In the broadest sense, dosimetry is done to measure and prevent or at least identify problems from significant or long-term exposure to harmful situations. More specifically, dosimetry refers to the study, measurement, method of measurement or instrument of measurement of radiation dose. Dosimetry often involves wearing a personnel badge, a dosimeter, that measures and monitors dose exposure. The absorbed dose in tissue resulting from exposure to ionizing radiation is reported in units of gray (Gy) for mass. Dose equivalent is reported in units of sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. In traditional measurement units that are still used, dose is often reported in rads and dose equivalent in rems, where 1 Gy = 100 rads and 1 Sv = 100 rems. For radiation workers or those who are routinely exposed to radiation, the yearly occupational dose limit is:
- 5.0 rems for whole body
- 15.0 rems for lens of the eye
- 50.0 rems for skin or an extremity
Legacy dosimeter types include film badge and quartz fiber designs. More recently, thermoluminescent dosimeters (TLDs) have been widely used as well. Like a film badge, a TLD is worn for a period of time (usually 3 months or less) and then is processed to determine the dose received.
The TLD measurement is based on a phosphor in a solid crystal structure. When a TLD is exposed to ionizing radiation, the radiation interacts with the phosphor crystal that releases light based on the trapped electrons subsequently being freed by stimulation heating it to a higher temperature. Since it is proportional to the radiation exposure, the released light is counted to determine how much exposure occurred. Thermoluminescent dosimeters can measure doses as low as 1 millirem. However, under routine conditions their low-dose capability is approximately the same as film badges. Accuracy and sensitivity to low doses are advantages of TLDs and since they are reusable, they have an additional advantage over film badges.
More recently, optically stimulated luminescence (OSL) dosimeters provide a very high degree of sensitivity by giving accurate readings as low as 1 mrem for x-ray and gamma ray photons. To provide their readings, OSL materials such as beryllium oxide ceramic contain defects in their crystal structure that trap electrons released by exposure to radiation. Analogous to TLDs, in the OSL dosimeter the stimulation occurs optically rather than thermally. Designed to provide X, gamma, beta and neutron radiation monitoring, OSL dosimeters provides whole-body dosimetry.
Unlike dosimetry that involves subsequent and separate processing, electronic personal dosimeters (EPDs) provide a continuous readout of cumulative dose, current dose rate and even warn the wearer with an audible alarm when a specified dose rate or a cumulative dose is exceeded. EPDs have long been used as secondary dosimetry for radiation workers for many years due to their ease of reading and alarm capability.
Part 2 of this two-part blog will provide a specific example of a newer EPD as well as discuss a dosimetry applied to an industry hazard other than radiation.
References
USDA | OHSEC | Radiation Safety Division | Dosimetry
Different Ways to Measure Radiation – Dosimetry Badge
NUREG/CR-6354, “Performance Testing of Electronic Personal Dosimeters.” (nrc.gov)
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