threshold dispersion of 75 eV FWHM in the low-bandwidth (1-MHz) mode of operation, thus enabling tight windowing of specific x-ray energies in x-ray spectroscopy experiments. Using the on-chip digital serial interface and external gating signals, the pixel counters can be configured to support high/low energy thresholding, or energy windowing, with individual bunch-gating using either two 15-bit counters or a single 30-bit counter. The DT-XPC ROIC supports a full-format frame rate of 90 Hz, assuming a 10-ms photon integration time, with a dead time of less than 1 ms. To support the development of very large format x-ray detectors, the DT-XPC ROIC is being designed to enable three-side buttablility of the ROIC, allowing the formation of 2 x N detector structures, with minimal dead space (2 pixels, or 200 µm) between individual sensors. Fabrication of the large format DT-XPC ROIC is planned at the end of 2015.

This study evaluated the use of Compton imaging technology using TlBr arrays to monitor prompt gamma rays emitted by B-10 in boron neutron capture therapy (BNCT) applied to a computerized human phantom. The Monte Carlo method, including particle-tracking techniques, was used for simulation. The distribution of prompt gamma rays emitted by the phantom during irradiation with neutron beams is closely associated with the distribution of the boron in the phantom. Maximum likelihood expectation maximization (MLEM) method was applied to the information obtained from the detected prompt gamma rays to reconstruct the distribution of the tumor with the boron uptake regions (BURs). The reconstructed Compton images of the prompt gamma rays were combined with the cross-sectional images of the human phantom. Quantitative analysis of the intensity curves showed that all combined images matched the predetermined conditions of the simulation.

In order to develop “tissue equivalent” radiation detector which does not disturb inspection result in Interventional radiography (IVR), the authors have been developing a new radiation sensor where organic photodiodes (OPDs) are fabricated directly on plastic scintillator. The fundamental possibility of applying the detector to X-ray measurements have been shown in the previous studies. In the present study, the authors have tested the OPD-based sensor for applying it to X-ray distribution measurement. The device structure of bulk hetero junction OPD was plastic scintillator plate (1 mm)/ IZO (200nm)/ PEDOT: PSS (30 nm) / PCBM: P3HT (300 nm) / Al (70 nm) with five sensing areas of 2 mm × 4 mm. The measured results have shown that the detector could measure X-ray distribution. By comparing with the simulated results, it has been shown that the measured results have reproduced the movements of the scintillation photons as reflected at the boundary of the scintillator. Further results of simulations and experiments with changing the reflection condition at the boundaries will be reported at the symposium.

The high resistance close to intrinsic CdZnTe (CZT) semiconductor is an excellent material candidate for high efficiency, high-resolution room-temperature (RT) nuclear radiation detectors thanks to its physical properties. Indeed, CZT crystals are available commercially and CZT detectors are steadily gaining acceptance in many medical, industrial, safeguards and scientific X- and ?-ray imaging and spectroscopic applications. The CZT detectors are favorable over other RT detector materials since they provide very good energy resolution and high stopping power. In Many CZT detection applications the detectors operate in varying environment with temperatures ranging from 10 ^oC and up to 35 ^oC and higher. Varying temperatures are known to affect the performance of CZT detectors. In this study we examine the effect of temperatures, in the above mentioned range, not only on standalone wafer crystals but also on wafers assembled on electronic boards using epoxy interconnections and under-fill. For each setup we examined also the effect of the contact type on the temperature influence. The results of ⁵⁷Co spectrum parameters and leakage current measurement show that at high temperatures the most significant influence on the detector performance is the leakage current. The current for Ohmic contacts increases linearly with temperature and as a result the energy resolution degrades as well. The peak efficiency of the measurement is marginally decreases. Effect on the charge collection efficiency is also seen by shift of the 122keV peak position, interestingly this shift was found to vary between wafers with different Zn concentration. For blocking contacts no effect on the leakage current and the spectrum parameters is observed. At lower part of the temperature range, the modules show degradation of detection quality, probably as a result of stress on the detector interconnections caused by the different thermal expansion of the CZT and the PCB on which the CZT was mounted on.

To improve CZT quality it is necessary to carefully measure the electrical parameters of crystals and detectors. The goal of this work is to study the temperature dependencies of the electrical properties of CZT crystals over a wide temperature range (290-420 ?) using Hall effect measurements. Cd(Zn)Te crystals, doped by In, were grown using the vertical Bridgman method. Usually the resistance of the as-grown crystals exceeded 10^10 Ohm-cm. The state of the defect-impurity system in as-grown samples is not an equilibrium one, because heating causes irreversible changes in the electrical characteristics. Therefore the samples were kept at 150 oC to reach stable values for the electrical parameters. The temperature dependence of the Hall constant for each sample has been described by its activation energy (Ea). At the point where a change in the Ea was observed, the nature of the carrier mobility temperature dependence also changed from "normal" (as a manifestation of scattering on lattice vibrations and ionized centers) at high temperature to exponential. Since the latter was a manifestation of the collective drift barriers caused by micro-inhomogeneities, only the high value of Ea (where there was no barrier component of charge-carrier mobility and the use of a classical method for parameter fitting) was correct. We identified Ea as the ionization energy of compensated deep donors (eD). For different samples the eDo values (at 0 K) were in the range from 0.50 to 0.78 eV, and the degree of compensation by donors [D+]/[D] varied from 0.3 to 0.98. The activation energy at low temperature (e1) can be described by the equation: e1 = eD-?eb, where eb is the height of drift barriers, determined from the temperature dependence of the carrier mobility, and ? has a value close to unity (0.5 for strong compensation and 2 for low compensation). The value of eb for the investigated samples lies within (0.05-0.35) eV range and probably depends on the crystal growth technology.

?Z? crystals were grown from a tellurium melt. Samples with regular geometric shape were prepared (4x4x2 mm3). Ohmic contacts were formed using indium. The goal was to investigate the influence of compensation on the detection properties of n-?Z? crystals. The crystal resistivity at 300 K was ~4x108 Ohm-cm for the beginning and ~8x10^8 Ohm-cm for the end of the ingot. Good agreement between calculation and experiment was obtained for Ed ~ 0.05-0.07 eV and Na/Nd = 0.99 for crystals near the beginning of the ingot, Ed ~ 1.0 eV and Na/Nd = 0.93-0.96 for crystals from the middle of the ingot, and Ed ~ 0.8-0.82 eV and Na/Nd = 0.03-0.05 from the end of the ingot, assuming the dark conductivity of the crystal is determined by a deep donor level. Such conductivity behavior can be explained if we assume that a deep level acts as an acceptor. However, in this case the compensation degree is opposite, i.e. for the mid-crystal Nd/Na is 0.03-0.05 and for the end of the ingot Nd/Na is 0.93-0.96. If the voltage switch (Vo) from an ohmic dependency to a quadratic one is independent or weakly dependent on temperature, then the crystal is slightly compensated. If the temperature dependence is large, then it is a highly compensated crystal. The relatively strong temperature dependence of Vo was obtained for a mid-ingot crystal. For samples near the end of the ingot, it was almost absent. According to these results one can make a clear conclusion about the donor nature of the deep level responsible for the dark conductivity in our investigated crystals. Detectors from the end of the ingot showed significantly better detection properties. Their energy resolution (FWHM) for Am-241 was equal to 4.5 keV. For mid-ingot detectors the FWHM was at least 2 times higher. Thus, CZT crystals with varying degrees of compensation and about the same resistivity showed different detection properties. The best detection properties were obtained for the crystals with a lower degree of compensation.

