Dosimetry devices are used to measure an individual’s exposure to radiation. Sometimes this is referred to as external dose. There are regulatory dose limits for the general public, and occupational workers. Federal guidance for emergency workers is included in state and local Radiological Emergency Plans.
Ideally, dosimetry should measure one’s radiation dose; however, the instrument measures all the radiation that it is exposed to. Some radiation passes through the person and is not absorbed by the body and does not contribute to the person’s radiation dose. Unless one’s exposure limit is exceeded, it is common practice to assume the dose is equal to the exposure recorded by the dosimeter.
There are several kinds of dosimeters: Self-Reading Dosimeters (SRD), Personal Radiation Devices (PRD), Thermoluminescent badge dosimeters (TLD) and Optically Stimulated Luminescence badge dosimeters (OSLD). The SRD allows an individual to observe his exposure at any time. The device is a small tube, slightly larger in diameter than an ink pen. It is an ion chamber that has a charge on its plates similar to a capacitor. When it is exposed to radiation, the plates discharge in proportion to the amount of radiation. Sometimes static electricity or dropping the SRD will discharge the plates causing the needle to go off scale and the user might be concerned that he has been in an extremely high radiation field. An emergency worker should always have a survey meter on if he thinks he is going to be exposed to radiation, and have a two man field team, so that one can check the other’s dosimeter if the SRD does accidentally discharge. SRDs should be tested annually for electrical leakage. There are now electronic versions available (e.g., PRDs) and they should also be calibrated annually.
Local jurisdictions located within the 10 mile EPZ of a nuclear power station are tasked with providing SRDs and TLD/OSLD badges, or equivalent dosimetry, to their designated emergency workers.
Office of Radiological Health | 109 Governor Street, 7th Floor | Richmond, VA 23219
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