INVESTIGATIONS OF THE EFFECTS OF UV IRRADIATION ON THE ETCHING BEHAVIOR OF CR-39 SOLID STATE NUCLEAR TRACK TSE CHUN CHUN

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1 INVESTIGATIONS OF THE EFFECTS OF UV IRRADIATION ON THE ETCHING BEHAVIOR OF CR-39 SOLID STATE NUCLEAR TRACK TSE CHUN CHUN MASTER OF PHIOLOSOPHY CITY UNIVERSITY OF HONG KONG MARCH 2007

2 CITY UNIVERSITY OF HONG KONG 香港城市大學 Investigations of the Effects of UV Irradiation on the Etching Behavior of CR-39 Solid State Nuclear Track Detector 紫外線對侵蝕 CR-39 固態核徑跡探測器表現之影響 Submitted to Department of Physics and Materials Science 物理及材料科學系 in Partial Fulfillment of the Requirements for the Degree of Master of Philosophy 哲學碩士學位 by Tse Chun Chun 謝珍珍 March 2007 二零零七年三月

3 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 (V b ) and track etch rate (V t ) 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/H 2 O. 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.

4 The variation of the bulk etch rate V b 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 V b 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, V b 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 V b. 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.

5 The effects on the track etch rate V t and the detector sensitivity (V t /V b ) 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 (V t /V b ) for CR-39 detectors with and without UV irradiation. These results show that both V b and V t increase but the detector sensitivity (V t /V b ) decreases. The enhancements of V b and V t are explained by the additional chain scission and the increased formation of hydroxyl group.

6 List of Contents List of Figures... i List of Tables... vii CHAPTER 1 CHAPTER 2 Introduction...1 Literature review Description of solid-state nuclear track detectors (SSNTDs) Different types of track-storing materials Applications of SSNTDs Measurement of radon gas concentrations Track formation mechanisms in SSNTDs Interaction of charged particles with matter Radiation damage in polymers Energy-Range Relationship Principles of Track Etching Procedures for revelation of tracks Definition of Bulk etch rate V b and Track etch rate V t Track etching geometry Huygen s principle in track development Three phases in the track development Track etching geometry for constant V t with normal incidence Measurements of Bulk etch rate V b and Track etch rate V t Effect of ultraviolet irradiation on the nuclear track recording properties of CR-39 SSNTDs Property modifications in UV irradiated polymeric track detectors...21

7 Probable mechanisms of photo-oxidation of polymeric track detectors Variations of V b and V t of polymeric films under UV irradiation...23 CHAPTER 3 Bulk Etching behaviour of CR-39 in NaOH/H 2 O Description of CR-39 SSNTDs Methodology Measurement of the bulk etch rate V b of CR-39 SSNTDs Optical absorption characteristics of CR-39 SSNTDs with the use of Fourier Transform Infrared (FTIR) spectroscopy Identifications of the etch products of CR-39 SSNTDs in NaOH/H 2 O Instrumentation Form Talysurf PGI Fourier Transform Infrared (FTIR) Spectroscopy X-ray powder diffraction (XRD) Results and discussion Determination of bulk etch rate of CR-39 SSNTDs in NaOH/H 2 O Analyses of FTIR spectra of CR-39 SSNTDs Correlation between the net absorbance A and film thickness t Mechanisms of chemical etching of CR-39 SSNTDs in NaOH/H 2 O Identification of etch products (sodium carbonate) by two different techniques Conclusions...49

