LET effects on the ESR/alanine signal: a spectral decomposition approach

Alanine dosimeters based on electron spin resonance (ESR) are widely used in medical dosimetry due to their reliability. Upon ionising radiation exposure, three main radicals are produced in the material (R1, R2 and R3). These radicals refer to the deamination of a protonated radical anion, the hydrogen subtraction from the central carbon, and an oxidation product, respectively [1]. Although the ESR/alanine signal response is linear over a wide range of doses for photons (i.e., low linear energy transfer (LET) radiation), for higher LET radiations, quenching effects and changes in the spectrum are observed. The latter, usually studied by means of the x/y ratio [2]. This study, therefore, aimed to investigate the influence of LET on the radiation-induced radicals in alanine using spectrum decomposition. For that, alanine pellets were irradiated with photons (6 MV X- rays) at the San Matteo Policlinico, and with proton and carbon ion beams at CNAO (both in Pavia, Italy). The latter two in a spread-out Bragg peak (SOBP) condition. Dose-averaged LET spanned from 0.5 up to 100 keV/μm, and doses between 8 and 40 Gy. The 3-component simulated spectrum was obtained using the EasySpin MATLAB [3], with the pepper function, including hyperfine interactions. Experimental ESR spectra were acquired with a 580 Bruker spectrometer (Germany) operating in the X-band at different microwave powers. Experimental spectra were fitted with those simulated using the least- squares method. The weight of each component and (in-)homogeneous broadenings were considered. The goodness of the fitting was evaluated by percentage root-mean-square-deviation (%RMSD). Results demonstrated that under low-microwave-power (< 3 mW) and low-LET (~0.5 keV/μm) conditions, R1 and R2 are the main contributors to the signal, whilst R3 becomes the predominant at high-LET and high-microwave-power conditions. Radicals’ microwave saturation behaviour and inhomogeneous broadening analyses suggested that radiation quality affects both the relative amounts of each radical and the environmental lattice surrounding it.