A CdTe detector equipped with Timepix can work in both counting mode and time over threshold (TOT) mode. When it works in TOT mode, the count recorded in each pixel represents the amount of collected charge. TOT mode provides an easy and direct way to measure the spectra of sources. A pixelated detector in TOT mode also makes it possible to observe and analyze charge sharing between pixels. The energy calibration of detector working in TOT mode is usually performed under the assumption that an isolated pixel with a nonzero count is free from charge sharing. We studied the calibration procedure on our detector which has a relatively small 55μm pixel size. The results showed that the energies of peaks of the spectra after calibration were constantly bigger than expected. We believed that it was caused by the fact that the charge sharing is not negligible for the isolated pixels when the pixel size is small.

In this work, we report on the results of electrical characterization of CdZnTe (CZT) detectors, with gold electroless contacts, grown by the boron oxide encapsulated vertical Bridgman technique (B-VB), currently produced at IMEM-CNR (Parma, Italy). The detectors, with different thicknesses (1 and 2.5 mm), have the same electrode layout: the anode is a central electrode (2 x 2 mm2) surrounded by a guard-ring electrode. The cathode is a planar electrode covering the detector surface (4.1 x 4.1 mm2). Current-voltage (I?V) characteristics were measured at different temperatures in order to study the charge transport and the electrical properties. These detectors were compared with the traveling heater method (THM) CdZnTe grown detectors (Redlen, Canada), fabricated with the same electrode layout and deposition. The results highlight the low leakage currents of the B-VB CdZnTe detectors even at high bias voltages: 38 nA/cm2 (T = 25?C) at 10000 V/cm. This feature allows high bias voltage operation, very important for high flux applications. These activities are in the framework of an Italian research project on the development of energy-resolved photon counting (ERPC) systems for high flux energy-resolved X-ray imaging.

Point defects are well-known crystal defects induced during the crystal growth. Possible causes for the generation of point defects are impurities and the dopant added to increase the resistivity of the bulk material. Different point defects have different roles in changing the material properties, such as resistivity, µt product and mobility of charge carriers. In this talk the research is focused on the effects of different types of point defects found in CZT crystals, such as A-centers, Cd vacancies, and deep traps of energy greater than 0.6 eV to 1.1 eV, on the electrical and electrical transport properties, and the performance of the detectors. Specifically the role of the deep traps in affecting the resistivity, leakage current at room temperature, and µt-product are explored experimentally. Current deep level transient spectroscopy (I-DLTS) is the main technique employed to identify the point defects, their density, capture cross-section, and trapping and de-trapping times for the charge carriers. In addition, the knowledge obtained from I-DLTS is related to the performance of the CZT detectors by measuring the detector response for a 241Am gamma-ray source.

Solution-grown Rubrene Crystals as Radiation Detecting Devices Leslie Carman (a), H. Paul Martinez (a), Lars Voss (a), Steven Hunter (a), Patrick Beck (a), Natalia Zaitseva (a) and Stephen Payne (a), Pavel Irkhin (b) andVitaly Podzorov (b). a) Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94551, USA b) Rutgers University, Newark, NJ 07102 USA There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5,6,11,12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high mobility. In this work, large, volumetric single crystals with volume ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. The response curve to pulsed illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport albeit larger. The high quality of the crystals together with the application of PEDOT/PSS on the graphite electrodes have led to the clear observation of alpha particles. Preliminary results with a 252-Cf source generate a small signal with the rubrene detector and may demonstrate the high-energy neutron detecting capability of this organic semiconductor.

CdZnTe and CdTe based semiconductor X-Ray and Gamma-Ray detectors have been intensively studied recently due to their promising potentials for achieving high-resolution, high signal-to-noise ratios and low leakage current, all are desirable features in applications ranging from medical diagnostics to homeland security. Understanding the atomic and electronic structures of the metal/semiconductor interfaces is essential for the further improvements of performance. Using density functional calculations, we systematically studied the stability, the atomic and electronic structures of the interfaces between CdTe (111) surfaces (Te-terminated) and the selected metals (Cu, Al and Pt). We also calculated the Schottky barrier height (SBH) by aligning the electrostatic potentials in semiconductor and metal regions. While comparing with our previous results on Cd-terminated interfaces, our calculations revealed significant differences between the two. While metals tend to deposit directly on reconstructed Te-terminated surfaces, they form a Te-metal alloy layer at the Cd-Terminated metal/CdTe interface. For both Te- and Cd- terminated interfaces, the Schottky barrier heights do not depend much on the choice of metals despite the large variation of the work functions. On the other hand, the interface structure is found to have large effect on the SBH, which is attributed to the metal induced states in the gap.

Filtered Back-Projection (FBP) is preferred in 4pi Compton imaging when the computational time is critical. In Compton imaging the result is usually reconstructed in a 4pi spherical space with far-field assumption, so the Spherical Harmonics Transform (SHT) is suitable for filtering. But the point spread function (PSF) of back-projected image is usually rotate-variant due to the geometry and attenuation of detectors, which limits the application of FBP with SHT. An approximate method is proposed that decomposes the transform matrix into several diagonal matrices and SHT matrices, introducing weighting factors to correct for attenuation without additional computational time. The results for a 2x2 3D position sensitive CdZnTe detector array show improved angular resolution and a rotate-invariant PSF. This method can also be applied to simplify the system matrix in different reconstruction algorithms such as MLEM.

In this paper, we report on the recent development of a 64-channel low-noise front-end readout ASIC for cadmium zinc telluride detectors aimed to X-ray and γ-ray imaging systems. There are 64 readout channels and 2 test channels in this ASIC. The readout ASIC is implemented in TSMC 0.35-μm mixed-signal CMOS technology, the die size of the prototype chip is 2.7 mm x 8.0 mm. At room temperature, the equivalent noise charge of a typical channel reaches 66 e- (rms) for the zero input capacitance with the average power consumption of 8 mW per channel. The linearity error is less than 1%. The overall gain of the readout channel is 200 mV/fC. The inconsistency among the channels is less than 2.86%. By connecting this readout ASIC to an 8 x 8 pixel CdZnTe detector, we obtained a γ-ray spectrum, the energy resolution of which is 4.5% at the 59.5-keV line of 241Am source.

