A new personal dosemeter for the individual monitoring of exposure to radon gas

Transcription

1 A new personal dosemeter for the individual monitoring of exposure to radon gas Rosabianca Trevisi a*, Paolo Orlando b, Giuseppe Arcovito b, Francesco Cardellini c, Elisabetta Fonti b, Claudia Orlando b and Marco D Alessandro a a Italian National Institute of Occupational Safety and Prevention (ISPESL) - Department of Occupational Hygiene Via Fontana Candida Monteporzio Catone (Rome), Italy b Università Cattolica del Sacro Cuore - L. go Francesco Vito Rome Italy c National Institute for Metrology of Ionising Radiations, ENEA Casaccia Research Centre, P. O. Box 2400, I Rome, Italy Abstract. Rn-Disk dosemeter, a new close-type radon device, is here proposed. Rn-Disk passive monitor consists in a diffusion chamber, made by conductive plastic material (polypropylene enriched with carbon), that holds CR-39 plastics solid-state track detectors (SSNTD). The Rn-Disk passive device (patent pending) has been designed and developed to assess radon exposure received by workers and members of the public. This passive device is an ON/OFF chamber, made of an upper and lower half which snap together during assembly. In particular, it has been designed to exclude radon progeny and dusts entry whilst it allows access of radon gas. The operation principle is based on the revolving cap capability to rotate between 0 and 180 toward its base. This paper includes a general description of Rn-Disk passive device; preliminary results of several type-tests and qualification tests, conducted in order to define the performances of the Rn-Disk dosemeter, when a CR-39 detector is enclosed in. Emphasis has been given to the investigation of Rn-Disk dosemeter performances in term of sensitivity, level of precision and accuracy of radon measurements, reproducibility, influence of environmental parameters (as temperature, humidity, dusts, etc.), etc. in accordance with ISO standards requirements. KEYWORDS: radon, passive dosemeter, personal dosimetry 1. Introduction Some of the most commonly used passive radon dosemeters consist in a track detector within a closed holder, which allows 222 Rn (radon) to diffuse into it, excluding radon decay products entry. Track detectors register only the signal related to alpha particles generated by radon inside the holder and decay products formed from it. Their increasingly use is due to their properties, such as simplicity, robustness, cheapness and easy of automation of track counting [1]. In last decades, many new radon monitoring devices have been developed in order to give practical solution to the problems connected with the estimation of individual dose of workers due to radon exposure, according to the request of ICRP (Publication n. 65) [2] and as implementation of EU BSS (Council Directive of the European Union 96/29/Euratom) within Europe [3]. On the basis of experimental results coming from radon monitoring in mines [4] and NRPB radon intercomparisons experience [5], general requirements of a personal radon dosemeters can be summarised as follow: * Presenting author, rosabianca.trevisi@ispesl.it 1

2 small holders (about 2 cm high), electrically conductive holders in order to prevent electrostatic charges deposition of radon decay products on the inner surface; independent response on the environmental conditions (including high humidity, altitude pressure, presence of mud, etc.). The experience gained with the implementation of EU BSS [3] highlighted the need for the availability of devices and procedures that carefully estimate the individual radon dose. This is especially required for those working conditions where the approach based on area monitoring of average radon exposure of workers is not adequate or sufficient. For most of the cases and in accordance with the current regulation, the assessment of the individual exposure to radon based on time-weighted area monitoring could be successfully performed. Conversely if an accurate values of the individual radon dose are needed, both the knowledge of spatial and temporal variation in radon levels and details on the exposure s time in each location are required. Dixon [6] showed that the choice between an area monitoring or a personal dosimetry depends on the characteristics of workplace and work organisation: in particular area monitoring is suitable when exposure is below the investigation level of interest, while personal dosimetry is preferred in case of high exposure level and/or workplaces with high radon level dishomogeneity. Starting from previous experiences on the application of NRPB/SSI device in area monitoring in workplaces, special efforts have been made towards the development of a new personal radon dosemeter with the same CR-39 detector used in the NRPB/SSI radon passive device. The development of Rn-Disk dosemeter is the result of a collaboration of three scientific Institutions: UCSC (Università Cattolica del Sacro Cuore), ISPESL (Italian National Institute of Occupational Safety and Prevention) and INMRI-ENEA (National Institute for Metrology of Ionising Radiations, ENEA Casaccia Research Centre). The present paper describes the new holder with its main characteristics and technical specifications as preliminary results of Rn-Disk qualification trials. 2. Rn-Disk dosemeter - Technical characteristics During last few decades, new radon monitoring devices have been designed and developed to solve practical problems occurring in the estimation of radon exposure in workplaces. Some of these new prototypes are passive devices based on the use of plastic damage track detectors (SSNTD), equipped with activation/deactivation ( ON/OFF ) systems. Passive radon devices equipped with ON/OFF systems are able to assure the measurement of actual radon levels [7, 8], or the estimation of individual dose due to radon during the working activity [9, 10]. In fact the ON/OFF system provides an easy solution to issues connected with uncontrolled exposure such as the estimation of radon transit exposure, the contribution during not-working time (night, week-end) and the response in case of multiple consecutive samplings. Rn-Disk is a new passive personal device designed to measure actual integrated exposure to radon. This new dosemeter (fig. 1) is a conductive plastic material (polypropylene enriched with carbon) holder, made of an upper half (a mobile semi-spherical cover cap) and a lower half (a fixed base) which snap together by means of a central pivot: the assembling is unique thanks to dedicated channels. Moreover, the geometry of the cover cap is such that once assembled with the base it creates an asymmetric diffusion chamber; in fact the cover cap is internally divided into two parts: one empty and one filled. The base has a slightly larger diameter to allow the 180 rotation of the cap. The 2

