G. Gerardi

High-rate x-ray spectroscopy in mammography with a CdTe detector: A digital pulse processing approach

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.

Time-dependent current-voltage characteristics of Al/p-CdTe/Pt x-ray detectors

Current-voltage (I-V) characteristics of Schottky Al/p-CdTe/Pt detectors were investigated in dark and at different temperatures. CdTe detectors with Al rectifying contacts, very appealing for high resolution x-ray and gamma ray spectroscopy, suffer from bias-induced polarization phenomena which cause current increasing with the time and severe worsening of the spectroscopic performance. In this work, we studied the time-dependence of the I-V characteristics of the detectors, both in reverse and forward bias, taking into account the polarization effects. The I-V measurements, performed at different time intervals between the application of the bias voltage and the measurement of the current, and the measured current transients show as the electrical instability manifests itself even in time intervals shorter (i.e., <1 s) than those are necessary to make evident the degradation of the spectroscopic properties. The results point out as this time interval is a critical parameter for correct investigations on...

Hard x-ray response of pixellated CdZnTe detectors

In recent years, the development of cadmium zinc telluride (CdZnTe) detectors for x-ray and gamma ray spectrometry has grown rapidly. The good room temperature performance and the high spatial resolution of pixellated CdZnTe detectors make them very attractive in space-borne x-ray astronomy, mainly as focal plane detectors for the new generation of hard x-ray focusing telescopes. In this work, we investigated on the spectroscopic performance of two pixellated CdZnTe detectors coupled with a custom low noise and low power readout application specific integrated circuit (ASIC). The detectors (10×10×1 and 10×10×2 mm3 single crystals) have an anode layout based on an array of 256 pixels with a geometric pitch of 0.5 mm. The ASIC, fabricated in 0.8 μm BiCMOS technology, is equipped with eight independent channels (preamplifier and shaper) and characterized by low power consumption (0.5 mW/channel) and low noise (150–500 electrons rms). The spectroscopic results point out the good energy resolution of both dete...

Direct Measurement of Mammographic X-Ray Spectra with a Digital CdTe Detection System

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 (1–30 keV). The DPP system performs a height and shape analysis of the detector pulses, sampled and digitized by a 14-bit, 100 MHz ADC. We show the results of the characterization of the detection system both at low and high photon counting rates by using monoenergetic X-ray sources and a nonclinical X-ray tube. The detection system exhibits excellent performance up to 830 kcps with an energy resolution of 4.5% FWHM at 22.1 keV. Direct measurements of clinical molybdenum X-ray spectra were carried out by using a pinhole collimator and a custom alignment device. A comparison with the attenuation curves and the half value layer values, obtained from the measured and simulated spectra, from an ionization chamber and from a solid state dosimeter, also shows the accuracy of the measurements. These results make the proposed detection system a very attractive tool for both laboratory research, calibration of dosimeters and advanced quality controls in mammography.

The PAPIA Image Analysis System

A pyramidally structured multiprocessor architecture for image processing is presented together with its different operating modes (SIMD and multi-SIMD). The main problems addressed in this paper are: the image input-output system via an active memory, the global and pyramid control unit, the programming environment and, naturally, the present state of the project.

High-rate dead-time corrections in a general purpose digital pulse processing system

The abilities on dead-time correction of a real-time digital pulse processing (DPP) system for high-rate high-resolution radiation measurements are presented. The DPP system, through a fast and slow analysis of the output waveform from radiation detectors, is able to perform an accurate estimation of the true input counting rate (ICR), a fine pulse height (energy) and shape (peaking time) analysis even at high ICRs.

Observation of the Crab pulsar, PSR 0531 + 21, at 0. 2-6. 0 MeV with the Figaro II experiment

Observations of the Crab pulsar (PSR 0531 + 21) in the energy range 0.2-6 MeV have been made with the balloon-borne Figaro II experiment. The gross appearance of the light curve is characterized by the well-known double-peak structure with a phase separation of 0.4. The data confirm that the second pulse is the dominant feature, and the interpulse region is more luminous than in other energy ranges. Above about 0.5 MeV, an extra structure was detected in the light curve, with two features at a phase lag of 0.4 as the main one. Phase-resolved spectroscopy indicates that the first peak spectrum is concave upward, and the second one is concave downward, while the averaged spectrum is well represented by a single power law with slope 2.2. The good timing accuracy of Figaro made it possible to measure a delay between gamma rays and radio waves of 300 + or - 70 microsec, that if produced by dispersion, implies a value of DM fitting very well the radio measurements 2 months before and after the observation. 29 refs.

Detection of a feature at 0. 44 MeV in the Crab pulsar spectrum with FIGARO II - A redshifted positron annihilation line

The balloon-borne experiment FIGARO II was launched from the base of Trapani-Milo on July 9, 1990 at 0433 UT and observed the Crab pulsar for about 5 hr. The light curve of the signal from PSR 0531 + 21 folded with the radio period shows clearly the known double-peak structure. The spectrum of the second peak, the dominant structure in the hard X-rays and low-energy gamma rays, is characterized by a feature which, if interpreted as an emission line, gives the intensity of (0.86 {plus minus} 0.33) {times} 10 to the {minus}4th photons/sq cm per sec at the energy of 0.44 {plus minus} 0.01 MeV at a confidence level of 99.6 percent. If this feature is the signature of positron annihilation close to the neutron star surface, redshifted by the intense gravitational field, it is possible to estimate the mass-to-radius ratio of the Crab neutron star and the positron production rate from one of the polar caps. The former is equal to 0.087 solar masses/km and the latter 8.2 {times} 10 to the 39th e(+)/s for a pencil-beam pattern, in reasonable agreement with some heuristic estimates based on the number of energetic particles radiating the optical pulses. 16 refs.

Energy resolution and throughput of a new real time digital pulse processing system for x-ray and gamma ray semiconductor detectors

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 higher flexibility, stability, lower dead time, higher throughput 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 shape and height analysis. X-ray and gamma ray spectra measurements with cadmium telluride (CdTe) and germanium (Ge) detectors, coupled to resistive-feedback preamplifiers, highlight the excellent performance of the system both at low and high rate environments (up to 800 kcps). A comparison with a conventional analog electronics showed the better high-rate capabilities of the digital approach, in terms of energy resolution and throughput. These results make the proposed DPP system 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.

M-VIF: A machine-vision based on information fusion

The authors describe a new architecture for machine vision, which is based on information fusion approach. Its general design has been developed by using a formal computation model that integrates three main ingredients of the visual computation: the data, the models, and the algorithms. The hardware design and the software environment of M-VIF are also given. The simulation of M-VIF is under development on the HERMIA-machine.