Wide dynamic range front-end electronics for gamma spectroscopy with a HPGe crystal of AGATA

Wide-range energy measurements (from 5 keV up to 50 MeV) have been carried out with an encapsulated 36-fold segmented HPGe detector of AGATA (Advanced GAmma-ray Tracking Array). The crystal is read out by 36 segment preamplifiers and 1 core preamplifier, connected to flash ADC\´s through 10-m shielded-twisted-pair cables. All preamplifiers are equipped with a de-saturation circuitry which swiftly resets the preamplifier when an energetic event or a burst of piled-up events put the ADC into saturation. By means of a TOT (Time Over Threshold) algorithm, it is possible to estimate the amplitude of the energetic events which exceed the typical dynamic range. The electronics automatically conditions the signal for standard pulse- height analysis if it is in the ADC voltage range and for Time- Over-Threshold (TOT) analysis if it exceeds the ADC range. A high-resolution spectroscopy has been achieved in both operation ranges. An unprecedented wide energy range of 5 keV to 50 MeV (on large pulser signal), or 80dB, has been obtained using a single acquisition chain, while maintaining a high energy resolution in the whole spectrum. The obtained resolution in "reset mode" was of <0.4% in all the tested reset range from 3 to 50 MeV. The TOT technique has been demonstrated using a ²⁴¹Am+Be source with Ni target generating high-energy gamma photons by neutron capture reactions. A remarkable resolution of 0.21% was obtained on the Ni spectrum line at the energy of 8.998 MeV.