Comparative investigations of the relative thermoluminescent efficiency of LiF detectors to protons at different proton therapy facilities

• The relative TL efficiency was determined for protons energy from 12 to 218 MeV.
• The differences of energy response between the two used TLDs are clearly visible.
• LiF:Mg, Ti efficiency shows a maximum of 1.2 for about 25 MeV protons.
• LiF:Mg,Cu,P efficiency decreases with decreasing proton energy.

Radiotherapy with proton beam is one of the most modern methods of cancer treatment. The basic dosimetric tools in proton therapy are ionization chambers. However, thermoluminescent detectors (TLDs) are also widely applied, being unrivalled in phantom measurements of spatial dose distribution. The relative thermoluminescent efficiency of LiF detectors for protons, which is important for radiotherapy and space dosimetry, have been studied several times, but the available experimental data concerning this subject are not straightforward. Some researchers show that the efficiency of LiF:Mg, Ti to protons with respect to gamma-rays, may significantly exceed unity (even more than 30%), however, many others show the efficiency close to unity or lower.In relation to this issue a research investigation concerning the efficiency of two commonly used TLDs: LiF:Mg, Ti and LiF:Mg,Cu,P, were started at the Institute of Nuclear Physics (IFJ PAN).The measurements were realized exploiting three separate proton beam accelerators normally used for cancer treatment. Two of them at the IFJ PAN Kraków using the 60 MeV and 230 MeV proton beams and another at the Helmholtz-Zentrum Berlin using the 68 MeV proton beam. The relative TL efficiency was determined for protons with energy ranging from 12 to 218 MeV. The results confirmed general energy dependence of two used TLDs. For LiF:Mg, Ti it was found that efficiency increases with decreasing proton energy and shows a maximum of 1.15 at about 20–25 MeV. For LiF:Mg,Cu,P the relative efficiency decreases systematically with decreasing proton energy and is below unity even for the highest studied proton energy.