Radon and Thoron Air Concentration Measurement

Contribution to the “Italy in Japan 2011” initiative Science, Technology and Innovation

Radon and Thoron Air Concentration Measurement Service (RS) M. Calamosca, S. Penzo, E. Consoli1

Abstract Since 2002 ENEA provides a Radon air concentration measurement Service for workplaces and dwellings monitoring. In the last decade the perception of the radon risk in causing lung cancer is worldwide increasing, also as a consequence of new epidemiological data. On the other hand the paucity of scientific data on the indoors thoron presence accounts for why this radioisotope looks less important to the human being health; otherwise in some far east and central Italian regions its radiological importance has been proved to be as significant as radon. Our Radioprotection Institute has developed a new thoron air concentration measuring device, based on twin alpha track CR-39 passive devices. As for radon alpha track detector passive device, the uncertainty related to the thoron measurement needs to be optimized and stated by the calibration step. The need of a System for Test Atmosphere to expose the measuring device to controlled atmospheres is fully realized for the 222Rn, but not for the 220Rn yet. Actually one of the Institutes, able to provide this metrological step, is the Environmental Radiation Effects Research Group (NIRS, Chiba, Japan), that yearly organizes an international inter-comparison on radon and thoron passive devices, and also periodic workshops on active measurement monitoring. Also because of the factual collaboration between our institutes, our thoron calibration facility has been significantly improved so to reduce the uncertainty associated to the thoron exposures. Keywords Radon, Thoron, NORM, Calibration Introduction The ENEA Radioprotection Institute (IRP), established in the ’60, has been primarily involved in R&D activities in the radioprotection field, with one of goals addressed to improve the quality of its measurement Services. The changes in our national regulatory standards, occurred in 2000, gave IRP the occasion to develop an own technical approach to tackle the goal to both qualify the radon measurement and to better assess the dose to worker and population exposed to natural ionizing radiation. In the last years the perception of the radon risk in causing lung cancer is worldwide increasing, also as a consequence of new epidemiological data [1]. On the other hand the paucity of scientific data on the presence of thoron in dwellings and workplaces is one of the reasons of why this radioisotope looks less important to the human being health, nevertheless somewhere its radiological importance has been proved to be at least as significant as radon. Recent proposed documents [1] have pointed out the thoron issue, and in particular how and why to measure it.

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ENEA- Radiation Protection Institute, [email protected]

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There is a general consensus, also outlined in the course of the Thoron 2010 workshop hold at NIRS [2], pointing out that the thoron issue involves both a problem of dose and underestimated risk, and a not well known contribution to the radon measurement signal with a potential to confound radon epidemiology. It must be stressed that the statement “the room thoron concentration is…” is meaningless if you want to assess the risk of such exposure. So we have decided to develop a regulated procedure to assess the whole risk associated with the inhalation of thoron and its decay products (often connected with a significant external exposure); of course, of primary importance in the procedure is the measurement of the thoron concentration at a fixed (defined) distance from the wall. In this context we say to refer about this measurement as an index of the thoron concentration. So we had to design a new alpha track detector thoron passive device (ATD-TnPD), to be used in a twin configuration coupled to our already patented radon device. To introduce the ATD-TnPD, it basically consists of a modification of the radon passive device (ATD-RnPD), which is supplied by our RS. The Radon air concentration measurement Service Since 2002 ENEA-IRP supplies a service for the evaluation of the 222Rn air concentration, in accordance with the Italian standards. In the qualification phase of the RS the design and characterisation of a new radon exposure holder was a primary concern. The new holder was designed relying on MCNPX Monte Carlo simulations of the detector response in different irradiation and readout conditions [3,4]. The last modification, patent MI2006A000703 [5,8], basically the insertion of a stainless steel shield between effective volume and detector (Figure 1), avoids the need of the transit detector, allows a direct background evaluation on the same detector and in the same etching conditions, detects the presence of spread defects not pointed out at the acceptance test and reduces the assembly cost. At least yearly the RS participates at one or more international intercomparisons [6,7] on nuclear tracks passive device, in order to check our quality standard. Table 1 shows all the results obtained with the IRP ATD-RnPD.

