EPR dosimetry of biohydroxyapatite below liquid nitrogen temperature

Hydroxyapatite, major component of all organic solid tissues, can be used as a sensitive biodosimeter based on the electron paramagnetic resonance (EPR) spectroscopy. However, the dosimetric signal of biohydroxyapatite overlaps with the so-called parasitic signals due to the close g factor values and broadening of the resonance line at room temperature. Moreover, the unsaturated parasitic signals possess the intensity comparable to the dosimetric resonance. All of these significantly complicates the dose determination and limits applicability mainly to the cases of relatively large accumulated dose. The negligibly saturated dosimteric spectrum can be at least partially separated at the liquid nitrogen temperatures (LNT) due to the strong saturation and suppression of the parasitic resonance lines and the linewidth shortening as shown in the present work. Moreover, the advances in the modern EPR equipment in the last two decades resulted in high sensitivity and stability of the signals measured. These are the key parameters along with the computer simulations for the precise dosimetric spectrum separation and processing. This could lead to the higher accuracy of the LNT EPR method proposed in the present work. To test the approach, the stepwise dose calibration of biohydroxyapatite over the range 0.5 - 20 Gy was made. The corresponding dosimetric signal measured at 70 K exhibited the linear dose response. The results suggest the applicability of the LNT EPR method in the retrospective dosimetry.