Fluke Biomedical 945A - Manual (Page 9)

Section 1 - Introduction
General Description
The Victoreen Model 945A is a wide-range area monitoring system with a detectable range of 10-1 to 107
mR/h. The system is comprised of the Model 977-210 Ion Chamber Detector, with local preamplifier, and
a Model 946A-200 Universal Digital Ratemeter (UDR). The detector and preamplifier are connected by
two five foot cables, encased in a flexible conduit. Other cable lengths, of up to 100 feet between
preamplifier and detector, are available. Both the detector and preamplifier are wall mounted in an area
designated by the customer. The 946A-200 Universal Digital Ratemeter is mounted in a rack in the
control room. A more detailed description of each piece of equipment can be found in the following
paragraphs.
Ion Chamber Detector/Preamplifier, Model 977-210
Ion chamber detectors are similar to capacitors, with two electrodes separated by a volume of air. In
order to operate, an electrical potential is applied between the electrodes. Thus, one electrode will have
a positive charge and the other a negative charge (relative to each other). Incident ionizing radiation will
cause the air in the volume between the electrodes to dissociate into positive and negative ions. Each
ion will be attracted to the electrode of opposite polarity. Ions that reach the electrode result in charge
transfer. The charge transfer causes a current to flow in the connecting wiring, representing the
measured amount of radiation intensity experienced by the ion chamber.
The Model 977-210 Wide-Range Ion Chamber Detector is a gamma sensitive radiation detection device,
which measures radiation in the range of 1 x 10-1 mR/h to 1 x 107 mR/h with an energy dependence of
less than ± 10% from 60 keV to 3 MeV. The pre-amp is housed in a gasket sealed enclosure.
Interconnection between the detector and pre-amplifier is accomplished via two five foot cables, encased
in a flexible conduit.
The wide range ion chamber detector design includes a single ion chamber with a volume of
approximately 1000 cc. The chamber walls are made of tissue equivalent plastic. An outer wall, made of
aluminum, is provided to protect the ion chamber and to mechanically interface with Victoreen’s Model
848-8 Field Test Source. The ion chamber is biased at a nominal 500 volts and produces an output
current proportional to radiation absorbed in the chamber. The current is approximately 8 x 10-11 A/R/h.
The ion chamber assembly makes use of a double seal design, where the actual ion chamber is back
filled with nitrogen at atmospheric pressure. The connector area is sealed against moisture and
particulate through the use of a sealed, liquid tight flexible conduit between the detector and preamplifier
electronics.
The preamplifier contains a microprocessor controlled, auto-zeroing, integrating electrometer, a
programmable gain amplifier, an analog - to - digital converter, an asynchronous serial communications
interface, an electronic check source, and the detector high voltage power supply. Communications with
the UDR is accomplished via the optically isolated Victoreen Serial Communication Loop driver/receiver
circuitry.
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