Fluke 90100 manual Model 35300A Ion Chamber Detectors, Air Density Corrections

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Theory of Operation

3

Overall Functional Description of the Model 35360A Tracker Display

Above values assume nominal electrical and radiological sensitivity, air density correction and front panel gain factors set to unity, and nominal value for the Display Units Conversion Factors. The following unity conversion factors were used; 3.52 R/nC, 0.879 rad/R, 0.00879 Gy/R, and 0.00879 Sv/R. The sensitivity values have been selected so the display resolution will not exceed the resolving capability of the A-D converter.

3.3 Model 35300A Ion Chamber Detectors

The Model 35300A Detector contains an array of four orthogonal placed ion chambers on 10 cm radii from a fifth centrally located ion chamber. The five ion chambers are of a vented, circular, parallel-plate configuration and are fully guarded for low leakage. The 35300A Detector connects to the 35360A TRACKER Display through a light weight interface cable.

Each of the 35300A Detector's five ion chambers may be depicted as containers of air, whose inside surfaces (bias plates) have been given a conductive coating. Located centrally in the air volume is a plastic disc (collector) which also has a conductive coating. The collector is electrically insulated from the bias plates and connected to the electrometer input through the 37783-15 detector interface cable.

Photons or electrons from the linear accelerator or Co60 source pass through and are absorbed into the chamber's materials and internal air volume. Absorption may also take place in external build-up materials when they are present. The primary absorption takes place in the chamber's windows and build-up materials when the incident photons or primary electrons interact with the atoms of these materials. When absorption occurs, high speed electrons are emitted from the interacting atoms. The high speed electrons spray into the ion chamber's air volume and produce many ionization tracks consisting of lower velocity electrons and "air" ions. The 300 V bias voltage separates the secondary electron cloud from the cloud of positive ions before recombination can occur. The sweeping of the electron cloud onto the ion chamber's collector produces a charge (or current) whose magnitude may be measured by the electrometer.

3.4 Air Density Corrections

For any vented ion chamber, the measurement of exposure (in roentgens) depends upon the mass of air from which the ionization charge is collected. The ion chamber calibration factors (ICCF's) supplied by Fluke Biomedical expresses this sensitivity in terms of R/nC. These ICCF's are specified for a temperature of 22 °C and a pressure of 760 millimeters of mercury (mm Hg). Since the 90100 TRACKER System may be used under ambient conditions which significantly differ from these, the corresponding changes in the sensitivity of the ion chamber must be accounted for. The air density correction factor (ADCF) has been defined as follows:

ADCF = (760 mm Hg / P) * (T / 295.15 K)

where the pressure "P" is expressed in mm Hg and the temperature "T" is in Kelvin.

When the user has customized the 35360A TRACKER Display to use other temperature and pressure units, the corresponding values entered by the user at the instrument's front panel are automatically converted to mm Hg and K before use in the above equation. The equations used for these conversions are listed in the following table.

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Contents Tracker System ModelFluke Biomedical Table of Contents Appendix a Using Build-Up Maintenance and ServiceTheory of Operation CustomizationIntroduction General InformationModel FeaturesSafety Terms Before You Start Safety PrecautionsSpecifications Manual AddendaMechanical Specifications Nominal Ion Chamber CharacteristicsRecommended Operation Conditions Ion Chamber Factory CalibrationDisplay Mode Measurements ModeModel 35360A Tracker Display Electrometer / A-D ConverterUnpacking and Inspection High Performance Battery ChargerInterface Cable Detector to Display EnvironmentOptional Equipment Standard System ContentsBlank Rear Panel Familiarization OperationDetector Input Powering the SystemDisplay. Be sure to use the correct end of the cable 3 RS-232 Computer Interface RJ-45 Style Positioning the Model 35300A Detector in the BeamOperation Positioning the Model 353300A Detector in the Beam Front Panel Controls Operation InstructionsReset PowerTest/Detector Units SelectPowering Up the Model 35360A Tracker Display Powering Down the Model 35360A Tracker DisplayUsing the TEST/DETECTOR Button Test/Detector Screen 2 Ion Chamber Calibration Factors Using the Units Select ButtonUsing the Gain Select Button Normal Mode Using the AIR Density ButtonAbsolute Measurements Using the Measure Select Button Normal ModePercent-of-Center Measurements Underranging and Overranging Conditions Battery and Bias Annunciators24. Positions of the Battery and Bias Annunciators Gain Setting ModeAborting the Fpgf Changes Saving the Fpgf ChangesBlank Theory of Operation Hardware Block Diagram of the Model 35360A Tracker Display Power Supply2 300-VOLT Electronic Bias Supply Front Panel SwitchesProcessor VFD DisplaySerial Communications Port ElectrometerMultiplexer and A/D Converter Air Density Corrections Model 35300A Ion Chamber DetectorsBasic Operating Equations Basic Operating Quantities Blank Maintenance & Service Problem TroubleshootingNo Display Flowchart Low battery Yes Annuciator displayed? Low Battery Indication at the Display35360A Tracker Bias Error LoBias or HiBias Indication at the DisplayLeakage Flowchart Dose Rate Measurement Zero Problem Flowchart Line Fuse ReplacementIf fuses continue to blow return instrument for repair Interface Connector Cleaning Cleaning the InstrumentReplaceable Parts Information Recalibration & Repair Information / Technical Assistance Replaceable Parts ListCustomization Connection to the Computer Make a Working DiskStarting the Customization Software Screen FormatsDisplay Customization Items Screen Appearance F2 Edit Item Editing F1 Help On-Line Help for using the Customization softwareF6 Print Report Printing F10 ExitDefinitions Charge Gain QG Setting the Electrometer Gains and OffsetsCurrent Gain, Current Zero IG and ADZ Ion Chamber Calibration Factor IccfElectrometer Charge Gains QG Electrometer Current Gain IGUsage Examples Changing the Power Down IntervalChanging the ION Chamber Calibration Factor Units Input Errors ErrorsBuild-Up, Peg, and Retainer Kit Options Using Build-UpNominal Number of build-up Solid Water Appendix Fluke Biomedical
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