62 research outputs found
High Resolution X-Ray Spectroscopy with Compound Semiconductor Detectors and Digital Pulse Processing Systems
The advent of semiconductor detectors has revolutionized the broad field of X-ray
spectroscopy. Semiconductor detectors, originally developed for particle physics, are now
widely used for X-ray spectroscopy in a large variety of fields, as X-ray fluorescence
analysis, X-ray astronomy and diagnostic medicine. The success of semiconductor detectors
is due to several unique properties that are not available with other types of detectors: the
excellent energy resolution, the high detection efficiency and the possibility of development
of compact detection systems. Among the semiconductors, silicon (Si) detectors are the key
detectors in the soft X-ray band (< 15 keV). Si-PIN diode detectors and silicon drift detectors
(SDDs), with moderate cooling by means of small Peltier cells, show excellent spectroscopic
performance and good detection efficiency below 15 keV. Germanium (Ge) detectors are
unsurpassed for high resolution spectroscopy in the hard X-ray energy band (>15 keV) and
will continue to be the choice for laboratory-based high performance spectrometers.
However, there has been a continuing desire for ambient temperature and compact
detectors with the portability and convenience of a scintillator but with a significant
improvement in resolution. To this end, numerous high-Z and wide band gap compound
semiconductors have been exploited. Among the compound semiconductors, cadmium
telluride (CdTe) and cadmium zinc telluride (CdZnTe) are very appealing for hard X-ray
detectors and are widely used for the development of spectrometer prototypes for medical
and astrophysical applications.
Beside the detector, the readout electronics also plays a key role in the development of high
resolution spectrometers. Recently, many research groups have been involved in the design
and development of high resolution spectrometers based on semiconductor detectors and
on digital pulse processing (DPP) techniques. Due to their lower dead time, higher stability
and flexibility, digital systems, based on directly digitizing and processing of detector
signals (preamplifier output signals), have recently been favored over analog electronics
ensuring high performance in both low and high counting rate environments.
In this chapter, we review the research activities of our group in the development of high
throughput and high resolution X-ray spectrometers based on compound semiconductor
detectors and DPP systems. First, we briefly describe the physical properties and the signal
formation in semiconductor detectors for X-ray spectroscopy. Second, we introduce the
main properties and critical issues of a X-ray detection system, highlighting the
characteristics of both analog and digital approaches. Finally, we report on the spectroscopic
performance of a high resolution spectrometer based on a CdTe detector and a custom DPP
system. As an application, direct measurements of mammographic X-ray spectra by using
the digital CdTe detection system are also presented
A digital approach for real time high-rate high-resolution radiation measurements
Modern spectrometers are currently developed by using digital pulse processing (DPP) systems, showing
several advantages over traditional analog electronics. The aim of this work is to present digital
strategies, in a time domain, for the development of real time high-rate high-resolution spectrometers.
We propose a digital method, based on the single delay line (SDL) shaping technique, able to perform
multi-parameter analysis with high performance even at high photon counting rates. A robust pulse
shape and height analysis (PSHA), applied on single isolated time windows of the detector output
waveforms, is presented.
The potentialities of the proposed strategy are highlighted through both theoretical and experimental
approaches. To strengthen our approach, the implementation of the method on a real-time
system together with some experimental results are presented. X-ray spectra measurements with a
semiconductor detector are performed both at low and high photon counting rates (up to 1.1 Mcps)
Accelerated Tests on Si and SiC Power Transistors with Thermal, Fast and Ultra-Fast Neutrons
Neutron test campaigns on silicon (Si) and silicon carbide (SiC) power MOSFETs and IGBTs
were conducted at the TRIGA (Training, Research, Isotopes, General Atomics) Mark II (Pavia, Italy)
nuclear reactor and ChipIr-ISIS Neutron and Muon Source (Didcot, U.K.) facility. About 2000 power
transistors made by STMicroelectronics were tested in all the experiments. Tests with thermal and fast
neutrons (up to about 10 MeV) at the TRIGA Mark II reactor showed that single-event burnout (SEB)
failures only occurred at voltages close to the rated drain-source voltage. Thermal neutrons did not
induce SEB, nor degradation in the electrical parameters of the devices. SEB failures during testing at
ChipIr with ultra-fast neutrons (1-800 MeV) were evaluated in terms of failure in time (FIT) versus
derating voltage curves according to the JEP151 procedure of the Joint Electron Device Engineering
Council (JEDEC). These curves, even if scaled with die size and avalanche voltage, were strongly
linked to the technological processes of the devices, although a common trend was observed that
highlighted commonalities among the failures of different types of MOSFETs. In both experiments,
we observed only SEB failures without single-event gate rupture (SEGR) during the tests. None of
the power devices that survived the neutron tests were degraded in their electrical performances.
