Nguyen Thi Thu Ha*, Trinh Van Giap, Nguyen Van Khanh, Le Dinh Cuong, Pham Bao Ngoc and Nguyen Thi Oanh
Institute for Nuclear Science and Technology (INST), Hanoi City, Vietnam
Received Date: April 26, 2017; Accepted Date: May 23, 2017; Published Date: June 16, 2017
Citation: Ha NTT, Giap TV, Khanh NV, Cuong LD, Ngoc PB, et al. (2017) Calibration Factor for LR 115 Type II Detectors Used to Measure Indoor 220Rn . J Environ Anal Toxicol 7:476. doi: 10.4172/2161-0525.1000476
Copyright: © 2017 Ha NTT, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Major aim of the paper is to determine calibration factor for LR 115 type II detectors used to measure indoor 220Rn . To determine calibration factor, LR 115 detectors were fixed on top and bottom inside urban cup chamber and sent to NIRS, Japan to 220Rn calibration exposure. After detectors have been calibration exposure at NIRS, authors carried out following all steps of procedure, which set up at laboratory in INST, Vietnam. This paper discusses the experimental method that is used in calculating the calibration factor of 220Rn inside the urban cup, calibration factors (CF) were determined by experimental with detectors placed at top and bottom inside urban cup were 0.023 ± 0.004 and 0.038 ± 0.007(tracks.cm-2/Bq.m-3.d), respectively. The curve between integrated 220Rn concentration and tracks density with factor R2=0.96 and 0.97 at top and bottom inside urban cup, respectively. In order to precision evaluation, authors used 20 couple of LR-115 detectors at 8cm detector-source distance with relative standard deviation less than 1.48%.
LR-115 type- II; Rn-220; Spark counter
Thoron ( 220Rn ) is a natural radon (222Rn) isotope with a relatively short half-life (55.6 s), comparing with 222Rn (3.82 day). Therefore, its indoor concentrations depend strongly on the distance from the point where 220Rn is emanated [1-3]. Therefore, short half-life 220Rn which makes accurate measurements of 220Rn concentrations extremely difficult to obtain. Nevertheless, the accurate measurement of 220Rn in the environment is an important task from the viewpoint of radiation protection . Determining the radiation hazard from 220Rn and its progeny has become imperative. Additionally, determining an accurate activity concentration for 220Rn and its progeny needs to concern calibration factor (CF).
Besides this large concern in 222R, importance of 220Rn has recently been recognized, 220Rn is present everywhere together with 222R, and a quantity of 220Rn is sometimes much larger than that of 222R at a certain position in dwellings Why was 220Rn underestimated in the past studies? The following reasons can be considered: there are many difficulties in measurement and calibration 220Rn , there are no epidemiological data related to 220Rn exposure and the 220Rn risk seems to be negligible.
What is a clear difference between mine studies and indoor studies on 222Rn? It is the presence of 220Rn . When considering a radon risk in indoor studies, special attention should be paid to 220Rn present together with 222Rn. However, recent statements about the contribution of 220Rn on the dose have to be reconsidered. Studies have shown that people living in environments with high 220Rn concentration in Brazil, China and India are receiving a high dose from 220Rn [2,5-9]. This is the reason why 232Th rich environment has to be taken into account in radiological point of view to prevent elevated risk originated from high 220Rn levels.
The aim of this study was to determine calibration factor CF to measure 220Rn used to LR-115 type II detector is placed inside Urban cup. Detectors are fixed on the top and bottom inside urban cup and sent to NIRS, Japan to 220Rn calibration exposure, after calibration exposure detectors sent back to Vietnam. Detectors were etched with 2.5 N NaOH solution following the etching protocols in INST’s lab, Vietnam. Tracks developed were counted using a spark counter. The CF (tr. cm− 2 per Bq.d. m− 3) for individual experiment was obtained by relating these tracks with the respective measured gas concentrations (Tables 1 and 2).
LR-115 type II detector: Using 12 μm thick LR-115 type II cellulose nitrate alpha track detectors manufactured by Kodak Pathe, France. The detectors consist of an active layer of red cellulose nitrate on a 100 μm clear polyester base substrate as shown in Figure 1. Detectors used with dimensions 3 × 3 cm.
