TY - JOUR
AU - Roca,, V
AB - Abstract The most used passive detectors for Radon measurement are the CR39s, both for the good stability of the material and for the practicality of use. But, commercial reading systems are expensive and not always fast. The aim of the present work was the development of a method for a rapid, efficient and economic evaluation of the result of the indoor Radon measurement performed with CR39 detectors. The analysis and acquisition of detector images were performed using a photo scanner and the free ImageJ software. Several groups of CR-39 detectors were exposed, developed and analysed. Calibration curve was obtained in a wide range of exposure values (200–12 000 kBq·h·m −3) to allow the procedure to be applied in all possible measurement environments. Furthermore, a statistical study was carried out on the shape and size of nuclear tracks after chemical development. The dependence of the track size on Radon exposure was effective in showing the trace saturation effect as well. INTRODUCTION Among the Radon and Thoron alpha particles monitors(1–6), solid-state nuclear track detectors (SSNTDs) are the most used nowadays(7, 8,), and the CR-39 detectors (made of poly-allyl-diglycol-carbonate) are the most widespread, both for the good stability of material and for the practicality of use(9). Ordinarily, the phases of the analysis of a CR-39 detector, after the exposition in a measurement environment, are two: the etching, to become visible the tracks thanks to an aggressive chemical solution; the image acquisition, the counting and analysis of the tracks, conducted using an optical microscope(8). The latter phase is usually very expensive and slow, because of the microscope cost and performance(8, 9,). Both low cost and quality-assured track evaluation methods are in great demand. Recently, some scanner-based SSNTDs evaluation methods have been introduced, which classifies tracks based on the morphological parameters of the applied image analyser software(9, 10). These methods need validations to be usable, reliable and for the verifiability of the measurement results. For this task, appropriate calibration of the measurement systems is required, mainly instructed to handle the formation of tracks of different shape. In the present study, a new scanner-based CR-39 track analysis methodology is developed, validated and appropriately calibrated through several European inter-comparison participation. The image acquisition of the CR-39 detector is performed using an Epson Perfection V800 Photo scanner; the counting and the analysis of the tracks are made using the image processing software ImageJ (free of charge). This tracks analysis methodology by scanner is efficient, fast and economic. This work is part of research conducted by the Environmental Radioactivity laboratory of University(11–15). MATERIALS AND METHODS In this study, 25 × 25 × 1.5 mm INTERCAST CR-39 detectors have been used(16). The acquisition area is 14 × 14 mm2. Each detector is placed within a PVC container (diameter 50 mm, height 20 mm) that acts as a diffusion barrier. The CR-39 will not be any use at continuous temperatures of ~40°C and above. Etching process After the exposure of the CR-39 detectors, they (placed in special grids) are etched in 25% weight/volume sodium hydroxide (NaOH) solution, at 98°C and 1.181 g·cm−3 density for 1 h. Then, the detectors are immersed in 2% weight/volume acetic acid (CH3COOH) solution for 30 min. At last, the detectors are rinsed in distilled water for 1 h. Scanner-based CR-39 track analysis method The procedure developed for the acquisition and analysis of tracks produced by Radon α-particle in CR-39 detectors, coming from several tests, is described. Image acquisition by scanner The acquisition of the CR-39 detectors images have been performed using the Epson Perfection V800 Photo scanner® with a double lens, able to acquire images transparently (Figure 1). The image acquisition parameters of the scanner have been set on the Epson Scan software associated with the device. The scanner operates with the following settings: Type: transmissive; Source: document table Auto exposure: photo; Colour: 16 bit grey; Resolution: 4800 dpi; Size: 14 × 14 mm2; Adjustments: unsharp mask, blacklight correction, dust removal, tone correction. Figure 1 Open in new tabDownload slide Image of a Radon exposed CR-39 detector acquired by scanner procedure (left), and the tracks analysis zoom in part of CR-39 performed by ImageJ software (right). Figure 1 Open in new tabDownload slide Image of a Radon exposed CR-39 detector acquired by scanner procedure (left), and the tracks analysis zoom in part of CR-39 performed by ImageJ software (right). Figure 2 Open in new tabDownload slide Calibration curves of the scanner-based CR-39 track analysis method for Radon exposure 200–12 000 kBq·h·m−3: track density vs exposure (a), exposure vs track density (b). Figure 2 Open in new tabDownload slide Calibration curves of the scanner-based CR-39 track analysis method for Radon exposure 200–12 000 kBq·h·m−3: track density vs exposure (a), exposure vs track density (b). Figure 3 Open in new tabDownload slide The calibration curves in Figure 2 for Radon exposure from 200 to 3000 kBq·h·m−3: tracks·cm−2 vs exposure (a), and exposure vs tracks·cm−2 (b). Figure 3 Open in new tabDownload slide The calibration curves in Figure 2 for Radon exposure from 200 to 3000 kBq·h·m−3: tracks·cm−2 vs exposure (a), and exposure vs tracks·cm−2 (b). In detail, the adjustments: Unsharp mask: making the edges of image areas clearer for an overall sharper image; Blacklight correction: removing shadows from image that have too much background light; Dust removal: removing dust marks; Tone correction: remapping the darks and the lights of image to have a good distribution of tones, with the right amount of contrast. Before each acquisition picture, the scanner surface was cleaned using alcohol. A grid placed inside the scanner allowed multiple acquisition of 28 detectors consecutively. Tracks analysis by free ImageJ software After the acquisition of the CR-39 detector images by scanner, the analysis of the images has been performed using the free image processing ImageJ software (https://imagej.nih.gov/ij). Two different analysis are performed: the counting of the tracks and the morphologic analysis of the tracks (Figure 1). The