Effects of radiations on the characteristics of alpha and fission tracks in CR-39 detectors

Abstract

The effects of neutron, gamma and alpha radiations on the alpha and fission fragment tracks registration and revelation properties of CR-39 detectors (CR-39 and CR-39(DOP) were studied. It was found that the ratio of the bulk etch rate of irradiated to unirradiated (V_G(irr.)/V_G(unirr.) detectors is linearly dependent on dose. An exponential decrease in fission track densities with increase in neutron fluence was observed. The ratio of V_G(irr.)/V_G(unirr.) was found to be high in CR-39 than that in CR-39(DOP) exposed to the same reactor neutron fluence. The decrease in fission track densities with increase in neutron fluence was observed to be faster in CR-39 than in CR-39(DOP). This indicates that doping with dioctyl phthalate improves the radiation resistance of CR-39 detectors. It was observed that in detectors exposed to an alpha flux of the order of 9.36 × 10⁶ / cm², the fission track density was reduced by 11% and thereafter it remained constant. The results also indicate that thermal neutron fluence up to 7.01 ×10¹¹ neutrons/cm² does not affect the alpha and fission track densities. I.R. spectra were also studied to find out the nature of chemical changes produced by these radiations on CR-39.     

Nonlinear absorption properties correlated with the surface and structural changes of ultra short pulse laser irradiated CR-39

We have investigated femtosecond laser irradiation effects on the surface topography, structural changes and nonlinear absorption properties of CR-39. For this purpose, a CR-39 target was exposed in air to 25 fs, 800 nm Ti: sapphire laser radiation at fluences ranging from 0.25 J cm⁻² to 3.6 J cm⁻². The surface of irradiated CR-39 probed by an Atomic Force Microscope (AFM) exhibits the formation of several topographical structures, like bumps, explosions and nano cavities. Raman spectroscopy is performed to explore chemical and structural modification of the irradiated target. The spectroscopy reveals changes such as cross linking, bond breaking, formation of new bonds etc. in the fundamental structure of the polymer after irradiation. In order to establish a correlation between morphological and structural changes with the changes in the nonlinear absorption of the irradiated CR-39, a Z-scan technique was employed. A comparison of experimentally obtained data from Z-scan measurements with our calculations predicts the dominance of three-photon absorption in the case of pristine CR-39, whereas for irradiated targets concurrence of three- and two-photon absorption is probable. Nonlinear absorption increases with increasing laser fluences and is well correlated by surface and structural changes revealed by AFM and Raman spectroscopy.     

Study of optical band gap, carbonaceous clusters and structuring in CR-39 and PET polymers irradiated by 100 MeV O ions

Commercially purchased CR-39 and PET polymers were irradiated by 100 MeV O⁷⁺ ions of varying fluences, ranging from 1×10¹¹ to 1×10¹³ ions/cm². The effects of swift heavy ions (SHI) on the structural, optical and chemical properties of CR-39 and PET polymers were studied using X-ray diffraction (XRD), UV-visible spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The XRD patterns of CR-39 show that the intensity of the peak decreases with increasing ion fluence, which indicates that the semicrystalline structure of polymer changes to amorphous with increasing fluences. The XRD patterns of PET show a slight increase in the intensity of the peaks, indicating an increase in the crystallinity. The UV-visible spectra show the shift in the absorbance edge towards the higher wavelength, indicating the change in band gap. Band gap in PET and CR-39 found to be decrease from 3.87 to 2.91 and 5.3-3.5 eV, respectively. The cluster size also shows a variation in the carbon atoms per cluster that varies from 42 to 96 in CR-39 and from 78 to 139 in PET. The FTIR spectra show an overall reduction in intensity of the typical bands, indicating the degradation of polymers after irradiation.     