3 volume of air, to which the detector is exposed, depends on the position of the cover cap respect to the base: by means a 180 rotation, the cap can reach two different positions labelled ON and OFF. This device is characterised by small dimension, light weight, presence of an eyelet that makes it easily portable. The system is equipped with an air gap that allows the diffusion of radon gas but excludes the entry of its decay products. The size of the air gap determines a diffusion time of about 10 minutes (τ) and assures the insensitivity of the device to 220 Rn (thoron) [11, 12]. The Rn-Disk device encloses a CR-39 (PADC, Columbia Resin 1939; LxWxH 37x13x1 mm) detector, similarly to the NRPB/SSI radon passive device [11, 13]. Figure 1: Rn-Disk Passive Device (Patent Pending) 3. Operating principle The operating principle of Rn-Disk device is based on the capability to vary the diffusion chamber volume at which CR-39 detector is exposed. In particular, once assembled, if the device is turned on the ON signed position, the empty side of the cap is positioned on the plastic detector and creates a diffusion chamber of an internal volume of 12.7 cm 3. Alternatively, if the device is switched to OFF position, the diffusion chamber is in opposite position in respect to the detector and the volume of air to which the detector is exposed is negligible: this rotating capability allows the selection of the OFF mode each time the measurement needs to be stopped or put on hold. In figure 2 the different position of Rn-Disk device cap are illustrated. In ON position the CR-39 detector is exposed to an air volume equal to diffusion chamber: inside the chamber, radon gas freely circulates and its derivatives alpha particle emissions can produce damage on detector surface as signal (latent tracks). Moreover, the presence of a safety release ensures the position of the cap and the integrity of the selected mode between the different phases. 3

4 Figure 2: Rn-Disk device in ON and OFF position the ON/OFF sign position 4. Procedure for detectors analysis This procedure is applied when CR-39 TASTRAK (TASL, UK) is employed as detector. After the exposure to radon gas, the CR-39 detectors are chemical etched in a water solution of potassium hydroxide 30% w/v at 80 C under constant stirring. The temperature control of the water bath is regulated with an accuracy of 1 C. More details about chemical etching have been already described elsewhere [13, 14]. The chemical treatment enhances the effect of the micro damages (latent tracks) generated on the detector surface by radon and its derivatives alpha particles, and allows an easy identification. The counting is performed manually by an optical microscope with medium low resolution (typically 5x or 10x) or by means automatic image scanning system. Track density (cm -2 ) is proportional to radon exposure (kbq h/m 3 ): the sensitivity (S) of the system expresses this relationship. 5. Characterisation of Rn-Disk passive dosemeter performances This paper describes the initial results of the qualification program in place as well as preliminary type tests performed in order to check the compliance with requirements of the radon measurement systems. The qualification trials consisted in exposure at different radon levels in calibration chamber under controlled environmental conditions. Rn-Disk device qualification trials have been executed at the National Institute for Metrology of Ionising Radiations (INMRI, Italy) facilities [15, 16, 17]. Table 1 summarises the environmental parameter ranges, such as temperature, humidity and atmospheric pressure recorded during the devices calibration. Table 1: Environmental conditions during Rn-Disk dosemeters calibrations Environmental conditions Range Temperature ( C) Relative humidity (%) Atmospheric pressure (kpa)

