Fluke Biomedical 10100AT - Manual (Page 50)

10100AT
Operators Manual
3-4
3.8 Multiplexer/A-D Converter
This block consists of an 8-channel analog multiplexer and 12-bit analog to digital converter, and
operates under direct control of the microcomputer. Its function is to convert the analog voltage signals
produced by the Electrometer and kV Signal Conditioning blocks into digital signals for use by the
microcomputer. In addition, signals from the Power Supply, 300 Volt Electronic Bias Supply, and the
Serial Communications Port are available for measurement. These measurements are used by the
microcomputer in its various control functions as described throughout this section. The A-D operates in
a bipolar mode, and all conversions are done relative to a 4.6 Volt full scale. The effective measurement
resolution is 2.25 mV and the A-D is capable of performing conversions at the approximate rate of 5 kHz.
3.9 kVp Input Signal Buffer/Divider
The Model 35050AT Dosimeter is equipped with a coaxial-BNC input connector for use in connecting to
the output of the Model 35080/80A/80B kV Divider. When a kVp divider is connected, the output of the
kV Input Signal Buffer/Divider is sampled at the rate of 5 kHz. During normal use, this block simply acts
as a unity gain buffer passing the kVp divider’s kV signal directly through to the A-D. When the kVp
divider is placed in battery check, an out of range voltage is detected and the kVp Input Signal
Buffer/Divider is switched to its 1/2 gain configuration. Thus, the battery voltage from the kVp divider is
brought back into the measurement range of the A-D. All normal kVp measurements are carried out with
this block in its unity gain configuration.
3.10 Ion Chambers
3.10.1 Basic Theory
The Model 96035B and Model 96020C Ion Chambers supplied with the Model 10100AT TRIAD Field
Service Kit are the Model 35050AT Dosimeter's sensors of exposure and exposure rate. The ion
chambers are of parallel plate geometry, constructed of air equivalent plastic, and vented to the
atmosphere.
They may be depicted as plastic containers of air, whose inside surfaces have been given a conductive
coating. Located centrally in the air volume is a plastic disc (collector) that is conductively coated. The
collector is electrically well insulated from the other surfaces, however connected to the electrometer
input.
X-ray photons (rays) from the generator pass through and are absorbed into the chamber's materials and
internal air volume. The primary absorption takes place in the windows, collectors, and wall because it is
of much higher density than air.
When absorption occurs (photons interact with atoms), high-energy electrons are emitted from the
interacting atoms. The high-speed electrons, spraying from the solid chamber materials into the air
volume, produce many ionized tracks consisting of lower velocity electrons and “air” ions.
It is the role of the bias voltage to separate and sweep these ion and electron clouds out of the air volume
and into the electrometer, before they recombine. The ion chamber, bias supply, and electrometer are
connected electrically in series. With both the electrometer and bias supply offering low impedances, the
ion chamber's charge or current is captured by the electrometer.
Thus, radiation is converted into electrical charge or current, by the international definition of exposure or
exposure rate, for measurement.