EPR Dosimetry: an update and prospective studies

Electron Paramagnetic Resonance (EPR) technique is useful to quantify the paramagnetic species in any matrix. The unpaired electrons present in paramagnetic materials have non - zero spin value, have an associated spin magnetic moment. When such a system is subjected to an external magnetic field, electronic Zeeman splitting of ground level state occurs. On application of suitable stimulant microwave energy, the electrons flip between the Zeeman levels of ground state, result in resonant absorption of the microwave energy. The intensity of resonant absorption signal is proportional to the concentration of the unpaired electrons in the irradiated material, could lead to possible use of such materials in EPR dosimetric applications. New materials were investigated for EPR dosimetry, wherein the radiation induced paramagnetic species retains the radiation signatures, lead to idea on radiation dose. Few of the materials have been identified as prospective EPR dosimeters. The radiation induced radical in Li2CO3 powder material being paramagnetic in nature (signals at g = 2.0036 and at g 2.0006) and radical concentration varying as a function of irradiation dose, led to its identification for possible use in EPR dosimetric applications. Besides, during the neutron irradiations, the reaction 6Li (n,α) 3H, led to the yield of radicals many folds higher compared to that of gamma irradiation. Thus, the commonly available Li2CO3 material has been assessed for the EPR dosimetric response in gamma and neutron environments. EPR investigation of Li2C2O4, Na2C2O4 mixtures was carried out to measure the radiation dose from γ photons and thermal neutrons in a mixed radiation field. A single line spectrum of CO2- radical at g = 2.0045 ± 0.0005 was found on gamma and neutron irradiations. Of all the mixture combinations, the 2:1 mixture was found more sensitive for gamma / thermal neutrons. Intensity of CO2- radical signal was found linear from 6 Gy - 11 kGy for gamma and 40 - 1500 kGy for thermal neutron flux. The radiation induced radical signal was found to be stable over a period of 300 days with marginal fading of < 1 %. The results of EPR dosimetry suggest that the Li2C2O4: Na2C2O4 mixture as the potential neutron dosimeter for high range dosimetry. The effect of gamma dose irradiation on sodium succinate was studied by EPR technique. It was observed that the radiation induced CO3- radical (g = 2.00357) as linear in signal - dose response, in 35 Gy - 4.4 kGy. Thus, sodium succinate powder samples could be used in EPR dosimetry, since CO3- radicals have been found stable for more than 6 months, post-irradiation. Solid State Nuclear Track Detector (SSNTD) material Tuffak polycarbonate film was studied for prospective EPR dosimetry, feasibility studies were carried out on gamma irradiated SSNTD film. The first derivative EPR spectra of irradiated Tuffak polycarbonate samples contained a singlet, signal at g = 2.00415. The signal was identified as CO33- from earlier reports. The EPR signal intensity of CO33- (g = 2.00415) was found linear in signal - dose response in 10 - 80 kGy. The present paper gives an update of newer EPR dosimetric materials that have been investigated, after a brief introduction to the basic principles of EPR. Further, prospective dosimetric materials with their suitability for applications in EPR dosimetry have been discussed. (author)