Angular Dependence

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

TLD chips of lithium fluoride (0.32 cm x 0.32 cm) are about 240 mg/cm2 thick. Significant

attenuation of the beta field takes place through the body of the chip. As a result, these types of TLD

chips under-respond to uranium decay betas by a factor of about 2.

Other TLD systems minimize this problem by adhering a thin layer of TL powder onto a plastic

backing. Current TLD personnel dosimeters typically use multiple detectors (typically, four) under

different filter thicknesses. The different responses of each element are used as input to an algorithm

which provides an estimate of the effective radiation energy and the doses at depths of interest.

Detectors that are very thin minimize energy-dependence. Film detectors demonstrate a high

energy-response dependence for low-energy photons, as well as beta energy-response dependence

(though the beta response is less variable than that of TLD chips).

Current systems could potentially provide accurate and precise information; however, their

complexity can lead to problems. Calibration of these systems should be performed by a person with

specific expertise in the detector's system and knowledge of badge response to high beta or mixed beta

and gamma radiation fields.

6.4.2 Angular Dependence

The dosimeter elements must be mounted in a badge or element holder. The assembled badge

usually displays severe angular dependence. Fortunately, in most cases, a worker's normal movements will

tend to average out some of this dependence. Some badge holder arrangements can flip the badge

completely over so that the "beta window" of the badge is facing the worker, not the source. The design of

the badge holder or strict administrative controls should be utilized to minimize this problem.

6.4.3 Dosimetry Practices

Beta and gamma fields in working areas should be well-characterized. See previous figures and

tables as examples. An attempt should be made to correlate survey instrument and dosimeter badge results.

Badge reading frequency should be long enough to accumulate a significant dose (100-mrem range) and

short enough to allow adequate control. Dosimeter change frequencies can vary with the specific work-site

conditions.

Although multiple badging is not usually necessary, it should be considered for use in very high

beta fields produced by separated uranium decay products. The dosimetry system used shall meet or be

specifically excepted from DOELAP standards (10 CFR 835.402(b)) and be specifically designated for

measuring both shallow and deep doses from uranium.

Dosimetry systems should be capable of providing routine results within a reasonable time

period. The system of badge collection and re-distribution should be well defined and minimize the

possibility of lost badges.

Badge reading systems should have established "action levels" to alert technicians or operators

of unusual results. Such results should include readings or TLD element ratios in excess of certain levels.

If possible, the system should automatically save glow curves of any unusual results.

The potential for badge contamination should be minimized. Where the potential for badge

contamination exists, badges should be frequently checked for contamination.

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