A new thoron atmosphere reference measurement system.

A new thoron reference ((220)Rn) in air measurement system is developed at the LNE-LNHB with the collaboration of the IRSN. This measurement system is based on a reference volume with an alpha detector which is able to directly measure thoron and its decay products at atmospheric pressure. In order to improve the spectrum quality of the thoron progenies, we have applied an electric field to catch the decay products on the detector surface. The developed system is a portative device which can be used to measure reference thoron atmosphere such as the BACCARA chamber at IRSN (Picolo et al., 1999). As this system also allows the measurement of radon ((222)Rn) in air, it was validated using the radon primary standards made at the LNE-LNHB. This thoron measurement system will be used, at IRSN, as a reference instrument in order to calibrate the thoron activity concentration in the BACCARA facility.

Influence of the type of CD case on the track density distribution in CDs exposed to thoron.

A novel approach for retrospective thoron (Rn-220) measurements in buildings was recently proposed. It employs CDs/DVDs as detectors, in which the alpha-tracks formed by thoron's progeny Po-212 are analyzed. Compact disks and DVDs that had been stored in their cases are suitable, because the case provides a fixed and reproducible geometry of the Po-212 source. Since the measurement and calibration procedures involve analysis of different pieces of the disk, it is important to test the homogeneity of the track density over the disk area. This report presents results of such a study, in which disks in different containers were exposed to thoron. In almost all disks, the track density was inhomogeneous, increasing significantly near the holes in the container through which thoron could enter. However, as demonstrated, in disks regularly used and randomly rotated in their containers, the track density is expected to homogenize. It is concluded that the homogeneity of the tracks should be tested in disks exposed to thoron in homes and should be estimated or compensated in calibration exposures.

The influence of thoron on instruments measuring radon activity concentration.

Thoron, the isotope 220 of radon, is a radionuclide whose concentration may influence the measurement of the activity concentration of (222)Rn in the air. If in the case of continuous and active sampling measuring instruments, using a pump for example, the influence of thoron on radon measurement is obvious and is taken into account in the apparatus, it is often assumed that in the case of a passive sampling, by diffusion through a filter for example, this thoron influence is negligible. This is due to the very short radioactive half-life of thoron, 55.6 s (3.82 d for (222)Rn), and the assumption that the diffusion time of thoron in the detection chamber is long enough beside that of the thoron half-life. The objective of this study is to check whether this assumption is true or not for different kinds of commercial electronic apparatus used to measure radon activity concentration from soil to dwellings. First of all, the devices were calibrated in activity concentration of radon, and then they were exposed to a controlled thoron atmosphere. The experiments concerning the thoron aimed to investigate the sensitivity to thoron in the radon measuring mode of the apparatus. Results of these experiments show that all devices have a very quick answer to thoron atmosphere, even though the sensitivities vary from one instrument to another. Results clearly show that this influence on radon measurement due to the thoron is observed also after the exposition because of the decay of (212)Pb and its progenies. In conclusion, the sensitivity to thoron in the radon measuring mode depends strongly on the type of instruments. The results of the present investigation show that for some apparatus, the influence of thoron cannot be disregarded especially when measuring radon in soil.

Development of a primary thoron activity standard for the calibration of thoron measurement instruments.

The LNHB and IRSN are working on a reference atmosphere for thoron ((220)Rn) instrument calibration. The LNHB, as the national metrology institute for activity measurement in France, has to create a new thoron reference standard in order to estimate with accuracy the thoron concentration of a reference atmosphere. The measurement system presented in this paper is based on a reference volume using an alpha detector, which is able to measure thoron and its decay products to define the thoron concentration of a thoron reference atmosphere. This paper presents the first results with this new system using a well-known radon ((222)Rn) atmosphere and a thoron ((220)Rn) atmosphere.