5 Rn-Disk dosemeters have been subjected to qualification tests in order to show that the design meets given performance requirements, such a: linearity of response, the level of precision, the level of accuracy. 5.1 Study on the linearity of response to radon exposure A calibration program has been performed in order to determine the response to radon of Rn-Disk measurement system. For this scope sets of 10 Rn-Disk dosemeters have been exposed to a series of known values of radon concentrations. The exposure trials have been carried out with increasing concentration level of radon. In particular sets of Rn Disk dosemeters have been exposed to radon values corresponding to 427, 1038, 2146 kbq h m -3, respectively. In every trial transit radon exposure has been estimated by means of a control dosemeters group. After exposure plastic detectors have been treated following standard protocols about handling, chemical etching, counting and data analysis. Experimental results are graphically reported in fig. 3 The net value of track density (cm -2 ) versus radon exposure (kbq h m -3 ) are plotted: net tracks density values have been obtained by subtracting mean transit exposure result. In the range between 100 and 2000 kbq h m -3 the linear behaviour is clearly displayed and the relative curve equation is reported: the slope equals to ,1 with a R 2 correlation of : the slope value represent the sensitivity (S) of the system, expressed in terms of m 3 /kbq h cm 2. Figure 3: Linearity of response of Rn-Disk dosemeter at different radon exposure level 7000 track density (cm 2 ) y = 2,6555x R 2 = 0, radon exposure (kbqh/m 3 ) 5

6 5.2 Quality assurance program US EPA describes a quality assurance program [18] as an integrated system or a program of activities involving planning quality control, quality assessment, reporting and quality improvement ensure that the product or service meets defined standards of quality. As part of the quality assurance program, in particular for homogeneity and reproducibility, blind tests have been executed at INMRI facilities. The tests comprised two sets of 10 Rn-Disk dosemeters exposed to a defined radon level unknown to the participants. Here below experimental results are analysed and discussed to determine precision, as an expression of the batch homogeneity, and accuracy, as an expression of reproducibility Estimation of the level of precision Replicated measurements, consisting in two or more simultaneous measurements, that can be used to estimate the precision error of the system and are very useful initially and/or whenever the measurements system is altered as stated by US EPA [18]. A program of performing replicated measurements allows to monitor the component of measurement error caused by random differences in devices and/or in the measurements process. To estimate the precision error of the measurement system, experimental data has been evaluated adopting the HPA (UK) procedure for the analysis of participants results during international intercomparison of passive radon dosemeters [5]: the level of precision has been expressed in terms of coefficient of variation, defined as standard deviation of results provided from a set of devices exposed to the same radon value divided by mean value. Table 2 summarises principal conclusions: in particular, the first column contains the reference radon exposure, the second the mean value of estimated radon exposure (expressed as arithmetic mean of experimental results from each set of 10 dosemeters) and in the last the coefficient of variation are reported. It can be observed that for exposure to 576 kbq h m -3 the estimated mean value was 604 kbq h m -3 with a coefficient of variation of 7.0. Moreover for exposure to 1578 kbq h m -3 the estimated mean value was 1582 kbq h m -3 with a coefficient of variation of 5.9. It can be pointed out that the achieved results are very good and demonstrate the homogeneity of the response of the new devices. Table 2: Estimation of precision error of Radon Disk measurement system Reference exposure Estimated exposure Coefficient of variation (kbq h m -3 ) (kbq h m -3 ) 576 ± ± ± ±

