Influence of Background Exposure on TLD Minimum Dose Detection and Determination Limits

Thermoluminescence dosemeters (TLD) are widely used to monitor personal doses due to photon fields in the radiation protection of workers. For these low dose range applications it is essential to decide whether a single TLD retains an occupational air kerma value above background or not. In other words, it is important to determine the detection limit LD and the determination limit LQ of the whole dosimetric system. LD is the smallest value of air kerma that can be detected at a specified confidence level, LQ is the minimum value of air kerma that can be measured with a given precision. ICRP 60 recommends an annual dose limit of 1 mSv for the public. Thus it is possible to take this value as the limit between public and exposed workers, as recommended by the European Community Directive and corresponding local laws. Therefore if NLD(t), the annual detection limit of the TLD system, is considerably less than 1 mSv, TLD evaluation can also be used to decide whether a worker can be considered exposed or not. N is the number of annual TLD readings, LD is the detection limit of the TLD system. Both N and LD depend on the monitoring interval time t. The dependence of LD and LQ on the background radiation level and on the monitoring intervals, i.e. on N, is investigated. Assuming that the background air kerma level is approximately constant, it can be shown that the shorter the time of exposure, the higher the annual detection limit. Furthermore, if one considers an annual dose limit of 1 mSv, LD is seen to be greater than the recording level Lr for short exposure times and the system is not able to evaluate it. A set of 15 TLD-100(LiF: Mg, Ti) dosimetric cards and a Harshaw 6000 TLD automatic card reader have been used in order to determine LD and LQ. It has been found that an increase of the monitoring time t from 30 to 180 days leads to a reduction of the annual detection limit from 0.32 ± 0.10 mGy to 0.100 ± 0.030 mGy. For t <180 days the recording level Lr is less than LD; for t <45 days the investigation level Li is less than LD.