P. Russo

EVALUATION OF ELEMENTAL AND COMPOUND SEMICONDUCTORS FOR X-RAY DIGITAL RADIOGRAPHY

Abstract We present a comparative study of the performance of elemental and compound solid state crystals of possible use in X-raY digital radiography. The general purpose EGS4 code was used to simulate photon-electron transport in the energy range 20 to 60 keV. The efficiency and the energy resolution, as a function of X-ray energy, are calculated and correlated to the different physical characteristics of the crystals considered.

Preliminary test of an imaging probe for nuclear medicine using hybrid pixel detectors

Abstract We are investigating the feasibility of an intraoperative imaging probe for lymphoscintigraphy withTc-99m tracer, for sentinel node radioguided surgery, using the Medipix series of hybrid detectors coupled to a collimator. These detectors are pixelated semiconductor detectors bump-bonded to the Medipix1 photon counting read-out chip (64×64 pixel, 170xa0μm pitch) or to the Medipix2 chip (256×256 pixel, 55xa0μm pitch), developed by the European Medipix collaboration. The pixel detector we plan to use in the final version of the probe is a semi-insulating GaAs detector or a 1–2xa0mm thick CdZnTe detector. For the preliminary tests presented here, we used 300-μm thick silicon detectors, hybridized via bump-bonding to the Medipix1 chip. We used a tungsten parallel-hole collimator (7xa0mm thick, matrix array of 64×64 100xa0μm circular holes with 170xa0μm pitch), and a 22, 60 and 122xa0keV point-like (1xa0mm diameter) radioactive sources, placed at various distances from the detector. These tests were conducted in order to investigate the general feasibility of this imaging probe and its resolving power. Measurements show the high resolution but low efficiency performance of the detector–collimator set, which is able to image the 122xa0keV source with

Spectroscopic performance of semi-insulating GaAs detectors for digital radiography

Abstract We studied pixel radiation detectors for X-ray radiography based on semi-insulating GaAs: in particular, we investigated both annealed and non-annealed contact deposition techniques for the ohmic contact and both ring-guarded and non-guarded Schottky contact, in order to reduce the leakage current and to increase the maximum applied electric field. Spectroscopic characterization with a 60xa0keV 241xa0Am source has been performed. Among these different detectors, the CCE can reach 99±6%, while the energy resolution Δ E / E can go down to 4.1±0.2%.

Use of silicon and GaAs pixel detectors for digital autoradiography

Solid state detectors made of Si (4.8/spl times/8 mm/sup 2/) and GaAs (6.4/spl times/8 mm/sup 2/) pixel matrices bump-bonded to the Omega2 and Omega3 electronic read-out systems, developed at CERN for H.E.P. experiments, have been used to obtain autoradiographic images of clusters of human epithelial cells and DNA fragments separated via electrophoresis, both labeled with /sup 32/P. The system has shown a good minimum detectable activity per unit area of 2.10/sup -4/ cps mm/sup -2/, and has proved linear for a count rate in the range 0.2-20 cpa, typical of autoradiography. The pixel dimensions are 75/spl times/500 /spl mu/m/sup 2/ (Si-Omega2) and 50/spl times/500 /spl mu/m/sup 2/ (GaAs-Omega3), respectively. We are able to clearly localize clusters of cells which have incorporated the radioactive tracer and DNA fragments on an electrophoretic gel on paper (blots).

Digital autoradiography with a Medipix2 hybrid silicon pixel detector

A new digital imaging system for beta and gamma autoradiography has been realized. The technological advancement of this imaging system is represented by the second-generation Medipix2 chip (a single photon/particle counting chip), developed at CERN in the framework of the Medipix2 European collaboration, as a successor of the Medipix1 chip. Medipix2 is realized in 0.25 /spl mu/m CMOS technology; it has a sensitive area of 14/spl times/14 mm/sup 2/ and a pixel pitch of 55 /spl mu/m, it has been bump-bonded to a 300 /spl mu/m thick silicon pixel detector. A dedicated electronic interface and a complete image acquisition and processing software allow to operate this hybrid pixel detector for digital autoradiography with acquisition time as long as several days. In a preliminary test of this imaging system for digital autoradiography we obtained a detection threshold of 1600 electrons (equivalent to about 6 keV in Si), a noise of 10/sup -3/ cps/mm/sup 2/, a minimum detectable activity of 0.32 Bq in 14 h for /sup 3/H and 0.012 Bq in 10 h for /sup 14/C. A preliminary spatial resolution test for /sup 14/C gives a FWHM resolution of 30 /spl mu/m. Real time images of /sup 3/H, /sup 14/C, /sup 125/I autoradiographic microscales are also presented.

