Answer to Question #7692 Submitted to "Ask the Experts"

When you look at dosimetry equipment (such as a MOSFET TN-1002) the sensitivity is defined in volt/Gy. This is not clear to me since Gy is J kg^-1 and the sensor can only measure photons/area of the sensor. How is the conversion done?

A MOSFET dosimeter works on the principle that irradiation with ionizing radiation produces changes in the charge carrier trapping such that a change in the threshold voltage necessary to induce source-to-drain current flow occurs following irradiation, compared to that required before irradiation. This change in voltage is generally proportional to dose. It is this change in threshold voltage that is actually being assessed following irradiation, and it is correlated with dose through an appropriate calibration process—i.e., a known dose (or series of doses) from a radiation source is delivered to a representative device and its response characteristics determined—i.e., the change in threshold voltage observed per unit dose to the medium of interest. The units of response are thus volts per gray, if absorbed dose is being evaluated. The particular voltage change induced by a given dose can vary considerably as the physical characteristics of the MOSFET device are changed. Detector area and other dimensions of the device, such as the thicknesses of the silicon and SiO₂ gate, will influence the sensitivity of the device. Orientation of the device with respect to the incident radiation can also influence response.

The fact that absorbed dose to the medium of interest has dimensions of J kg^-1 is unrelated to the quantity measured by the MOSFET device, change in threshold voltage, in units of volts or millivolts. Such situations are common among many dose-measuring instruments. For example, a Geiger-Mueller detector records the rate at which discharges are occurring in a gas-filled tube and the detector yields counts per minute. It is only through the calibration process whereby a given count rate is correlated with dose rate to a medium of interest.

I hope this is helpful to you.

George Chabot, PhD, CHP