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Personal Dosimetry Techniques (TLD)

Personal Dosimetry Techniques (TLD)

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  • 1. E-8. Personal Dosimetry Techniques (TLD)Hideaki OMURANuclear Technology and Education Center,JAERIC O N T E N T SIntroduction1. Principle of measurement2. Outline of TLD(UD-5120PGL)system2.1 Composition2.2 Measured data and correction3. Outline of operating procedure4. Cautions for use4.1 Handling of the reader4.2 Handling of TLD element5. Outline of Experiment6. Apparatus7. ProceduresPreparation for measurement8. Primary Handling of the TLD Reader8.1 TLD Reader and Quality Assurance8.2 Practice to make measurement by the TLD reader and preparation ofTLD elements8.3 Irradiation of TLD elements8.4 Measurement of exposure dose8.5 Using ICRU type phantom and irradiation of TLD elements8.6 Comparison of experimental results between the measurementwithout and with phantom9. Determination of Personal Dose equivalent
  • 2. 1IntroductionThe determination of occupational dose is composed of two components,one is external exposure, the other is internal exposure from radionuclidesentering into the body. The effective dose equivalent from externalexposure used to compare with the legal dose limit is generally obtainedfrom a device worn outside of the workers body.Thus, the reading from such device must relate to the effective doseequivalent. Since 1985 the ICRU had put out a report (ICRU Report 39)designated to "the operational quantities". The set of quantities are usedfor calibration of external radiation measuring devices to the estimation ofEffective Dose Equivalent, HE. The Personal Dose Equivalent, Hp(10), isone of the quantities which is used for calibrating personal dosimeters.The Personal Dose Equivalent, Hp(d), is the dose equivalent in softtissue below a specified point on the body at an appropriate depth, d. Forweakly penetrating radiation, a depth of 0.07 mm for the skin and 3 mm forthe eye are employed. The personal dose equivalent for those depths isthen denoted by Hp(0.07) and Hp(3), respectively. For strongly penetratingradiation, the depth of 10 mm, Hp(l0), is employed. Hp(d) can be measuredwith a detector which is worn at the surface of the body and covered with anappropriate thickness of tissue-equivalent material. The calibration of thedosimeter is generally performed under simplified conditions on anappropriate phantom.1. Principle of measurementThermoluminescent dosimeters (TLD) utilizes the effect in somematerials which can store the energy of ionizing radiation in excited stateelectrons in the abnormality of lattices. When a Thermoluminescentelement is heated, its radioactive energy accumulated inside is emitted inthe form of light. By detecting this light, the dose can be measured. Theamount of light is proportional to the energy deposited. TLD is widely usedfor personal dosimeters as well as for area monitoring. Example of TLDmaterials are compounds of LiF, Li2B4O7, BeO, CaF2 and CaSO4, doped withvarious activators. LiF, Li2B4O7 and BeO are nearly tissue-equivalent forphotons while CaF2 and CaSO4, although far from tissue-equivalent, havethe advantage of being 20 to 30 times more sensitive.
  • 3. 22. Outline of TLD (UD-5120PGL) system2.1 CompositionThe TLD system is composed of UD-5120PGL, TLD element, fingerholder, and annealing oven.(1) The UD-5120PGL is a thermoluminescent dosimeter readercomprising a microcomputer.(2) The TLD element is available in CaSO4 type, BeO type, etc. and ispresented in glass capsule seal shape and film shape.(3) The finger holder is offered in various types to be suited to versatileapplications, purposes and element shapes.(4) The annealing oven is an electric heating oven for removing theresidual dose when re-using the element.2.2 Measured data and correctionThe data displayed in the display unit and the data printed out by thebuilt-in printer after measuring the TLD element by this reader aredirect reading values.In this reader, the direct reading value is stored, and is arithmeticallyoperated in terms of output correction factor (correction factor applied toall measuring elements), element sensitivity correction factor for eachTLD element, and background value (BG), and the processed data isprinted out again or transmitted to an external system.
