## 3 Results and Discussion  
### EPR Spectroscopy  
There are no paramagnetic centers in nominally pure samples (HA and OCP); therefore, no EPR signals in CP materials can be observed. An X-ray or \(\gamma\)-irradiation procedure performed on the samples leads to the formation of radiation centers that can be successfully detected by EPR (Figure 1). As we can see, the acquired EPR spectra from the irradiated samples have quite a strong anisotropy, with a characteristic splitting into hyperfine structures. Powder spectra of the radiation-induced centers (nitrate radicals) in CPs are very often described by a Spin-Hamiltonian of axial symmetry:  
\[H = g_{||}\beta B_{2}S_{z}+g_{\perp}\beta\big{(}B_{x}S_{x}+B_{y}S_{y} \big{)}+A_{||}S_{z}I_{z}+A_{\perp}\big{(}S_{x}I_{x}+S_{y}I_{y}\big{)} \tag{2}\]  
where \(g_{||}\) and \(g_{\perp}\) are the main components of the \(g\) tensor, \(A_{||}\) and \(A_{\perp}\) are the main components of the hyperfine tensor, \(B_{\rm i}\), \(S_{\rm i}\), and \(I_{\rm i}\) are the projections of the external magnetic field strength, electronic (\(S=1/2\)), and nuclear (\(l=1\)) spins, respectively, onto the \(i=\{x,y,z\}\) coordinate axis, and \(\beta\) is the Bohr magneton. The Spin-Hamiltonian parameters for the same center may differ significantly, depending on the type of sample. The spectra of the CP samples synthesized from the nitrogen-containing reagents (see Equation (1)) can be described with the parameters that are shown in Table 1.  
The shape of the EPR line and the splitting values for HA and OCP are very different. This may be due to the different atomic environments, the degree of distortion of the crystal lattice, or the level of delocalization of the paramagnetic center [55]. Differences in the parameters of the Spin-Hamiltonian should be useful for further analysis of crystal structures of samples doped with various cations. It should be noted that the EPR spectra of CP samples differ in the degree of anisotropy in the experiment on the high-frequency part of the spectrometer (Figure 1b). Experimental measurements in two or more different frequency ranges allow for a description of the obtained results more accurately, i.e., unequivocally identify the paramagnetic centers and determine the parameters of the main interactions. The W-band EPR spectrum of OCP is more difficult to interpret, despite the fact that the values of the hyperfine interaction (\(A_{||}\) or \(A_{\perp}\)) are the smallest for the calcium phosphates group. It was concluded that in the irradiated HA samples, EPR is mainly due to the stable NO\({}_{3}^{2-}\) ions, preferably substituting one distinct PO\({}_{4}^{3-}\) position in the HA structure (substitution of the B-type), while there are several non-equivalent positions for the nitrate radical in the OCP structure. In our paper [55], we found that the distribution of \(A_{\perp}\) for TCP is larger than that of the HA. We \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \(g_{\bf xx}\) & \(g_{\bf yy}\) & \(g_{\bf zz}\) & \(A_{\perp}\) (\({\bf mT}\)) & \(A_{||}\) (\({\bf mT}\)) \\ \hline jawbone & 2.0026 (2) & 2.0008 (2) & 1.996 (1) & - & - \\ HA & 2.006 (1) & 2.006 (1) & 2.002 (1) & 3.37 (5) & 6.65 (4) \\ OCP & 2.0014 (3) & 2.0014 (3) & 2.0035 (5) & 2.08 (5) & 2.50 (5) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The Spin-Hamiltonian parameters of the X-ray irradiated samples. Figure 1: EPR spectra of the irradiated samples in pulse mode for the (a) X-band and (b) W-band frequency ranges at T = 297 K.  
suggested that it could be due to the three different positions for PO4 group substitution in the TCP structure, as well as due to the various mechanisms for charge compensation. Since hydroxyapatite is an inorganic mineral component of bone tissue, we studied the biogenic material after X-ray irradiation. As an example, the jawbone of a Vietnamese mini-pig was chosen for the EPR study. This sample, like synthetic hydroxyapatite, does not contain its own paramagnetic centers.