thermoluminescent dosimeters ( tlds) from the institute of physics, krakow, poland

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Thermoluminescent Dosimeters ( TLDs) from the Institute of Physics, Krakow, Poland. Adam Thornton. Thermoluminescent Dosimeters. What is a TLD? How TLDs work Reading TLDs and taking measurements Examples of ‘ glowcurves ’ Analysing the data TLD response in different conditions - PowerPoint PPT Presentation


Thermoluminescent Dosimeters from the Institute of Physics, Krakow, Poland

Thermoluminescent Dosimeters (TLDs) from the Institute of Physics, Krakow, PolandAdam ThorntonThermoluminescent DosimetersWhat is a TLD?How TLDs workReading TLDs and taking measurementsExamples of glowcurvesAnalysing the dataTLD response in different conditionsTLDs in mixed fieldsWhy we use them and where they are usedH4IRRAD results (preliminary)Conclusions about using TLDs at CERNInformation on the new cyclotron at the IFJ(some Polish required)What is a TLD?

What is a TLD?

What is a TLD?A slide containing pellets of variously doped Lithium Floride phosphorsCommon variations used:LiF:Mg,Ti [N or 7] MTSLiF:Mg,Cu,P [N or 7]MCPThe N and 7 stand for which lithium is used in the sampleN -> Natural, a combination of lithium 6 and 7.7 -> Only lithium 7 is usedThe material has thermoluminescent properties after exposure to radiationEach type has a different sensitivity (efficiency) to different types of radiationFor example, lithium 7 is not sensitive to thermal neutrons, but lithium 6 is [this difference can be used to work out the thermal neutron dose, by simply subtracting one form the other]TLDs can be calibrated in specific radiation fields and this information can then be used to determine the dose absorbed by the material [TLDs from the IFJ Krakow are calibrated using gamma source Co60]Designed for personal dosimetryHow TLDs workThe one trapping one recombination centre model:Electrons/hole pairs become excited when exposed to radiationIf the electron is given enough energy, it moves into the conduction bandWhen the electron tries to return to the ground state, there are two possibilities:It returns directlyIt gets trapped in an imperfection within the crystal structure (deliberately made from the doping process)When the sample is heated, the electron receives enough energy to break from the trap and recombine with the hole -> this process emits lightThis light can be measured by a photomultiplier and the TLDs exposure to radiation can be calculatedThe trapped energy states can last for up to 2 years(?) which make them a good a passive measuring device for radiationSensitive between Gy to MGy

How TLDs work

Reading the TLDsThe TLDs are heated to 100oC for 10 mins to remove low TL peaks in glow curveAll TLDs read at 2oC/s, (with argon gas environment)First the calibration detectors are read (1Gy gamma)Background TLDs are read with high photomultiplier sensitivity and temperatures between:100oC to 400oC for MTS (7 and N)100oC to 270oC for MCP (7 and N)Experiment TLDs read, sensitivity depends on expected dose better accuracy achievable on manual reader if estimated dose is known, using the same temperatures as beforeAfter reading, the TLD signal is reduced, so can only be read once (some studies into new methods of secondary readings using UV light, not yet successful)

Reading the TLDs

Reading the TLDs (Glowcurve)

Peak normalised to 220oCReading the TLDs (Glowcurve)

Peak normalised to 220oCAnalysing TLDsExport glow curve data from tool to data fileData normalised to 220oCThe integral is taken: 100oC to 248oC for MTS 100oC to 270oC for MCP Take average of calibration data (with SD)Take average of backgroundFrom the raw data: Dose = counts / (cali - BG)Each TLD has an individual response factor (IRF) which is determined after reading:Annealing, exposing all slides to the same dose and comparing each with the mean of all detectors. The data is then compensated.Correction function is used on all those with dose above 1Gy, as above this the signal to dose ratio is no longer linearTLD Response

TLD Response

TLD Response

TLD Response

TLD Response

Dose >1Gy is non linearTLD Response

Results corrected for non linearity

TLD ResponseResults corrected for non linearity

TLD Response Summary up to 1 Gy linear from 1 Gy to 1kGynonlinear, but correctable(however from around 0.6 kGy uncertainties grow strongly -> especially for MCP)

> 1 kGy UHTR method may be used (for MCP)(but up to around 3 kGy high uncertainties)UHTR (Ultra high temperature ratio) This is the ratio between the total integrated TL signal for temperatures above the temperature T(x), to the total integrated TL signal. A good value for T(x) has been found at 250 Celsius.20TLD Response Summary

UHTR (Ultra high temperature ratio) This is the ratio between the total integrated TL signal for temperatures above the temperature T(x), to the total integrated TL signal. A good value for T(x) has been found at 250 Celsius.21TLDs in mixed fieldsCERF 2007 (B. Obryk et al.)mGy to 150GyGood agreement with simulationsComparison with alanine also showed agreement, TLDs more accurate at low dosesThermal and epithermal efficiency better for MTS than MCP (reconfirmation)Conclusion: TLD can be used in a mixed field environment, but further calibration required

Various 2009 (B. Obryk et al.)Further tests with high dose and mixed field (more high dose)Defect clusters proposed as reason for strange MCP behaviour at high doseConclusion:Further research required

Why do we use TLDs?Used in along side other detector types for additional comparisonSensitive to small doses, more so than the other kinds of active detectorsNot effected by electric/magnetic fieldsSmall size and mobile so can be placed anywhereComparing the dose absorbed by LiN and Li7, the thermal neutron dose can be calculated (simple subtraction)TLDs at CERNCurrent locations used:LHC (all around the machine, normally in pairs (in front and behind shielding))CNGS (on all PMI positions, including target gallery side)H4IRRAD (various in shielded and non-shielding positions, attached to PMI, Radmon and BLM for comparison)

H4IRRAD ResultsG:\Projects\R2E\Monitoring\TLD\H4IRRAD\TLD_Results_Final.xlsH4IRRAD ResultsTLD Results Comparison (Gy)DetectorTLDDoseDoseTLD dose(Sim)(Detector)MCP-NMCP-7MTS-NMTS-7H4RAD0243881.781.703. wall120.88H4RAD03 rack38.72H4RAD042926715.1912.7011.4515.968.08H4RAD0543583.373.834.372.506.832.90BLM vertical2926965.1547.1259.8758.6562.9945.64BLM horizontal164.01129.92PMI2926871.2261.8858.3862.4349.02H4IRRAD ConclusionsIndividual response factors still need to be determined (first attempt failed due to residual dose after annealing) Results will be more accurateMTS-N measured dose close the values from Fluka (H4RAD02 position not good)More thorough comparison with simulations to be performed on Barts returnReasonable agreement with BLM (more detailed comparison needed)Conclusions and further workCalibration work in mixed field, beneficial to us and BarbaraUsing more closely the simulations, Radmon and BLM data to determine the doseThis leads to more accurate results for the LHC TLDsWhen using TLDs, try doing placing them with slide number in order (avoids complications, low dose on lowest numbers)No need for background (they have at the insitute)

New CyclotronFollowing slide from:Witold Mczyski(Wizka protonow i infrastruktura dla bada podstawowych w CentrumCyklotronowym Bronowice)

Many thanks to Markus Brugger, Barbara Obryk, Wojciech Gieszczyk and the rest of the section in the IFJ dosimetry service and EN/STI/EET group

Questions? (I dont speak Polish)


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