Detectors and Instrumentation
Research focus
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The research topic aims at studying and developing new detectors and instrumentation for radiation measurement. In the period 2003-2006 the research was focused to the design and characterization of innovative semiconductor detectors and of low noise readout electronics as well as the development of complete detection systems for imaging and spectroscopy of x- and gamma-rays, ionizing particles, neutrons. Applications fields are in synchrotron experiments, medical imaging, astrophysics, material analysis, microdosimetry. One major activity is the development of the controlled drift detector, a new silicon imager invented by this group (UE and US patents, 1997) which combines excellent position and energy resolution of x-rays with fast readout. The research focussed on the optimization of the detector technological process and characterization of the ultimate performances and limitations of different layout architectures and operating modes. Main investigated issues are the extension to larger active area (3 cm sq.) as well as pixel scaling down to 50 ?m, charge confining mechanisms, on-chip JFETs for signal amplification and reset, self timing. The research is carried out in the frame of a strong collaboration lasting more than 15 years with Brookhaven National Laboratory, Instrumentation Division (USA), Max Planck Institute Semiconductor Laboratory in Munich (D) as well as with the Dept. of Electronics, Politecnico di Milano. A link with Sincrotrone ELETTRA (Trieste) has been established to qualify these detectors for high resolution x-ray imaging and spectroscopy applications [F1]. A second detector development was focussed on silicon detectors for microdosimetry and neutron spectrometry [F2]. The detector is a monolithic silicon telescope consisting of a DE and an E stage fabricated on a single silicon wafer. The DE stage (thickness 1.9 mm) is suitable for silicon microdosimetry (mainly for hadrontherapy) when coupled to a tissue-equivalent converter. The main issues of silicon microdosimetry have been faced: the minimisation of field-funnelling, the corrections for the tissue non-equivalence of silicon, the minimisation and control of the sensitive volume and the minimisation of the read out noise.A pixelated device constituted by a matrix of cylindrical DE stages (from 10 up to 500 ?m in diameter) was designed (and fabricated by STMicroelectronics) in the the framework of the research project: “Nuovo rivelatore di particelle nucleari telescopico monolitico e sue applicazioni”, MURST, FAR 14, starting Apr 2003 and ending Apr 2007. The same telescope can be applied as a neutron spectrometer when coupled to a polyethylene converter. The DE-E scatter plot acquired with coincidence techniques allows separating neutrons from the secondary electron component of the radiation field which otherwise overwhelms the recoil proton distribution. The present detection system can measure neutron spectra in the Research Assessment Exercise (2003 – 2006) -- Dipartimento di Ingegneria Nucleare - CeSNEF (DIN) 48 energy range 0.3 MeV – 8 MeV and reduction of the minimum detectable energy is under investigation. The activity on readout electronics was centered on the development of a low noise VLSI preamplifier with innovative architecture which matches the requirements of low capacitance detectors (less than 0.1 pF), wide range of shaping times, enhanced slew-rate requirements. The research activity also dealt with electronic devices on high resistivity silicon substrate for X-ray spectroscopy systems as well as new filtering techniques – both in analog and digital domain - to precisely measure the amplitude and/or the time-of-arrival of detected pulses and to the development of multi-channel digital acquisition systems. The development of an innovative Compton scatter detector for SPECT imaging (Single Photon Emission Computed Tomography) was the goal of an international project [F4]. The proposed scatter detector – i.e. the core of the Compton telescope system - is based on a specially designed multi-linear silicon drift detector in which a precise event trigger (few ns) is extracted from the current induced on the field electrodes by the initial separation of e-h pairs. High resolution images of the ionization tracks and the ionization profile of Compton electrons in a single silicon layer were experimentally recorded for the first time. A second project for nuclear medicine applications [F3] was focussed on the development of an innovative, solid state detector for scintimammography. Small silicon photodiodes coupled with CsI scinitillators assembled in a pixellated matrix were proposed for ? -ray detection for 99mTc radioisotope. The modular scintigraphic head was equipped with readout chips for analog signal processing, conversion and data transfer to a dedicated acquisition unit. In the last months of 2006 the group has been involved in the challenging detector development for the new European X-ray Free Electron Laser (XFEL). The XFEL machine will produce very short (1 fs) and intense X-ray pulses (103-105 photons per pulse) in the 6-15 keV energy range with repetition rate of 5 MHz which demand significant innovation as no detector technology is presently adequate. The detector proposed by our group, based on a monolithic 512x512 detector module with frame readout shorter than 1 ?s and on suitable on-chip electronics, was selected for the submission of a full proposal. We have the full responsibility of the detector WP and this will become the most important activity for the next 4 years.
Dipartimento di afferenza
Docenti afferenti
Full Professors
Stefano Agosteo
Andrea Castoldi
Associate Professors
Alberto Fazzi
Vincenzo Varoli
Assistant Professors
Andrea Pola