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• 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)

What is a TLD?

What is a TLD?

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

How TLDs workThe 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

How TLDs work

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)

Reading the TLDs

Reading the TLDs (Glowcurve)

Peak normalised to 220oC

Reading the TLDs (Glowcurve)

Peak normalised to 220oC

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

TLD Response

TLD Response

TLD Response

TLD Response

TLD Response

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LiF:Mg,Cu,P Co-60 data

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• Dose >1Gy is non linear

TLD Response

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Measured data points Fitted empirical function Linear trend

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• Results corrected for non linearity

TLD Response• Results corrected for non linearity

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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 -> especially for MCP)

• > 1 kGy UHTR method may be used (for MCP)(but up to around 3 kGy high uncertainties)

TLD Response Summary

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

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 LiN and Li7, the

thermal neutron dose can be calculated (simple subtraction)

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)

H4IRRAD Results

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

H4IRRAD Results

TLD Results Comparison (Gy)Detector TLD Dose Dose TLD dose (Sim) (Detector) MCP-N MCP-7 MTS-N MTS-7H4RAD02 4388 1.78 1.70 3.29 1.27 6.26 1.28H4RAD03 wall 120.88 H4RAD03 rack 38.72 H4RAD04 29267 15.19 12.70 11.45 15.96 8.08H4RAD05 4358 3.37 3.83 4.37 2.50 6.83 2.90BLM vertical 29269 65.15 47.12 59.87 58.65 62.99 45.64BLM horizontal 164.01 129.92

PMI 29268 71.22 61.88 58.38 62.43 49.02

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)

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)

New Cyclotron

Following slide from:Witold Męczyński(Wiązka protonow i infrastruktura dla badań podstawowych w Centrum Cyklotronowym 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 don’t speak Polish)