MICRODOSIMETRIC INTERPRETATION OF THE ANOMALOUS PHOTON ENERGY RESPONSE OF ULTRA-SENSITIVE LIFMG,CU,P TL DOSIMETERS

An anomalous relative response, S(E), showing a minimum at a photon en ergy E=100 keV has been measured for ultra-sensitive LiF:Mg,Cu,P TL de tectors. The relative TL efficiency, eta(E)=S(E)/mu'(E), where mu'(E) is the ratio of the energy-dependent mass energy absorption coefficien ts of the LiF:Mg,Cu,P and of air, was found to decrease with decreasin g photon energy, with a local minimum of eta=0.73 at E=83 keV. This an omalous response is explained as an ionisation density effect. The TL efficiency of LiF:Mg,Cu,P is known to decrease strongly with increasin g ionisation density, e.g. for alpha particles and thermal neutrons. W ith increasing photon energy, the density of ionisation, expressed by the mean lineal energy y(F) or by LET. decreases, except for photons o f energies between 50 and 120 keV, the transition region from the phot oelectric process to Compton scattering. Due to the changing mode of p hoton interaction and the corresponding changes of the photon induced secondary electron spectra in LiF, a local maximum of y(F) for about 8 0 keV photons arises which agrees with the local minimum in TL efficie ncy, eta. eta was found to be a linear function of y(F) over the photo n energy range considered.