The TlBr detector shows high gamma-ray detection efficiency because of its high atomic numbers and high density. However, the TlBr planer detector with more than 1mm thick can hardly operate with high energy resolution because the mobility-lifetime product (µt) of holes is not so high. In order for the sensitivity of the TlBr to be further improved, a thicker detector is required to be realized. In order to effectively read out the information of deposition energy in the TlBr detector, we study a novel digital signal processing technique. In conventional detectors, induced charges are accumulated for sufficient time to collect whole carrier information. However, in detectors with low carrier mobility, slow carriers, such as holes, may be captured before achieving to electrodes. One of the methods to eliminate information of slow carriers is a single polarity charge sensing technique, such as Frisch grid detector. For TlBr detector, capacitive Frisch grid technique was applied and excellent energy resolution was achieved. However, capacitive Frisch grid type detectors show asymmetric peak shape because of its imperfection in single polarity charge sensitivity. In this paper, we propose a novel digital signal processing technique extracting information of early carrier movement before carriers are captured. The increase rate of induced charges, or induced current, in a planner detector consisting of parallel plate electrodes is proportional to the total produced charge because carrier velocity and weighting potential are constant. The induced current in early period of a signal pulse is proportional to total produced charge or deposition energy. In order to confirm the feasibility of our proposed technique, we conducted basic experiments. We confirm that the induced current spectrum shows clearer and more symmetric peak shape than the conventional pulse height spectrum.

CdTe Semiconductor detectors have been widely used for ?-ray and X-ray detection and/or imaging applications. CdTe detector has high detection efficiency due to high atomic number (Cd: 48, Te: 52). Moreover, CdTe detector can be operated at room temperature due to large band gap energy (1.44 eV). However, CdTe Schottoky detectors have a problem of polarization phenomena in operation. It is consider that the main cause of polarization phenomena is the change of internal electric field. The change of internal electric field affects carrier transportation. We have developed a measurement device to evaluate carrier transportation in CdTe diode. The device can measure the energy spectrum (pulse height) and the rise-up time distribution in real-time online. We have evaluated the effect of polarization phenomena to carrier transportation with our device. The rise-up time distribution can be used as the indicator of the carrier transportation time. In this study, we have investigated the effect of polarization phenomena to the rise-up time and the peak value using our system. 241Am and 57Co radioisotopes were measured at room temperature (around 297 K). The detector is In/Ti//CdTe//Pt Schottoky CdTe diode of 4 x 4 x 0.5 mm. The CdTe detector was connected charge sensitive preamplifier (CLEAR PULSE 5102) and our measurement device. Reverse bias of 100V was applied to CdTe detector during measurement. In this study, there are few relations between rise-up time and peak height. On the other hand, we have found the strong relations between rise-up time and full width at half maximum (FWHM) have same tendency. We will discuss polarization mechanism by using the relations.

To limit errors such as patient mispositioning, evolution of patient or/and tumor morphology and treatment planning errors in the dose delivery, it is necessary for patient positioning systems and position verification systems composed of X-ray imaging devices. But, the radiation therapy requires beam energies high enough to treat tumors, and the main interactions of the incident photon are Compton scattering at these energies. Currently, using these characteristics of radiation, we are developing a Compton camera for incident beam monitoring during radiation therapy. The beam monitoring system is composed of a two double-sided silicon strip detectors (or CdZnTe strip detectors) for the photon tracking, a scintillation detector for the energy determination of the incident photon, and radiation signal conditioning circuits such as preamplifier and shaping amplifier, etc. In this study, the preamplifier for 32 channel double-sided silicon strip detectors was developed and characterized. The preamplifier was designed using commercially available operational amplifier having low noise, low power consumption, high open loop gain, high gain bandwidth product characteristics, and low input capacitance. To minimize 1/f noise at the input stage of the preamplifier, a JFET was used at the input of the operational amplifier. Matching well with double-sided silicon strip detectors and having a high signal-to-noise ratio and an appropriate count rate, the feedback resistance and capacitance were optimized. The measured sensitivity is 3.3 mV/fC, RMS noise 300 electrons FWHM, rise time of the output pulse 10 nsec and decay time of output pulse 200 usec. For this detector and preamplifier combinations, the spectroscopy was performed using Ba-133 calibration source. The energy resolution of the measured spectra for the 31 keV are 2.12 keV FWHM at room temperature.

In this work, we present the results of spectroscopic investigations on CdZnTe (CZT) detectors grown by the boron oxide encapsulated vertical Bridgman technique (IMEM-CNR, Parma, Italy). The detectors, with different thicknesses (1 and 2.5 mm), are characterized by the same electrode layout (gold electroless contacts): the anode is a central electrode (2 x 2 mm2) surrounded by a guard-ring electrode, while the cathode is a planar electrode covering the detector surface (4.1 x 4.1 mm2). The results of electrical investigations point out the low leakage currents of these detectors even at high bias voltages: 38 nA/cm2 (T = 25?C) at 10000 V/cm. The time-stability and the spectroscopic response of the detectors, at different temperatures and fluxes, were investigated. 241Am spectra were measured up to 1 Mcps. The detectors were compared with the traveling heater method (THM) CZT grown detectors (Redlen), fabricated with the same electrode layout. These activities are in the framework of an Italian research project on the development of energy-resolved photon counting (ERPC) systems for high flux energy-resolved X-ray imaging.

Charge collection efficiency (CCE) of CdTe/CdZnTe radiation detectors is affected by the internal electric field, trapping and detrapping of charged carriers on shallow and deep defects or band-bending close the surface due to the contact preparation or surface processing. CCE for strongly absorbed radiation can be also affected by the surface recombination of generated carriers. Transient current technique (TCT) is a powerful method for determination of transport properties of CdTe/CdZnTe detectors and for better understanding of the charge collection process. From the shape of the current waveform the drift mobility, shape of the internal electric field, trapping and detrapping times of charged carriers, surface recombination velocity and other transport parameters can be evaluated. In this work laser-induced TCT is used for detector characterization. Our experimental setup consists of various pulsed laser diodes (LD) with wavelengths for above and below band-gap illumination. CdZnTe detectors were excited by pulsed LD using short excitation pulses and the repetition rate in the range of 1-1000 Hz. Laser pulse creates electron-hole pairs and drifting carriers generate a current pulse that is amplified by current sensitive preamplifier and recorded by digital sampling oscilloscope. The drift velocity and the trapping and detrapping times of generated charged carriers are evaluated from the analysis of bias dependence of the current pulse shapes based on the Shockley-Read-Hall trap assisted recombination model (SHR) after various surface processing and metallization methods. Modified SHR model is also used for characterization of the surface recombination velocity expecting two dimensional density of traps only exist at the surface.

The European Spallation Source (ESS), situated in Lund, Sweden, is going to become the world's brightest and strongest source of neutrons by 2025. The purpose of this research is to expose silicon pixel detector technology to the world of neutron scattering and to open up new avenues of possibilities for neutron instrument design. Silicon pixel detectors characterization is a collaborative ongoing project between European Spallation Source (ESS), Sweden, SINTEF in Oslo and University of Bergen (UiB) in Bergen, Norway. The silicon pixel array detectors are fabricated at SINTEF and coated with neutron converter material B4C at Linköping, Sweden. These detectors primarily aim for the needs and requirements of a highly innovative spectrometer named CAMEA – Continuous Angular and Multiple Energy Analyzer proposed by ESS. It is a cold-neutron inverse-geometry time-of-flight spectrometer. Combining indirect time-of flight with multiple consecutive analyzer arrays, this instrument will provide massive flux on the sample and strongly enhanced efficiency in detecting neutrons scattered in the horizontal scattering. The combination yields a spectrometer with completely unprecedented performance – with gains from 2 up to 4 orders of magnitude compared to current state of the art. The experimental work running at UiB includes electrical measurements on sensor level, investigation of different methods for deposition of neutron converter material, sensor response using alpha, proton and neutron source, radiation hardness and detector’s lifetime. A systematic, reliable and reproducible characterization procedure is identified and will be discussed. Moreover, preliminary measurement results from the first prototypes will be presented. The work is supported by Norwegian Research Council.

