Book excerpt: Historically, X-rays have been used in catalysis, imaging, and other applications, and the interaction of X-rays with nanomaterials is a growing field. In particular, the enhancement of ionizing effects of X-rays in the presence of nanostructures is of great interest in science, technology, and medicine. In clinical radiotherapy, gold nanoparticles at tumor sites effectively lower the X-ray dose required for treatment by minimizing ionization damage to healthy tissue. So far, radiosensitization by gold nanoparticles has been studied by fluorescence specotroscopy, cell studies, and theoretical simulations. However, more rigorous quantification methods are needed to understand the mechanism of enhancement of X-ray ionization effects. This study quantifies the enhancement of X-ray ionizing effects by gold nanoparticles with electron paramagnetic resonance spectroscopy, or EPR spectroscopy, a technique to measure the concentrations of free radicals, and investigates physical enhancement and posits a theory for its mechanism. The precision and specificity of EPR spectroscopy eliminate many of the difficulties associated with conventional methods of quantification, which fail to distinguish between enhancement of physical origin and enhancement due to chemical and biological mechanisms. The EPR spin trap 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) was selected to detect radicals generated from X-ray irradiation of gold nanoparticles in solution, and scavenging experiments were performed to identify the radicals. It appeared that hydroxyl radicals and superoxide radicals contributed approximately equally to the enhancement, which was 0.7-fold per weight percent of gold in water, compared to the theoretical physical enhancement of 0.69-fold per weight percent, and demonstrated a linear dependence on gold nanoparticle concentration, thus providing evidence for physical enhancement.