Fluke Biomedical 30-471 - Manual (Page 9)

Operation
Operation
2
2-1
Section 2
Operation
2.1 Operation
The solid-state detectors are p-n junction style diodes that are operated with zero applied bias voltage.
This mode of operation is known as the photovoltaic mode. In this mode the diode becomes a self-
generating device, not requiring a power supply. When the diode's sensitive region is introduced to
radiation, it produces a current output. Based on the internal construction of the detector, this output
current is either positive or negative.
Within these detectors a silicon conductor chip is utilized. By using this chip, it minimizes the detectors
leakage current. Since the leakage current has been reduced and due to the detectors great resistance, a
stable output is achieved. This output is exceptionally linear throughout the specified range of the
detector. With radiation fields that cover the specified range of the appropriate diode detector, the
detector's reproducibility is better than 0.2%. This output remains constant as long as the dose and diode
position remain constant. Changes in this output only become noticeable when the source to detector
distance (SDD) is altered.
The isotropic characteristics of these detectors allows for measurements to be made with little
consideration of the detectors orientation within the radiation field. The reason for this is that the
detector's sensitive area always faces the radiation field regardless of the detectors position. This
characteristic allows for an excellent geometric response. However, ideally the detector should be placed
in the radiation field so that the field totally encompasses the detector. A raised cross hair has been
placed on top of the detector to allow for ease of alignment. They can be applied to the patient with tape,
and the rugged waterproof housing allows ease of cleaning with no degradation to the detector.
The sensitivity variation with temperature (SVWT) coefficient which is taken into account for thermal
transfer from the patient is < 0.5% per degree Celsius. The semiconductor material within the detector is
influenced by temperature. The effect of temperature is to introduce or remove more thermoenergy to the
system. More energy bounces more charge carriers out of the centers, and thus increases the signals.
The rate of change in sensitivity with temperature depends on the number of generation-recombination
centers.