RADMON meeting, 22 July 2004 Use of TLD at CERN 1

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 2

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 3

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 4

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 5

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 7

Energy response to photons

RADMON meeting, 22 July 2004 Use of TLD at CERN 8

Energy response to neutrons

Li-6: σth = 945 barn, Li-7: σth = 0.033 barn

RADMON meeting, 22 July 2004 Use of TLD at CERN 9

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)

RADMON meeting, 22 July 2004 Use of TLD at CERN 10

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 11

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)

RADMON meeting, 22 July 2004 Use of TLD at CERN 12

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

RADMON meeting, 22 July 2004 Use of TLD at CERN 13

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:

RADMON meeting, 22 July 2004 Use of TLD at CERN 14

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