Experimental determination of parameters of tracks on solid state nuclear track detectors from alpha particles with different incident energies and angles

Abstract

The use of solid state nuclear track detectors (SSNTDs) has already become a well-known technique which has been widely applied in monitoring concentrations of radon gas by recording their emitted alpha particles. The development of methods for long-term integration measurements of radon progeny concentrations in air based on alpha spectrometry employing SSNTDs, however, are still being explored. To develop the method, determining the dependencies of track parameters on incident energies and angles of alpha particles would be an essential procedure. In the present study, the main task is to determine the major axes and minor axes for the tracks of alpha particles with different incident energies and angles for SSNTDs (CR-39 and LR 115 detectors) experimentally. For this purpose, the first step is the experimental determination of alpha particle energy losses in air. Measurements of the track parameters critically depend on the thickness of the removed layer of the SSNTDs during etching. However, based on observations using scanning electron microscope (SEM) and surface profilometry (using Form Talysurf), inhomogeneities in the thickness of the unetched and etched LR 115 detectors have been identified. These features are studied and the results show that LR 115 detectors cannot be assumed to be homogeneous in general. In order to obtain a large set of precise data for the major axes and minor axes of the alpha tracks, fast and precise measurements of the thickness of the active layers of the SSNTDs are needed. It is found that the thickness of the removed layer of an etched LR 115 detector can be measured accurately and rapidly using Energy Dispersive X-Ray Fluorescence (EDXRF). It is also found that stirring during the etching process can increase the bulk etch rate of the LR115 detector. The latter can help shorten the etching time and obtain the major axes and minor axes of the alpha tracks more efficiently. For the main task, the major axes and minor axes for the tracks of alpha particles with different incident energies and angles on the CR-39 and LR 115 detectors have been determined. The tracks on the CR-39 detectors were systematically irradiated by alpha particles with energies from 1 to.5 MeV and incident angles from 40° up to 90° and with specific thickness of removed layers. A set of new coefficients for a chosen V function (which is the ratio between the track etch rate Vt to the bulk etch rate Vb) by fitting the experimental data with the data generated with a track growth model incorporating the chosen V function. For the tracks on the LR 115 detectors, the experimental procedures were similar to those for the CR-39 detectors (i.e., incident energies from 1 to 5 MeV and incident angles from 30° up to 90°, and with specific thickness of removed layers). The functional form of the Durrani-Green's V function (Durrani et al., 1999) with a set of new coefficients was used to fit the parameters. The experimental data were found to fit the respective models of V function satisfactorily. The respective V functions were then used to generate databanks for the lengths of major and minor axes for tracks in both CR-39 and LR 115 detectors. In the last step of the present study, an interpolation program written in the Fortran language is used to deduce the incident energies and angles of the alpha particles from given major and minor axes by using the databanks. This is an important step in developing the technique of alpha spectroscopy.

Date of Award	16 Feb 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Ngok Peter YU (Supervisor)