Cadmium zinc telluride (CdZnTe or CZT) has been considered to be the most potential material for fabrication of room temperature nuclear radiation detectors, owing to its large band gap, high absorption efficiency as well as excellent electron transport properties. Due to the growing application of CdZnTe detectors in astrophysics missions, investigation on the influence of radiation damage induced by abundant charged particles in space appears to be necessary since it is important to radiation protection. In this work, we studied the bias-dependent influence of 2 MeV proton induced radiation damage on overall spectroscopy performance as well as local charge collection efficiency via ion beam induced charge (IBIC) technique. Detector response under different applied biases show that there is little change for peak channels after point injections of 2 MeV proton beam with doses up to 1.27 × 10^13 p/cm^2, whereas corresponding energy resolution deteriorates severely under low biases. Further analysis on the charge collection efficiency (CCE) distribution reveals that diameter of low CCE regions induced radiation damage decreases significantly with increasing applied biases, which is attributed to reduced carrier trapping resulting from the suppression of electron cloud size. Interestingly, CCE mapping proves that intrinsic defects embedded in the near-surface region of CdZnTe crystal are activated by the radiation damage, and deteriorate the overall CCE uniformity significantly under low biases.

Charge collection efficiency (CCE) and mobility-lifetime product (µt) are important material parameters which may decide on the applicability of the detectors. The higher CCE and µt detector has the better material it is. Because of this various techniques are used for better identification of these transport parameters. One of the most widespread method is to determine them via Hecht equation from collected charge voltage dependency. In the present work we studied a different ways for determination of the Hecht relation on CdTe and CdZnTe based radiation detectors. The aim was to find optimal conditions to precisely determine µt product of the samples. We used classical alpha particle induced transient charge technique (a-TChT) based on irradiation of samples with alpha particles and then we compare it with gamma ray induced transient charge technique (?-TChT), laser induced (of different wavelengths) transient charge technique (L-TChT), photocurrent-voltage dependency and laser induced transient current technique (L-TCT). Each method have some negatives (e.g. plasma effect in alpha TChT) and some positives (e.g. small surface recombination in gamma TChT), so we needed to use modified Hecht relation for better determination of µt parameter.

Due to its wide band gap (2.68 eV) and high stopping power, thallium bromide (TlBr) is being investigated as a room temperature semiconductor gamma-ray spectrometer. When cooled to -20°C, performance of better than 1% FWHM at 662 keV has been observed on 5x5x5 mm³ pixelated TlBr detectors. The room-temperature lifetime (under continuous application of applied bias) of thick TlBr detectors operated at high fields (~2000 V/cm) has been limited to weeks or months due to polarization caused by ionic conduction. Previous work has shown that this degradation process is limited to the surface and that surface preparation techniques can extend the lifetime of thin planar TlBr devices at room temperature. In this work, the lifetime and stability of large 5x5x5 mm³ pixelated TlBr arrays is presented. Detectors performance is compared for different surface preparation techniques showing improved stability with new surface preparation techniques. Additionally, we use depth sensing techniques to track the depth-dependent photopeak centroids over time and conclude that once surface degradation effects are mitigated, TlBr performance can improve over time at room temperature. The improvement is likely similar to conditioning at -20°C in which the electric field stabilizes and becomes more uniform.

The particle detector based on high purity epitaxial 4H-SiC reveals promising properties in detection of various type of ionizing radiation. In this work we focused on detection of fast neutrons generated by D – T (deuterium – tritium) nuclear reaction. The high quality liquid phase epitaxial layer of 4H-SiC was used as a detection region. The thickness of the layer was 105 um and the diameter of circular Au/Ni Schottky contact was 1.4 mm. Detectors base on 4H-SiC were tested for fast neutron detection generated by D – T reaction. Energies of detected neutrons varied from 16.0 MeV till 18.3 MeV concerning to accelerative potential of deuteron which rose from 600 kV up to 2 MV. The detection of fast neutrons in SiC detector is primarily caused by the elastic and inelastic scattering on silicon or carbide component of detector material. Other possibility which increases the detection efficiency is using a fast neutron conversion layer. In our experiments we glued on detector Schottky contact HDPE (high density polyethylene) conversion layer to transform fast neutrons to high energy protons, which are detected by 4H-SiC detector. We used various thicknesses of HDPE up to 1.3 mm. The comparison of detection properties between detector with and without HDPE conversion layer was accomplished. The alpha particles of nuclear reactions between neutrons and atoms of semiconductor material were observed. We also performed the simulation of fast neutron detection by SiC detector using MCNPX (Monte Carlo N-particle eXtended) code to optimize detection efficiency.

In our previous research [1], we have optimized the bulk semi-insulating (SI) GaAs detectors for fast-neutron detection. Their active area of 7.4 mm2 was created by a multi-pixel structure of Schottky contact, which was coated by a HDPE (High Density PolyEthylene) conversion layer, where neutrons transfer their kinetic energy to hydrogen atoms through elastic scattering. Detectors were tested at a 239Pu-Be source of fast neutrons with continuous spectrum of neutrons up to 12 MeV and with their mean energy of about 4 MeV. In this paper, we have examined SI GaAs detectors at a different type of neutron source to study their detection properties. The detectors were exposed to mono-energetic neutrons with kinetic energy of 16.8 MeV generated by deuterium – tritium nuclear reaction. The MCNPX (Monte Carlo N-particle eXtended) code has been used to support the analysis of the experiment. First, the influence of HDPE layer thickness on the relative detection efficiency of fast neutrons was studied. The conversion efficiency of layer is influenced by two phenomena, the elastic recoil rate of neutrons and the projected range of recoiled protons in HDPE layer. The theoretical optimum thickness of the conversion layer was determined to 1.9 mm using the MCNPX code. The results from simulation were compared with the experimental data, where the thickness of the conversion layer varied from 50 µm to 1300 µm. The MCNPX code was used also to model the detector response to used fast neutrons concerning the effect of various thicknesses of the HDPE conversion layer. Finally, the effect of active detector thickness modified by detector reverse bias on detection efficiency was studied. [1] A. Sagatova et al., JINST 8 C03016 (2013).

In this paper the fabrication and characterization of diamond detectors and their application in a coincidence system are reported. Two diamond detectors were used in a coincidence system in a way to reject gamma rays and count the beta particles at elevated temperature (~150 ºC). Diamond crystal were commercially acquired from Element Six.

