Timing properties of transmission avalanche counters at moderate specific ionization

Abstract

The subject of investigation is a large-area parallel plate avalanche counter (PPAC). The counter timing properties when detecting α-particles of low energy are analyzed at n-heptane vapour pressures ranging from 5 to 30 Torr. Under typical measurement conditions, a time resolution of 380 ps has been achieved at these pressures; there is no evidence for further improvement at higher pressures. Considered are the TOF test setup efficiency and the electron drift velocity in n-heptane under relatively high electric fields. Besides, the variability of the basic physical processes contributing to the detection is evaluated and the effect of the fine energy structure of the divergently emitted α-particles upon the PPAC actual time resolution is investigated. A large dynamic range of the PPAC energy resolution was noticed; the contributions of individual side effects to the detector timing properties are evaluated. The empirical relations resulting from the investigation lead to a safe method of estimating the plateau width on the time resolution curve of transmission avalanche counters.