Yields of formation and scissions at ether bonds along nuclear tracks in CR-39

Yields of CO₂ gas formation and the scissions at ether bonds have been determined along ion tracks in CR-39 plastics. Thin CR-39 films with thickness were irradiated by 130 MeV C-13 ions at 45 incidence up to , and then IR spectra were obtained without removing the samples from the vacuum chamber. The absorption band for CO₂ was observed clearly around , whose absorbance increased proportional to the fluence up to 3×10¹² ions/cm² and then saturated. Using a mole absorption coefficient for gaseous CO₂, the yield along the track was determined to be molecules/μm. This means that the G-value of CO₂ formation in CR-39 was about 3.6 molecules/100 eV. The scission yield of ether bonds was found to be 11,000 scissions/μm. This corresponds to a G-value of about 5.5 scissions/100 eV. These values suggest that the parts between the two carbonate ester bonds in each repeat unit of CR-39 can be segmented into two CO₂ and other small molecules along the nuclear tracks.     

Ion beam modification of surface properties of CR-39

The effects of ion-beam bombardment on the physical and chemical properties of poly(allyl diglycol carbonate) (CR-39) polymer have been investigated. CR-39 samples were bombarded with 320 keV Ar and 130 keV He ions at fluences ranging from 1 × 10¹³ to 2 × 10¹⁶ ions/cm². The nature and extent of radiation damage induced were studied by UV–VIS spectrometry, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, as well as Vickers' hardness measurements. In addition, the effect of ion fluence on the wetting properties of ion-beam bombarded CR-39 polymer was determined by measuring the contact angle for distilled water. UV–VIS spectra of bombarded samples reveal that the optical band gap decreases with increasing ion fluence for both Ar and He ions. In the FTIR spectra, changes in the intensity of the bands on irradiation relative to pristine samples occurred with the appearance of new bands. XRD analyses showed that the degree of ordering of the CR-39 polymer is dependent on the ion fluence. Changes of surface layer composition and an increase in the number of carbonaceous clusters produced important change in the energy gap and the surface wettability. The surface hardness increased from 10.54 MPa for pristine samples to 28.98 and 23.35 MPa for samples bombarded with Ar and He ions at the highest fluence, respectively.     

N+ Implantation-Induced Surface Hardening of Poly (Allyl Diglycol Carbonate) Polymer

The effect of 100 keV N⁺ ions implantation on the surface structure and hardness of poly (allyl diglycol carbonate) (CR-39) polymer was studied. The surface hardness of virgin and implanted CR-39 specimens was determined using a Knoop microhardness test. The surface hardness was found to be enhanced after implantation, e.g., becoming eight times higher at a load of 9.8 mN, for a dose of 2 × 10¹⁶ ions cm^?2. The change in bonding and surface structure of the CR-39 polymer due to implantation was studied using the specular reflectance Fourier transform infrared (FTIR) technique. The disordering produced in the implanted matrix was estimated using the Urbach edge method from the UV-Visible absorption spectra. The relationship of surface hardening with the chemical and structural changes was explored     

A study of the effect of gamma and laser irradiation on the thermal,optical and structural properties of CR-39 nuclear track detector

A comparative study of the effect of gamma and laser irradiation on the thermal, optical and structural properties of the CR-39 diglycol carbonate solid state nuclear track detector has been carried out. Samples from CR-39 polymer were classified into two main groups: the first group was irradiated by gamma rays with doses at levels between 20 and 300 kGy, whereas the second group was exposed to infrared laser radiation with energy fluences at levels between 0.71 and 8.53 J/cm². Non-isothermal studies were carried out using thermogravimetry, differential thermogravimetry and differential thermal analysis to obtain activation energy of decomposition and transition temperatures for the non-irradiated and all irradiated CR-39 samples. In addition, optical and structural property studies were performed on non-irradiated and irradiated CR-39 samples using refractive index and X-ray diffraction measurements. Variation in the onset temperature of decomposition T _o, activation energy of decomposition E _a, melting temperature T _m, refractive index n and the mass fraction of the amorphous phase after gamma and laser irradiation were studied.

It was found that many changes in the thermal, optical and structural properties of the CR-39 polymer could be produced by gamma irradiation via degradation and cross-linking mechanisms. Also, the gamma dose has an advantage of increasing the correlation between thermal stability of the CR-39 polymer and bond formation created by the ionizing effect of gamma radiation. On the other hand, higher laser-energy fluences in the range 4.27–8.53 J/cm² decrease the melting temperature of the CR-39 polymer and this is most suitable for applications requiring molding of the polymer at lower temperatures.     