8 CHAPTER 4 Effects of UV irradiation on the etching behavior of CR-39 SSNTDs Introduction Methodology Preparation of UV-irradiated samples of CR-39 SSNTDs Determination of bulk etch rates of the UV-irradiated CR-39 SSNTDs Computational studies with the GAUSSIAN 98 computational package Chemical modification analyses Physical modification analyses Instrumentation UV-Visible (UV-VIS) spectroscopy X-ray photoelectron spectroscopy (XPS) Dynamic Mechanical Analyzer (DMA) Nano-indenter Results and discussion Computational studies of bond dissociation energy Photo-oxidation mechanism of CR-39 SSNTDs at short UV wavelength Analysis of photo-oxidation by infrared spectroscopy Identification of oxidation products by derivatization reactions Surface analysis of untreated and UV-treated CR-39 SSNTDs by XPS UV-Vis analyses...91

9 Study of bulk etch rates of short-wavelength UV-irradiated CR-39 SSNTDs Explanation of increased bulk etch rate of UV-irradiated CR-39 SSNTDs at the beginning of UV exposure by determination of chain scission through IR analysis Explanation of saturation of bulk etch rate at prolonged exposure time by determination of crosslinking with analyses of mechanical and thermal properties Photo-oxidative degradation with depth for short-wavelength UV-irradiated CR-39 SSNTDs Photo-oxidation at different wavelengths Conclusions CHAPTER 5 Effects of UVC irradiation on alpha-particle track parameters in CR-39 SSNTDs Introduction Methodology Sample preparation and irradiation Measurement of the bulk etch rate V b Measurements of diameters and depths of alpha-particle tracks Determination of track etch rate V t and derivation of V function (V t /V b ) Results and discussion Chemical modifications of CR-39 SSNTDs by alpha particles and UV photons...123

10 Variation of bulk etch rate V b of UV-irradiated CR-39 detector Variation of diameters and observed depths of alpha-particle tracks Variation of track etch rate V t Conclusions REFERENCES Appendix A: Specification of Form Talysurf PGI (Taylor Hobson) Appendix B: Specification of Nanoindenter (Hysitron TriboScope) Appendix C: Stopping Range of alpha particle in CR-39 detector by SRIM-2003 Appendix D: The energies of alpha particles after traveling different air distance by using α spectroscopy system

11 i List of Figures Fig The breaking of the polymeric bonds by a charged particle....9 Fig The variation of the linear rate of energy loss de/dx as a function of the α-particle residual energy in a medium (Durrani, 1997)...10 Fig Geometry of the track development when the incidence is normal with respect to the detector surface and V t is constant...12 Fig Side view of the etched track. The etched cone of the charged particle is described by Huygen s principle (Enge, 1980)...13 Fig The evolution of an etch pit profile with prolonged etching...14 Fig The geometry of an etched track under normal incidence with constant V t (adopted from Durrani and Bull, 1987)...15 Fig Longitudinal section of an etched track when V t is variable Fig Repeating unit of polymer structure of CR-39 detector...26 Fig CR-39 SSNTDs with thickness of 1000 µm and a dimension of 3 cm 3 cm Fig A part of CR-39 detector was masked by epoxy before etching in NaOH/H 2 O Fig Determination of the difference in the heights of the exposed part and the masked part of the detector by using the Form Talysurf PGI...28 Fig Preparation of fine products of dried etchant before XRD analyses...30 Fig A typical commercial Form Talysurf PGI surface profilometer: (a) photo; (b) schematic diagram...32 Fig Schematic diagram of an FTIR spectrometer...33 Fig Characteristic stretching and bending vibrations in plane and out of plane...34

12 ii Fig Spectral correlation chart in the mid-infrared region for some representative vibrations...35 Fig Two-dimensional view of the geometry arrangement of the X-ray powder diffractometer Fig The scattered X-rays from the crystal when the Bragg s law is applied...37 Fig The relationship between the removed layer thickness (µm) and the etching time for different temperatures of the etchant solution (50, 60, 70 and 80 o C) in 6.25 N NaOH/H 2 O...38 Fig The data and the corresponding linear regression lines for etching the CR-39 SSNTDs in 6.25 N NaOH aqueous solution at 50, 60, 70 and 80 o C Fig The FTIR spectra of CR-39 detectors with the thickness of 5, 17, 100 and 1000 µm in percentage transmittance (%T). The absorption of radiant energy is therefore represented by a trough in the curve Fig Calibration curve of the thickness of the CR-39 film Fig FTIR spectra in transmission mode: (a) Experimental result of dried solute of 6.25 N NaOH/H 2 O after the reaction with CR-39 detector at 70 o C, (b) Result of computer-assisted identification of sodium carbonate...46 Fig XRD spectrum of the mineral Thermonatrite reported in literature Fig XRD spectrum of the crystalline etch products of CR-39 detectors in NaOH/H 2 O Fig Schematic representation of force-displacement curve for a nano-indentation procedure...58

