A Versatile Low-Noise Wide-Range Charge-Sensitive Preamplifier for HPGe Detectors

A fast low-noise hybrid charge-sensitive preamplifier for HPGe detectors, has been designed for the new array detector "GALILEO" under construction at INFN Legnaro National Laboratory (Italy). The preamplifier includes an over-threshold fast-reset circuitry that greatly reduces the dead time due to preamplifier and/or ADC saturation. A 1-bit digital output signal is provided in LVDS or CMOS format, whose width yields an indirect precise measurement of the large over-threshold signals. Combining the under- and over-threshold working modes an unprecedented spectroscopic dynamic range is obtained of 10(5), from 0.1 fC to 10 pC, i.e. from similar to 2 keV to 180 MeV of equivalent energy in germanium. A fast analogue signal is optionally provided as a trigger for ancillary detectors. The preamplifier is optimized to feature low noise, fast rise time and wide amplitude range when connected to anode detector electrodes. It includes a fast differential buffer for low-noise signal transmission to a remote ADC, with a preset common-mode DC component, that can be adjusted to match the ADC requirement. The rise time, as measured at the test bench, is as fast as 7.5 ns, with a detector capacitance of 23 pF and with a 5m 50 Omega twisted-pair cable connecting the preamplifier to the ADC. With its input transistor working at 150K the preamplifier features an intrinsic noise of similar to 90 electrons r. m. s., i.e. similar to 600eV fwhm in germanium, and a noise slope of 8eV/pF.