Reduction of background in CR-39 SSNTD using chemical pre-etching methods

Presently, CR-39 Solid State Nuclear Track Detectors (SSNTD) is being used for Fast Neutron Personnel Monitoring in India. Individual detectors are cut manually from CR-39 sheets. Laser-cutting of the sheets will ensure precision and also save on the laborious efforts involved in sheet cutting. It was observed that there is a slight increase in background tracks due to laser-cutting. Studies were carried out on the effect of chemical pre-etching at higher temperatures in properties of CR-39 such as the background, sensitivity and minimum measurable dose. For this purpose a pre-etch step was included in the normal processing of the CR-39 foils in an elevated temperature electrochemical cell (ETECE cell) and the duration for pre-etch step at elevated temperature was optimized. Samples of CR-39 exposed to dose equivalents ranging from 1 mSv to 10 mSv using Am-Be neutron source were processed by both the methods, under normal processing using ECE cell and in the optimized chemical pre-etch followed by normal processing. The dose responses of CR-39 processed by both methods are compared in this paper.      

Atomic force microscopy, Raman spectroscopy and nonlinear absorption properties of femtosecond laser irradiated CR-39

Investigations have been performed to explore ultrashort laser irradiation effects on the surface topography as well as structural and nonlinear absorption properties of a polymer CR-39. For this purpose, a CR-39 target was exposed in air to 25 fs, 800 nm Ti:sapphire laser radiation at fluences ranging from 0.25 J cm⁻² to 3.6 J cm⁻². The surface features, structural changes and nonlinear absorption were explored by AFM, Raman Spectroscopy and a Z-scan technique, respectively. Several topographical structures like bumps, explosions and nano cavities have been observed on the irradiated surface. Raman spectroscopy reveals changes in the fundamental structure of the polymer after the irradiation. Nonlinear absorption data contained by the Z-scan technique predict the dominance of three-photon absorption in case of pristine CR-39. Furthermore, nonlinear absorption (three or two photon) increases with increasing laser fluences and is well correlated with surface and structural changes revealed by AFM and Raman spectroscopy.     

Electrochemical etching technique for neutron dosimetry

In the present work we have employed allyl diglycol carbonate (CR-39) and cellulose triacetate (CTA) plastic for detection of neutron recoil tracks without radiator. For CR-39, the results reveal that registration efficiency is a function of duration of chemical pre-etching and the best results are obtained with chemical pre-etching of 3 hours. It was also investigated that the ac field strength of 28 kV/cm having 2.5 kHz frequency was optimum for revelation of tracks. Interestingly the sensitivity is fluence dependent and it was constant up to a fluence of about 10⁸ n/cm². The sensitivity abruptly decreased with increased fluence. At optimum experimental conditions the minimum detection limit for CR-39 was found to be 0.47 mSv. For CTA, the tracks have been revealed by electrochemical etching (ECE) only and the minimum detection limit was found to be 0.85 mSv at optimum experimental parameters.      

Effect of Transferred Electronic Energy Density on Optical,Electrical and Structural Properties of Polyallyl-Diglycol Carbonate (CR-39) Polymer

Commercially available polyallyl-diglycol carbonate (CR-39) track detectors have attracted wide interest in many fields of science and technology. This is because of their low cost, relatively easy handling, and being more similar to human tissue than other passive detectors. After the Fukushima accident, there was a need to study the impact of the released alpha particles from radionuclides. In this study, CR-39 polymers were irradiated with α-particles with different linear energy transfers 11.11, 19.70, and 28.77 eV/Å, at fluences of 49,490/cm² and 2,482,763/cm². The modifications in the optical, electrical, and structural properties induced by the radiation were measured. The results showed that low values of transferred electronic energy density along the alpha particle tracks led to a slight increase in the optical band gap energy and a small reduction in the conductivity. The amorphous nature of the CR-39 samples was not affected by the changes of the transferred electronic energy density.     