This paper reports on recent efforts to develop a uranium oxide?based solid-state neutron detector. The semiconductive characteristics of UO₂ and other oxides of uranium have long been recognized, but few attempts have been made to exploit these properties to fabricate uranium oxide?based semiconductor devices. The large amount of energy (approximately 165 MeV) released in the neutron-induced fission of a uranium nucleus indicates these materials may be useful for neutron detection. We report here on recent efforts to develop a direct-conversion neutron detector from uranium oxide-based Schottky, pn, and resistive devices. Experimental tests of the neutron sensitivity of successful devices have been attempted and the results are presented.

CdTe/CZT is now a material consolidated for the detectors realization operating at room temperature, which find a large variety of applications in astrophysics, medical imaging and security. An Italian collaboration, involving the CNR/IMEM and INAF/IASF institutes, was born several years ago with the aim to develop a national capability to produce CZT detectors starting from the material growth to the final detection device. The collection efficiency of the charge carriers affects some important features of these detectors, such as the pulse height, energy resolution, photopeak efficiency. In fact the low mobility of the charge carriers (particularly the holes) and trapping/detrapping phenomena can degrade the CdTe/CZT detector response, depending on the distance between the charge formation position and the collecting electrodes. Two kinds of techniques can be used to improve both the collection efficiency and the energy resolution, based on the optimization of the electrode geometry and/or signal compensation methods. We have implemented a biparametric method that uses a twin pulse shaping active filter to analyze the same signal from the detector: one “slow”, which is proportional to the energy of the incident photon, and one “fast”, which depends on the position of the interaction with respect to the collecting electrode. We present this biparametric technique applied on planar CZT detectors grown by the Vertical Bridgman method at CNR/IMEM, the experimental results obtained as a function of the bias voltage, photon energy, shaping time pairs and the compensated spectra. Furthermore, this technique could be implemented in an array of detectors, whose front-end electronics is composed of ASICs, where the shaping time can be selected for each channel, like the RENA-3 IC (NOVA R&D).

A new low-power application specific integrated circuit (ASIC) for Cadmium Zinc Telluride (CZT) detectors for single-photon emission computed tomography (SPECT) application is being developed at BNL. As the first step, a 32-channel prototype ASIC was designed and tested recently. Each channel in the ASIC has a preamplifier followed by CR-RC³ shaping circuits and three independent energy bins with comparators and 16-bit counters. The ASIC was fabricated with TSMC 0.35-µm complementary metal–oxide–semiconductor (CMOS) process and tested in laboratories. The power consumption is less than 1 mW/ch with a 2.5-V supply. At a channel gain of 400 mV/fC and peaking time of 500 ns, an equivalent noise charge (ENC) of 350 e- was measured at room temperature, while the crosstalk rate was less than 0.3%. The 10-bit DACs for global thresholds have an integral nonlinearity (INL) less than 0.56% and differential nonlinearity (DNL) less than 0.33%. In the presentation, we will report the detailed test results for this ASIC.

Thallium Bromide (TlBr) is a wide bandgap, compound semiconductor with high gamma-ray stopping power and promising physical properties. However, performance degradation and the eventual irreversible failure of TlBr devices can be caused by the electro-migration of Tl+ and Br- ions to the electrical contacts across the device. This phenomena is enhanced by the interaction of radiation with TlBr devices where the collision of the irradiating particle (alpha and gamma) a lattice atom, imparting sufficient energy to knock it out of the lattice. This, so-called, displacement effect was prominently observed in long term evaluation of TlBr devices. Under continuous alpha and gamma irradiation, the TlBr devices degraded more rapidly than those that were intermittently irradiated. The extent of radiation damage also depends on factors such as temperature, applied bias, incident energy and flux.

A series of Cadmium Zinc Telluride (CZT) growths with 10% zinc have been conducted in a 4 inch modified vertical Bridgman furnace in order to optimize the quality of CZT crystals derived from seeded growths. This involved re-designing certain physical aspects of the experimental furnace setup as well as manipulation of different growth parameters. Two specific difficulties in fabricating detector grade CZT were addressed: controlling initial nucleation sites and inhibiting melt to ampoule wall adhesion. Poor thermo-physical properties of CZT render as grown ingots prone to low single crystal yield. Initial nucleation sites are of extreme importance due to their extended influence on the entire crystal. Seeded growth may be able to decrease grain boundaries by preferentially directing nucleation thereby increasing single crystal yield. Growths have been performed with seed crystals using a seed pocket containing quartz ampoule. However, incorporation of a seed pocket resulted in unwanted nucleation sites at the boundary between the seed pocket and the region above it. By removing this seed pocket we hoped to eliminate unwanted nucleation sites while deriving a preferential grain structure from our seed crystal. Melt to ampoule wall adhesion has also been known to cause unwanted nucleation centers and thus poor single crystal yield. Promising results have been obtained in the implementation of pyrolytic Boron Nitride (pBN) crucibles to decrease melt to ampoule wall adhesion in non-seeded growths. We have applied pBN to seeded growths by using a pBN crucible inserted within the quartz ampoule. Discussion will be presented on the single crystal yield as well as the electrical properties and the performance of these ingots as radiation detectors.

Proximity charge sensing is a relatively new technique that has two distinct advantages over directly-deposited electrodes. The first advantage is that it eliminates the need to connect wires to the directly-deposited electrodes on the semiconductor crystal, which simplifies the fabrication process of the whole system. Second, it can be used to improve the position sensitivity of the device via signal interpolation. Proximity-charge sensing has previously only been implemented in Ge and Si-based detectors. Though the energy resolution of HPGe detectors has not yet been surpassed, room-temperature semiconductors such as CZT offer a significant advantage over HPGe since they do not require expensive, high-maintenance cooling systems. This study explores the use of proximity charge-sensing on CZT crystals. The weighting potentials of proximity-sensing and directly-deposited electrode designs were generated using ANSYS Maxwell and quantitatively compared using a figure of merit (FOM). Proximity-sensing designs invariably show a better FOM than directly-deposited designs in these simulations. The most promising proximity-sensing design is being fabricated and characterized on a CZT crystal, and its performance compared to directly-deposited electrodes.

Large volume single crystals of Cd_0.9Zn_0.1Te (CZT) have been grown by vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). The grown semi-insulating CZT crystals have shown high promise for high-resolution room-temperature radiation detectors due to their high dark resistivity (~4x10¹⁰ Ω-cm), good charge transport properties [(μτ)_e = (6-8) x 10^-3 cm²/V], and relatively low cost. The grown CZT single crystals (~2.8 cm diameter and >10 cm long) have demonstrated a very low radial Zn concentration deviation, low dislocation densities and Te precipitate/inclusions, and high infrared transmission. Details of the CZT single crystal growth, their physical and chemical analysis, surface processing, nuclear radiation detector fabrication, and testing of these devices are presented.

