Introduction - FAST ComTec CSPA10 User Manual

Introduction

1 Introduction
FAST ComTec's CSPA10...13 is a single channel charge sensitive preamplifier module intended for use
with various types of radiation detectors including semiconductor detectors (e.g. Si, CdTe and CZT), P-I-N
photodiodes, avalanche photodiodes (APDs), and various gas-based detectors. The CSPA10...13 is one
of a series of four charge sensitive preamplifiers offered by FAST ComTec, which differ from each other
most notably by their gain. A guide to selecting the best charge sensitive preamplifier for your application
can be found on the next page or at our web site: http://www.fastcomtec.com. As with all FAST ComTec's
preamplifier modules, the CSPA10...13 is housed in a small shielded metal case with a input for plug-in
power supply.
CSPA1x is a successor model of CSP1x, which includes easier supply with classic plug-in power supply.
Other improvements include integrated protection circuit and better variability.
The FAST ComTec Model CSPA10...13 Charge Sensitive Preamplifier is designed for optimum
performance with Silicon Surface Barrier (SSB) detectors. Operating as a charge to voltage converter, the
unit accepts charge carriers produced in the detector during each absorbed nuclear event. The output
then provides a voltage in direct proportion to the collected charge at the rate of 1.4V per picocoulomb.
This translates to a gain of 62mV per MeV (see table) for room temperature silicon detectors.
For typical use with positively biased SSB detectors, the extremely linear energy output provides a
negative polarity pulse ideal for energy spectroscopy.
The high charge rate capability of the design is evidenced by an energy rate capacity of greater than 10 5
MeV per second when used with silicon detectors. In order to take full advantage of such a high count
rate capabilty, a main amplifier with a correspondingly high count rate ability, such as the FAST ComTec
Model CSA4, should be used.
The basic operation of the preamplifier is indicated in the functional schematic diagram below, see Fig.
1.1. The first stage acts as an operational integrator which produces an output potential proportional to
the accumulated charge on the feedback capacitor Cf. The integrator drives the output buffer with a gain
of +2.
The noise contribution of the preamplifier is only 1.7 KeV, FWHM, (Si) with an increase of less than 34 eV
for each picofarad of additional source capacitance. To reduce noise injected through the HV input, a
decoupling network is utilized for filtering the detector bias voltage.
To reduce the possibility of transmission line reflections when using long lengths of coaxial cable, the
energy output is provided with a series resistor, eliminating the necessity of terminating the line at the
receiving end.
For those applications utilizing lower voltage detectors with vacuum chamber feedthrough connectors or
direct detector cabling inside a chamber, the Model CSPA10...13-1B provides a BNC for the detector and
HV bias input. Necessary power is provided by any FAST ComTec main amplifier through the five foot
compatible cable furnished with the preamp.
6 ComTec GmbH
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