Canberra 2002C Cooled FET Germanium Detector Preamplifier
This is a Canberra 2002C cooled FET germanium detector preamplifier with user manual. The Canberra Model 2002C is a low noise, cooled FET, high speed preamplifier designed for high resolution gamma spectroscopy and timing measurements using cooled Ge semiconductor detectors. The preamplifier converts the ionization charge developed in the detector during each absorbed nuclear event to an output pulse whose amplitude is proportional to the total charge accumulated in that event. The pulse decays exponentially with a time constant of 50 ps (nominal) to segregate successive events in high count rate applications. The preamplifier includes a low noise FET input circuit optimized for the ultra-high source impedance of germanium detectors. The charge amplifier and buffer stages have been designed for both the low noise and high speed performance needed for precise energy and timing spectroscopy. In addition, a special circuit monitors the activity of the detector, and warns when improper operating conditions exist. A high voltage filter insures a noise-free power source for the detector. The block diagram in Figure 1 below shows a functional breakdown of the preamplifier. The first stage serves as an integrator yielding an output voltage proportional to the accumulated charge from the detector. A differentiator follows the first stage and includes a calibrated pole/zero network to provide the unipolar signal with an accurate return to baseline. The second amplifier serves as an output buffer and allows preamp conversion factors of 100 mV/MeV or 500 mV/MeV, jumper selectable. Separately terminated Energy and Timing outputs are available. A calibrated offset adjustment insures a nominal quiescent dc output of zero volts. To reduce noise, the Model 2002C input circuits are cooled by mounting them inside the cryostat, creating an integral detector/preamp assembly. Using a Canberra Model 2020 Spectroscopy Amplifier set at 4 μs unipolar semi-Gaussian shaping, noise behavior is summarized in Table A.1 below. In order to optimize the signal-to-noise ratio for low energy sources, the gain can be switched to the higher setting. The fast rise time of the 2002C is maintained over a wide range of detector capacitances, making the preamp an excellent choice for timing measurements. Timing analysis can be done using Canberra Timing-Filter Amplifiers, Constant Fraction Discriminators and Time to Amplitude Converters as required. The count rate capability of the Model 2002C has been demonstrated in excess of 200,000 counts per second using a Co gamma energy source (1.33 MeV peak). In order to take advantage of the high-countrate capability of the 2002C, a Canberra high-count-rate main shaping amplifier is recommended. A source which is too active may lead to detector-preamp overload. The Model 2002C includes a red High Rate warning LED which glows when the high rate condition is approached. A test input is provided to assist system setup and as a diagnostic aid. The nominal voltage gain through the preamplifier test input is 1X for the output scale factor of 100 mV/MeV and 5X for the output scale factor of 500 mV/MeV. The output of the charge integrator is available at a rear panel test jack for detector/preamplifier troubleshooting. The V-l characteristics of the detector can be readily checked by measuring the test-point voltage as a function of detector bias voltage. For use with cryostats equipped with a temperature sensor, the Model 2002C includes circuitry to monitor detector temperature. Two temperature status LEDs and a High Voltage Inhibit output to control the high voltage power supply are included. Power for the Model 2002C is usually supplied from the associated Canberra pulse shaping amplifier. The power lines are filtered within the Model 2002C to provide high noise immunity.
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