Bare-track detectors

Yonehara, H., H. Kimura, M. Sakanoue, E. Iwata, S. Kobayashi, K. Fujimoto, T. Aoyama, and T. Sugahara, Improvement in the Measurement of Radon Concentrations by a Bare Track Detector, this volume (1987). 

The accuracy of the measurement of radon concentrations with bare track detectors was found to be unsatisfactory due mainly to the changes of the deposition rate of radon progeny onto the detector as a result of air turbulence. In this work, therefore, a method was developed which can correct the contributions of the deposition to the track densities by classifying the etched tracks according to their appearance, i.e. round or wedge shaped. Using this method, about 30% improvement in the error of measurements was achieved. The calibration coefficient ob tained by experiment was 0.00424 tracks/cm /h/(Bq/m ), which agreed well with the calculated value. Comparison was also made of the present method with other passive methods, charcoal and Terradex, as to their performance under the same atmosphere. 

Ikebe et al., 1984) are useful for precise measurements of low level radon, they are too expensive to make long-term measurements at numerous locations. The bare track detectors had been studied before the two other types of detectors were developed (Rock et al., 1969 Alter and Fleischer, 1981). The devices are suitable for measurements in a large number of dwellings, because the detectors are very inexpensive and can be sent and set up easily. The materials used for alpha particle detection are an allyl diglycol... 

YONEHARA ET AL. Improving Bare- Track-Detector Measurements... 

The sources of a-rays which produce the tracks on the bare CR-39 detectors are divided into airborne activity and activity deposited on the surface of the detectors. The relationship between time-averaged radon concentration (Cq) and the track density (T) on the bare track detector is represented by... 

In order to assess the accuracy of the present method, we compared it with two other methods. One was the Track Etch detector manufactured by the Terradex Corp. (type SF). Simultaneous measurements with our detectors and the Terradex detectors in 207 locations were made over 10 months. The correlation coefficient between radon concentrations derived from these methods was 0.875, but the mean value by the Terradex method was about twice that by our detectors. The other method used was the passive integrated detector using activated charcoal which is in a canister (Iwata, 1986). After 24 hour exposure, the amount of radon absorbed in the charcoal was measured with Nal (Tl) scintillation counter. The method was calibrated with the grab sampling method using activated charcoal in the coolant and cross-calibrated with other methods. Measurements for comparison with the bare track detector were made in 57 indoor locations. The correlation coefficient between the results by the two methods was 0.323. In the case of comparisons in five locations where frequent measurements with the charcoal method were made or where the radon concentration was approximately constant, the correlation coefficient was 0.996 and mean value by the charcoal method was higher by only 12% than that by the present method. 

Measurements were made using two types of passive track-etch alpha dosimeters. One of them was the bare detector of CR-39. After exposure these dosimeters were etched by 30 % NaOH at 70°C for 5 hours. The number of pits was scored under a microscope with a television camera in Shiga University of Medical Science. Methods of calibration and adjustment for deposition of radon daughters introduced by Yonehara (Yonehara et al., 1986) were adopted. The second detectors were Terradex type SF (Alter and Price, 1972). These detectors consist of a plastic cup, covered by a filter to allow entry only of gases, with a track-etch detector inside. The reading of results was carried out by Terradex Corp. in Walnut Creek, California, U.S.A.. The measurements of radon concentration were carried out by both methods in each location, except for Hokkaido where the measurements were done only by Terradex. However, the data obtained by CR-39 detectors will be mainly presented in this paper, because the two methods did not give identical results as separately reported in this proceedings by Yonehara et al. (Yonehara et al., 1986). 

On other hand, we found a good correlation between the results by the present method and those by the Terradex detector. However, the mean value obtained by the Terradex detector were about twice those by the present method. The reasons for this significant difference are unknown and may be due to errors in the calibration experiments and in the conditions during the measurements. In the calibration experiments the effect of existence of thoron in the chambers could be one of the reasons. As regards the condition in the measurements, methods for subtraction of background tracks and deposition of dust onto the bare detector could be candidates. However, we do not have enough data to determine the reasons for some of this difference. 

Using the present method, the bare CR-39 detector becomes useful for the long-term measurements of radon concentrations in various dwellings. The present method is useful for laboratories which wish to measure radon concentrations at low cost. The practical calibration coefficient Kc equals to 0.00424 tracks/cm /h/(Bq/m ). 

Two types of track-etch monitor occur, open and closed types. In the open type, the SSNTD is not contained in a volume and is exposed to the air as a bare foil. This detector will register the alpha radiation from the Rn and RnD in the air, and the track density on the foil represents the sum of these activities. However, the Rn signal will be much larger than the signal from the RnD, except at very high levels of RnD (high F factor), and the track density has to be interpreted in terms of this ratio, which is typically unknown. In close monitors the SSNTD is enclosed in a closed container into which Rn diffuses through a filter. This prevents the entry of RnD and dust particles into the chamber, and the foil is then sensitive only to the alpha radiation from Rn and RnD formed in the container. There is a repeatable equilibrium between the isotopes in the container, and calibration provides the relationship between the Rn concentration and the track density on the foil. A typical track-etch radon monitor of the closed type is shown in Fig. 9.27. 

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