Urban cup chamber: It is commonly named as the “Karlsruhe” with inner base radius of 2.35 cm, top radius of 3.35 cm and height of 4.8 cm. There are holes in the brim of the chamber, which allow air to flow in and out of the chamber. LR-115 detectors are fixed in top and bottom inside urban cup as shown in Figure 2.
Spark counter system: Spark Counter Model 9201 T-G has been researched and manufactured since 1994 at INST, Vietnam.
Etching system: The system as shown in Figure 3. The etching solution NaOH 2.5 M was maintained at 60 ± 1°C in a water bath. The water bath was sheltered by a plastic cover to reduce water loss. The region of damage around each track was extended by the etchant so that the tracks became visible as small holes on the surface of the detector. After 1.5 h, chemical etching the films are rinsed in running cold water for 30 minutes, dried, and put back into a plastic bag for protection.
220Rn calibration system at NIRS, Japan: The NIRS developed a compact 220Rn calibration chamber. The chamber consists of four components: exposure system, calibration system, monitoring system and humidity control system. A 150L stainless steel vessel is used as the exposure chamber. In order to make a homogeneous distribution of 220Rn concentration, an inner fan is installed. Lantern mantles are used as the practical 220Rn source. The 220Rn concentration is measured using a single scintillation cell method .
The experimental arrangement is shown in Figure 4, authors prepared fifteen detectors LR-115 with dimensions 3 × 3 cm and six urban cup chambers. Twelve Detectors is fixed in top and bottom inside six urban cup chambers, three retained detectors to background counter. Then chambers with detectors is wrapped by polyethylene and sent to NIRS, Japan to calibration exposure. Procedure of calibration exposure is helped by Sorimachi et al. [6-9]. After calibration, exposure detectors were sent back to Vietnam. Detectors were etched with 2.5 N NaOH solution at constant temperature 60 ± 1°C following the etching protocols which set up at laboratory in INST, Vietnam. Tracks developed were counted using a spark counter is shown in Figure 5. The CF (tr. cm− 2 per Bq d m− 3) for individual experiment was obtained following equation (1). The experimental results were shown in Tables 1 and 2.
H: Exposure duration (h)
CF: Calibration factor (tr.cm-2 per Bq.m-3.d)
Table 1 summarizes the results of the average CF obtained from a few experiments with varying concentrations and exposure durations for LR-115 type II detector is placed in top inside urban cup was 0.023 ± 0.004 (tracks.cm-2/Bq.m-3.d). Relationship between integrated thoron concentration and track density at top inside urban cup as shown in Figure 6 with R2=0.96.
Table 2 summarizes the results of the average CF obtained from a few experiments with varying concentrations and exposure durations for LR-115 type II detector is placed in bottom inside urban cup was 0.038 ± 0.007 (tracks.cm-2/Bq.m-3.d). Relationship between integrated thoron concentration and track density at bottom inside urban cup as shown in Figure 7 with R2=0.97.
A practical methodology has been developed to determine average calibration factors (CF) for LR-115 type II detector. Precision evaluation authors used 20 couples of LR-115 detector at 8cm detector-source distance with relative standard deviation less than 1.48%.
The results of the average CF obtained from a few experiments with varying concentrations and exposure durations for LR-115 type II detector is placed in top inside urban cup was 0.023 ± 0.004 (tracks. cm-2/Bq.m-3.d). Relationship between integrated 220Rn concentration and track density at top inside urban cup as shown in Figure 5 with R2=0.96.
The results of the average CF obtained from a few experiments with varying concentrations and exposure durations for LR-115 type II detector is placed in bottom inside urban cup was 0.038 ± 0.007 (tracks. cm-2/Bq.m-3.d). Relationship between integrated 220Rn concentration and trackdensity at bottom inside urban cup as shown in Figure 6 with R2=0.97.
The authors are grateful to Dr. Trinh Van Giap, the Board of Institute for Nuclear Science and Technology(INST) for professional advice on several aspects of the 220Rn measurements. Authors are also grateful to Dr. Sorimachi et al. for their help during calibration exposure of detectors.