The State of California's Health and Human Services Agency, Department of Health Services, states that personnel monitoring devices should fulfill at least four criteria:
- Provide a record of the quantity of exposure.
- Provide a measurement of the accumulated exposure over a specific period of time, mainly per month.
- Provide some indication of the type and quality of the incident radiation and the rate at which it was received.
- Provide a legally acceptable record of personnel exposure.
It should be noted that the personnel monitoring device should be kept in the same location on the body, and that location should be noted. Remember, however, that the real exposure the individual receives may be many times the amount recorded, if significantly more radiation is received at a location remote from the monitoring device.
So, what are acceptable monitoring devices? There are only two: the film badge or the thermoluminescent dosimeter (TLD). Just to make a note: The pocket chamber or dosimeter is not considered legal for monitoring purposes, because it does not provide a permanent record. It can, however, be used in addition to the film badge or TLD, especially for personnel who find themselves in the unfortunate circumstance of taking films during pregnancy.
Let's compare your two options: The film badge is lightweight, and the holder has various filters embedded that act as filters of low-energy X and gamma rays, and beta particles. The attenuation of radiation that occurs from the various filters results in different film densities when the film is developed. The exposure is calculated by comparing these densities. The film becomes a permanent record and can be reviewed if necessary.
The TLC is a dosimeter, usually one that uses lithium fluoride in the form of solid chips. Ionizing radiation interacts with lithium fluoride crystals; some electrons of the crystal are raised to higher energy states of excitation, in which they become trapped. Later, when the crystals are heated in a special measuring device, these trapped electrons return to their normal energy levels and in the process, they emit light. The amount of light emitted is proportional to the amount of radiation to which the crystal was exposed. The average accuracy of TLDs is about 9 percent.
Did you know that the accuracy of a film badge can be as poor as 25 percent? The problem with the TLDs is that they are more expensive than film badges, and once the TLD exposure has been read, it cannot be read again. Therefore, this record is not truly permanent.
Regarding the pocket dosimeter or the pocket ionization chamber, it provides the user with an immediate indication of the exposure received, so if you need immediate information (e.g., if the operator is possibly pregnant), one can monitor oneself very carefully with the use of a pocket ionization chamber and a film badge. Or, if you need to know just how much ionizing radiation a patient may be receiving from an X-ray series, this can be a very useful device. Remember, it is not acceptable for legal monitoring purposes, because it does not provide a permanent record and is a subjective evaluation - generally by the person wearing it. It also has a limited dose range and is sensitive to mechanical shock.
Just for your review:
The 10 CFR 20, section 20.1201, mandates the following annual occupational dose equivalent limits:
Whole body exposure - 5 rem
Skin and extremities - 50 rem
Lens of the eye - 15 rem
If you want more information, it is available through the State of California - Health and Human Services Agency, Department of Health Services, Radiologic Health Branch (www.dhs.ca.gov/rhb/).
Deborah Pate, DC, DACBR
San Diego, California
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