Use of TLD at CERN
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Transcript of Use of TLD at CERN
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Use of TLD at CERN
Marco Silari, Jacques Wolf (SC/RP)
• Working principles• Evaluating the exposure• TLD response to gammas and neutrons• The TLD service of the RP group• Experimental procedure
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Exposure to radiation
• Thermoluminescent dosimeters are inorganic crystals exhibiting high concentration of trapping centers in the gap between valence and conduction bands
• Electrons are elevated from the valence to the conduction band by the incident radiation and are then captured at one of the trapping centers
• Holes can also be trapped in an analogous process• The TLD material functions as an integrating
detector in which the number of trapped electrons and holes is a measure of the number of electron-hole pairs formed by the radiation exposure
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Evaluating exposure• After the exposure, the trapped
carriers can be measured by heating the TLD
• At a temperature that is determined by the energy level of the trap, the trapped electrons can pick up enough thermal energy so that they are re-excited back to the conduction band
• The liberated electrons then migrate to near a trapped hole, where they can recombine with the emission of a photon
• Alternatively, if the holes are released at a lower temperature, they may migrate to a trapped electron and their recombination also results in a radiated photon
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Evaluating exposure• The emitted photons are in the visible region of the e-m
spectrum and their total number is an indication of the original number of electron-hole pairs created by the radiation
• TLD readers derive a signal by heating the dosimeter and reading the light output via a photomultiplier. The light yield is recorded as a function of sample temperature in a glow curve
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TLD response vs dose
• The response curve of a TLD material as a function of absorbed dose shows three regions• a region in which the response is linear • a region of supralinearity, in which the
sensibility increases (but the reliability decreases)
• a saturation region (defines maximum detectable dose = 80% of saturation dose)
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Lithium Fluoride
• Almost negligible fading at room temperature• Low atomic number, which does not differ
much from that of air or tissue energy deposited in LiF is correlated to gamma dose
• Constant response over a wide range of photon energies
• Because natural lithium contains 7.4% Li-6, LiF dosimeters are sensitive to slow neutrons via (n,α) reaction
• The response to neutrons can be enhanced by using lithium enriched in Li-6, or suppressed by using lithium consisting entirely of Li-7
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Energy response to photons
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Energy response to neutrons
Li-6: σth = 945 barn, Li-7: σth = 0.033 barn
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The TLD service of the RP group
At CERN we use Harshaw TLD 600 (95.6% Li-6) and TLD 700 (0.01% Li-6)
Size of the dosimeter: 3.1 mm x 3.1 mm x 0.9 mm (23 mg)
Dose range: 10 µGy – 100 Gy (in practice: 10 µGy – a few mGy)
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The TLD service of the RP group
One automatic reader, ALNOR (10 µGy – 2 Gy)
One semi-automatic reader, HARSHAW, for high doses
Cross-calibration of TLDs with the environmental monitoring stations
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The TLD service of the RP group
Number of measurements in 2003:
211 gamma measurement locations (1688 TLD readings)
442 neutron measurement locations (10608 TLD readings)
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Experimental procedure (1)
• 2 readings with ALNOR by heating the TLDs with a jet of N gas at 340 degrees
• thermal treatment of 16 hours at 80 degrees to reduce fading
• installation, exposure to radiation, recuperation
• thermal treatment of one hour at 80 degrees to eliminate instable peaks at low temperature and short-lived
• first reading with ALNOR at 340 degrees• second reading with ALNOR at 340 degrees for
background subtraction
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Experimental procedure (2)
• thermal treatment of 16 hours at 80 degrees
• irradiation with Cs-137 source in the calibration lab, 1 mSv
• thermal treatment of 1 hour at 80 degrees
• reading with ALNOR at 340 degrees for background subtraction
To correct the individual gamma response of each TLD, the following procedure is applied:
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Conclusions
• How many measurement positions are needed by the LHC experiments? If the number is significant (i.e., above a few percent of our present workload), the required manpower and resources have to be addressed
• To be used for high-level dosimetry, some investigation on their response to the various types of radiation is required
• For neutron monitoring in the environment, the TLDs are used within a polyethylene moderator, whilst in the LHC experiments they should be used bare
• The RP group still has to take a decision on whether or not to maintain the TLD service or to outsource it