13 iii Fig Schematic diagram of dual beam type UV-visible spectrometer Fig Origin of absorption in relation to molecular orbitals...60 Fig Schematic diagram of an X-ray photoelectron spectroscopy XPS system...61 Fig Schematic diagram of dynamic mechanical analysis instrumentation...64 Fig (a) The behavior of a viscoelastic material under oscillatory stress where the stress leads the strain by a phase angle (delta δ). (b) Complex modulus E* deconvoluted into storage E and loss modulus E...65 Fig A schematic diagram (a) and a photo (b) of a nano-indentation apparatus...68 Fig (a) Typical force displacement curve showing measured and derived parameters; (b) Schematic of indentation showing the displacements observed during an indentation experiment...69 Fig Repeating units of CR-39 detector with numbering Fig The FTIR spectra for the 5 µm CR-39 film before and after 20, 40, 100, 200, 300 and 400 h of UVC exposure in the presence of oxygen: (a) cm -1 (hydroxyl vibration region) and (b) cm -1 (C=O stretching vibration region), (c) cm -1 (C-O stretching vibration region)...81 Fig Subtracted FTIR spectra between the non-irradiated and the irradiated sample for 40, 100, 200, 300 and 400 h. (a) cm -1 (hydroxyl + hydrocarbon stretching vibration region), (b) cm -1 (C=O stretching vibration region) and (c) cm -1 (C-O stretching vibration region)...82

14 iv Fig FTIR spectra of (a) virgin sample and (b) UV-irradiated sample for 400 h before and after treatment with NH Fig Subtraction of the initial spectra of (a) virgin sample and (b) UV-irradiated sample for 400 h from the spectra recorded after NH 3 derivation Fig FTIR spectra of (a) virgin sample and (b) UV-irradiated sample for 100 h before and after treatment with 2,4- dinitrophenylhydrazine Fig XPS spectrum obtained from the unexposed CR-39 detector...87 Fig The variation of O/C ratio as a function of UVC exposure time Fig C1s peak for CR-39 detectors (a) before and (b) after UV treatment..89 Fig UV-Vis spectra of CR-39 detectors under UVC irradiation...93 Fig Induced changes in optical absorbance as a function of wavelength for the irradiated samples...93 Fig Representative example for Tauc s plot to derive the optical gap energy E g from the UV-Vis spectrum data: un-irradiated CR-39 detector Fig (a) The removed layer thickness and (b) the bulk etch rate V b of CR-39 detectors for different UVC exposure durations where represents etching time of 0.5 h, represents etching time of 1 h and represents etching time of 3 h...97 Fig The variation of net absorbance for different UVC exposure durations Fig The variation of the hardness and reduced modulus as a function of the UVC exposure duration. ( represents the hardness and represent the reduced elastic modulus)...102

