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Thermoluminescent Dosimeters ( TLDs) from the Institute of Physics, Krakow, Poland

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

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Thermoluminescent Dosimeters ( TLDs) from the Institute of Physics, Krakow, Poland

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  1. Thermoluminescent Dosimeters (TLDs) from the Institute of Physics, Krakow, Poland Adam Thornton

  2. 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 • TLDs in mixed fields • Why we use them and where they are used • H4IRRAD results (preliminary) • Conclusions about using TLDs at CERN • Information on the new cyclotron at the IFJ (some Polish required)

  3. What is a TLD?

  4. What is a TLD?

  5. What is a TLD? • A slide containing pellets of variously doped Lithium Floride phosphors • Common variations used: • LiF:Mg,Ti [N or 7] MTS • LiF:Mg,Cu,P [N or 7]MCP • The N and 7 stand for which lithium is used in the sample • N -> ‘Natural’, a combination of lithium 6 and 7. • 7 -> Only lithium 7 is used • The material has thermoluminescent properties after exposure to radiation • Each type has a different sensitivity (efficiency) to different types of radiation • For 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 dosimetry

  6. How TLDs work The one trapping – one recombination centre model: • Electrons/hole pairs become excited when exposed to radiation • If the electron is given enough energy, it moves into the conduction band • When the electron tries to return to the ground state, there are two possibilities: • It returns directly • It 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 light • This light can be measured by a photomultiplier and the TLDs exposure to radiation can be calculated • The trapped energy states can last for up to 2 years(?) which make them a good a passive measuring device for radiation • Sensitive between μGy to MGy

  7. How TLDs work

  8. Reading the TLDs • The TLDs are heated to 100oC for 10 mins to remove low TL peaks in glow curve • All 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 before • After 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)

  9. Reading the TLDs

  10. Reading the TLDs (Glowcurve) Peak normalised to 220oC

  11. Reading the TLDs (Glowcurve) Peak normalised to 220oC

  12. Analysing TLDs • Export glow curve data from tool to data file • Data normalised to 220oC • The integral is taken: • 100oC to 248oC for MTS • 100oC to 270oC for MCP • Take average of calibration data (with SD) • Take average of background • From 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 linear

  13. TLD Response

  14. TLD Response

  15. TLD Response

  16. TLD Response

  17. TLD Response • Dose >1Gy is non linear

  18. TLD Response • Results corrected for non linearity

  19. TLD Response • Results corrected for non linearity

  20. TLD Response Summary • up to 1 Gy linear • from 1 Gy to 1kGy nonlinear, but correctable (however from around 0.6 kGy uncertainties grow strongly -> especiallyfor MCP) • > 1 kGy UHTR method may be used (for MCP) (but up to around 3 kGy high uncertainties)

  21. TLD Response Summary

  22. TLDs in mixed fields • CERF 2007 (B. Obryk et al.) • mGy to 150Gy • Good agreement with simulations • Comparison with alanine also showed agreement, TLDs more accurate at low doses • Thermal 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 dose • Conclusion: Further research required

  23. Why do we use TLDs? • Used in along side other detector types for additional comparison • Sensitive to small doses, more so than the other kinds of active detectors • Not effected by electric/magnetic fields • Small size and mobile so can be placed anywhere • Comparing the dose absorbed by LiNand Li7, the thermal neutron dose can be calculated (simple subtraction)

  24. TLDs at CERN • Current 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)

  25. H4IRRAD Results G:\Projects\R2E\Monitoring\TLD\H4IRRAD\TLD_Results_Final.xls

  26. H4IRRAD Results

  27. H4IRRAD Conclusions • Individual response factors still need to be determined (first attempt failed due to residual dose after annealing) Results will be more accurate • MTS-N measured dose close the values from Fluka (H4RAD02 position not good) • More thorough comparison with simulations to be performed on Bart’s return • Reasonable agreement with BLM (more detailed comparison needed)

  28. Conclusions and further work • Calibration work in mixed field, beneficial to us and Barbara • Using more closely the simulations, Radmon and BLM data to determine the dose • This leads to more accurate results for the LHC TLDs • When 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)

  29. New Cyclotron Following slide from: WitoldMęczyński (Wiązkaprotonow i infrastrukturadlabadańpodstawowych w Centrum CyklotronowymBronowice)

  30. 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 don’t speak Polish)

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