To develop a measurement protocol which was optimised for the lower outdoor concen- trations, the following issues were addressed:
• minimising the background track density on the unexposed CR-39 detectors
• the choice of method used to measure track density (manual or automated track counting)
• minimising the uncertainty in the background track density
During the production of CR-39 detectors a certain number of alpha tracks may be laid down; these are called background tracks. The detectors are manufactured and cut from sheets of PADC, and the background track density has been found to vary both within sheets and between sheets [107]. Furthermore the manufacturer (TASL Ltd., UK) states that the background track density is different for the top and bottom surface of a detector, so the top (identified by the 6 digit serial number) is always used when counting tracks. The background track density and the associated uncertainty ultimately
8 hr etch 16 hr etch 24 hr etch Manual counting
ATD1(tracks/cm2) 12.4 20.0 27.0
Standard error in count, % ± 11.4 ± 8.9 ± 7.7
Limit of detection (tracks/cm2) 41.9 47.6 67.4
Automatic counting
ATD (tracks/cm2) 33.9 43.3 64.8
Standard error in count, % ± 6.8 ± 6.1 ± 4.9
Limit of detection (tracks/cm2) 79.3 79.3 137.9
Table 3.1: A comparison of background counts as a function of counting method and etch time.
1Average track density
limit the lower detection limit of the detectors. Detectors with a low background count were requested for this project from the manufacturer. These detectors are manufactured in a low radon environment (<5 Bq/m3) and only briefly exposed while they are being taken out of the oven in which they are cured. They are then wrapped up in radon-proof bags for storage prior to being sent to the customer [108].
The limit of detection (LoD) of a detector (CR-39 in this study) is the lowest quantity of a substance that can be recorded reliably or with a statistical certainty. It is typically deduced by measuring blank samples and calculating the LoD as the mean + (2 x standard deviation) [109]. In this study a more conservative statistical approach was taken and the LoD was calculated as the mean + (3 x standard deviation), corresponding to a 99.7% confidence in the calculation. To investigate the effect of pre-etching on the detection limit and determine if it is possible to ’etch-out’ background tracks through a longer etch time, 30 detectors were etched in sets of 10 for 8 (single etch), 16 (double etch) and 24 (triple etch) hours. The detectors were then counted both automatically and manually to investigate which was the more appropriate method; the results are shown in table3.1. Figure3.6depicts the change in alpha track size with increasing etch time.
Manual counting for all etch times gave a lower background count. The larger back- ground automatic count is due to aberrations in the plastic which the automated system counted as alpha tracks. The longer etch time did not etch-out background tracks but
(a) 8 hour etch.
(b) 16 hour etch.
(c) 24 hour etch.
instead, along with enhancing alpha tracks, enhanced any aberrations which can be indistinguishable from true tracks (figure3.8a). This gave rise to the increase in track density with etch time. The quoted standard errors in the counts are ultimately due to Poisson counting statistics. As can be seen from table 3.1, for all etch times the manual method gave a lower detection limit; hence it was decided to use the manual counting method for both the remainder of the uncertainty minimisation study and the outdoor study. Along with these 30, now pre-etched, detectors, an additional 30 un-etched detectors were exposed to a known radon concentration for 19 hours in a walk-in radon chamber.
This chamber consists of a purpose-built stainless steel, walk-in room filled with radon from a radium-226 source in which the radon concentration is about 2,500 Bq/m3 (figure 3.7). The radon chamber is located at the EPA, Dublin, Ireland, and it is used to calibrate their CR-39 detectors and for research projects. It has a glass observation window and entry to the chamber is by an airlock two-door system. It can take one month for the radon concentration in the chamber to build up from zero; therefore the source is normally left unshielded and the radon gas is in equilibrium. The radon concentration in the chamber is monitored using an Atmos 12 dpx; this continuous monitor is the EPA’s traceable reference detector and is detailed in Chapter 4.
An exposure time of 19 hours was chosen because an exposure to about 2,500 Bq/m3 for 19 hours, equates to approximately the same track density (140 tracks/cm2) that an exposure to 5 Bq/m3 over 12 months would yield. 5 -10 Bq/m3 was the expected outdoor radon concentration range. Following exposure, the previously etched 8, 16 and 24 hours sets of detectors were etched for a further 8, 16 and 24 hours respectively and the previously un-etched detectors were etched in groups of 10 for 8, 16 and 24 hours.
At the longer etch times of 16 hours and 24 hours, distinguishing between true alpha tracks and aberrations in the plastics became difficult (figure 3.8a). Also, as the longer etch time did not etch-out background tracks, an 8 hour pre-etch and an 8 hour post- etch time were used as the optimised conditions for this study. A pre-etch gave the advantage of easy distinction between background tracks and tracks laid down following the pre-etch (figure3.8b) and, by counting the detectors after the pre-etch, a background concentration could be defined for each individual detector.
Figure 3.7: EPA radon chamber.
(a) Aberrations in a 24hr etch. (b) 8 hour pre and post-etch.
8 hr etch 16 hr etch 24 hr etch Pre & Post-etch
Sensitivity (tracks.m3/cm2.kBq.h) 4.5 4.7 4.3
Post-etch
Sensitivity (tracks.m3/cm2.kBq.h) 4.4 5.3 4.8
Table 3.2: Sensitivities for various etch times.
The effect of the longer etch time on the sensitivity factor was also investigated by exposing 6 sets of 5 detectors (3 sets pre-etched and 3 sets not) in the radon chamber for 1 week and the results are reported in table 3.2. The detectors are most sensitive for a 16 hour post-etch, however, the difference in sensitivity over an 8 hour etch was small and did not outweigh the advantage of the better specificity between tracks and background at the shorter 8 hour etch time.