A Risetime Discriminator for Low Level Tritium Counting

This paper describes the design of a risetime discriminator circuit (RTD), with associated anticoincidence/coincidence (AC/C) logic and its application to the counting of low-activity tritium samples in gas proportional counter systems. The circuitry, packaged in a double-width NIM, extracts signal risetime information which is used in conjunction with the output of an annular guard counter to electronically reject background counts due to high energy charged particles (μ-mesons), and gamma-ray photons. The use of the guard and AC/C logic without RTD is effective in reducing the background due to charged particles because of the high probability of coincidental detection by the guard and proportional counters. However, the reduction of backgromd related to gamma-ray photons by simple AC/C techniques is ineffective because of the small probability of a coincidence event occurring in the two counters. The RTD circuit utilizes the property that gamma photons and high energy pmesons produce ionization tracks in the proportional counter that are typically longer and have lower specific charge density than those tracks produced by low-energy (tritium) beta particles. Thus, these events have longer charge collection times and result in slower signal risetimes at the output of a charge-sensitive (integrating) preamplifier. The RTD circuit differentiates the wide-band preamplifier signal which produces a pulse with amplitude proportional to the signal risetime. The derivative is then peak-detected, stretched, delayed and inverted. An amplitude comparison is then made between the stretched derivative and the linear input signal using a linear summation amplifier.