EPR Dosimetry – An Update and Prospective Studies S. Murali1a, T. S. Sudheera, S. Thanamania, D. P. Ratha, J. A. Sapkala, V. Natarajanb, M. K. Bhideb, D.N. Sharmaa. a. Radiation Safety Systems Division, b. Radio Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai – 400 085, India. Abstract. 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, 6 3 the reaction Li (n,α) H, 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 = 3− 3− 2.00415. The signal was identified as CO3 from earlier reports. The EPR signal intensity of CO3 (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. KEY WORDS: Electron Paramagnetic Resonance Dosimetry, Radicals, Signal – Dose, kGy. Author’s Email: [email protected] 1. Introduction Dosimetry is an area of interest to Health Physicists, Medical Physicists, accidental/high dose assessments during radiation incidences and during planned high irradiation for industrial products, pasteurisation of food materials, spices, radiation sterilisation of medical products etc., at industrial irradiators. Varieties of techniques are presently available for the dosimetry depending upon the: requirements of dose measurement ranges, convenience of dose measurements, dosimetric material characteristics and reproducibility of the dose measurements. The interaction of radiation with matter is best understood from the study of the end products generated. A large number of radiation dosimetric processes have a focused interest in the unpaired electrons / free radicals generated as radiation damages in such substances. Thus the study of changes in the concentrations of unpaired electrons / free radical as the radiation induced damages is found to be useful in the dosimetric studies[III]. Hence the technique wherein the concentration of the unpaired electrons could be easily quantified using the associated paramagnetic property may have applications in the field of dosimetry. 2. Electron Paramagnetic Resonance (EPR) technique Electron Paramagnetic Resonance (EPR) is the study of direct transitions between the electronic Zeeman levels of ground state. EPR is a method of magnetic resonance under suitable microwave stimulation, enabling to detect the unpaired electrons by their magnetic moment[II]. In the absence of an externally applied magnetic field, the spin magnetic moments have no preferred direction, are randomly oriented. If a uniform magnetic field is imposed, the components of spin angular momentum Ms get resolved due to the interaction of spin magnetic moment (of unpaired electrons) with the external magnetic field. Each unpaired electron has a spin angular momentum associated with it, due to the spin quantum number s = ½. The magnitude of the spin momentum for the unpaired electron would be Ms = + ½ or Ms = - ½, depending upon the orientation of the spin angular momentum. In an external magnetic field the electrons have a resolved component of either Ms = + ½ or Ms = - ½ in the direction of the applied field. The application of external magnetic field divides the unpaired electrons into two groups, those electrons with magnetic moments lined up parallel to the magnetic field have the energy reduced by an amount ½ gβH while those with magnetic moments lined up anti-parallel to the magnetic field have the energy increased by the same amount ½ gβH[IX]. The basic principle of the electron resonance technique is to apply the external electromagnetic radiation and arrange the frequency υ, to be such that hυ is equal to the energy difference between the two groups of electrons. The transition of electron from an energy level to another energy level can be induced by the external electromagnetic perturbation of suitable frequency and polarization. A photon of energy hυ = gβH is absorbed, it results in the transition by satisfying ∆ms = ± 1 and ∆mI = 0. EPR spectral absorption indicates that the unpaired electron in the lower energy level is more numerous than in the higher level. The population between the two energy levels is given by −−∆∆∆∆ΕΕΕΕ / kT N2 = N1 exp k – Boltzmann constant, T – Temperature in Kelvin[IX]. Since resonant absorption takes place at γγγ, in presence of the external electromagnetic radiation either of the states will be equally populated. The microwave power is generally kept low to avoid the saturation of the signal. The difference between N1, N2 can be made as large as possible applying large magnetic field H. The energy level population explains the necessity of the values of the operational parameters. With reference to the equation on population of states the parameters that can vary is ‘H’ the externally applied magnetic field. Thus, the value of the magnetic field at resonance determines the value of the g factor. 2.1 EPR dosimetry Electron Paramagnetic Resonance (EPR) technique finds application in the field of dosimetry i. due to paramagnetic species formed by irradiation of the material, with concentration of paramagnetic defect centers is proportional to the dose delivered to the specimen and ii. due to the EPR signal intensity of the resonance peak is proportional to the unpaired electrons / cc of the specimen. Hence, the signal intensity is used as a function of the radiation dose delivered to the sample in EPR dosimetry. Besides, EPR is a non-destructive, non-invasive method, highly sensitive technique (responsive for spin concentration ≥ 109 -1011/ cc)[III]. The dose measurements can be repetitive with dosimetric signal accumulated and can be used at wide range of temperatures. EPR read-out technique is based on the quantitative measure of the absorption signal of irradiated dosimeter material. Free radicals in organic materials correspond to trapped electrons or rupture of covalent bonds. The advantage of EPR dosimetry is its ability to formulate dosimeters equivalent to mammalian tissues, water and products processed by irradiation. Besides EPR spectroscopy has proven potential for dosimetry of high-level radiation. New materials / common lab reagents were investigated for EPR dosimetry, wherein the radiation induced paramagnetic species retains the radiation signatures, lead to idea on radiation dose. 3. Experimental The prospective dosimetric samples for EPR dosimetry were chosen such that the radiation signatures in the form of paramagnetic radicals are available in the commonly available reagents / chemicals in the laboratory. Powder samples about 100 mg, heated for moisture removal and in select