Physics and Engineering of Radiation Detection

11.4. Active Dosimetry 643

Table 11.4.1: Typical uncertainties in air filled ionization chambers used in dosime-
try.

Source Uncertainty (%)

Measurement of sensitive volume (each dimension) 0.05

Humidity 0.05

High voltage ground shifts 0.02

Inhomogeneity of electric field 0.02

Pulse height measurement 0.02

Ion collection 0.10

Charge measurement (capacitance uncertainty) 0.05

W-value for dry air 0.20

Stopping power ratio 0.40

Perturbatioin correction factor 0.15

filled counterparts. Most importantly, due to their small sizes they can be used in
in vivo^2 dosimetry, something that is highly desired in radiation therapy.
We will not discuss here the construction details and working principles of com-
mon semiconductor dosimeters based on junction diodes since similar devices have
already been discussed in the chapter on solid state detectors. However, a relatively
newer form of semiconductor dosimeter, namely the MOSFET device, will be dis-
cussed in some detail in the next section. Later on we will also discuss diamond
dosimeter, which is now gaining popularity due to its radiation hardness.

B.1 MOSFETDosimeter......................

MOSFET is an acronym of Metal Oxide Semiconductor Field Effect Transistor.
This technology has existed for a number of years but its use has been limited to
electronic devices. Its possible role as a radiation dosimeter was investigated a few
years ago. It was found that it could not only work as an efficient dosimeter but
also provided many advantages over conventional dosimeters. Ever since, its use has
considerably increased in the field of radiation therapy.
The simplified diagram of a p-channel type MOSFET dosimeter is shown in
Fig.11.4.4. Such a device is fabricated on an n-type silicon substrate having a typical
thickness of 500μm.

(^2) In vivomeansin live.