A Novel electronic chain for high level neutron coincidence counters

This paper describes a counting chain (charge preamplifier, shaping amplifier and threshold discriminator) devoted to He-3 neutron detectors. The preamplifier adopts an innovative circuit with the gate of the input JFET floating and a DC feedback loop that stabilizes the output voltage acting on the input cascade second transistor. Static and dynamic analysis, including the effects of the detector bias network, is developed and reported. The shaping amplifier is a fifth-order quasi-gaussian OPAMP active filter that minimizes the number of components. Shaping time constants ranging from 1 mu s down to 100 ns are explored in order to maximize the count rate capability, maintaining nevertheless neutron discrimination. Experimental signals and spectra show that 100 ns shaping time is a lower limit for the usual He-3 proportional detector. Since this counting chain is characterized by single 5V supply operation, high counting rate, small size and low cost, it is well suited for high efficiency neutron well detectors where a large number (10 - 100) of counting tubes are used (High Level Neutron Coincidence Counter HLNCC).