Low noise, dual gain preamplifier with built in spectroscopic pulser for highly segmented high-purity germanium detectors

We present a fast and very low noise hybrid front-end electronic device consisting of a complex charge sensitive preamplifier with a built in fast reset circuitry and a very accurate spectroscopy pulser. The front-end electronics was developed for the central electrode of 36-fold segmented, high purity germanium detectors in the frame of AGATA array of detectors. The intrinsic energy resolution in conjunction with a highly segmented detector is less then 900 eV for central electrode with a detector capacity of 3c 47 pF. The dynamic range of the central charge sensitive stage is as large as 100 dB, ranging from 0.1 fC to 10 pC, i.e. from 3c2 keV to 180 MeV in terms of photon energy. To achieve such an unprecedented wide dynamic range, we split the whole spectroscopic range in four sub-ranges, namely 0-5 MeV, 0-20 MeV, 5-180 MeV and 20-180MeV, respectively. The first two ranges are measured with standard pulse-high spectroscopy techniques. Thereto, the analog electronics was designed with two conversions gains, 200 mV/MeV and 50mV/ MeV, respectively to match the 1 V input dynamic range of the very fast pipe line ADC used by the digitizer units. To extend the pulse-height spectroscopy range well beyond the fast ADC analog input range, a new method was implemented and at a certain threshold we are switching from a standard pulse height analysis method to a time over threshold technique TOT. In order to achieve that, adjacently to the pole-zero cancellation circuit, a fast-reset circuit controlled by a fast comparator and an accurate zero crossing detector has been developed. Using this technique we can achieve an energy resolution of 0.04% at equivalent gamma energies of 180MeV. A programmable spectroscopic pulser has been designed and implemented in the central electrode (core) front-end electronic to achieve 'self-calibration standards' in both time and amplitude domain, which was needed to characterize such a complex, highly segmented high purity solid state detector