## 3 Results and Discussion
Figure 3: Evolution of the ESR spectra recorded at different times in (**a**) non-chemically and (**b**) chemically treated fingernail samples exposed to 20 Gy of \({}^{137}\)Cs \(\gamma\)-rays following storage in the vacuum chamber (2% air humidity level, 20 \({}^{\circ}\)C temperature). The notations NT\(-\)BKG and T\(-\)BKG correspond to the background signal before irradiation for non-chemically treated samples and after chemical treatment for those chemically treated, respectively. IR represents \(\gamma\)-ray irradiation. Results shown in Figure 5 indicate the efficacy of DTT on the stability of RIS in chemically and non-chemically treated samples at two different storage conditions (vacuum chamber and freezer). ESR signals were measured as the peak-to-peak intensity in the magnetic field range from 328.9 mT to 329.7 mT. All experimental data points were normalized to the first ESR signal of the samples measured soon after exposure to 20 Gy (this corresponds to the fourth and second ESR measurement for chemically and non-chemically treated samples, respectively, as illustrated in Figure 1. Measurements of the RIS were also taken up to 7 days in irradiated fingernail samples collected from the same individual. The dotted lines with unfilled and filled box markers correspond to non-chemically treated samples stored in the vacuum chamber and freezer, respectively. Solid lines with unfilled and filled circle markers represent chemically treated samples stored in the vacuum chamber and freezer, respectively. As shown in Figure 5, it can be argued that the effect of DTT is more pronounced in fingernail samples stored in the freezer than those in the vacuum chamber. It may be observed that the signal intensities in chemically treated samples stored in the freezer increased after 24 h but remained almost unchanged for up to 7 days after irradiation, while the corresponding signals in the vacuum-stored chemically treated samples continued to increase for the same measurement times. Another interesting observation is that the behavior of the signals obtained in non-chemically treated samples correlated well with the measured signals in chemically treated samples for both storage conditions and  
Figure 4: Evolution of the ESR spectra recorded at different times in (**a**) non-chemically and (**b**) chemically treated samples exposed to 20 Gy of \({}^{137}\)Cs \(\gamma\)-rays following storage in the freezer. The NT\(-\)BKG, T\(-\)BKG, and IR have the same notations as in Figure 3.  
Figure 5: ESR signal peak-to-peak amplitude (App) changes with time in chemically and non-chemically treated samples stored at two different conditions (vacuum chamber and freezer). All experimental points from each experiment were normalized with regard to the first measurement data soon after the exposure to 20 Gy of \({}^{137}\)Cs \(\gamma\)-rays. hence the freezer-stored samples (with 3% air humidity at -20 \({}^{\circ}\)C temperature) were deemed to be significantly more stable than the vacuum-stored samples (with 2% air humidity at 20 \({}^{\circ}\)C temperature). This observation broadly supports the finding of [PERSON] et al. [31], that persistent ESR signals can be achieved if samples are kept at low (sub-zero) temperature, although the air humidity level was not mentioned in their experimental work. Next, a comparison was made for background signals measured from non-chemically treated unirradiated fingernail (control) samples stored in the vacuum chamber and freezer, as presented in Figure 6. ESR signals were measured as the peak-to-peak amplitude (App) of the main background-singlet of the spectra from the (a) vacuum-stored and (b) freezer-stored samples.