Implementing variable energy positron lifetime spectroscopy based on secondary electron timing from thin carbon foil in transmission geometry

Abstract

Positron annihilation lifetime spectroscopy (PALS) is a notoriously difficult technique to apply on a slow positron beam because no natural “start signal” is available for marking the entry of the positron into the sample. A new variable energy positron annihilation lifetime spectroscopy (VEPALS) system (otherwise known as a lifetime beam) based on secondary electrons (SEs) emission in the forward geometry from a carbon foil (C-foil) was constructed. In this system, a C-foil was installed on a stainless steel supporting mesh and a tungsten needle with the length of 1 cm was connected at one of its end on the centre of the foil while the other end touches the target sample. The slow positron passes through the C-foil thus generating SE in the forward direction. The detected SE is treated as a “start signal” of an electronic clock. The clock is stopped by the detection of the annihilation gamma radiation originating from the annihilation of the positron with an electron inside the sample.

SE emission in the forward geometry from the C-foil with thicknesses from 1.0 – 5.0 𝜇g/cm2 induced by the bombardment of 1 – 20 keV positron beam was studied with the aid of a micro-channel plate (MCP). The electronic stopping power, the material dependence parameter, the maximum scattering angle of the emitted SE and the energy distribution of SEs were presented. The results were analyzed with some calculations and Monte Carlo simulations. The SE yield drops to a constant value for positron beam energy above 15 keV. The maximum SE scattering angle was about 60°.

In order to test the efficiency of this needle-foil device without the target sample, the yield of SE and the transmission coefficient of positrons were studied as a function of the incident positron energy ranging from 1 – 20 keV, the electric potential of the annular electrode and three different column lengths of the annular electrode. The spot size of the transmitted positrons and the diameter of the SE ring were also investigated.

A circular single crystal p-type silicon (Si) sample was used to test the performance of the VEPALS system. A conventional positron lifetime spectrum of Si was obtained and compared with results from the VEPALS system. The positron lifetime spectra of the Si were studied for annular electrode potential ranging from 1.5 – 3.0 kV. The 1.5 kV annular electrode provided the optimal positron lifetime of Si. The Si positron lifetime spectra were further studied as a function of positron beam energy for the 1.5 kV annular electrode. The analyzed spectra have a bulk defect-free lifetime component of 231 ps with a more than 66% intensity. This defect-free lifetime component intensity could be higher than 80% for a 15 keV positron beam.

The unwanted PALS events and the satellite peak of the PALS were examined and explained in terms of a few positrons being scattered to the extent that they miss the sample and strike the MCP directly. A positive bias potential at the MCP grid was provided for eliminating the satellite peak.