An electronic personal dosimeter is modern dosimeter, which can give a continuous readout of cumulative dose and current dose rate, and can warn the person wearing it when a specified dose rate or a cumulative dose is exceeded. EPDs are especially useful in high dose areas where residence time of the wearer is limited due to dose constraints.
Types of EPDs
EPDs are battery powered and most use either a small Geiger-Mueller (GM) tube or a semiconductor (Si chip) in which ionizing radiation releases charges resulting in measurable electric current.
- G-M counter. A Geiger counter consists of a Geiger-Müller tube (the sensing element which detects the radiation) and the processing electronics, which displays the result. G-M counters are mainly used for portable instrumentation due to its sensitivity, simple counting circuit, and ability to detect low-level radiation. Because of the large avalanche induced by any ionization, a Geiger counter takes a long time (about 1 ms) to recover between successive pulses. Therefore, Geiger counters are not able to measure high radiation rates due to the “dead time” of the tube.
- Semiconductor Detector. Semiconductor detectors are based on ionization in a solid (e.g. silicon) and include different types of solid-state devices with two terminals called diodes. For example a silicon diode, which has a p-i-n structure in which the intrinsic (i) region is sensitive to ionizing radiation, particularly X rays and gamma rays. Under reverse bias, an electric field extends across the intrinsic or depleted region. In this case, negative voltage is applied to the p-side and positive to the second one. Holes in the p-region are attracted from the junction towards the p contact and similarly for electrons and the n contact.
- Scintillation Detector. Some EPDs use a scintillating crystal such as sodium iodide (NaI) or cesium iodide (CsI) with a photodiode or photomultiplier tube to measure photons released by radiation.
For example, the DMC 3000 is an electronic radiation dosimeter, EPD, that provides dose and ambient dose rate readings for deep dose equivalent Hp(10). It is one of the most used EPDs on the market. It uses a Si chip detector with gamma sensitivity of 180 cps/R/h. This electronic personal dosimeter has the following characteristics:
- Energy response (X-ray and gamma) from 15 keV to 7 Mev.
- Dose measurement display range: between 1 μSv and 10 Sv.
- Rate measurement display range: between 10 μSv/hr and 10 Sv/h.
See also: The Radiation Dosimeters for Response and Recovery Market Survey Report. National Urban Security Technology Laboratory. SAVER-T-MSR-4. <available from: https://www.dhs.gov/sites/default/files/publications/Radiation-Dosimeters-Response-Recovery-MSR_0616-508_0.pdf>.