## 3 Results and Discussion
### EPR Spectroscopy
However, after X-ray exposure, radiation-induced centers are formed. The EPR spectra of samples in two frequency ranges are shown in Figure 1. The signal is attributed to the stable carbonate radical, which is identified by the degree of rhombic anisotropy (shape of line) and the values of the \(g\)-factor (see Table 1). As we can see, all three values of the \(g\)-factor can be accurately determined due to the high spectroscopic resolution (at W-band), while for the X-band, it is possible to distinguish only perpendicular (\(g_{\text{xx}}=g_{\text{yy}}=g_{\perp}\)) and parallel orientations (\(g_{\text{zz}}=g_{\parallel\,1}\)). At the same time, there is no evidence of a nitrate radical. Figure 1 clearly shows that the presence of free radicals in the irradiated sample depends on the type of biomaterial under study. Relaxation characteristics were also investigated for the CP samples. The obtained data are shown in Table 2. As we can see, the relaxation times T1 (spin-lattice or longitudinal relaxation) and T2 (spin-spin or transverse relaxation) differ depending on the type of CP material. This difference may indicate that the radiation-induced centers occupy various positions in the crystal lattices of HA, OCP, and biogenic compounds (jawbone of a Vietnamese mini-pig). Because of the difference in spin-spin relaxation times, we can conclude that there is an unequal distribution of free radicals for various CP materials. Therefore, a full set of spectroscopic values and dynamic characteristics of radiation-induced centers will allow us to unambiguously determine the nature of EPR signals in poorly studied CP materials (amorphous, biphasic, and doped materials). Samples irradiated by X-ray and \(\gamma\)-radiation were additionally explored in the Q-band microwave range. Experimental results are presented only in continuous wave mode at room temperature. Figure 2 shows the difference in the EPR spectra between the types of irradiation for the same type of material. The nature of the paramagnetic centers can be dependent on the type of radiation exposure. The influence of the type of radiation (and other radiation characteristics) on the EPR spectrum was extensively discussed in the literature [56; 57]. However, for the OCP sample, the difference in the EPR spectra according to the type of irradiation is not observed. It may serve as additional proof to the conclusion made in ref. [54] that, in contrast to HA, the radiation-induced radicals in OCP are stabilized not in the apatite, but in the hydrated layers of OCP. The results obtained in the Q-band frequency range provide additional information about the spectroscopic values of the spin system. But as we can see in the case of irradiated calcium phosphates, not all available frequency ranges are suitable for the analysis of powdered materials. The anisotropy of the \(g\)-factor for nitrate radicals in high magnetic fields is comparable to or even greater than the hyperfine interaction splitting, while at the X-band, the converse can be observed. Therefore, measurements of the nitrogen radical in an intermediate frequency range lead to the formation of a complex EPR spectrum that is difficult to interpret. Nevertheless, in other cases, a comparison of the EPR spectra makes it possible to clearly determine the origin of the radiation-induced paramagnetic centers and calculate the parameters of the Spin-Hamiltonian. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Material** & **Spin–Lattice Relaxation** & **Spin–Spin Relaxation** \\  & **T1 (\(\mu\)s)** & **T2 (\(\mu\)s)** \\ \hline HA & 28.5(5) & 3.0(1) \\ OCP & 60.7(8) & 0.68(5) \\ jawbone of a mini-pig & 17.7(2) & 1.31(8) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relaxation characteristics of radiation-induced paramagnetic centers in different matrices at room temperature T = 297 K, in the X-band frequency range \(\
u_{\text{MW}}\) = 9.6 GHz. Values for CP materials were measured at B0 (see Figure 1a).