hePhysicsofRadiology-chapter5-3 放射物理 英文版 教学.ppt

MeasurementofRadiation Instrumentation TechniquesPartThree BMECollegeShermanSheen 辛学刚 ThePhysicsofRadiology solidstatedetectors thediode Shortresponsetime Enormousdynamicrange Highsensitivity Flexibleforms ThermoluminescentDosimetry TLD PRECISIONOFTLDANDDOSERANGE MATERIALSFORTLD USESOFTLD CLINICALDOSIMETRY PERSONNELMONITORING DOSECALIBRATIONSERVICES ChemicalDosimetry A isthechangeinabsorbanceat followingadoseDtotheferroussulphate isthemolarextinctioncoefficientoftheferrousion isthedensityofthesolutioninkg m3 Gistheyieldinnumberper100eV Example Itisrequiredtodeterminetheaveragedosetoasampleofwaterinacomplexlyshapedradiationcontainernearacobaltsource ThisismosteasilydoneusingtheFrickedosimeter ThecontainerisfilledwithFrickesolutionandexposed Asampleofthewell mixedirradiatedsolutionisthenplacedinaquartzcuvetteof2cmpathlengthanditsabsorbancemeasuredagainstthestocksolutionat304nm Supposethemeasuredabsorbanceis0 360 Calculatethedose FilmasaDosimeter radiantenergy crystalsofsilverbromide latentimage Filedeveloped filmfixed negativefilmresults areasthathavebeenexposedtoxraysappeardark DirectMeasurementofAbsorbedDose TheCalorimeter SummaryofCalibrationProcedures ExposureRateDeterminations Selectasuitabletreatmentapplicatororreferencefieldsize forexample10by10cm anddirectthebeaminsuchawayastoavoidscatteringfromnearbyobjects b Selectfilter tubecurrent mA andvoltage kV anddeterminethehalf valuelayerforthesesettingsofthemachine c Selectasuitablepracticaldosimeterthathasanexposurecalibrationfactortraceabletoastandardizationlaboratory seesection4 09 forthehalf valuelayeroftheradiationbeamtobemeasured Theionizationchambershouldbeonethathasbeendesignedspecificallyfortheenergyofradiationbeingmeasured Itmaybenecessarytointerpolatebetweentwocalibrationfactorstoobtaintherequiredone d Fastenthedosimeterinaclampsothatitspositionisaccuratelyknownandcanbereproducedforrepeatedreadings Thecenterofthesensitivevolumeoftheionchambershouldbeatthecenterofthefieldandevenwiththeendofthetreatmentconeifthereisone orelseatthereferencedistancechosenforthecalibration Sometimesthisisnotpossible inwhichcaseaninversesquarecorrectionwillhavetobemade e Setthetimertoanappropriatevalue turnonthemachine andbringittotheoperatingpointselectedinb Opentheshutter orinthecaseofacobaltunitsimplyturnthebeamon Iftheionchamberisoperatedasanexposureratemeasuringinstrument itsreadingcannowbetakendirectly Ifitistobeoperatedasanexposuremeter timer settingsshouldbechosensothatasetofreadingsareobtainedatbothrelativelyhighscalereadings sayabout3 4scale andrelativelylowscalereadings Theexposureratemaythenbeobtainedbytakingdifferencesofsetsofreadingstodeterminetheslopeofthe readingversustime curve Severalsetsofreadingsshouldbeaveragedtogive M t the readingperunittime Alternatively sincemanycomputersandcalculatorsareequippedwith leastsquaresroutines itisconvenienttotakeaseriesofreadingsMiattimestextendingacrosstherangeofthedosimeteranddeterminetheslope M t oftheMversustline Thislatterproceduregivesinformationabouttheprecisionofdeterminingthisslope itcanalsogiveinformationabouttheshuttertimingerror f Duringtheexposure foranxraymachineitmaybenecessarytoadjusteitherthekVormA Ifthisisnotpossibletheaveragetubecurrentandvoltageoverthetimeoftheexposureshouldberecorded g Measuretemperatureandpressure Theexposurecalibrationfactorasissuedbythestandardizationlaboratoryisvalidonlyforthetemperatureandpressurestatedbythem Itisusually22oCand101 3kPabutthisshouldbeconfirmed h Thecorrectedexposurerateis where M tistheaverageuncorrectedexposurerateasobtainedinparagraphe fistheSSD xisthedistancefromtheSSDtothecenterofthechamber PandTarethepressureandtemperatureofthegasintheionchamberwhenthemeasurementismade andTxisthetemperatureforwhichthecalibrationfactor Nx applies i Anumberofothersmallcorrectionsmayberequired Efficiencyofioncollectionandtheproblemsofleakage stemionizationarediscussed Adosimetershouldalsobecheckedforconstancyofsensitivity andifithaschangedsincethecalibrationwasperformed allowancemustbemade Someinstrumentsareequippedwithadevicecontainingalong livedradioactiveisotopeforthispurpose Alternatively carefulmeasurementsofacobaltunitcanbeused j Theseprocedures carefullycarriedout shouldallowanoutputcalibrationfortheconditionsmeasured toanaccuracyofbetterthan 2 5 I7 DeterminationofAbsorbedDoseinaPhantom Thephantomshouldbeclampedtothetreatmentunitorlocatedinsuchawaythatitspositioncanbereproducedforrepeatedmeasurementsandthedistancesfromthesourcetotheionchambershouldbeknownwithaprecisionofatleast 2mm Anappropriatephantommustbechosen Theseprocedures care