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La conoscenza della distribuzione energetica e del rateo di fluenza dei fasci X è essenziale nei controlli di qualità in medicina diagnostica, sia in termini dosimetrici che della qualità delle immagini. Gli spettri energetici possono essere utilizzati per stime accurate delle tensioni dei tubi (kVp), per la correzione di distorsioni dovute al beam-hardening e per la corretta implementazione delle nuove tecniche dual-energy [1]. In mammografia, gli spettri energetici possono essere usati per stimare l’esposizione, il kerma in aria e la distribuzione energetica della dose assorbita nei tessuti, superando gli inconvenienti dovuti alla dipendenza energetica della risposta dei dosimetri (a stato solido, camere a ionizzazione), tipicamente utilizzati in ambiente clinico. La calibrazione dei dosimetri, che generalmente comporta procedure dispendiose in termini di tempo e di complessità, diventa critica nei controlli di routine in ambiente clinico. Tipicamente, la stima degli gli spettri diagnostici viene effettuata attraverso simulazioni numeriche basate su modelli semi-empirici [2] e metodi Monte Carlo [3]. Nei controlli di qualità di routine, informazioni incomplete su alcuni parametri caratteristici dei tubi X (angolo dell’anodo, filtri, tensione effettiva, etc.), potrebbe compromettere l’accuratezza e la precisione negli spettri. La misura diretta degli spettri rappresenta sicuramente il migliore approccio, attualmente limitato dalla complessità della procedura e dai flussi elevati dei fasci X clinici (ratei di fluenza > 106 fotoni/mm2 s). A causa dei flussi elevati dei fasci X diagnostici, la misura diretta degli spettri energetici ne rappresenta attualmente una grande sfida: i flussi elevati rendono significative le distorsioni dovute ai tempi morti e al pile-up sia nel conteggio che nella misura dell’energia. In tale contesto, saranno presentate le prestazioni di un sistema portatile innovativo per la misura diretta di energia e rateo di fluenza di fasci X diagnostici, anche in condizioni di flusso elevato. Il sistema, sviluppato presso il Dipartimento di Fisica e Chimica dell’Università di Palermo, è basato su rivelatori a semiconduttore di nuova generazione (CdTe) ed elettronica digitale innovativa basata su tecniche di digital pulse processing (DPP). Saranno presentate le prestazioni spettroscopiche del sistema con sorgenti monoenergetiche e tubi X di laboratorio. I risultati della misura diretta di spettri X diagnostici ed il confronto con altri sistemi dosimetrici (dosimetri a stato solido, camere a ionizzazione) saranno inoltre mostrati.

Digital pulse processing (DPP) techniques are increasingly used in the development of modern spectroscopic systems. DPP systems, based on direct digitizing and processing of detector signals (preamplifier output signals), ensure higher flexibility, stability, lower dead time, higher throughput and better energy resolution than traditional pulse processing systems. In this work, we present our progress in the development of DPP systems for high-rate high-resolution radiation measurements. An innovative digital system, able to perform multi-parameter analysis (input counting rate, pulse height, pulse shape, event arrival time, etc.) even at high photon counting rates is presented. Experimental results with CdTe detectors (planar and pixel detectors) are presented, at both low and high counting rate environments (up to 2 Mcps).

New generation spectroscopy systems have advanced towards digital pulse processing (DPP) approaches. DPP systems, based on direct digitizing and processing of detector signals, have recently been favoured over analog pulse processing electronics, ensuring better 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 shape and height analysis. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors, coupled to resistive feedback preamplifiers, will be presented, both at low and high rate environments (up to 1.1 Mcps). The proposed DPP system is a very attractive tool for both laboratory research and for the development of advanced detection systems for high-rate-resolution spectroscopic imaging, recently proposed in diagnostic medicine, industrial imaging and security screening.

