Ionization-cluster distribution of α particles in nanometric volumes of propane

The electron collector is a system of electrodes that encloses an almost wall-less cylindrical volume whose height equals its diameter of 3.7 mm (SV). The α-particles of a collimated narrow beam pass at a distance “d” from the sensitive volume (SV) and are detected by a solid-state detector (SSD); the low-energy secondary electrons produced by each α-particle within the SV are collected to a single-electron counter (SEC), which consists of a drift column and a multi-step avalanche chamber (MSAC), and are detected as a function of their arrival time at the MSAC. More details about the apparatus can be found elsewhere [1]. Measurements were performed at 300 and 350 Pa of propane, corresponding to simulated diameters of the SV (at 20 °C, density 1.0 g/cm) of 20.6 nm or 24.0, respectively, for distances “d” ranging from 0 to 70 nm aside the track. In evaluating experimental results, the detector efficiency must be taken into account. The final mean total counting efficiency ε for single electrons which start within the sensitive volume SV and are detected at the MSAC, was estimated to be about 20% at 300 Pa and about 28% at 350 Pa. To test the reliability of experimental results, a Monte Carlo code was developed able to simulate the production of secondary electrons by impact ionization of α-particles in propane at low gas pressure and to follow the histories of these electrons down to the ionization threshold energy of 11.08 eV for propane. The structure of α-particle track segments at an energy of a few MeV is almost exclusively based on their ionization cross section, on the spectral and angular distribution of secondary electrons produced by impact ionization, and on the properties of secondary electron degradation. As no experimental ionization cross sections of α-particles at energies of a few MeV in propane were available, the determination of the integrated ionization cross section σion(Tα) was based on the analytical model of Rudd et al.. The simulation of the energy and angular distributions of electrons liberated by impact ionization of the alpha particles was performed applying the semiempirical Hansen-Kocbach-Stolterfoht (HKS) model. More details on the adopted procedures are given in [2].