11.4. Active Dosimetry 645
MOSFET dosimeters also have some disadvantages, some of which are listed below.
Sensitivity to Bias Instability:MOSFET devices are very sensitive to drifts
in bias voltage and therefore require highly stable power supplies.Radiation Damage:Being semiconductor devices, MOSFET dosimeters are
highly susceptible to the damage caused by the radiation. Therefore if they
are used in high radiation fields, their response may change after irradiation.Temperature Dependence:Like all semiconductor devices, their response
is vulnerable to small drifts in temperature. However, specially designeddouble
MOSFET devices have shown good temperature stability.B.2 DiamondDosimeter
Diamond is a radiation hard substance, which has made it a material of choice in
for building detectors used in hostile radiation environments. The detectors based
on CVD diamonds have been discussed in the chapter on solid state detectors. The
dosimeters based on CVD diamond have the same type of structure but they are
smaller in size. Therefore we will not discuss the construction details of such detec-
tors but it is certainly worthwhile to discuss their advantages.
Radiation Hardness: Diamond has high radiation hardness, which makes
it suitable for use in very high radiation environments. The particle beams
generally used in radiation dosimetry are fairly intense and therefore the use
of diamond based dosimeters is gaining interest in this field.Stability against Water:Diamond does not absorb water, which is a added
advantage for their use inin vivodosimetry.Good Temperature Stability:Diamond dosimeters have shown very good
temperature stability, with a typical value of less than 0.1% per degree Celsius.
This makes them superior to conventional MOSFET dosimeters and compara-
ble to double MOSFET dosimeters.
Good Isotropy:Their behavior is fairly isotropic, making them easy to use
in practical applications.
Good Energy Response: The energy response of diamond dosimeters is
fairly linear and therefore they can be used over a large dynamic range without
the need of correction factors.Small Size: Typical diamond dosimeters are fairly small, of the order of a
fewmm^3. This allows them to measure dose without appreciable affecting the
radiation field.Good Spatial Resolution:Due to their small size, diamond dosimeters show
very good spatial resolution.Tissue Equivalence: Since diamonds are based on carbon, they have good
tissue equivalence. This makes them advantageous over semiconductor dosime-
ters, which require energy corrections due to their tissue nonequivalence. Of
course this only holds for medical dosimetry applications.