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L- α -alanine has aroused considerable interest for use in radiation EPR dosimetry and has been formally accepted as a secondary standard for high-dose (kGy) and transfer dosimetry of high-energy photons and electrons. In this work, we extended the investigation of the energy response of alanine EPR films in the low energy range for X-photons (1–30 keV). Electron Paramagnetic Resonance (EPR) measurements were performed on Kodak BioMax alanine films exposed to low-energy X-rays from a Cu-, W- and Mo-targets tube operating at voltages up to 30 kV. Films were chosen because of the low penetration of the soft X-rays used. The response of alanine to low-energy X-rays was characterized experimentally and the relative response (with respect to high energy photons) was found to be between 0.8 and 0.9 for Cu- and W-tube X-rays, and 1.0 for Mo-tube X-rays. The attenuation profiles were investigated and it was found that 1 mm of film material reduces the intensity of the X-ray-beam by about 70%, 50% and 40% for Cu-, W- and Mo-tube X-rays, respectively. Monte Carlo simulations were performed to model the energy release as well as the depth dose profiles for the various radiation beams used. These data are considered relevant for dosimetric applications in low energy beams such the high-gradient treatment fields used in monoenergetic microbeam radiation therapy (MRT) with synchrotron radiation as well as in brachytherapy with low energy sources, for instance 169Yb.

Al/CdTe/Pt detectors are very attractive devices for high-resolution X-ray spectroscopy, even though they suffer from polarization phenomena, which cause a progressive time degradation of the spectroscopic performance. In this work we investigated on the time dependence of the electric field of an Al/CdTe/Pt detector under optical perturbation by means of Pockels effect measurements. A tunable laser with wavelengths ranging within 700−1000 nm and a 940 nm light emitting diode (LED) were used. The measurements of both the electric field profile and the total current were used to better understand the effects of the optical perturbation on polarization phenomena. The results point out as the above band-gap light, due to the trapping of photo-generated holes at the anode (the Al/CdTe blocking contact), brings to a reduction of the negative space charge caused by the bias voltage (bias induced polarization) and the LED irradiation (radiation induced polarization). The reduction of the negative space charge ensures a quite stable and uniform electric field distribution, typically termed depolarization. Conversely, optical perturbation with sub-band-gap light enhances the polarization with the formation of two oppositely charged regions within the detector.

Nowadays, CdZnTe (CZT) is one of the key materials for the development of room temperature X-ray and gamma ray detectors and great efforts have been made on both the device and the crystal growth technologies. In this work, we present the results of spectroscopic investigations on new boron oxide encapsulated vertical Bridgman (B-VB) grown CZT detectors, recently developed at IMEM-CNR Parma, Italy. Several detectors, with the same electrode layout (gold electroless contacts) and different thicknesses (1 and 2.5 mm), were realized: the cathode is a planar electrode covering the detector surface (4.1×4.1 mm2), while the anode is a central electrode (2×2 mm2) surrounded by a guard-ring electrode. The detectors are characterized by electron mobility-lifetime product (µeτe) values ranging between 0.6 and 1·10−3 cm2/V and by low leakage currents at room temperature and at high bias voltages (38 nA/cm2 at 10000 V/cm). The spectroscopic response of the detectors to monochromatic X-ray and gamma ray sources (109Cd, 241Am and 57Co), at different temperatures and fluxes (up to 1 Mcps), was measured taking into account the mitigation of the effects of incomplete charge collection, pile-up and high flux radiation induced polarization phenomena. A custom-designed digital readout electronics, developed at DiFC of University of Palermo (Italy), able to perform a fine pulse shape and height analysis even at high fluxes, was used. At low rates (200 cps) and at room temperature (T=25 °C), the detectors exhibit an energy resolution FWHM around 4% at 59.5 keV, for comparison an energy resolution of 3% was measured with Al/CdTe/Pt detectors by using the same electronics (A250F/NF charge sensitive preamplifier, Amptek, USA; nominal ENC of 100 electrons RMS). At high rates (750 kcps), energy resolution values of 7% and 9% were measured, with throughputs of 2% and 60% respectively. No radiation polarization phenomena were observed at room temperature up to 1 Mcps (241Am source, 60 keV), while a detector collapse characterizes the samples at T Comparison with two traveling heater method (THM) grown CZT detectors (REDLEN, Canada), fabricated with the same electrode layout, is also presented. 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.