Neutron response study using poly allyl diglycol carbonate

The results of an experimental work aimed at improving the performance of the CR-39 nuclear track detector for neutron dosimetry applications are reported. A set of CR-39 plastic detectors was exposed to ²⁵²Cf neutron source, which has the emission rate of 0.68 × 10⁸ s⁻¹, and neutron dose equivalent rate 1 m apart from the source is equal to 3.8 mrem/h. The detection of fast neutrons performed with CR-39 detector foils, subsequent chemical etching and evaluation of the etched tracks by an automatic track counting system was studied. It is found that the track density increases with the increase of neutron dose and etching time. The track density in the detector is directly proportional to the neutron fluence producing the recoil tracks, provided the track density is in the countable range. This fact plays an important role in determining the equivalent dose in the field of neutron dosimetry. These results are compared with previous work. It is found that our results are in good agreement with their investigations.      

Experimental study of laser induced decomposition based processing of a brittle plastic,CR-39 (allyl diglycol carbonate)

A cw CO₂ laser, coupled with an astigmatism free beam focussing mirrors arrangement is used for processing a brittle plastic, CR-39 without producing cracks, vents or chips. The processing is based on the formation of volatile products of laser-induced decomposition in the plastic. Threshold fluence for the decomposition (found to be independent of the power density and beam residence time) in CR-39 at=10.6m is determined to be 25 J cm^–2 and the decomposition threshold power density for cw irradiation 2.1±0.5 W cm^–2. The depth and width of the tapered laser processed region are observed to increase with power density and beam residence time. The widths attain a steady state value of 1 mm at beam residence time above 65 ms, for a fixed power density (2.5×10⁴ W cm^–2) and sheet thickness (250 m). Taper angle of the edges decreases with increasing power density and/or beam residence time. The heat affected zone (measured in crossed polarisers) around the processed region is found to extend with increasing beam residence time but remains unaffected on changing power density. The results are discussed in terms of the optical and thermophysical properties of CR-39 and the parameters of the interacting laser beam.     

Swelling and structure of radiation induced near-surface damage in CR-39 and its chemical etching

Systematic measurements of swelling of CR-39 nuclear track detector (NTD) due to irradiation with fission fragments and alpha particles over a wide range of fluences from ²⁵²Cf are presented here. Precisely designed and optimized exposure and chemical etching experiments were employed to unfold the structure of radiation induced surface damage. Delays in the startup of the chemical etching of latent tracks in low radiation fluence detectors are measured and are found to contain important information about structure of the surface damage. Simple atomic scale pictures of radiation induced surface damage and its chemical etching are developed using measurements of radiation induced swelling of CR-39 detectors and nuclear track parameters. The computer code SRIM2010 was utilized for the calculations of basic features of latent tracks of fission fragments and alpha particles in CR-39. Another computer code TRACK_VISION was used to compute parameters of etched tracks. Computations and experimental findings in the paper coherently compose a realistic picture of radiation damage.     

Color changes in CR-39 nuclear track detector by gamma and laser irradiation

A study of the effect of gamma and laser irradiation on the color changes of polyallyl diglycol (CR-39) solid-state nuclear track detector was performed. CR-39 detector samples were classified into two main groups. The first group was irradiated with gamma doses at levels between 20 and 300 kGy, whereas the second group was exposed to infrared laser radiation with energy fluences at levels between 0.71 and 8.53 J/cm². The transmission of these samples in the wavelength range 300–2500 nm, as well as any color changes, was studied. Using the transmission data, both the tristimulus and the coordinate values of the Commission Internationale de l'Eclairage (CIE) LAB were calculated. Also, the color differences between the non-irradiated samples and those irradiated with different gamma or laser doses were calculated. The results indicate that the CR-39 detector acquires color changes under gamma or laser irradiation, but it has more response to color changes by gamma irradiation. In addition, structural property studies using infrared spectroscopy were performed. The results indicate that the irradiation of a CR-39 detector with gamma or laser radiations causes the cleavage of the carbonate linkage that can be attributed to the ?H abstraction from the backbone of the polymer, associated with the formation of CO₂ and ?OH with varying intensities.     

High Energy Charged Particle Registration in CR-39 Polycarbonated Detector

Track etch rate characteristics of CR-39 plastic detector exposed to ²⁸Si ions of 670 MeV energy have been investigated. Experimental results were obtained in terms of frequency distribution of the track diameter, track density and bulk etching rate. A dependence of the mean track diameter on energy was found. The application of the radiation effect of heavy ions on CR-39 in the field of radiation detection and dosimetry are discussed. Results indicated that it is possible to produce etchable tracks of ²⁸Si in this energy range in CR-39. We also report the etching characteristics of these tracks in the CR-39 detector.     