15 v Fig Dynamic mechanical thermogram of blank CR-39 detectors Fig Schematic illustration of the effect of the degraded surface layer on the penetration of oxygen into the near surface layers: (a) crosslinked surface, (b) chain scission surfaces Fig The changes of the removed layer thickness of CR-39 detectors exposed to UVC for 10, 15 and 30 h as a function of the etching time Fig The calculated results from differentiation of changes of the removed layer of CR-39 detectors which have been exposed to UVC for 15 h Fig The variation of net absorbance for different UVA+B exposure durations Fig The bulk etch rate of CR-39 detectors for different UVA+B exposure times where the etching time was 3 h Fig The variation of net absorbance with different UVA exposure time..110 Fig The bulk etch rate of CR-39 detectors for different UVA exposure times where the etching time was 3 h Fig Experimental setup for alpha irradiation with normal incidence. A collimated beam of 241 Am α-particle was incident on CR-39 detectors after traversing different distances in air using several collimators of different thicknesses Fig The emission spectrum of a UV lamp with the peak at 257 nm Fig (a) Top view of a track opening and (b) longitudinal section of an etch pit in a CR-39 detector generated by 5 MeV alpha particles. Etching conditions: 6.25 N NaOH/H2O, at 70 o C for 8 h

16 vi Fig The illustration of the software ImageJ with the image of calibration ruler and cross-section of an etched pit Fig Geometry of the track development of 5 MeV alpha particle for an unexposed CR-39 detector when the incidence is normal with respect to the detector surface. Etching condition: 6.25 N NaOH/H 2 O, at 70 o C for (a) 6 h, (b) 10 h Fig Variation of the bulk etch rate of the CR-39 detector irradiated with UVC for 10 h as a function of etching time in 6.25 N NaOH/H 2 O Fig Variation of the 5 MeV alpha-particle track diameter as a function of (a) etching time and (b) removed layer thickness in CR-39 detectors with different UV treatments Fig Variation of observed 5 MeV alpha-particle track depth as a function of (a) etching time and (b) removed layer thickness in CR-39 detectors with different UV treatments Fig The experimentally obtained evolved profiles of an 5 MeV etch pit with prolonged etching in 6.25 N NaOH for (a) unirradiated, (b) pre-uv alpha-particle irradiated, and (c) post-uv alpha-particle irradiated CR-39 detectors Fig Variation of V t /V b as a function of the residual range of alpha particles in CR-39 detectors with different UV treatments Fig The variations of V t as a function of the residual range of alpha particles in CR-39 detectors with different UV treatments

17 vii List of Tables Table 3.1. The coefficients (A and B) for the linear regression equations (y = A + Bx) for the relationship between the removed layer thickness y (µm) and the etching time x (t) for different temperatures (50, 60, 70 and 80 o C) without stirring, and the corresponding determined etching rates (µm /h) in 6.25N NaOH/H 2 O Table 3.2. The coefficients (C and D) for the linear regression equation ln(v b ) = (C+D/T) for different etchant concentrations (4, 6.25 and 10 N) Table 3.3. The wave numbers of the more significant absorption peaks in the FTIR spectrum of CR-39 detector, and the corresponding assignment of the function groups and the modes of vibration...43 Table 4.1: Description of the wavelengths and corresponding energies of 3 different ultraviolet sources used in the present experiments...52 Table 4.2. Calculated bond dissociation enthalpies (kcal/mol) of CR-39 detector Table 4.3. XPS elemental composition on the un-exposed and UV-exposed CR-39 detectors...88 Table 4.4. Deconvoluted peak assignments with the corresponding theoretical binding energy and bond type of C 1s peak Table 4.5. The relative peak area calculated from the C1s XPS spectra of the virgin and UV-exposed CR-39 detectors...90 Table 4.6. Optical band gap energy E g of the virgin CR-39 and UV-irradiated CR-39 detectors...95 Table 4.7. Glass transition temperature on loss modulus and loss tangent curves as a function of UVC exposure time

CITY UNIVERSITY OF HONG KONG 香港城市大學

CITY UNIVERSITY OF HONG KONG 香港城市大學 Functionalized Luminescent Platinum(II) Molecular Hosts: Synthesis, Spectroscopic Characterization and Sensing Applications 功能性發光二價鉑分子主體 : 合成光學表徵與傳感之應用 Submitted to