Fig. 1 - IRP radon holder images, with the stainless steel screen and the CR-39 detector

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Intercomparison

Average COV Precision

Average COV Accuracy

Sum

Correction factor

NRPB 2004 NRPB 2005 APAT 2006 HPA 2006 HPA 2007 HPA 2008 HPA 2009 HPA 2010 Bfs 2010 NIRS 2010

8,5% 9,2% 5,5% 6,0% 6,5% 9,0% 7,3% 5,6% 3,5% 10,4%

9,6% 3,8% 10,6% 12,4% 3,6% 7,8% 2,8% 3,7% 6,5% 7,3%

18,1% 13,0% 16,0% 18,4% 10,1% 16,8% 10,1% 9,2% 10,0% 17,7%

1,096 1,031 0,894 0,876 1,036 0,961 1,026 1,028 0,944 0,932

Table 1 - ENEA RS results at the international intercomparisons

The thoron air concentration measurement service A new thoron ATD-TnPD [9] has been designed and calibrated (Figure 2). The purpose was to develop a twin device, without changing own radon holder geometry. To reduce the ATD-TnPD diffusion time a sufficient number of openings were worked out on the sidewalls of the holder and covered by a screen wire tissue. To reduce the ATD-TnPD response to alpha particles emitted by 222Rn and 218Po the thickness of the window, facing the CR-39 detector surface, has been slightly increased.

Fig. 2 - The ENEA twin ATD in its proper working position

The ATD-TnPD response has been calibrated at PTB, at NIRS and by our thoron calibration circuit [10], that we are going to briefly describe. The 220Rn exposure apparatus (figure 3) consists of a closed-circuit composed by a circulating pump, the thoron source, the exposure chamber, the active thoron monitor and the pneumatic radon cleaner circuit [10,11]. The total airflow is set at 18.0 l min-1. The active thoron monitor (TnM), inserted at the exit of the exposure chamber, is a modified version of the classic two filters method, i.e. primarily characterized by the absence of the second filter. Its length was reduced to optimize the growth of 216Po at its operating flow. The assessment of the TnM response factor is fundamental to evaluate the mean thoron concentration within the exposure chamber. It was determined both directly and by comparison with a RAD 7 monitor.

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Fig. 3 - Schematic diagram of the thoron exposure apparatus circuit

Conclusions Recent data have shown that doses due to thoron and its progeny can no longer be considered negligible, so that to more accurately assess the health impact to indoor thoron there is a need to improve the passive methods to measure thoron and its decay products. There is also a urgent demand of calibration facilities and of shared measurement protocols. Our work follows this general requirement, showing “de minimis” a step forward a more comprehensive characterization of our thoron calibration apparatus, but also some interesting new instrumentation designed to be used in laboratory but that could be adapted for an in field use.

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References 1.

World Health Organization. WHO handbook on indoor radon - A public health perspective. ISBN 978 92 4 154767 3. WHO, Geneva, 21 September (2009).

2.

Tokonami. S., J. McLaughlin, L. Tommasino and H. Harley. International workshop on environmental thoron and related issues. Rad. Prot. Dosim. Vol. 141, No. 4, p. 315, 2010.

3.

Calamosca, M., Penzo S. and G. Gualdrini. “Experimental determination of CR-39 counting efficiency to alpha particles to design the holder of a new radon gas dosemeter”, Radiat. Meas., 36, 217-219, 2003.

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Calamosca, M., Penzo, S. and G. Gualdrini. “The features of the new radon gas CR-39 dosemeter developed at the ENEA IRP”, Radiat. Meas., 36 221-224, 2003.

5.

Calamosca M. e S. Penzo. “Dispositivo di campionamento passivo per rivelatori di tracce nucleari per la valutazione della esposizione di radon in aria senza impiego di rivelatore per le esposizioni di transito”. Brevetto (MI2006A000703) depositato in data 10/4/2006.

6.

Calamosca M. and S. Penzo. The technical framework supporting the accreditation action of the ENEA ION-IRP radon service, Proc. 11th Int. Congress IRPA, ISBN 84-87078-05-2, 2004.

7.

Calamosca M., Penzo S., Rossetti M. and A.O. Mustapha. “Meeting new challenges in radon measurements service with solid state alpha track analysis” Rad. Prot. Dosim. Vol. 125, No. 1-4, pp. 576-580, 2007.

8.

Calamosca M. and S. Penzo. “Are ageing and fading really a problem for the quality of the passive radon measurements?”, Radiat. Meas., 43 S422-S426, 2008.

9.

Calamosca M. and S. Penzo. “A new CR-39 nuclear track passive thoron measuring device”, Radiat. Meas. Vol. 44, N. 9-10, 2009.

10. Calamosca M. and S. Penzo. “How to assess the sensitivity and related uncertainty of a new solid state passive thoron measuring device”, Appl. Radiat. and Isotopes 67, 854859, 2009. 11. Calamosca M. and S. Penzo. “The ENEA-IRP Thoron calibration facility” Rad. Prot. Dosim. Vol. 141, No. 4, pp. 468-472, 2010.

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