EPR characterization of the mixed radiation field of a boron neutron capture therapy irradiation facility: a dual natural lithium formate/L-alanine dosimeter

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Electron paramagnetic resonance (EPR) is a non-destructive technique widely used to estimate doses of ionizing radiation [1] in radiological emergencies, medical therapies, industrial processes, etc.An EPR dosimeter detects and quantifies point paramagnetic defects, usually free radicals, that are created by ionizing radiation (photons, neutrons, electrons) incident on an adequate material. The intensity of the EPR signal is a reliable measure of the number of paramagnetic centers induced, which is proportional to the received dose.Several compounds [2] can be used as EPR dosimeters: L-alanine , hidroxyapatite, 2-methyl alanine, lithium formate, glucose, etc. The election depends on the sensitivity, time stability, linearity, and the dose interval of interest (from mGy to kGy).L-alanine [3] is widely used in industrial medium dose applications (50 Gy–10 kGy) or high dose applications (10–50 kGy). The widespread use of alanine in low-dose (below 5 Gy) biomedical applications is due to its tissue equivalent properties. The induced free radicals responsible for the five-line EPR spectrum are very stable and produce a signal that can remain unchanged for years. Its response is reasonably linear in a wide range of doses. The optimal range of detection for alanine commercial sensors is 10 Gy–100 kGy [4]. It is almost insensitive to thermal neutrons and for this kind of radiation it is usual to dope alanine with boron or lithium, with a large neutron capture cross section, to enhance the response [5].Lithium formate monohydrate (abbreviated ‘LiFo’) is EPR sensitive and has several advantages [6]. LiFo’s atomic composition is closer to that of water, what makes it more similar to organic tissues, has a higher sensitivity (5–6 times), and a simpler (single line) EPR spectrum tan alanine’s. LiFo is receiving considerable attention due to its ability to capture thermal neutrons through the reaction 6Li + n → 4He + 3H. The cross section for thermal neutrons capture of the 6Li isotope (natural abundance 7.5%) is roughly 2 104 times larger than the majority isotope 7Li. Thus, the irradiation of LiFo with thermal neutrons produces the emission of alpha particles and tritium atoms which generate paramagnetic defects that could be detected and quantified by EPR [7]. This is relevant for medical applications that use mixed radiation fields of photons and termal neutrons, as Boron Neutron Capture Therapy (BNCT).An adequate characterization of a BNCT mixed field implies the simultaneous determination of the gamma photon dose and the thermal neutron fluence in a certain position, which is crucial to manage the therapy [8]. A dual dosimeter that would combine the EPR readings of natural Li and 6Li-enriched LiFo was proposed in the past [7].In this work we investigate the factibility of a L-alanine/LiFo dosimeter without requiring 6Li isotopic enrichment. The γ-doses in our BNCT source go from ~7 Gy to 150 Gy, while the thermal neutron fluences range is 1012 - 2 1013 neutrons cm−2. For calibration purposes we used a 60Co auxiliar source ranging from 0.1 to 50 kGy γ-doses.We used commercial L-alanine pellets and specially prepared (milling and sievieng) natural lithium formate powder samples of specific granulometry, that were irradiated in the 60Co irradiation plant (PISI) and in the mixed flux of the BNCT facility (Fig. 1) placed in the RA6 experimental reactor (Bariloche, Argentina).The EPR study of the irradiated samples was performed by determining the intensity of the spectrum relative to a reference standard constituted by Mn2+ impurities diluted into a MgO single crystal. As expected, L-alanine has revealed to be largely insensitive to thermal neutrons in the investigated range. On the contrary, the EPR intensity of irradiated natural LiFo powders is clearly sensitive to thermal neutrons and has a linear dependence on the γ-dose (Fig. 2).Based on these results we propose a dual dosimeter by combining L-alanine pellets and LiFo powders that would allow to determine the γ-dose and thermal neutron fluence in a selected position. Moreover, we demonstrate that the 6Li enrichment that has been proposed in the past to enhance the performance of Li-based EPR dosimeters is not crucial here. The natural isotopic abundance is enough to obtain a satisfactory sensibility to thermal neutrons in our BNCT irradiation facility for fluences > 1012 neutrons cm-2This work was supported by Conicet, ANPCyT and UN Cuyo (Argentina). The technical support from R. Benavídes, C. Pérez and M. Guillén is deeply acknowledged. **