In this work we report on the results of electrical characterization of new CdZnTe detectors grown by the Boron oxide encapsulated Vertical Bridgman technique (B-VB), currently produced at IMEM-CNR (Parma, Italy). The detectors, with gold electroless contacts, have different thicknesses (1 and 2.5 mm) and the same electrode layout, characterized by a central anode surrounded by a guard-ring electrode. Investigations on the charge transport mechanisms and the electrical contact properties, through the modeling of the measured current–voltage ( I – V ) curves, were performed. Generally, the detectors are characterized by low leakage currents at high bias voltages even at room temperature: 34 nA/cm 2 ( T =25 °C) at 10,000 V/cm, making them very attractive for high flux X-ray measurements, where high bias voltage operation is required. The Au/CdZnTe barrier heights of the devices were estimated by using the interfacial layer-thermionic-diffusion (ITD) model in the reverse bias voltage range. Comparisons with CdZnTe detectors, grown by Traveling Heater Method (THM) and characterized by the same electrode layout, deposition technique and resistivity, were also performed.

Al/CdTe/Pt detectors are very attractive devices for high-resolution X-ray spectroscopy, even though they suffer from bias-induced time instability (polarization). Polarization phenomena cause a progressive time-degradation of the spectroscopic performance of the detectors, due to hole trapping and detrapping from deep acceptor levels that directly control the electric field distribution. In this work we present experimental investigations on the electric field profile of planar Al/CdTe/Pt detectors by means of Pockels effect measurements. The time/temperature dependence of the electric field was investigated in a long time window (up to 10 h) and the correlation with the reverse current transients was also studied. Two energy levels (0.62 eV and 1.16 eV) of the deep hole traps were measured, in agreement with our previous results obtained through electrical and spectroscopic approaches.

Photon counting detectors with energy resolving capabilities are desired for high flux X-ray imaging. In this work, we present the performance of a pixelated Schottky Al/p-CdTe/Pt detector (4×4) coupled to a custom-designed digital readout electronics for high flux measurements. The detector (4×4×2 mm 3 ) has an anode layout based on an array of 16 pixels with a geometric pitch of 1 mm (pixel size of 0.6 mm). The 4-channel readout electronics is able to continuously digitize and process the signals from each pixel, performing multi-parameter analysis (event arrival time, pulse shape, pulse height, pulse time width, etc.) even at high fluxes and at different throughput and energy resolution conditions. The spectroscopic response of the system to monochromatic X-ray sources, at both low and high rates, is presented with particular attention to the mitigation of some typical spectral distortions (pile-up, baseline shifts and charge sharing). At a photon counting rate of 520 kcps/pixel, the system exhibits an energy resolution (FWHM at 59.5 keV) of 4.6%, 7.1% and 9% at throughputs of 0.9%, 16% and 82%, respectively. Measurements of Ag-target X-ray spectra also show the ability of the system to perform accurate estimation of the input counting rate up to 1.1 Mcps/pixel. The aim of this work is to point out, beside the appealing properties of CdTe detectors, the benefits of the digital approach in the development of high-performance energy resolved photon counting (ERPC) systems for high flux X-ray imaging.

Digital pulse processing (DPP) systems, based on direct digitizing and processing of detector signals, have recently been favoured over analog electronics, ensuring higher flexibility, stability, lower dead time and better spectroscopic performance. In this work, we present the performance of a new real time DPP system for X-ray and gamma ray semiconductor detectors. The system is based on a commercial digitizer equipped with a custom DPP firmware, developed by our group, for on-line pulse height and shape analysis. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors highlight the excellent performance of the system both at low and high rate environments (up to 800 kcps). These results make the proposed DPP system a very attractive tool for both laboratory research and the development of advanced detection systems for high-rate-resolution spectroscopic imaging, recently proposed in diagnostic medicine, industrial imaging and security screening.

