An electronic personal dosimeter is a modern dosimeter that can give a continuous read-out of cumulative dose and current dose rate and 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 the residence time of the wearer is limited due to dose constraints.
MOSFET dosimeter is a small portable device for monitoring and direct reading radiation dose rate. Since it is based on the MOSFET transistor, the metal-oxide-semiconductor field-effect transistor (MOSFET), the principle of operation is similar to semiconductor detectors. MOSFET dosimeters are now used as clinical dosimeters for radiotherapy radiation beams. Their main advantage is their physical size, which is less than 4 mm2. In radiation therapy dosimetry, MOSFET dosimeters often replace TLD dosimeters since they offer immediate read-out.
Principle of Operation of MOSFET Detectors
The operation of MOSFET detectors is summarized in the following points:
- Ionizing radiation enters the sensitive volume of the detector and interacts with the semiconductor material.
- Particle passing through the detector ionizes the atoms of the semiconductor, producing the electron-hole pairs. Electron-hole pairs are generated within the silicon dioxide by the incident radiation. Electrons, whose mobility in SiO2 at room temperature is about 4 orders of magnitude greater than holes, quickly move out of the gate electrode. In contrast, holes move in a stochastic fashion towards the Si/SiO2 interface, where they become trapped in long-term sites, causing a negative threshold voltage shift (∆VTH), which can persist for years.
- The difference in voltage shift before and after exposure can be measured and is proportional to dose.
Advantages and Disadvantages of Electronic Personal Dosimeters
- EPDs can display a direct reading of the detected dose and dose rate in real-time.
- EPDs have a dose rate alarm and a dose alarm, which warn the person wearing it when a specified dose rate or a cumulative dose is exceeded.
- The dosimeter can be reset, usually after taking a reading for record purposes, and thereby re-used multiple times.
- EPDs can measure a wide radiation dose range from routine (μSv) levels to emergency levels (hundreds mSv or units of Sieverts) with high precision.
- EPDs are generally the most expensive dosimeters.
- EPDs are generally large.
- EPDs measure and record radiation exposure due to gamma rays, X-rays, and sometimes beta particles. For neutrons, TLDs are more capable.