Dose thresholds and quality assessment by statistical analysis of routine individual monitoring TLD data

Customers of individual monitoring services often use the reported dose data not only to verify compliance with legal dose limits, but also to assist in implementing the ALARA principle. Since ICRP, in Report 60, recommended lowering the annual dose limit by, on average, a factor of 2.5, the ALARA goals tend to be reduced as well and often approach natural background levels. This implies that the doses to be measured can easily be of the same order as the threshold of measurability of the dosimetry system. If the net occupational dose is determined by subtracting the contribution from natural background radiation, the standard deviation at the 0.00 mSv level provides important information. In our case this standard deviation appears to be 0.013 mSv for fortnightly dosemeters. This includes a variation of 0.01 mSv (one SD) due to variations in the natural background from place to place in The Netherlands. The standard deviation appears to be almost constant up to dose's of about 0.20 mSv. This implies that the thresholds as mentioned in ANSI N13.11 (appendix D) are: critical lever, L(C) = 0.02 mSv and detection level L(D) = 0.04 mSv for alpha and beta = 5%.