Speaker
Alberto Gola
(Politecnico di Milano and INFN Italy)
Description
The scope of the DRAGO project, supported by Italian INFN, is the realization of a
high resolution, compact gamma ray imager, based on the Anger camera principle. In
this configuration, the light generated by a unique scintillator is read by an array
of 77 Silicon Drift Detectors. In order to locate the position of interaction of the
photon inside the scintillator it is necessary to make an amplification and
filtering of the detector signals followed by a processing of the acquired data.
The electronics readout and processing system can be divided in two separate parts:
the analog front end and the DAQ board.
The analog front end is composed of 80 readout channels divided in 10 CMOS chips,
realized in the 0.35um AMS technology, each one processing 8 channels. Each analog
channel of the circuit includes a low-noise preamplifier, a 6th order semigaussian
shaping amplifier with four selectable peaking times from 1.8us up to 6us, a peak
stretcher and a baseline holder. The integrated time constant used for the shaping
are implemented by means of a recently proposed ‘RC’ cell. This cell is based on the
de-magnification of the current flowing in a resistor R by means of the use of
current mirrors. The 8 analog channels of the chip are multiplexed to a single
analog output. A suitable digital section provides self-resetting of the channels,
trigger output and the programming of independent threshold on the analog channels
by means of a programmable serial register and 3bit DACs. The energy resolution
measured using a single channel of the chip with a Silicon Drift Detector Droplet
(SDD3) is of 128eV at 6keV with the detector cooled at -20°C.
The multiplexed outputs signals are the sent from the CMOS circuit to the
acquisition system. For each gamma event, the acquisition system performs the A/D
conversion of all the signals of the array and sends them to a host PC, where the
position reconstruction is executed on-line. The DAQ board contains 10 ADCs, each
one dedicated to a single ASIC of the analog section. When a gamma event takes
place, each ADC converts the sequence of signals coming from the analog multiplexer,
with a resolution of 13 bit (ENOB). The burst conversion rate of the board is 50
Ms/s and is limited essentially by the analog section; this gives a dead time of
about 2us per event. The converted data are stored in a FIFO memory, for
buffering, and then are transferred to the host PC via a USB 2.0 bus. The data
processing for the reconstruction of the position of interaction of the event in the
Anger Camera is based on a correlation filter implemented in C++.
Author
Carlo Fiorini
(Politecnico di Milano and INFN Italy)
Co-authors
Alberto Gola
(Politecnico di Milano and INFN Italy)
Marta Gaia Zanchi
(Politecnico di Milano and INFN Italy)
Matteo Porro
(Max Planck Institut Halbleiterlabor, Munich, Germany)