Different surface treatments such as chemical etching in bromine-containing solutions and nanosecond laser annealing were applied to provide the appropriate surface states of the CdZnTe (CZT) crystals prior to formation of metal-semiconductor contacts and electrode deposition. The effect of chemical etching and irradiation with nanosecond laser pulses of energy density below and above the melting threshold on the characteristics of the surface region of different CZT solid solutions were studied. Depending on laser energy density, number of pulses and irradiation ambient conditions, photoelectric, electrical and optical properties of CZT crystals could be changed. Laser processing of CZT samples with the certain energy densities resulted in an increase in the photosensitivity and transformation of photoconductivity (PC) spectrum profile. This was attributed to improvement of homogeneity in the crystal surface area and equalization of structural characteristics in the subsurface region and bulk of the samples. An increase in the PC signal, especially in the high-energy part of the spectrum, was evidence of a decrease in the surface recombination rate because of laser annealing of structural imperfections, in particular desorption of contaminations and oxides. A shift of the maximum and red edge of the PC spectrum to higher energies was attributed to formation of a surface layer with a wider bandgap. Irradiation of chemically etched CZT crystals with the certain energy density resulted in an increase in the transmittance in the near-IR region that was associated with annihilation of point defects in the CZT bulk and/or gettering them by extended defects. Nanosecond laser irradiation of CZT samples pre-coated with an electrode metal film allowed to modify I-V characteristics because of changing the surface states of the metal-CZT interface. The laser-assisted technique of the surface processing of CZT has been developed for fabrication of X-ray and ?-ray detectors.

Amorphous selenium (a-Se) alloy materials doped with lithium (Li), boron (B), arsenic (As), and chlorine (Cl), were synthesized for room temperature radiation detection applications using an optimized alloy composition for enhanced charge transport properties. A two-step synthesis process has been implemented to first synthesize the a-Se (As) and a-Se (Cl) master alloys from zone-refined Se (~7N), and then synthesize the final mixed optimized alloys. The alloy material was used for thin-film deposition on oxidized aluminum (Al) and indium tin oxide (ITO) coated glass substrates. Material purity, morphology, and compositional analysis of the a-Se alloy materials were investigated by glow discharge mass spectroscopy (GDMS), x-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). Different metals of various work functions (Ni, W, Au, Pd, In, Cu, Sn, Cu, Ag, Al, Cr, Zn and Mo) were selected for Schottky barrier detector performance studies. Current-voltage (I-V), capacitance voltage (C-V), and current transient measurements were performed at different temperatures to investigate the metal contacts and contact stability characteristics. Single and multi-element detectors with and without various blocking contacts (electron and holes) have been fabricated and tested and the results show promising characteristics for x-ray and high energy nuclear radiations with its high dark resistivity (~10¹² Ω-cm) and large-area scalability.

The energy-imaging integrated deconvolution (EIID) algorithm is an effective method in gamma-ray Compton imaging. In some applications, the event number is limited, so penalties are needed to regularize the reconstruction. In this paper, the edge preserving and object sparsity priors are employed based on the conventional EIID likelihood function, and Chambolle Pock (CP) algorithm is used to optimize them. Different CP based penalized algorithms are realized and experiments on the Polaris-II 3D CZT system are performed. The results show that CP based penalized algorithms performs much better in local smoothing, boundary preserving and background denoising, but it converges more slowly than EIID.

Travelling Heater Method (THM) grown Cadmium Zinc Telluride (CdZnTe) is a promising material for the fabrication of X-ray detectors for medical, security and dental imaging. These applications demand the detectors to function under high values of X-ray flux (10' to 100' of Mcps/mm^2) with stable and uniform performance. CdZnTe detectors have traditional suffered from high levels of crystal defects formed during the growth process, which lead to a number of undesirable detector characteristics such as reduced detection efficiency, non-uniformity, instability and signal polarization at higher flux values. We have greatly reduced these issues in our modern THM-grown CdZnTe crystals for X-ray detection. We discuss the use of various material and device characterization tools (e.g. X-ray Diffraction, Infra Red mapping, Photo Induced current transient spectroscopy, Pockels, contactless resistivity mapping and photo-current mapping) to investigate the effect of crystal defects on device performance under high flux X-ray. Furthermore, we discuss the performance of our small pixel detectors (< 200 mm pitch) in terms of edge pixel vs. central pixel behavior, and sidewall leakage current.

VAD_UM v1.2 is a low noise digitizer ASIC developed in University of Michigan for 3-dimensional position-sensitive room-temperature pixelated semiconductor detector such as CdZnTe. It has been demonstrated that VAD_UM ASIC is capable of providing sub-pixel position resolution at 122 keV for coded aperture imaging. However, the image quality, especially signal-to-noise ratio was found poorer than expectation. This article presents the improvement of the image quality using mask-antimask and moving mask techniques. Moving mask technique is a new technique combining traditional coded aperture imaging with time coded aperture imaging together. It was proposed to improve image quality using position-sensitive detectors if the source was stationary. Significant improvement of signal-to-noise ratio has been gained using both techniques. Position resolution as good as 3 mm for a source placed 44 cm away has been experimentally achieved.

Amorphous selenium (a-Se) unique properties made it commercially viable x-ray to charge transducer used in radiation medical imaging detectors. One of the most important properties of any photoconductor used in radiation detectors is electron-hole pair creation energy (Wehp). For its practical relevance it is important to understand the mechanism of recombination behind the x-ray generated charge in a-Se. Indeed, it was shown that electron-hole pair creation energy (Wehp) in a-Se does not follow the general so-called Klein rule for semiconductors. Moreover, it was shown that the value of Wehp decreases with the increase in electric field. It is believed that the underlying mechanism for such dependence is that the application of high electric field to a-Se suppresses recombination of photogenerated charge carriers thus improving charge collection and decreasing Wehp. Clarification of the effect of electric field on electron-hole creation energy required Pulse Height Spectroscopy (PHS) and charge collection (CC) experiments over a much broader range of electric fields than it is possible with plain a-Se layers due to the technical complications associated with operating a-Se at electric fields higher than 30 V/µm. Here we show that electron-hole pair creation energy Wehp decreases linearly with field all the way up to avalanche threshold of 80 V/µm and saturates at ~ 9 eV at higher fields. In addition we measured Wehp in a wide range of temperatures (193K - 330 K) and show that it decreases with increasing temperature. In addition, in order to describe theoretically a passage of an energetic primary electron through a-Se layer and secondary EHP creation through multiple random collisions we performed a Monte Carlo simulation (based on Geant4). Our experimental data combined with Monte Carlo Simulation allow us to expand our knowledge on recombination mechanism in a-Se and develop modified columnar recombination mechanism in a-Se based x-ray detectors.

Lithium indium diselenide is a relatively new thermal neutron sensitive semiconductor detector. This new detection material has been shown to respond well to thermal neutrons and alpha particles with a pulse height above the gamma-ray-induced signal. However, no quantification of the expected detector response to thermal and fast neutron and gamma-rays for different crystal dimensions and charge collection properties has taken place. We report on the results of various simulation techniques utilized to provide the LISe detector response function for various radiation stimuli.