tracks counting is performed through the sequential application of the following functions provided by the ImageJ software: Accurate Gaussian Blur filter: convolution with a Gaussian function for smoothing of each pixel of the image; Bandpass filter: deconvolution of large structures (shading correction) and small structures (smooth-ing) by Gaussian filtering in Fourier space, according to spatial frequency; Make binary: converts the image to black and white (white is the background colour, black is the fore-ground colour); Find stack maxima: determination of the local maxima in a binary image. The morphological analysis of the geometry shape of the tracks is performed through several functions implemented in ImageJ that provide for each track: Area (μm2); Circularity (4π·Area·Perimeter−2) that indicate how a track shape approaches a circle; Aspect ratio: (major axis/minor axis) that represents the relationship between tracks width and height. RESULTS AND DISCUSSION CR-39 background The background of not-exposed CR-39 detector was analysed using the same procedure for exposed detectors, described in the previous section. The inspection was performed on 10 detectors: the average background tracks number is 2170 ± 257, and the average number of tracks·cm−2 is 1107 ± 131. Each measurement performed with CR-39 detector is corrected for that background. Ageing and fading effects are negligible(16). Validation and calibration The proposed scanner-based CR-39 track analysis methodology has been appropriately validated through several different exposure measurements performed: (1) in our Radon Exposure Chamber using a pylon RNC-RN-1025 226Ra source(17–19); (2) by National Institute of Ionizing Radiation Metrology of the National Agency for New Technologies, Energy and Sustainable Economic Development (INMRI-ENEA) in Rome; (3) during European intercomparison campaigns of Federal Office for Radiation Protection (BfS) in Berlin during 2014 and 2015, and of Italian Association for Radiation Protection (AIRP) in Lurisia spa complex during 2014, where our analyses were performed using the proposed scanner-based CR-39 track analysis method and the other participants used optical microscope method. The proposed method is validated and calibrated in the range of exposure values from 200 to 12 000 kBq·h·m−3, in order to allow the procedure to be applied in all possible measurement environments. Calibration curves were obtained and displayed in Figure 2, fitted by Matlab® procedure(20–23): track density (tracks·cm−2) vs exposure (1) and vice versa (2). The limits of detection are not computed experimentally but could only be extrapolated from calibrations curves. The repeatability and reproducibility of the proposed method are ensured by using of six CR-39 detectors at each Radon exposure. The track density increase according to the equation y = 1910.7·ln(x) − 10 519 (R2 = 0.98) with the increasing of the Radon exposure. The exposure increases according to equation y = 254.7·e0.0005·x (R2 = 0.98) with the increasing of the tracks·cm2. Considering the range of Radon exposure from 200 to 3000 kBq·h·m−3, suitable for indoor places and workplaces, the calibration curves can be approximated by linear functions (Figure 3). Within this limited exposure range: the tracks·cm−2 linearly increase, according to equation y = 1.69x + 292.71 (R2 = 0.98), with the increasing of exposure; the exposure linearly increases with the tracks·cm2 increasing, according to equation y = 0.54x − 151.44 (R2 = 0.97). Morphological analysis of the tracks A large investigation was carried out to study the dependence among the shape and the size of nuclear tracks on the Radon exposure. Figure 4 reports three fitted graphs representing the values of area (μm2), circularity and aspect ratio vs the Radon exposure values. The average area of the tracks increase according to the equation y = 24.7·e7E−5x (R2 = 0.93) with the increasing of the exposure, caused by overlapping effect of the tracks(8, 24). Within the limited exposure range 200–3000 kBq·h·m−3, the average area of the tracks results independent of the exposure (linear behaviour). Figure 4 Open in new tabDownload slide Morphological analysis of the tracks produced by Radon α-particle in CR-39 detectors, vs the Radon exposure from 200 to 12 000 kBq·h·m−3. The graphs show the area (a), the circularity (b) and the aspect ratio (c) vs Radon exposure. Figure 4 Open in new tabDownload slide Morphological analysis of the tracks produced by Radon α-particle in CR-39 detectors, vs the Radon exposure from 200 to 12 000 kBq·h·m−3. The graphs show the area (a), the circularity (b) and the aspect ratio (c) vs Radon exposure. The average circularity of the tracks linearly decreases according to the equation y = −2E−5·x + 0.94 (R2 = 0.97) with the increasing of the exposure. The average aspect ratio of the tracks linearly increases according to the equation y = 3E−5·x + 1.32 (R2 = 0.96) with the increasing of the exposure. Considering the limited exposure range 200–3000 kBq·h·m−3, the average circularity and aspect ratio indicate that tracks are almost circular. With the increasing of the Radon exposure, the track density increases (Figure 1), and, overlapping effect of the tracks occurs making tracks no longer circular(8, 24). CONCLUSIONS The proposed procedure developed for the acquisition and analysis of tracks produced by α-particle of Radon and its daughters in CR-39 detectors, is efficient, fast and economic and has provided effective results during intercomparison participation around Europe. The acquisition of detector images was performed using Epson Perfection V800 Photo scanner and the tracks analysis of the detectors was performed using free ImageJ software. The methodology was validated and calibrated for exposure range of 200–12 000 kBq·h·m−3: up to 3000 kBq·h·m−3 the track density increases linearly with the increasing of the exposure, while there is no linear trend over 3000 kBq·h·m−3. 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TI - ANALYSIS BY SCANNER OF TRACKS PRODUCED BY RADON ALPHA PARTICLES IN CR-39 DETECTORS
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncaa140
DA - 2020-11-27
UR - https://www.deepdyve.com/lp/oxford-university-press/analysis-by-scanner-of-tracks-produced-by-radon-alpha-particles-in-cr-Wwksj7gK0M
SP - 154
EP - 159
VL - 191
IS - 2
DP - DeepDyve
ER -