Radiation detectors based on Cadmium Zinc Telluride (CZT) compounds are becoming popular solutions thanks to their high detection efficiency, room temperature operation, and to their reliability in compact detection systems for medical, astrophysical, or industrial applications. However, despite a huge effort to improve the technological process, CZT detectors’ full potential has not been completely exploited when both high spatial and energy resolution are required by the application, especially at low energies (<10 keV), limiting their application in energy-resolved photon counting (ERPC) systems. This gap can also be attributed to the lack of dedicated front-end electronics which can bring out the best in terms of detector spectroscopic performances. In this work, we present the latest results achieved in terms of energy resolution using SIRIO, a fast low-noise charge sensitive amplifier, and a linear-array pixel detector, based on boron oxide encapsulated vertical Bridgman-grown B-VB CZT crystals. The detector features a 0.25-mm pitch, a 1-mm thickness and is operated at a −700-V bias voltage. An equivalent noise charge of 39.2 el. r.m.s. (corresponding to 412 eV FWHM) was measured on the test pulser at 32 ns peaking time, leading to a raw resolution of 1.3% (782 eV FWHM) on the 59 keV line at room temperature (+20 °C) using an uncollimated ²⁴¹Am, largely improving the current state of the art for CZT-based detection systems at such short peaking times, and achieving an optimum resolution of 0.97% (576 eV FWHM) at 1 µs peaking time. The measured energy resolution at the 122 keV line and with 1 µs peaking time of a ⁵⁷Co raw uncollimated spectrum is 0.96% (1.17 keV). These activities are in the framework of an Italian collaboration on the development of energy-resolved X-ray scanners for material recycling, medical applications, and non-destructive testing in the food industry.

A study of the reset transient of charge-sensitive amplifiers operating with a pulsed switch in parallel to the feedback capacitance is presented. Analytical models have been developed for amplifiers in both linear and slew rate regimes during the reset phase. The models predict the time interval required to reset the amplifier that, in most cases, significantly differs by the simple discharge of the feedback capacitance through the switch resistance and strongly depends on the open-loop gain and bandwidth of the core amplifier and on the input capacitance. The models also quantitatively predict spurious voltage transients at the preamplifier's input and possible ringings at the output, as a function of the main general parameters of the circuit. A case application study of the presented model is proposed for low-capacitance detectors, such as semiconductor drift detectors or small pixel detectors.

The current noise power spectral density (NPSD) of a silicon low-gain avalanche diode (LGAD) has been measured at room temperature under different bias conditions. The device has been biased in its whole operating range up to close the junction breakdown with dark currents between 100 pA and 3 nA. The NPSDs have been measured in a bandwidth from 10 Hz up to a maximum frequency ranging from 10 kHz up to 3 MHz, depending on the noise level. The NPSDs have been found white in all the operative conditions with no evidence of a 1/ f component. An empirical law is proposed to describe the NPSD with high accuracy over four orders of magnitude from 10 ^{$-$ 28} to 10 ^{$-$ 24} A ² /Hz. The excess noise factor $\zeta$ has been experimentally determined from the NPSD and accurate measurements of the multiplication gain M . A good agreement with McIntyre’s theory has been found only at higher gain values, but for M $<$ 30, the excess noise has been found to be accurately predicted by the M ⁿ law with n $=$ 0.632 $\pm$ 0.014. An extremely low value of $\zeta$ $=$ 1.06 $\pm$ 0.01 has been measured at M $=$ 5, opening perspectives to the development of ultralow-noise radiation and particle detecting systems for both timing and spectroscopy applications.

A wideband low-noise transimpedance amplifier (TIA) is designed for the measurement of the noise power spectral density (NPSD) of the current of low-gain avalanche diodes (LGADs). The design focuses on minimization of system background noise and maximization of operating bandwidth. The diffusive line effect of the high value resistor used in the feedback network has been analyzed, and a compensation of its effect has been implemented. With a transimpedance of 200 MΩ, a background white noise of 9.5 fA/√(Hz) and an input current dynamic range up to 20 nA are achieved. The useful bandwidth ranges from 1 kHz up to 3 MHz depending on the noise level to be measured. An effective method for calibrating the system and measuring its transfer function with high accuracy over the full bandwidth is described. The designed TIA enables an accurate characterization of LGADs noise, which is a crucial step for the technology qualification and for the prediction of the performance of these devices as radiation and particle detectors.

A Monte Carlo simulator is presented to evaluate the spectroscopic response of High-Z semiconductor pixel radiation detectors starting from first principles physical laws. It performs simulation in three different domains: electrostatic fields, photonmatter interaction, and photo-generated charge-carriers transport. A sampling algorithm is proposed to address the issue of fast and accurate computations of charge cloud dynamics in presence of both diffusion and electrostatic repulsion effects, avoiding direct numerical integration of the induced current equation stated by the Shockley-Ramo theorem under a constant electric field hypothesis. The simulator is written as an Object-Oriented Programming (OOP) source code repository, relying on the integration between MATLAB® and COMSOL® Multiphysics, and can be run with one executable script. The simulator architecture is presented, followed by a detailed explanation of all the physical models and implemented simulation strategies. The simulator is validated with experimental energy spectra acquired with a radiation detection system based on CdZnTe pixel detectors and ultra low-noise front-end electronics with state-of-the-art energy resolution.