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Theory of Operation
Introduction
2
2-1
Section 2
Theory of Operation
2.1 Introduction
The Model 35040 Therapy Dosimeter is a radiation measurement instrument intended for use in
calibration procedures for high-energy accelerators,
60
Co, and brachytherapy. Basic theory of operation
of the instrument is explained in the following paragraphs. For information on radiation dosimetry and ion
chamber measurement theory, the reader should refer to one of the standard radiological physics texts.
2.2 Functional Description
The Model 35040 Therapy Dosimeter is a portable instrument that measures charge and current.
Calibrated ion chamber factors convert charge and current to directly measure radiation dose and dose
rate in the user specified radiological units. The reading may be corrected for air density by the user
entering ambient temperature and barometric pressure readings. The electronic bias setting is selectable
from the front panel.
Figure 2-1. Hardware Block Diagram of the Model 35040 Therapy Dosimeter
AC Line
POWER
SUPPLY
BATTERY
PACK
FUSE
POWER
SWITCH
CONTACTS
FRONT PANEL
SWITCHES
POWER
CONTROL
BATTERY
STATUS
PROCESSOR
SELECTION
SWITCH
CONTACTS
VFD
DISPLAY
SERIAL
COMM.
PORT
RS-232
COMPUTER
INTERFACE
CONNECTION
HIGH VOLTAGE ISOLATION
ELECTRONIC
BIAS SUPPLY
BIAS
BIAS CONTROL
CONVERSION
CONTROL
CONVERSION
RESULTS
MULTIPLEXER AND
A/D CONVERTER
ELECTROMETER
ION
CHAMBER
CONNECTOR