Ph.D Thesis

Ph.D StudentBuchbinder Lotem
SubjectAdvanced Electron Spin Resonance in Tooth Enamel for
Retrospective Biodosimetry
DepartmentDepartment of Chemistry
Supervisor PROF. Aharon Blank
Full Thesis textFull thesis text - English Version


Large-scale triage after major radiological events, such as nuclear-reactor accidents or radiological terrorist attacks, requires a method of dose estimation called retrospective biodosimetry (RBD), able to detect doses in the range of 0.5-6 Gy. A well-known method for RBD is electron spin resonance (ESR), which can be used to detect and measure radiation-induced defects (stable free radicals) in the enamel layer of the teeth. The concentration of these defects is linearly correlated with dose in the applicable range.

Despite its great potential, and proven results when applied to extracted teeth, ESR still struggles to provide accurate in vivo readings. This is mainly because all available ESR-based RBD methods rely on quantitative signals for calculating the concentration of radicals in tooth enamel in order to evaluate the dose, which requires accurate knowledge of the volume of the measured enamel?not a viable option in live subjects.

This study examined radiation-induced paramagnetic defects in the enamel layer of the human tooth using advanced ESR methods, with the ultimate goal of supporting the development of an innovative practical RBD device for in vivo use. We used advanced pulsed ESR techniques, such as ESR relaxometry and dipolar ESR spectroscopy, to better understand the spectroscopic and physical characteristics of these paramagnetic defects, as well as the distribution of defects in the enamel layer, their interaction with the enamel layer, and the interactions between them.

Measurements of irradiated quartz and teeth were carried out using a lab-built (non-commercial) pulsed ESR system, operating at ~10 GHz with a dedicated probe-head, and using a commercial pulsed ESR spectrometer operating at ~35 GHz.

Relaxation time measurements (T1 and T2) show that T2 of tooth enamel is in the order of a few hundred nanoseconds and is dose-dependent, at room temperature and at lower temperatures, with lower doses having a longer T2, while T1 did not show dose dependence. Furthermore, T2 distributions were much narrower at low temperature (85K) than at room temperature, indicating that at room temperature, the radicals’ motion in the matrix and their interaction with each other affect relaxation times, while motion is reduced at lower temperatures, revealing differences between doses more clearly. In this study we were also able to measure for the first time double electron-electron resonance (DEER) signals of teeth at room temperature and to detect dose effect on DEER background decay. Using an arbitrary wave generator unit in our lab-built system, we were able to significantly extend the DEER evolution times monitored and gather extensive data for analysis. Spin concentration was also estimated from instantaneous diffusion effects and showed higher local concentration than the overall average concentration (obtained from calibrated continuous-ESR measurements), which supports the hypothesis of non-uniform radical formation in the enamel layer during irradiation.

The findings from these innovative techniques provide us with a better understanding of the characteristics of radicals in tooth enamel and may be used for the development of an RBD device to estimate spin concentration and evaluate dose without prior knowledge of the measured enamel volume.