Over the last decades, CdTe detectors are widely used for the development of room temperature X-ray and gamma ray spectrometers. Typically, high resolution CdTe detectors are fabricated with blocking contacts (indium, aluminum) ensuring low leakage currents and high electric field for optimum charge collection. As well known, time instability under bias voltage (termed as polarization) is the major drawback of CdTe diode detectors. Polarization phenomena cause a progressive degradation of the spectroscopic performance with time, due to hole trapping and detrapping from deep acceptors levels. In this work, we studied the polarization phenomenon on new Al/p-CdTe/Pt detectors, manufactured by Acrorad (Japan), through electrical and spectroscopic approaches. In particular, we investigated on the time degradation of the spectroscopic response of the detectors at different temperatures, voltages and energies. Current transient measurements were also performed to better understand the properties of the deep acceptor levels and their correlation with the polarization effect.

The high performances of CdTe detectors for X-ray and gamma ray spectroscopy are already well known. Among the traditional semiconductor spectrometers, CdTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact detection systems. In this work, we investigated the performance of a CdTe detector coupled with a custom digital pulse processing (DPP) system for X-ray spectroscopy. The DPP method, implemented on a PC platform, performs a pile-up inspection and a pulse height analysis of the preamplifier output pulses, digitized by a 14-bit, 100 MHz ADC. The spectroscopic results point out the excellent performance of the digital spectrometer both at low (150 cps) and at high photon counting rate (up to 370 kcps), confirming the high potentialities of CdTe detectors coupled with DPP systems. The detector and the DPP system were developed by our collaboration as a spectrometer prototype for direct measurement of diagnostic X-ray spectra in the 1–40 keV energy range.

The spectroscopic performances of a CdZnTe detector (crystal size: 5×5×0.9 mm 3 ) with five electrodes (cathode, anode and three steering electrodes) were studied. The anode layout, which consists of a circular electrode ( φ =80 μm) surrounded by two ring electrodes (gap=100 μm; radial width Δ r =100 μm) and by one electrode that extends to the edge of the crystal, is mostly sensitive to the electron carriers, overcoming the well known effect of the hole trapping in the measured spectra. We report on the spectroscopic response of the detector at different bias voltages of the electrodes and at various photon energies ( 109 Cd, 241 Am and 57 Co sources). The CdZnTe detector exhibits excellent energy resolution (0.87 keV FWHM at 22.1 keV, 1.15 keV FWHM at 59.5 keV and 1.91 keV FWHM at 122.1 keV; working temperature −10 °C) and low tailing (FW.1M to FWHM ratio of 1.87 at 22.1 keV, 1.93 at 59.5 keV and 2.01 at 122.1 keV). This study stresses on the excellent spectroscopic properties and the stability of the CdZnTe detectors equipped with a custom anode layout. Furthermore, the obtained results make such detectors very attractive candidates as X-ray spectrometers mainly for medical applications.

Pile-up distortion is a major drawback in X-ray spectroscopy at high count rate. Pulse width narrowing with shaping techniques can lead to the reduction of the pile-up distortion, but a low shaping time reduces the noise filtration and leads to a poor energy resolution. Thus, only a best compromise solution between the pile-up and the noise requirements is achievable. The hardware manipulation needed to adjust the parameters of the traditional electronic shaping amplifiers makes it uneasy to tests various settings in different conditions. Digital techniques can help to overcome such difficulties. A digital signal processing and analysis system for X-ray spectroscopy is described in this paper. The system processes the output signal of a Charge Sensitive Preamplifier (CSPA) connected to an X-ray semiconductor detector. The output CSPA signal is registered with a high speed ADC (sampling rate up to 100 MS/s, 14 bit resolution) and a fully digital shaping is performed off-line by a dedicated software instead of analog electronics. The software carries out the rest of data analysis (peak detection, pile-up recognition, rejection or correction) and finally calculates the photon count, constructs the spectrum and extracts other useful information. We processed the output signals of an Amptek ® XR-100 T CdTe detector used with mammographic X-ray beams. We developed the system with the LabVIEW ® platform. The software is characterized by a user friendly GUI which make it easier to change settings than traditional electronics.