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 -> 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.

21. TLD 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.

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
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 LiN and 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 TLD Results Comparison (Gy) Detector TLD Dose TLD dose
(Sim)
(Detector)
MCP-N
MCP-7
MTS-N
MTS-7
H4RAD02
4388
1.78
1.70
3.29
1.27
6.26
1.28
H4RAD03 wall
120.88
H4RAD03 rack
38.72
H4RAD04
29267
15.19
12.70
11.45
15.96
8.08
H4RAD05
4358
3.37
3.83
4.37
2.50
6.83
2.90
BLM vertical
29269
65.15
47.12
59.87
58.65
62.99
45.64
BLM horizontal
164.01
129.92
PMI
29268
71.22
61.88
58.38
62.43
49.02

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: Witold Męczyński (Wiązka protonow i infrastruktura dla badań podstawowych w Centrum Cyklotronowym Bronowice)

32. 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)