On the development of a solid-state, low dose EPR dosimeter for radiotherapy

To the extent investigated in the papers comprising this thesis, lithium formate apparently retains the good properties of alanine while offering the increased sensitivity (6-7 times) required for clinical use. However, in the same manner as alanine was tested for various properties before accepted for routine use, lithium formate must withstand far more attention before it can be concluded that it is superior to alanine as an EPR dosimeter. There are five major issues to be investigated: 1) The dependency of fading properties with humidity, before irradiation and during. 2) Storage after irradiation. 3) The effect of making dosimeters with a binder, e.g. paraffin, so as to increase mechanical stability of the dosimeters and to reduce adverse effects of humidity. 4) The effect of UV- and visible light exposure on irradiated lithium formate dosimeters at ambient temperature and variable humidity. 5) Dependency studies regarding photon energies ranging from low-energy X-rays through Co60 gamma-irradiation to high-energy (MeV) X-rays (brehmstrahlung). 6) Dependency studies using radiation exhibiting different LET values, comprising ultra-low-energy photons or electrons and particles like protons, a-particles, neutrons and other ions. Some work on these issues is already in progress in our laboratories, e.g. a continuing ENDOR study on the structure of the radiation-induced radicals. Complementary to this, interesting properties of lithium formate with respect to neutron irradiation have been discussed. This is due to the fact that Li6 has a very large cross section for neutron capture (at low neutron energies) thereby producing alpha-particles with high LET values. Thus, dosimetry in mixed fields (gamma, neutron) should be possible using either ''normal'' lithium formate or eventually Li6 substituted lithium formate where the isotope ratio should be varied to an optimum value for the actual purpose. Much more work in this direction is needed. With the prospect of further increasing the sensitivity of lithium formate with respect to isotopic substitution and eventually trace metal doping, more detailed measurements to examine the relaxation times in lithium formate are necessary. Such measurements may well be made using single crystals. Simulations of the saturation behaviour based on the theory by Martian, can be compared to pulsed EPR measurements, the most acknowledged technique for precise determination of relaxation times in a given system.4 papers with the following titles are included: 1) EPR dosimetric properties of formats. 2) Electron paramagnetic resonance (EPR) dosimetry using lithium formate in radiotherapy. Comparison with thermoluminescence (TL) dosimetry using lithium fluoride rods. 3) Radiation-induced radicals in lithium formate monohydrate LiHCO2H2O and ENDOR studies of' X-irradiated crystal and polycrystalline samples. 4) Estimation of' X-ray beam quality by electron paramagnetic resonance (EPR) spectroscopy