7 5.2.2 Estimation of the level of accuracy ISO standard on passive radon monitoring devices requires [19] that the reference radon exposure will fall in an interval minor of ± 25% of the estimated value. Starting from this requirement, experimental data have been analysed in order to check if this criterion is met. In Table 3 principal conclusions are summarised: in particular, in the first two columns, the reference radon exposures during the blind test and the mean value of estimated radon exposure (expressed as arithmetic mean of experimental results from each set of 10 dosemeters) are respectively reported. In the last column the percentage differences between the above values are given. It can be observed that for exposure to 576 kbq h m -3 the percentage difference was equal to 4.6 % while for exposure to higher level the percentage difference was 1.5%. Table 3: Estimation of level of accuracy of Radon Disk measurement system Reference exposure Estimated exposure % Difference (kbq h m -3 ) (kbq h m -3 ) 576 ± ± ± ± Rn-Disk dosemeters have successfully met the ISO requirement. Authors want to point out that both results are very satisfactory since the % difference is below 5% Study of Response of Rn-Disk dosemeter in the OFF position The protocol related to calibration program required the exposure of a small group of Rn-Disk dosemeters in OFF position in each trial with the aim to study the response of the new device in OFF position. The ON/OFF operating principle of Rn-Disk device could reduce the risk of uncontrolled exposure prior and after use. This characteristic could allow a simplification of laboratory and storage protocols as well as quality control procedures. Table 4 summarises the experimental results: the analysis of data reveals that the inactivation of Rn- Disk dosemeters in OFF mode is not complete but it averages around 10% the exposure of the ON mode. It is important to stress that the background noise registered on the different prototypes in the OFF mode at each exposure level does not vary in percentage. Additional studies to reduce the background noise are still ongoing. Table 4: Response of Rn-Disk dosemeter in the OFF position Reference exposure (kbq h/m 3 ) Rn-Disk (OFF) exposure (kbq h/m 3 ) % Exposure recorded by Rn-Disk (OFF) 427 ± ± ± ± ± ±

8 3. Conclusion and perspectives Rn-Disk personal dosemeter is a new passive device designed for the monitoring of the individual radon exposure. It possesses innovative and unique capabilities with the presence of an ON/OFF system, that allow to evaluate the real worker exposure. This dosemeter represents a valuable technical support in applications where the average radon concentration, deriving from the mathematical integration over time, can lead to an over or under estimation as well as those cases where worker protection is required. A research program has been designed and is being executed: preliminary results have been shown. Requirements, such as linearity of response, level of precision and level of accuracy have been verified. The reported results demonstrate that the Rn-Disk personal dosemeter ensures reliability and reproducibility combined with easiness of use while additional efforts to reduce the background noise are still ongoing. These peculiar characteristics open to additional development for application and refinements beside the design of protocols for its proper use in working environment. The experience encourages the authors to go on in their studies and to investigate those occupational situations in which the Rn-Disk passive monitor can be suggested as suitable device for area and/or personal dosimetry of radon, according to the legal obligations connected with the implementation in EU Countries of the Basic Safety Standards. REFERENCES [1] TOMMASINO, L., CHEROUATI, D.E., SEDIEL, J.L., MONNIN, M., A plastic bag sampler for passive radon monitoring. Nucl. Tracks Meas. 12 (1986) [2] International Commission on Radiological Protection (ICRP). Protection against 222 Rn at home and at work. ICRP Publication 65, Ann. ICRP 23, Oxford, UK, Pergamon Press (1993). [3] Council Directive 96/29-EURATOM of May laying down basic safety standards for the protection of the health of workers an the general public against the dangers arising from ionizing radiation. Legislation. Official Journal of the European Community L. 159 vol. 39 (1996). [4] FRANK, A.L., BENTON, E.V., A diffusion chamber radon dosimeter for use in mine environment. Nucl. Instrum. Methods, 109 (1973) [5] HOWARTH, C.B., MILES, J.C.H., Results of the 1999 European Community intecomparison of passive radon detectors. REPORT EUR EN (2000). [6] DIXON, D.W., A stratified approach to individual dosimetry for radon daughters in mines. Radiat. Prot. Dosim., 82 (1999) [7] CALAMOSCA, M., PENZO, S., GUALDRINI, G., The features of the new radon gas CR-39 dosemeter developed at the ENEA Institute of Radioprotection. Rad. Meas. 36 (2003) [8] CALAMOSCA, M., PENZO, S., GUALDRINI, G., Experimental determination of CR-39 counting efficiency to α particles to design the holder of a new radon gas dosemeter. Rad. Meas. 36 (2003) [9] SCIOCCHETTI, G., COTELLESSA, G., SOLDANO, E., PAGLIARI, M., A new technique for measuring radon exposure at working places. Rad. Meas. 36 (2003) [10] SCIOCCHETTI, G., COTELLESSA, G., SOLDANO, E., PAGLIARI, M., A novel approach for testing passive radon monitors with an exposure standard based on alpha track detector. Rad. Meas. 40 (2005)

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