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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/5073

Title: Investigations of the effects of UV irradiation on the etching behavior of CR-39 solid state nuclear track detector
Other Titles: Zi wai xian dui qin shi CR-39 gu tai he jing ji tan ce qi biao xian zhi ying xiang
紫外線對侵蝕 CR-39 固態核徑跡探測器表現之影響
Authors: Tse, Chun Chun (謝珍珍)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Irradiation
Nuclear track detectors
Particle track etching
Ultraviolet radiation
Notes: CityU Call Number: QC793.3.T67 T74 2007
Includes bibliographical references (leaves 136-146)
Thesis (M.Phil.)--City University of Hong Kong, 2007
vii, 146 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: During the last three decades, a number of papers have been published to discuss the effects of UV exposure on solid-state nuclear track detectors (SSNTDs). However, most of them differ in finer details, e.g., the particular wavelength and the extent of irradiation. For this reason the conclusions drawn are also sometimes quite different. The present study confirms that UV exposures can drastically change the bulk etch rate (Vb) and track etch rate (Vt) of CR-39 detectors at short UV wavelength. In this thesis, the changes in the chemical, physical and mechanical properties of CR-39 detectors under UV irradiation were studied together with the corresponding photo-degradation mechanisms in order to explain the variation of the etching behavior of CR-39 detectors in NaOH/H2O. The energy transfer to CR-39 detectors by UV irradiation leads to excitation and ionization of the molecular chains, hence to radical formation, main chain scission, and also to cross-linking of polymeric chains. On extended exposures to UV light, CR-39 detectors slowly degrade, turning progressively yellow and changes of its chemical and physical properties occur. The chemical modifications were studied by Fourier Transform Infrared (FTIR) spectrometry, UV-Visible (UV-VIS) spectrometry and X-ray photoelectron spectroscopy (XPS) whereas the physical modifications were determined by dynamic mechanical thermal analysis (DMA) and nano-hardness measurements. The variation of the bulk etch rate Vb of CR-39 detectors under UV irradiation with different wavelengths have been investigated in this thesis. It has been found that UVA causes no changes in both the net absorbance in IR spectra and the Vb values. Both UVA+B and UVC irradiation caused scission of the chemical bonds, which was also manifested by the faster chemical etching rates. The scission process was revealed by the decreasing net absorbance at particular wavelengths in the infrared spectra. However, for prolonged UVC exposures, Vb will no longer increase because of formation of a cross-linked layer on the surface of the CR-39 detectors. The cross-linking process was indicated by the increased hardness and elastic modulus determined with a nanoindenter and the increased glass transition temperature. In the early stage of UVC irradiation, the chain scission leading to free radical formation is the pre-dominant process which induced an increased Vb. For prolonged UVC exposures, the free radicals created by scission may lead to intermolecular crosslinking which will hinder the penetration of oxygen into the bulk polymer and will thus retard the degradation rate. Oxidation of track detectors induced by UV-light results in the formation of oxygen-containing function groups. The FTIR spectral analysis has been made to investigate the structural changes induced in the CR-39 detectors due to UV irradiation of the polymers. The changes have been determined from the relative increase or decrease in the intensity of the peaks associated with the functional groups present in the CR-39 detectors. Photoproducts such as aldehydes, ketones, carboxylic acids and esters, which were produced during the UV irradiation, have been identified by FTIR analyses. The effects on the track etch rate Vt and the detector sensitivity (Vt/Vb) of the CR-39 detector from UV irradiation at short wavelengths have also been investigated. The measurements of the diameters and depths of alpha-particle tracks were used to derive a V function (Vt/Vb) for CR-39 detectors with and without UV irradiation. These results show that both Vb and Vt increase but the detector sensitivity (Vt/Vb) decreases. The enhancements of Vb and Vt are explained by the additional chain scission and the increased formation of hydroxyl group.
Online Catalog Link: http://lib.cityu.edu.hk/record=b2217895
Appears in Collections:AP - Master of Philosophy

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