Thermoluminescent Dosimeters
(TLDs) from the Institute of
Physics, Krakow, Poland
Adam Thornton
Thermoluminescent Dosimeters
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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?
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A slide containing pellets of variously doped Lithium Floride phosphors
Common variations used:
– LiF:Mg,Ti
– LiF:Mg,Cu,P
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[N or 7]
[N or 7]
MTS
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
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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]
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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 work
The one trapping – one recombination centre model:
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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)
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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
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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
• Dose >1Gy is non linear
LiF:Mg,Cu,P
Co-60 data
LiF:Mg,Ti
1000
linearity
sublinearity
saturation
and sensitivity decrease
10000
Measured dose, Gy
100
10
saturation
supralinearity
linearity
and sensitivity decrease
Measured dose, Gy
Co-60 data
1000
100
10
1
1
1
10
100
Real dose, Gy
1000
10000
1
10
100
Real dose, Gy
1000
10000
TLD Response
• Results corrected for non linearity
LiF:Mg,Cu,P
LiF:Mg,Ti
Co-60 data
10000
Measured data points
Fitted empirical function
Linear trend
10000
Measured data points
Fitted empirical function
Linearity trend
1000
Real dose, Gy
1000
Real dose, Gy
Co-60 data
100
100
10
10
1
1
1
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Measured dose, Gy
100
1000
1
10
100
Measured dose, Gy
1000
TLD Response
• Results corrected for non linearity
LiF:Mg,Cu,P
LiF:Mg,Ti
Co-60 data
Co-60 data
10000
10000
10
100
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correction range
Real dose, Gy
correction range
Real dose, Gy
100
high uncertainty
1000
1000
1
1
1
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Measured dose, Gy
100
1000
1
10
100
Measured dose, Gy
1000
TLD Response Summary
• up to 1 Gy
• from 1 Gy to 1kGy
linear
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 nonshielding 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
H4RAD02
TLD
Dose
Dose
(Sim)
(Detector) MCP-N
4388
1.78
H4RAD03 wall
120.88
H4RAD03 rack
38.72
TLD dose
1.70
H4RAD04
29267
15.19
H4RAD05
4358
3.37
3.83
65.15
47.12
164.01
129.92
BLM vertical
BLM horizontal
PMI
29269
29268
71.22
MCP-7
MTS-N
MTS-7
3.29
1.27
6.26
1.28
12.70
11.45
15.96
8.08
4.37
2.50
6.83
2.90
59.87
58.65
62.99
45.64
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)