  • 4. 3Thermoluminescent Dosimeters System UD-5120PGL General specificationItems ContentsDose display range 0.1μSv to 999Sv (0.01 mR to 99.9 kR)Measuring rangeUD-200S (CaSO4:Tm) X and γ-rays1μSv to 200mSv ( 0.1 mR to 20 R)Display8 digits, 4 range automatic changeoverwith range-over displayMeasuring time Standard 10 secondsHeater temperature Approx. 430 degree Celsius fixedPhotomultiplier Voltage Approx. 560V fixedCalibrationAutomatic sensitivity correction by built-i ncalibration light source (CAL) measurementDark Current CompensationDark current of photomultiplier isautomatically compensated (manualcompensation is possible by selection).Memory of correction valueOutput correction factor (energy, fadingcorrection) 1 point. Element sensitivitycorrection factor of each TLD element 999dataMeasured data memory Measured data before correction 99 dataID number 001 to 999, 3 digitsPrinter 17 digits discharge built-in printerExternal outputTransmission output: RS-232CGlow output: 5-volt full scaleOperating temperature,Humidity0 to 40 degree Celsius,10 to 80%RHPower source 100 V± 10%, 50/60 Hz, 350 WDimensions493 mm (wide) x 366 mm (deep) x 273 mm(high)Weight A pprox. 24 k g
  • 5. 4Fig. 2 Glow curve for CaSO4:Tm Element
  • 6. 5
  • 7. 63. Outline of operating procedurePreparation for measurement of TLD elements by this reader and normalmeasuring operation are outlined as below.Preparation for measurement normal measuring operationTurn on power Draw out sliderConfirm and set the date Press RESET keyRaise temperature Insert TLD elementReady to measure Push in sliderSelect TLD element type Press READ keyCalibration light sourceMeasurement processing(CAL measurement)End of measurementDisplay ofMeasured valueDisplay and set IDnumberOutput to printerand external unit
  • 8. 74. Cautions for useThe operating temperature of this reader is 0 to 40 0C. Preferably, use atthe ambient temperature of 15 to 25 0C .4.1 Handling of the reader(1) Make sure to supply the specified power source as defined on the nameplate.(2) Avoid damp, dusty or vibrating places.(3) When moving or transporting the reader, be careful not to give a shock.(4) Since a high voltage power source is built in, do not touch the internalparts while the power is on.(5) Various variable resistors for adjustment provided in the case, havebeen correctly adjusted before shipping. Do not touch them.(6) After turning off the power, cool for more than 30 minutes beforeputting the vinyl cover on it.(7) If the reader is shut down for a long period or when used in a dampplace, water drops may be collected on the convergence glass surface ofthe slider. Wipe off with a dry gauze, and push in the slider and pressthe READ key (this process is called empty running). By repeating thisempty running, dry the convergence glass sufficiently before use.(8) If any abnormality occurs, an error code is shown on the display unit,and the error message is printed out.For details, see the error code list elsewhere in the manual.4.2 Handling of TLD element(1) Do not touch the TLD element by hand.(2) Deposit of flammable dust or fat from fingertip may burn when heating,which may result in an error.(3) If the element (contained in glass capsule) is contaminated, clean theglass tube with ethyl alcohol (reagent first grade or special grade) and agauze, and dry it at ordinary temperature.(4) Do not expose the element directly to sunlight.(5) Anneal the element in the annealing oven after measurement orbefore use.
  • 9. 85. Outline of Experiment1) Practice on the method of primary handling of the TLD reader.2) Basic treatment of the TLD element.3) Reading of background dose by the TLD reader.4) Annealing of the TLD element.5) Attachment of TLD elements to the phantom.6) Irradiation of the radiation to TLD elements.7) Reading of exposure dose by the TLD reader.8) Estimate the Calibration Factor (K).6. Apparatus1) Radiation sources: Cs-137.2) TLD elements UD-200S and TLD reader Panasonic UD-5120PGL.3) Annealing oven.4) Ionization chamber type survey meter.5) ICRU Type Phantom.6) (Tape measure.)7) (Stop Watch.)7. ProceduresPreparation for measurement1) Turn on the power of the TLD reader.2) Prepare the TLD elements.3) Put the Annealing oven in operation.4) Prepare an ionization type survey meter.5) Set the ICRU type phantom at the place of irradiation.6) Place correctly the Cs-137 source.7) (Prepare a tape measure.)8) (Prepare a stop watch.)9) Prepare to use alcohol and gauze.
  • 10. 98. Primary Handling of the TLD Reader8.1 TLD Reader and Quality Assurance1) Calibration by the Internal Light Source① Draw out the slider and rotate 90 degrees clockwise. CAL will bedisplayed.② By pushing the READ key, calibration measurement will beautomatically started for 10 times. Reader calibration factorRDR.C. F. is displayed and printed out. If the value has notchanged from the previous reading, that indicates the stability ofthe reader.③ Rotate the slider 90 degrees anti-clockwise and insert the slider.2) Handling of TLD elementsIn advance of an actual irradiation, a quality assurance isrecommended using TLD elements irradiated to a certain dose bystandard radiation field. However, for the practice, the following steps① to ③ are carried out by any of the TLD elements.① Draw out the slider, take off one side of TLD elements by its plasticcap and set it at the appointed position on slider, then insert it.② Push READ key, a blast of hot air will blow to the TLD element for10 second then the result will be displayed and printed out.③ Record the results in data sheet.④ Compare the results with the standard irradiation values bydrawing a calibration curve.8.2 Practice to make measurement by the TLD reader and preparationof TLD elements1) Insert a TLD element (not irradiated) in TLD reader and then readbackground radiation dose of each element and record it.2) Make measurement twice for each TLD element, and confirm anindication of the second measurement is 0 mSv.3) Repeat measurements over 10 elements.4) Anneal all TLD elements, for the sake of correctness , by using theAnnealing oven.5) If the TLD elements was dirty, then clean them by using a piece ofgauze soaked with alcohol.
  • 11. 108.3 Irradiation of TLD elements1) Place the Cs-137 source at the position to irradiate.2) Using ionization chamber survey meter to measure the radiation doserate, decide place and time to irradiate considering the radiation doset o be obtained.3) Irradiate TLD elements.8.4 Measurement of exposure dose1) Read TLD elements by the reader and take record of the measuredvalues.2) Confirm and record the result of exposure in the unit of mSv forinstance of measurement for each TLD element.3) Calculate an apparent calibration factor (k) as a ratio of the measuredvalue of TLD divided by that value of the ionization chamber surveymeter.4)Anneal the TLD element by Annealing oven.8.5 Using ICRU type Phantom and irradiate the TLD elements1) Irradiate the indicated number of TLD elements at the same distancefrom the source.2) Read the TLD elements by the reader and record the measuredvalues .3) Confirm and record the result of measurement for each TLD element.4) Calculate the calibration factor (k) as a ratio of the measured value bythe TLD to that value by the ionization chamber survey meter.8.6 Compare the experimental results between the measurementswithout and with phantom9. Determination of Personal Dose Equivalent, by TLD.1) Record the results in data sheet.2) Fill the work sheet and compare the results.