Fluke Biomedical 30-471 - Manual (Page 17)

Theory Of Operation
Theory of Operation
3
3-1
Section 3
Theory of Operation
3.1 Theory of Operation
Semiconductors have been actively used as a solid medium for radiation monitoring since the 1960's.
Early versions of semiconductor detectors were referred to as crystal counters. Today they are known as
diode detectors or solid-state detectors.
The main advantage of using a semiconductor for a radiation detector is its ability to generate a large
number of information carriers for a given incident radiation event. The information carriers that we are
concerned with are the electron-hole pairs created by the impinging radiation through the detector. When
a charged particle passes through the semiconductor it generates electron-hole pairs along its path. The
amount of charge collected by this event is related to the energy loss of the charged particles passing
through the semiconductor.
It is the charge carriers’ interaction with the generator-recombination centers that cause variations with
dose rate. At low dose rates, electrons and holes recombine and are ejected from the centers at some
rate. As the dose rate becomes high, the center starts to become saturated by electrons or holes. The
carriers are then able to proceed farther then they were previously. Increasing the path length of the
carriers in effect increases the sensitivity of the detector.
By placing a detector, along with a farmer chamber, at different distances from a radiation source, the
determination of variation with dose rate can be accomplished. Any variations of the detector with respect
to the farmer chamber reflects that detector's variation with dose rate. It was determined that the farmer
chamber accurately recorded the relative change in dose with distance. Both sets of measurements were
normalized to 100 SSD. The following graphs show the results to the measurement made.
Diode (type 472 [5–11 MV phontons]) output farmer chamber
vs. SSD “Dose Rate Dependence”
SSD (cm)
Normalized reading (d
iode
/far
mer chamber)