SO3- radicals for EPR dosimetry: X- and Q band EPR study and LET dependency of crystalline potassium dithionate

The structures of the free radicals formed by the irradiation of potassium dithionate (K2S2O6) with Co-60 gamma-rays and N-14(7+) ions were investigated by EPR to further examine a recently proposed LET effect in this material. Two types of SO3- radical ions were identified in X-irradiated single crystals by measurements at X- and Q bands. One of these (S1) exhibited S-33 hyperfine couplings A(perpendicular to) = 12.49, A(parallel to) = 15.60 mT, the other (S2) A(perpendicular to) = 11.29, A(parallel to)= 13.92 mT. The g-factors were nearly isotropic, g(perpendicular to) = 2.0010, g(parallel to) = 2.0003; and g(perpendicular to) = 2.0026, g(perpendicular to)= 2.0008, respectively. The S-33 hyperfine coupling tensors and g-tensors were axially symmetric about the trigonal < c >-axis, coinciding with the direction of the S-S bonds of the two nonequivalent S2O62- ions in the crystal. A model for the radiation damage was proposed in which the SO(3)(-)radical ions retain the orientation of the SO3 groups, aligned along the trigonal axis. The structure of a third main radical species (S3) with g(perpendicular to) = 2.0026, g(parallel to) = 2.0052 could not be unambiguously assigned, due to undetected S-33 features. The relative integrated intensities of S1, S2 and S3 depended on the radiation quality and were approximately estimated as 0.18: 0.65:0.17 for Co-60 gamma-rays and 0.47: 0.38: 0.15 for N-14(7+) ions. Additional weak lines on the low field side of the main signal were tentatively attributed to SO2- radical ions. An even weaker strongly anisotropic pair of lines was attributed to SO3- radical pairs separated by 0.93-0.95 nm along the trigonal axis. (c) 2013 Elsevier Ltd. All rights reserved.