A study of the worst-case bias condition (gate and/or drain) during irradiation was performed
Electrical Characterization of CdTe pixel detectors with Al Schottky anode
Pixelated Schottky Al/p-CdTe/Pt detectors are very attractive devices for high-resolution Xray
spectroscopic imaging, even though they suffer from bias-induced time instability (polarization). In
this work, we present the results of the electrical characterization of a (4x4) pixelated Schottky Al/p-
CdTe/Pt detector. Current-voltage (I-V) characteristics and current transients were investigated at
different temperatures. The results show as deep levels play a dominant role in the charge transport
mechanism. The conduction mechanism is dominated by the space charge limited current (SCLC) both
under forward bias and at high reverse bias. Schottky barrier height of the Al/CdTe contact was
estimated by using the thermionic-field emission model at low reverse bias voltages. Activation energy
of the deep levels was measured through the analysis of the reverse current transients at different
temperatures. Finally, we employed an analytical method to determine the density and the energy
distribution of the traps from SCLC current-voltage characteristics
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 resistivefeedback
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
Progress in the Development of CdTe and CdZnTe Semiconductor Radiation Detectors for Astrophysical and Medical Applications
Over the last decade, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors have attracted increasing interest as X-ray and gamma ray detectors. Among the traditional high performance spectrometers based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact and reliable detection systems. In this paper, we review the current status of research in the development of CdTe and CdZnTe detectors by a comprehensive survey on the material properties, the device characteristics, the different techniques for improving the overall detector performance and some major applications. Astrophysical and medical applications are discussed, pointing out the ongoing Italian research activities on the development of these detectors
Charge Transport Properties in CZT Detectors Grown by the Vertical Bridgman Technique
Great efforts are being presently devoted to the development of CdTe and CdZnTe detectors for a large variety of applications, such as medical, industrial, and space research. We present the spectroscopic properties of some CZT crystals grown by the standard vertical Bridgman method and by the boron oxide encapsulated vertical Bridgman method, which has been recently implemented at IMEM-CNR. By this technique the crystal is grown in an open quartz crucible fully encapsulated by a thin layer of liquid boron oxide. This technique prevent the crystal-crucible contact allowing larger single grains with lower dislocation density to be obtained. Several mono-electrode detectors were realized with two planar gold contacts. The samples are characterized by an active area of ≈4x4 mm2 or ≈7x7 mm2 and with thickness ranging from 1 to 2 mm. The charge transport properties of the detectors have been studied by mobility-lifetime (μτ) product measurements, carried out at the European Synchrotron Radiation Facility (Grenoble) in PTF configuration, where the impinging beam direction is orthogonal to the collecting electric field. We have performed several fine scans between the electrodes with a beam spot of 10x10 μm2 at different energies from 60 keV to 400 keV. In this work we present the test results in terms of μτ product of both charge carriers and an evaluation of the spectroscopic response uniformity across the sensitive volume of tested samples
Study of the spectral response of CZT multiple-electrode detectors
Cadmium zinc telluride (CZT) is a promising material for room temperature X-ray and gamma-ray detectors. The high atomic number and the wide band-gap give high quantum efficiency and good room temperature performances. Due to hole trapping, particular electrode structures have been developed to provide single-charge carrier collection (electrons), exploiting the excellent charge transport properties of the electrons. In this work, the spectroscopic performances of two CZT detectors (CZT1: 5 mm times 5 mm times 0.90 mm; CZT2: 4.8 mm times 5 mm times 0.55 mm) with five electrodes (cathode, anode and three steering electrodes) were studied. The anode-collecting electrode, surrounded by three steering electrodes (biased for optimum charge collection), is mostly sensitive to electron carriers, overcoming the effects of hole trapping in the measured spectra (hole tailing). We investigated on the spectroscopic response (241Am source; 59.5 keV) of the detectors at different bias voltages of the electrodes. The detectors exhibit excellent energy resolution (CZT1: 2.0% FWHM at 59.5 keV; CZT2: 1.7% FWHM at 59.5 keV; working temperature -10degC) and low tailing (CZT1: FW.1M to FWHM ratio of 1.93 at 59.5 keV; CZT2: 2.35 at 59.5 keV). This study stresses on the excellent spectroscopic properties of the CZT detectors equipped with a custom anode layout, making them very attractive candidates as x-ray spectrometers mainly for medical applications
Recent advances in the development of high-resolution 3D cadmium zinc telluride drift strip detectors
In the last two decades, great efforts have been made in the development of 3D cadmium-zinc-Telluride (CZT) detectors operating at room temperature for gamma-ray spectroscopic imaging. This work presents the spectroscopic performance of new high-resolution CZT drift strip detectors, recently developed at IMEM-CNR of Parma (Italy) in collaboration with due2lab (Italy). The detectors (19.4 mm × 19.4 mm × 6 mm) are organized into collecting anode strips (pitch of 1.6 mm) and drift strips (pitch of 0.4 mm) which are negatively biased to optimize electron charge collection. The cathode is divided into strips orthogonal to the anode strips with a pitch of 2 mm. Dedicated pulse processing analysis was performed on a wide range of collected and induced charge pulse shapes using custom 32-channel digital readout electronics. Excellent room-Temperature energy resolution (1.3% FWHM at 662 keV) was achieved using the detectors without any spectral corrections. Further improvements (0.8% FWHM at 662 keV) were also obtained through a novel correction technique based on the analysis of collected-induced charge pulses from anode and drift strips. These activities are in the framework of two Italian research projects on the development of spectroscopic gamma-ray imagers (10-1000 keV) for astrophysical and medical applications
- …