**3.1.8. Electron paramagnetic resonance**

The concept of electron paramagnetic resonance (EPR) spectroscopy is very similar to more familiar nuclear magnetic resonance (NMR). Both methods deal with the interaction be‐ tween electromagnetic radiation and magnetic moments. In the case of EPR, the magnetic moments arise from electron rather than nuclei. The term EPR was introduced as a designa‐ tion taking into account contributions from electron orbital as well as spin angular momen‐ tum. The term electron spin resonance (ESR) was also widely used because in most cases the absorption is linked primarily to the electron-spin angular momentum [82],[83]. EPR spec‐ trum is a diagram in which the absorption of microwave frequency radiation is plotted against the magnetic field intensity [83].

The technique of electron paramagnetic resonance spectroscopy may be regarded as the consequence of the STERN–GERLACH experiment. They showed (in 1920) that an electron magnetic moment in an atom can take on only discrete orientation in a magnetic field, despite the sphericity of the atom. Subsequently, UHLENBECK and GOUDSMIT liked the electron magnet‐ ic moment with the concept of electron spin angular momentum. In hydrogen atom, there is additional angular momentum arising from the proton nucleus. BREIT and RABI described the resultant energy levels of hydrogen atom in a magnetic field. RABI et al [81] studied the transition between levels induced by an oscillating magnetic field, and this experiment was the first observation of magnetic resonance. The first observation of electron paramagnetic resonance peak was made in 1945 by ZAVOISKY, who detected the radiofrequency absorption line from CuCl2·2H2O sample using the radiofrequency (RF) source at 133 MHz [82].

The major components of EPR spectrometer are shown in **Fig. 6**. The microwave bridge supplies the microwaves at controlled frequency and power, which are transmitted to the sample cavity via the waveguide. The sample cavity is placed perpendicular to applied magnetic field, which can be varied in controlled way. In addition to this main magnetic field, a controlled but smaller oscillating magnetic field is superimposed on the cavity via the Zeeman modulation frequency. The ideal way to perform the experiment would be to apply a fixed magnetic field and vary the microwave frequency. However, microwave generators are only tunable over very limited ranges. Thus, the microwave frequency is fixed and applied magnetic field is varied. The magnetic field is applied until it reaches the value at which the sample will absorb some of the microwave energy, i.e. and EPR transition occurs [84],[85].

**Fig. 6.** The block diagram for typical continuous wave EPR spectrometer [84].

Electron paramagnetic resonance (EPR) spectrum of X-irradiated sodium and carbonate containing synthetic apatites has been studied by MOENS et al [86]. Observed spectra were decomposed in terms of five theoretical curves representing O– radical, two CO3– radicals (surface and bulk) and two CO2− radicals (surface and bulk). These species were also descri‐ bed in A-type and B-type carbonate-apatites [87],[88], tooth enamel [89],[90],[91],[92], [93] and bone [94],[95], apatites, renal stones [96], etc.
