Ultra-fast low-noise preamplifier for bulky HPGe gamma-ray sensors

We present a fast low-noise hybrid charge-sensitive preamplifier for germanium position-sensitive gamma-ray detectors. In conjunction with a bulky 36-fold segmented detector it provided a record resolution of 1.71/1.77 keV fwhm on the 1.17/1.33 MeV 60Co lines. The preamplifier 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 50ohm twisted-pair cable connected at its output. The dynamic range of the preamplifier input stage is as large as 92 dB, ranging from 0.275 fC to 9.9 pC, i.e. from 5 keV to 180 MeV in terms of photon energy. On signals larger than 2 MeV a fast reset (~10 MeV/µs) is enforced to reduce the system dead time. An estimate of the amplitude of such large signals can be derived from the reset time, still obtaining a high resolution. Using this technique we achieved an energy resolution of 0.3% at 16.7 MeV.