1,721,033 research outputs found
Response of semi-insulating GaAs detectors to near-infrared picosecond light pulses
No full textSemi-insulating gallium arsenide (SI-GaAs) detectors, in the form of Schottky diodes, have been irradiated from the front or from the back contact or from the side, with light pulses from picosecond laser source, tunable in the range of
780–1020nm and focused to a 50–100 um spot on the surface of the detector. Energy per pulse was in the range of
4–25 pJ, with a 10 or 100 Hz repetition rate. Each pulse mimics the interaction ofa single energetic particle with the
substrate, but with a photon mean free path modulated in the range of 1 mm–1 cm, depending on the light wavelength.
The SI-GaAs substrates have a thickness of 200 um (VGF grown), or 600 um (LEC) or 1000 um (VGF). The time
response ofthe detectors, analysed with the Transient Current Technique (using a fast digital sampling oscilloscope) in
terms of peak signal amplitude and charge rise time, has been evaluated as a function of: reverse voltage bias
(50–1200 V), light wavelength (780–1020 nm), side position between the electrodes, position on the front contact. The
results are: (1) the shape ofthe transient current signal may show the presence of two peaks, about 1 ns apart; (2) the
charge rise time, considered to be dominated by the charge collection time, is between 2 and 13 ns; (3) the charge rise
time, as due to electron drift or hole drift in the region between the contacts, has been measured as 2–5 ns, also
dependent on the applied bias; (4) the transient current (exponential) decay time, for front or back irradiation, increases
with the bias voltage up to 2–20 ns and these data are considered to be consistent with the electron detrapping time; (5)
under reverse bias, for 1mm pad side and a large ohmic contact on the back, the electric field extends around the front
contact up to about 200 um
Response of semi-insulating GaAs detectors to low energy protons
No full textThe performance of semi-insulating GaAs detectors, grown with the LEC technique, has been studied by irradiating
the Schottky diodes with 2 and 2.4 MeV monoenergetic protons in a pencil beam with sub-millimeter width (70 or
600 um). The beam was moved across the surface of the front (Schottky) contact, in order to investigate the uniformity of the detection characteristics over the sensitive area ofthe diodes, and to study the electric field behavior around the Schottky contact. For each scanning position, a pulse-height spectrum was measured. Then, the charge collected and the energy resolution were obtained as a function of the irradiation position both on the contact and outside it. The data show that
- the best spectroscopic response occurs for the beam with 70 um width,
- when the beam is incident onto the contact, the energy resolution is between 1% and 5%, and the variation ofthe
charge collected, for different irradiation position, is less than 30%,
- when the beam is incident onto the border of the contact (substrate is irradiated), the spectrum is degraded and no
clear peak is present,
- collection of charge still occurs at distances up to about 500 um from the border of the Schottky contact (for a pixel
size of 200 um), or up to about 200 um (for a pixel diameter of 3 mm),
- saturation ofthe curve collected charge vs. reverse bias voltage occurs at about 100 V for both 2 MeV
(range=32 um) and 2.4 MeV protons (range=41 um)
Imaging performance of single-element CdZnTe detectors for digital radiography
No full textWe measured the charge collection efficiency and energy resolution at 60 keV and 122 keV for single-element Cd0.8Zn0.2Te semiconductor detectors 2x10x10 mm(3) irradiated transverse to the electric field, in view of their possible use for digital radiography. Charge collection efficiency, energy resolution and charge collection time have been measured as a function of the bias voltage. The detector's imaging performance was evaluated, using a scanning procedure, by measurements (at 60 keV) of the line spread function and edge response function of the imaging system; metallic test objects and bone tissue were imaged. Image contrast values as low as C=10% were obtained for a minimum 5:1 S.N.R.
Portable system for imaging of α, β and X-ray sources with silicon pixel detectors and Medipix 1 read out
No full textA portable system has been developed for the control and the readout of a 64×64 pixel silicon hybrid detector (1.1×1.1 cm2 active area), bump-bonded to the Medipix1 readout chip. This system was originally designed for biomedial imaging applications and is based on standard commercial digital I/O boards and a compact dedicated interface (p-MUROS), connected to the portable PC. The Medipix1 chip is hosted in a 257 Pin Grid Array chip carrier directly placed on the p-MUROS board. This board is the simplified version of the MUROS1 readout board already developed in the framework of the Medipix collaboration. User-friendly software system (Medisoft 3.3) has been written to provide control of the system and image acquisition, display and processing. This complete hardware/software imaging system has been used with laboratory α, β and γ sources for test purposes and represents the portable version of a desktop imaging system developed for β-autoradiography. The system description and complete imaging tests are presented. Additional applications include the monitoring of the flux and the energy spectrum of X-rays tubes
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