The thallium chalcohalide compounds Tl6SeI4 and Tl6SI4 are promising materials for efficient room-temperature ?-rays detection due to their suitable bandgaps and high densities. However, a detector-grade is the prerequisite to make these materials responsive to heavy rays. Impurities can lead to charge trapping and scattering, which will kill most of photoinduced electrons and holes. In this work, multiple technologies including evaporation method in a bent tube, horizontal zone refining and vertical narrow zone refining were developed to purify elemental (Se and S), binary (Tl2Se, Tl2S and TlI) and ternary precursors (Tl6SeI4 and Tl6SI4), aiming at a future application of these chalcohalide detectors. Based on effectiveness assessments of these purification steps by impurity analysis, the optimized purification technology for achieving a detector-grade purity was implemented. The crystals were grown by conventional Bridgman method with the purified raw materials. In order to evaluate the quality of obtained crystals, systematic measurements were performed to determining the stoichiometry, defects and impurities. Significant enhancement of photoresponse upon Co-57 radiation was found after purification of raw materials.

A novel kind of detector based on polycrystalline grade diamond substrate and reduced graphene oxide (RGO) contacts is presented. This detector combines some of the good qualities of diamond (like radiation hardness and almost unique combination of electric, thermal and optical properties) with low-Z contacts. This characteristic together with the possibility of patterning the electrodes with standard lithographic techniques, make this detector particularly suitable for X-ray beam monitors where the intensity and the position of the photon beam needs to be measured with minimal effect on the beam itself (i.e. in-line and highly transmissive measurement). The steps needed to realize our novel detectors together with a new GO rapid thermal reduction process are described and the results of preliminary X-ray tests reported.

To date, high stopping power semiconductor radiation detectors that operate at room temperature have yet to achieve performance approaching the energy resolution of cooled high purity germanium detectors (HPGe). Large bandgap semiconductors with lower noise tend to have low Z and thus poor efficiency for stopping gamma radiation. On the other hand, high Z semiconductor materials, which are efficient at stopping gamma radiation, present low bandgaps resulting in large dark currents and require cooling to achieve good energy resolution. In this work, we present the concept for a high efficiency, room temperature semiconductor detector that has the potential to rival the energy resolution of HPGe detectors. This is achieved by combining a high-Z absorber with a low-Z and low noise junction using a newly developed crystal growing method that incorporates interfacial misfit arrays. Here we describe the detector concept and the considerations for the development and testing of such a detector. We also present results from our first successfully grown test sample of GaSb/GaAs crystal.

The X-ray transmission radiography is one of the commonly used non-destructive techniques for the inspection of the inner structure of painted arts. This method can help with the process of the evaluation and restoration of the paintings even for features smaller than 1 mm if the quality of the radiographs is sufficiently high. The use of single particle counting hybrid pixel detectors of Medipix/Timepix family brings several advantages to the field of X-ray imaging. Combined with micro-focus X-ray source they provide wide dynamic range together with high spatial resolution in the micrometre range. The limitation given by their small sensitive area was recently overcome by assembling large area imaging detector WidePIX which is composed of a matrix of Timepix readout chips equipped with edgeless silicon sensors. The precise modular architecture of this device minimizes the insensitive areas and gaps between individual tiles to few microns. The overall sensitive area of used WidePIX_10×5 detector is 14.3 × 7.15 cm² divided into 3.25 mega pixels. The effective field of view of an imaging system can be further increased by scanning large objects in multiple sub-acquisition mode with different detector positions. The Timepix detector with proper per-pixel energy calibration provides spectroscopic capability with energy resolution at level of keV in each pixel. The X-ray transmission images measured in several energy windows can be used for basic material identification without degradation of favourable high spatial resolution in large area. The visualization of information obtained from the variations in energy windows is done by pseudo-colouring. The differences between individual images in energy windows are used as colour channels in RGB colour space. The method and its interpretation was tested in the field of cultural heritage and the results obtained with the painting Passion of Christ by Alessandro Varotari are presented.

Semi-insulating CdTe semiconductor is the most efficient and important material for room temperature solid state X-ray and gamma-ray detectors. In order to form effective electrical contacts (either Ohmic or diode-like), it is important to control the state and characteristics of the surface region of CdTe crystals before electrode deposition. Low temperature photoluminescence (PL) as a non-destructive and highly sensitive technique was used for characterization of detector grade CdTe crystals after irradiation with nanosecond laser pulses. High-resistivity Cl-compensated p-like CdTe(111) single-crystal samples obtained from Acrorad were subjected to chemical polishing etching to remove oxides and surface disturbed layers. YAG:Nd and KrF lasers with wavelength 532 nm and 248 nm, emitting single pulses with duration of 7 ns and 20 ns respectively, were used as irradiation sources to modify the surface state and structure of the surface region of CdTe crystals. The dependences of PL intensity and spectrum profile on laser processing energy density, measurement temperature, and excitation power were studied. The evolution of the PL spectra of CdTe crystals subjected to nanosecond laser irradiation was attributed to transformation of point defect structure in the surface region. The relative redistribution of the band intensities in three PL regions, particularly a decrease in the intensity in the deep level and edge regions and increase in the exciton band intensity after laser irradiation with the certain energy densities has demonstrated the possibilities to efficiently modify the surface state and increase the structural perfection of the CdTe surface region. Application of different laser wavelengths allowed us to excite and study thinner and thicker CdTe surface layers. The obtained PL data have been used to choose the optimal regimes of laser processing of CdTe crystals for formation of appropriate electrical contacts and fabrication of detectors.

The principally improved technique of the laser-induced doping has been studied by irradiation of the In/CdTe structures from the CdTe side using pulses of the first harmonic of a YAG:Nd laser (? = 1064 nm, t = 8 ns) with intensity varied in the range 1.5-6 MW/cm² in water at room temperature. A relatively thick (0.1-2 µm) In dopant film was evaporated on the CdTe(111)B crystal surface in vacuum. Extreme conditions in the confined area at the CdTe-In interface under laser irradiation resulted in formation of the diode structure with a high barrier height. The computer simulation of laser heating of the In film deposited on the CdTe surface was performed. The calculation results were used to choose the optimal temperature regimes for achieving the effective In doping of thin and thicker CdTe layers near the CdTe-In interface under laser irradiation through the CdTe bulk. After laser-induced doping and prior to vacuum deposition of an Au electrode, the samples were dipped into aqueous H₂O₂ solution and then rinsed in methanol. The formed M-p-n In/CdTe/Au diodes were tested by electrical and spectral measurements. There were strong correlations between the properties of the obtained diodes (I-V and spectral characteristics) and laser processing conditions (laser intensity, number of pulses, thickness of the deposited In film, etc). The best results (steep rectification, low leakage current and high energy resolution in radioisotope spectra) for the In/CdTe/Au diodes were obtained when CdTe-In interface was subjected to irradiation with laser pulses of intensity that led to heating of the entire thin (0.3-0.5 µm) In film up to the melting point as it was supposed according to the calculations. Satisfactory characteristics of the In/CdTe/Au diodes were achieved also when a thicker (about 2 µm) In film was used. In this case, the In layer near the CdTe-In interface was under the melting temperature while the external In film surface remained at room temperature.

