A low-noise, low stored energy preamplifier for high-voltage radiation detectors

A low-noise preamplifier with very low stored energy has been developed for use with high-voltage radiation detectors. The input will withstand 3.5 kV, has a physical capacitance to ground of 200 pF, and an effective “small signal” capacitance of ∼30 nF. When connected to a high-voltage electrode, the low physical input capacitance to ground limits the stored energy and helps protect the detector system should an accidental discharge occur. The large effective input capacitance minimizes the voltage transient on the high-voltage electrode that occurs during charge collection, thus lowering the charge that is capacitively coupled to surrounding electrodes from this transient. If the surrounding electrodes are part of a position sensing readout, then this coupled charge is distributed uniformly among readout electrodes and is added to the signal charge induced by the motion of charge carriers containing position information. This introduces an error into the position centroid calculation. The preamplifier presented here greatly reduces this effect while keeping stored energy low. Equivalent input noise charge (ENC) is 256 electrons RMS when using a 250 ns unipolar shaping amplifier and no input capacitance. With input capacitances of 18 and 223 pF, the noise increases to 361 and 1766 electrons RMS, respectively.