Answer to Question #8865 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

The following question was answered by an expert in the appropriate field:

Q

Can we use LiF TLD-100 for high-energy beams, for example, up to 1.2 MeV?

A

TLD-100 is certainly usable for high-energy photon measurements. LiF TLD-100 does exhibit some energy dependence in its dose response, with the largest variations occurring at rather low energies. If you are making dose measurements, the relative dose response depends, in part, on what energy has been used in calibrating the dosimeters. If the dosimeters have been calibrated with a relatively high energy source, such as 137Cs (662 keV), the response at 1.2 MeV will be very close to that at the calibration energy (i.e., TLD [thermoluminescent dosimetry] light output per unit tissue dose will be very close at both energies). If you attempt to measure doses at low-photon energies (below 100 keV or so) you will find that the response of the dosimeter, calibrated at 137Cs energies, is higher at the lower energies, often being 30 to 40 percent higher in the 10s of keV region. The TLDs have often been used for measurements of high photon energies available, for example, in medical therapy installations where photon energies may exceed 10 MeV. Many experimentalists have found that the TLD-100s, calibrated at 60Co energies (average of 1.25 MeV), yield a somewhat depressed response to the higher-energy photons, responses often being 8 to 10 percent lower than that observed at 60Co energies.

George Chabot, PhD, CHP

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