United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

Determination of Radiation Exposure from Dosimeters

HPPOS-186 PDR-9111210292

Title: 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

Applicability: All

Page Last Reviewed/Updated Thursday, March 29, 2012