GaAs pixel radiation detector as an autoradiography tool for genetic studies

Abstract We present an autoradiography tool to be used mainly for genetic studies. It performs a quantitative analysis of radioactivity and can follow a dynamic process. We designed several applications, in particular one aimed at detecting hybridization of radio-labeled DNA fragments with known DNA-probes deposited on a micro-array. The technique is based on GaAs pixel array detector and low threshold, large dynamic range and good sensitivity integrated electronics developed for medical applications, suitable to detect markers (gamma or beta) such as 14C, 35S, 33P, 32P, 125I, even at very low activities. A Monte Carlo simulation of β− detection in GaAs is presented here in order to study the spatial resolution characteristics of such a system. For several biological applications, the electronics is required to perform at high temperatures (from 37° to 68°): we present here studies of noise and minimum threshold as a function of the temperature.

Gallium arsenide pixel detectors for medical imaging

Gallium arsenide pixel detectors processed on a 200 μm Semi-Insulating (SI) Hitachi substrate were bump-bonded to the Omega3 electronics developed at CERN for high energy physics [1]. The pixel dimensions are 50 μm × 500 μm for a total of 2048 cells and an active area of ∼0.5 cm2. Our aim is to use this system for medical imaging. We report the results obtained after irradiation of the detector with different X-ray sources on phantoms with different contrasts. The system showed good sensitivity to X-rays from 241Am (60 keV) and 109Cd (22.1 keV). It is also sensitive to β− particles from 90Sr as well as from 32P which is used as a tracer for autoradiography applications. The inherent high absorption efficiency of GaAs associated with the self-triggering capabilities of the pixel readout system reduced considerably the acquisition time compared with traditional systems based on silicon or emulsions. The present configuration is not optimised for X-ray imaging. The reduction of the pixel dimensions to 200 μm × 200 μm together with the integration of a counter in the pixel electronics would make the detector competitive for applications like mammography or dental radiology. For certain applications in biochemistry, such as DNA sequencing, where good spatial resolution is required only in one direction, the present setup should allow the best spatial resolution available up to now with respect to other digital autoradiographic systems. DNA sequencing tests are now under way.

AUTORADIOGRAPHY WITH SILICON STRIP DETECTORS

A digital autoradiography system based on double sided silicon strip detectors (1.6 × 1.6 mm2 active surface with 100 μm pitch) has been developed and successfully tested with beta-emitting tracers. It is shown here that the system is able to perform imaging of organic material with specific sensitivity as small as 0.002 nCi/mm2, and to record activity measurements with good linearity in the range 0.002–20 nCi/mm2. Autoradiographic images of clusters of mammary cells marked with ortho-(32P)phosphate, obtained with an exposure time of about 10 min are presented.

First X-ray images with a double-sided microstrips silicon crystal. A novel detector for digital radiography?

In the framework of an experimental program to study the possibility of using a silicon crystal for X-ray detection in the diagnostic energy range (10-100 keV), the authors present the first experimental results obtained with a silicon detector (300 mu m thick, 1.4*1.4 cm2 surface) with microstrips deposited on each side: 25 mu m spacing on the junction side and 50 mu m spacing on the ohmic side: 25 mu m spacing on the junction side and 50 mu m spacing on the ohmic side; the read-out pitch is 100 mu m for both sides. Energy and spatial resolutions have been measured by means of collimated radioactive sources: 241Am (E gamma =60 keV) and 109Cd (E gamma =24 and 88 keV). A very good linearity in the response to the photon energy has been measured. Tantalum wires, i.e. high contrast objects, have been used as phantoms when exposing the detector to the 60 keV photons; the authors present the images thus obtained.

GaAs pixel arrays for β imaging in medicine and biology

Abstract We present an autoradiography system (BETAview) for a quantitative analysis of radioactive markers in biological samples; this system is also able to monitor a dynamic process. It is based on a solid-state pixel array detector, bump bonded to low threshold, integrated electronics developed for medical applications (Medipix). The present assembly is equipped with a 200xa0μm thick GaAs square detector, about 1xa0cm side, segmented into 64×64 170-μm size square pixels; simulation and experimental studies show good linearity and high sensitivity to β sources. Imaging tests with β sources of medical and biological interest such as 14C, 35S, 32P are reported.