This paper reported extensive reliability tests on CdZnTe detectors for medical imaging and security applications. The devices under test (DUT) included bare CZT detectors, detector assemblies with print circuit board (PCB) carriers, and modules with electronics. The CZT detectors with various anode patterns and physical configurations were attached to anode and cathode PCB carriers in house. Multiple reliability test protocols were designed to evaluate stabilities and reliabilities of the DUTs. They included burn-in screening tests, highly accelerated stress tests, and mechanical shock and vibration tests. Detector leakage current-voltage (IV) curves and radiation spectra responses under Co-57 and Cs-137 were used to compare performance changes. Over 100 detectors in various configurations were tested. Through performance comparison and root cause analysis of the degraded ones, detector fabrication processes, and attachment and ruggedization designs were optimized. The well-designed, fabricated and attached detector assemblies and modules will be reliable in medical imaging and security field applications. Key words: CZT, radiation detector, reliability, temperature cycling, vibration and shock test

The measurement of the polarization status of cosmic sources high-energy emission is today recognized as a key observational parameter to understand the active production mechanism and its geometry. Therefore, a mandatory requirement for new instrumentations operating in this energy range will be to provide high sensitivity for polarimetric measurements. In this framework, we have presented the concept of a small high-performance imaging spectrometer optimized for polarimetry between 100 and 600 keV suitable for a stratospheric balloon-borne payload and as a pathfinder for a future satellite mission. The detector with 3D spatial resolution is based on a CZT spectrometer in a highly segmented configuration designed to operate simultaneously as a high performance scattering polarimeter. Herein, we report on the results of a Monte Carlo study devoted to optimize the configuration of the detector for polarimetry. In particular, we have studied the polarimetric sensitivity for two different 3D spatial resolution scales (2 and 0.5 mm) and for different spectroscopic response. Furthermore, to assess the reliability of the numerical model, we compare Monte Carlo results with experimental data obtained with the high performance CZT spectro-imager module Caliste.

TlBr is a material of interest for use in room temperature gamma ray detector applications due to its wide bandgap, 2.7 eV, and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of ~1 % at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize at room temperature using conventional configurations. Much progress has been made to increase the usable room temperature lifetime from days to many months. Continued improvement of room temperature, high-resolution gamma ray detectors based on TlBr requires further understanding of the degradation mechanisms which include polarization, increasing dark current over time, diffusion of contact metal into the bulk and Tl rich surface dendritic growth. Digitization of preamp waveforms is powerful tool for characterizing transport properties as a function of depth. In this paper we will present results to characterize and analyze detectors using waveform digitization, scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy, in order to better understand failure mechanisms and improve detector performance and longevity.

Conventional near-field coded aperture imaging relies on detecting a complete mask pattern magnified and projected onto a position-sensitive detector plane. The best achievable spatial resolution is therefore limited by the position resolution and extent of the detector. We present a method for shifting the coded aperture mask between successive measurements such that the near-field spatial resolution is no longer detector-limited. A relatively small detector plane was used to sample a large and complex mask pattern by dividing the measurement into smaller parts. The technique has the added advantage for array systems such that artifacts due to gaps between detectors can be avoided.

A simulation study of the potential performance of a proposed indirect-conversion silicon neutron detector was performed. In contrast to previous work on this topic, which focused on ¹⁰B, and ⁶Li-based converters, the neutron sensitive converter material was ²³⁵U. We investigated two different detector geometries: ²³⁵U-filled pores in a silicon matrix and silicon pillars surrounded by ²³⁵U. The potential detector performance was studied for a broad parameter space, looking at different pore/pillar sizes and depths. It was shown that ²³⁵U offers a credible alternative to other detector materials, offering similar detection efficiencies with potentially superior noise suppression.

BiSI nanostructures were synthesized by solvothermal method with Bi₂S₃ and I₂ as precursors. The solvents used were either water or ethylene glycol, citric acid was used as capping agent. Nanostructures were characterized by powder X-ray diffraction. Samples prepared with ethylen glycol show BiSI diffraction peaks, whereas Bi₂S₃ peaks were found in the diagrams of samples prepared with water as solvent, possibly due to an incomplete reaction of the sulphide with the iodine. Samples were also observed under Transmission Electron Microscopy (TEM) to characterize size and morphology of nanostructures; both syntheses yielded nanorods in the range of 100-200 nm mean width, but in the synthesis with ethylene glycol round nanoparticles were also detected. Nanostructures from the synthesis with ethylene glycol and citric acid yielded the best results and were then chosen to fabricate pellets by pressure sintering. These pellets were then used to assemble detectors by depositing Au as electrodes, attaching Pd wires with Aquadag, and then encapsulating the electrode with acrylic. The microstructure of the pellet was characterized by Scanning Electron Microscopy (SEM). Electric properties and response to a ²⁴¹Am source of 0,4 mR/h were measured. The detector yielded a resistivity of 2x10¹⁰ O.cm, and showed a linear response to radiation. This finding opens a new field of work, with the possibility of constructing detectors without the need of a complicated process such as crystal growth. As future work, it is needed to improve the pellets fabrication process, and the procedures for detectors construction.

Abstract – Detecting thermal neutrons is vital to safeguard nuclear materials, but is challenged by the recent shortage of 3He. Thus, alternative materials are highly demanded. Among them, solid-state detectors with potential advantages of compact size, light-weight, easy maintenance and low cost, are of special interest. Boron compound semiconductors are excellent candidates because the B-10 isotope has large thermal neutron capture cross-section (3840 barns) and is readily abundant (20% of natural boron). Because of this, we are investigating the feasibility of icosahedral boron arsenide (B12As2) materials as thermal neutron detectors. Primarily we focused on thin-film materials grown by chemical vapor deposition (CVD) process and electron beam physical vapor deposition (EBPVD) process. For material characterization, we measured the photocurrent response of the materials with blue light and used Hall-effect measurements to determine the charge carrier type and its mobility. We also fabricated planar devices and conducted electrical characterization by measuring low- temperature, and above- room temperature I-V characteristic plots. In this presentation, we will discuss the characterization of the materials and devices and present our test results.

Future high-energy space telescopes missions require further analysis of orbital environment induced activation and radiation damage on main instruments. Such kind of studies are being performed in the framework of the selected Horizon 2020 AHEAD (Activities in the High Energy Astrophysics Domain) project and of M4 ESA submitted proposals, such as ASTROGAM. At typical orbital altitudes (~ 550 km) the proton flux can induce an activation background noise specially in high atomic number detection materials, such as CdTe that require further radiation effects analysis. Also, the detector performances, its energy resolution, peak channels or leakage currents can be seriously deteriorated by particle irradiation. In order to optimise the operational performances of future CdTe based gamma-ray space telescopes under orbital radiation environment, a series of tests were performed on a detection plane prototype at the ICNAS (Instituto de Ciências Nucleares Aplicadas à Saúde), Coimbra, Portugal. A 8×8 ACRORAD CdTe prototype was tested under a proton beam up to 15 MeV energy and a flux equivalent to a typical orbital proton flux (~ 0.5 protons/cm².s.MeV). The activation gamma-rays emitted by isotopes decays induced by the proton beam were measured and registered its time evolution. Radiation damage effects were analysed through energy resolution deterioration as proton dose was increased. Obtained results will be analysed and discussed in the context of radiation environment inflight conditions and space telescopes’ gamma-ray detection planes optimization.