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 ∼ 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 ∼2 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.