What is the difference between Electronic Personnel Dosimeters and TLDs or Film Badges?
![Text Box: There are many differences. Each dosimeter has its own characteristics, and needs to be dealt with independently, even in the manner in which they are calibrated. In general, TLDs and film can be thought of as "passive dosimeters" while electronic personnel dosimeters (EPDs) are "active dosimeters". Passive dosimeters do not provide a real-time indication of radiation exposure but must be returned to a central laboratory for processing and analysis. Active dosimeters can give an indication of real-time radiation exposure both in terms of total dose accumulated since the start of the wearing period and the instantaneous dose rate experienced by an individual wearing the EPD. Passive dosimeters are generally used for relatively long periods of time (monthly, quarterly, even annual) while active dosimeters are used for very short periods such as a single work shift in order to give a very rapid indication of the individuals occupational radiation exposure experience.
TLDs accumulate dose from the time they are annealed, prior to shipment, until the time processed (heating to about 300 degrees C). Therefore, all exposure, occupational, natural background and transit dose (higher natural background) is accumulated. Similarly, Film accumulates dose until it is returned for processing. In both cases the Control Badge is used to distinguish between natural (background) and doses accumulated as a result of one's work with radiation sources. Through the use of dose calculation algorithms, TLDs and film badges can measure and report radiation doses attributed to a variety of radiation types and energies - gamma, beta, x-ray and neutron.
Electronic Personnel Dosimeters (EPDs) [sometimes also known as Alarming Personnel Dosimeters (APDs)] measure radiation exposure only while turned on. By their design, EPDs typically respond well to high energy photons such as those from 137Cs or medium to high energy x-ray but do not respond well to low energy x-rays Whether or not an EPD responds to beta particles or to neutron radiation depends on the model of EPD being used.
One advantage of an EPD is the ability to respond to radiation levels considerably lower than is possible with a TLD. Also, unlike TLDs, EPDs can be configured to display the instantaneous exposure rate as well as the accumulated exposure and can be set to provide audible and/or visual alarms when pre-defined set points are exceeded. EPDs do not need to be returned to a central laboratory for processing each month or quarter, but they do need to be returned for re-calibration at an interval recommended y the manufacturer (usually annually).
Because of their design, the response of an EPD does not exactly match the response shown by a TLD when exposed to the same radiation field for the same length of time. Typically depending on the energy of radiation EPDs will tend to under-respond when compared to TLDs. For this reason it is very important to have a good understanding of the radiation fields being detected by the EPD and to develop a protocol for comparing results to the dose of record reported by the TLD.](image7092.png)