Use of chemical etching of CR-39 foils at elevated temperature for fast neutron personnel monitoring in India

CR-39 Solid State Nuclear Track Detecting foils (SSNTD), along with 1 mm thick polyethylene radiator, sealed in triple laminated pouches, are used for country wide Fast Neutron Personnel Monitoring in India. With the present system of processing by elevated temperature electrochemical etching (ETECE) and evaluation using automatic image analysis, only 16 foils are processed at a time and it is useful over the dose equivalent range 0.2 mSv to 10 mSv. It has been reported that, by processing CR-39 of good detection efficiency by chemical etching at elevated temperature, more numbers of foils can be processed simultaneously. In the present study, CR-39 foils from Pershore Moulding (UK) have been chemically etched using 7 N KOH under various conditions of temperature and etching durations and evaluated using high magnification microscope. The duration of chemical etching, has been optimized at a constant temperature of 60°C for chemical etching process. The characteristics of the chemically etched CR-39 foils are compared with the characteristics of the CR-39 foils processed by the existing system of ETECE and the detailed results are presented in the full text of the paper. It has been observed that by chemical etching process, the dose equivalent range of CR-39 foils can be extended above 60 mSv.      

Efficient alpha particle detection by CR-39 applying 50 Hz-HV electrochemical etching method

Alpha particles can be detected by CR-39 by applying either chemical etching (CE), electrochemical etching (ECE), or combined pre-etching and ECE usually through a multi-step HF-HV ECE process at temperatures much higher than room temperature. By applying pre-etching, characteristics responses of fast-neutron-induced recoil tracks in CR-39 by HF-HV ECE versus KOH normality (N) have shown two high-sensitivity peaks around 5–6 and 15–16 N and a large-diameter peak with a minimum sensitivity around 10–11 N at 25°C. On the other hand, 50 Hz-HV ECE method recently advanced in our laboratory detects alpha particles with high efficiency and broad registration energy range with small ECE tracks in polycarbonate (PC) detectors. By taking advantage of the CR-39 sensitivity to alpha particles, efficacy of 50 Hz-HV ECE method and CR-39 exotic responses under different KOH normalities, detection characteristics of 0.8 MeV alpha particle tracks were studied in 500 μm CR-39 for different fluences, ECE duration and KOH normality. Alpha registration efficiency increased as ECE duration increased to 90 ± 2% after 6–8 h beyond which plateaus are reached. Alpha track density versus fluence is linear up to 10⁶ tracks cm⁻². The efficiency and mean track diameter versus alpha fluence up to 10⁶ alphas cm⁻² decrease as the fluence increases. Background track density and minimum detection limit are linear functions of ECE duration and increase as normality increases. The CR-39 processed for the first time in this study by 50 Hz-HV ECE method proved to provide a simple, efficient and practical alpha detection method at room temperature.     

Measurements of neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets bombarded by 0.5 and 1.0 GeV protons

Measurements have been carried out on neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets and the surrounding paraffin moderators bombarded by 0.533 and 1.0 GeV protons. CR-39 detectors were deployed on the surfaces of targets and moderators to record the neutrons produced in the targets. The measurements show that:

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Gamma-photon induced modifications in Polyvinylchloride (PVC) film

Gamma photon induced modifications in Polyvinylchloride (PVC) polymer have been studied in the dose range of 10¹-10⁶Gy at room temperature. Polyvinylchloride films have been irradiated with different doses of gamma radiation from a ⁶⁰Co source. To monitor the chemical and structural changes caused by gamma radiation IR and UV-Vis spectroscopy of pristine and irradiated PVC have been performed and using the spectral data some changes in the properties have been investigated. The spectral studies have indicated that at a dose of 10⁶Gy scissioning of the C-a bonds takes place. This scissioning of the chain initiates decomposition at a lower temperature. Thermal stability of the polymer reduces due to irradiation but the decomposition patterns remain the same. The optical band-gap is found to decrease due to irradiation at doses bigher than 10³Gy.