Determination of Radiation Exposure from Dosimeters
See the memorandum from W. P. Ellis to G. L. Snyder (and others) dated March 25, 1977.
When both a direct reading dosimeter (DRD) and a film or TLD badge are worn, the film or TLD reading is normally considered the dose of record. If a film or TLD badge is exposed when not worn, it may be appropriate to use a DRD reading. The purpose of the badge dosimeter is to measure the radiation dose received by the individual who wears it.
For example, if a badge dosimeter shows a reading of 3.5 rem for a month or quarter, the nuclear industry and NRC have historically accepted this as proof that the individual received a radiation dose of 3.5 rem if one cannot show that the exposure to the badge most likely occurred when the employee was not wearing it.
Although all facts surrounding an overexposure should be established, the inspector does not need to establish additional proof that a radiation exposure occurred.
However, if there is cause to believe that the individual was not exposed, it is incumbent on the licensee to demonstrate or provide evidence that the exposure to the badge dosimeter did not constitute a valid exposure to its user.
NRC does not take the position that badge readings are not accepted as valid exposures of personnel if there is not other positive proof to support the finding; rather, in the interest of safety, we must accept the badge readings as valid radiation exposures of personnel unless the licensee can provide reasonable evidence to the contrary.
A second point of concern is the consideration of DRD values versus the film or TLD badge in establishing an individual's radiation dose. Generally, the DRD has not been accepted by the nuclear industry or NRC as the dosimeter of record. It is true that on some occasions when a film or TLD badge was inadvertently exposed while not used by the designated user, the DRD has been used as the best evidence of the individual's exposure.
However, there are too many variables involved to use the DRD in lieu of the film or TLD badge. Therefore, the DRD is considered to be a control device (i.e., only an indicator of the estimated dose). The DRD as a general rule is highly energy dependent. Many such dosimeters are made of metal or other materials with high atomic numbers which absorb many of the low energy photons.
Consequently, we find that the film or TLD readings are higher than the DRD for the same exposure to multi-energy photons. The DRD may show a lower radiation exposure than the film or TLD because of the error in numerous readings at the start and end of each work period. On the other hand, the exposures estimated from DRDs could also establish error on the high side, dosimeters can drift or discharge when bumped and are not considered reliable even to the extent of their limited ranges.
When exposure data is collected for an individual by both DRD and either film or TLD badge, the dose as determined from the film or TLD should be accepted as the individual's exposure of record.
Often a licensee will explain that the DRD readings were 2.5 rem (at the control point) and the film or TLD readings was 3.3 rem or some similar value. The latter reading is the most representative of the individual's exposure to radiation if all other factors are equal. This is frequently the source of failure to make adequate survey or evaluation of the radiation levels which results in exposure to individuals in excess of the regulatory limits.
We cannot accept the licensee's explanation of error in calculations of the estimated dose from DRDs as reasons to forgive failure to make proper evaluations of such potential exposures.
Finally, questions concerning exposures that resulted from licensed byproduct material and other unlicensed sources of ionizing radiation such as x-ray or radium were answered. If any part of an individual's exposure results from licensed byproduct materials, the NRC has jurisdiction for taking enforcement actions for the total exposure. If an individual were to receive 3 rem from x rays and 0.3 rem from gamma rays emitted by cobalt-60 for a total of 3.3 rem in a single quarter, the NRC would issue a citation for a radiation dose of 3.3 rems and indicate that it exceeds the permissible quarterly limit.
[Note: The "new" 10 CFR Part 20 does not have quarterly or other limits covering periods of less than a year. In order to maintain compatibility with ICRP recommendations for dose limitation, the quarterly limit has not been kept and only annual dose limits are stated in 10 CFR 20.1201 (a). The annual limit for the whole body is 5 rem (0.05 Sv). If an individual were to receive a "deep-dose equivalent" of 5 rem from x rays and 0.3 rem from gamma rays emitted by cobalt 60 for a total of 5.3 rem in a calendar year, the NRC would issue a citation for a dose equivalent to the whole body of 5.3 rems and indicate that it exceeds the permissible annual limit.]
Regulatory references: 10 CFR 20.101, 10 CFR 20.201, 10 CFR 20.1201, 10 CFR 20.1501
Subject codes: 8.1, 8.3