Recently, Al/CdTe/Pt detectors have been proposed for the development of high resolution X-ray spectrometers. Due to the low leakage currents, these detectors allow high electric fields and the pixellization of anodes with the possibility to realize single charge carrier sensing detectors. In this work, we report on the results of electrical and spectroscopic investigations on CdTe diode detectors with Al/CdTe/Pt electrode configuration (4.1×4.1×0.75 and 4.1×4.1×2 mm3). The detectors are characterized by very low leakage currents in the reverse bias operation: 0.3 nA at 25 °C and 2.4 pA at -25 °C under a bias voltage of -1000 V. The spectroscopic performance of the detectors at both low and high photon counting rates were also investigated with a focus on the minimization of time instability, generally termed as polarization, looking for the optimum bias voltage and temperature. Good time stability, during a long-term operation of 10 h, was observed for both detectors at -25 °C and by using an electric field of 5000 V/cm. The 2 mm thick detector exhibited good energy resolution of 6.1%, 2.5% and 2.0% (FWHM) at 22.1 keV, 59.5 and 122.1 keV, respectively. Performance enhancements were obtained by using digital pulse processing techniques, especially at high photon counting rates (300 kcps). The 2 mm thick detector, after a digital pulse shape correction (PSC), is characterized by similar performance to the thin detector ones, opening up to the use of thick CdTe detectors without excessive performance degradations. This work was carried out in the framework of the development of portable X-ray spectrometers for both laboratory research and medical applications.

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 and lower dead time. We present our research activities on the development of X-ray and gamma ray spectrometers based on semiconductor detectors and DPP systems. We developed off-line and real-time DPP systems able to perform precise height and shape analysis of detector pulses. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors highlight the excellent performance of the systems both at low and high rate environments (up to 800 kcps).

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors for x-ray detectors have experienced a rather rapid development in the last few years. Among the traditional x-ray detectors based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show higher detection efficiency at high energies and good room temperature performance and are well suited for the development of compact detection systems and pixel arrays for simultaneous measurements of photon interaction position and energy. This chapter is an introduction to the physics and the technology of CdTe and CdZnTe pixel detectors for x-ray spectroscopy and imaging. The physical properties of CdTe and CdZnTe and the device fabrication technology are presented. A detailed discussion on physical processes and signal formation in CdTe and CdZnTe pixel detectors follows the introduction. We describe the electrical, the spectroscopic and the spatial properties of different CdTe/CdZnTe pixel detector configurations. Finally, we present the performance and the test results of recently developed pixel detector prototypes designed for astrophysical applications.

Purpose:Direct measurement of mammographic x-ray spectra under clinical conditions is a difficult task due to the high fluence rate of the x-ray beams as well as the limits in the development of high resolution detection systems in a high counting rate environment. In this work we present a detection system, based on a CdTe detector and an innovative digital pulse processing (DPP) system, for high-rate x-ray spectroscopy in mammography. Methods: The DPP system performs a digital pile-up inspection and a digital pulse height analysis of the detector signals, digitized through a 14-bit, 100 MHz digitizer, for x-ray spectroscopy even at high photon counting rates. We investigated on the response of the digital detection system both at low (150 cps) and at high photon counting rates (up to 500 kcps) by using monoenergetic x-ray sources and a nonclinical molybdenum anode x-ray tube. Clinical molybdenum x-ray spectrum measurements were also performed by using a pinhole collimator and a custom alignment device. Results: The detection system shows excellent performance up to 512 kcps with an energy resolution of 4.08% FWHM at 22.1 keV. Despite the high photon counting rate (up to 453 kcps), the molybdenum x-ray spectra, measured under clinical conditions, are characterized by a low number of pile-up events. The agreement between the attenuation curves and the half value layer values, obtained from the measured spectra, simulated spectra, and from the exposure values directly measured with an ionization chamber, also shows the accuracy of the measurements. Conclusions: These results make the proposed detection system a very attractive tool for both laboratory research and advanced quality controls in mammography.

We show the results of some numerical simulations trying to reproduce the QPO behavior in black hole and neutron star sources. Our simulations are based on the idea that a nearly periodic luminosity oscillation can be obtained from a perturbation of the source gravitational field with a sinusoidal time behavior. We find that some specific features of the QPO phenomenon can be described by this simple model, but the required amplitude of the gravity perturbation is more than 1% of the unperturbed gravitational field. If the hypothesis is formed that such a perturbation is due to a density fluctuation (of the accretion disk or the source itself, in the case of the neutron star) going around the source gravity center, a minimal perturbation amplitude of the 1% appears too large to be considered physically realistic in the known framework of compact sources. Our conclusion is therefore that this model needs further theoretical study and probably additional ‘ingredients’ and hypothesis.