RUI: Low-Energy Electron Scattering From Uracil and Thymine

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Since the early 2000's it has been understood that free electrons in biological environments are able to interact with, and cause damage to, DNA (deoxyribonucleic acid). This is in spite of the generally slow speeds of these electrons. This finding carried significant implications in various fields of biomedicine, in particular those that employ radiation as a therapeutic measure. This is due to the fact that such radiation creates free electrons within the body, which effectively constitutes a secondary radiation dose. However, to this day the effect of these electrons is largely excluded from most dosage calculations, in part due to the paucity of fundamental data describing how free electrons interact with molecules that make up DNA. Experimental physics techniques to measure electron interaction "cross sections," a quantity that describes the chemistry of a molecule by characterizing its physical size and shape, are well established for many targets. These techniques require molecules to be in the gas phase, however, whereas commercial samples of most biological molecules are solids (powders) at room temperature. While changing a substance from solid to gas is possible by simply heating it, this approach is incompatible with other crucial aspects of most standard experimental techniques. The research supported by this project will commission a new apparatus that will employ new measurement techniques, compatible with heated samples, to make measurements of electron interactions with DNA molecules. Improving our fundamental understanding of electron interactions with biological molecules will allow the effects of radiation on biological material to be more accurately included into radiation dosage calculations, which may ultimately enable the radiation to be better targeted to where it is needed most.This project will commission a new electron scattering spectrometer for the purpose of making measurements of the total, total elastic, and elastic differential cross sections for the nucleobases, with a view to making measurements of uracil and thymine within the time frame of the current project. Under this project, a beam of electrons is sent through a large volume of a heated gas cell containing a vapor of the target under consideration. The use of a large volume cell, rather than a traditional crossed beam technique, allows for a sufficient number of electron-target interactions to achieve reasonable statistics in the signal, and removes complications with normalizing the measurements since the target volume and density are easily characterized. However, the approach defeats using the detector location to determine the final state momentum of the scattered particle, as the scattering location is not well localized. Instead, this work will employ a modified version of the "magnetic beamline" technique, pioneered at the University of California San Diego for studies of positron interactions with atoms and molecules, whereby a strong magnetic field guides all scattered electrons along the spectrometer axis to a detector, and a retarding electric field is used to determine their momentum components parallel to and perpendicular to the spectrometer axis, thus determining their final state momentum. In addition to making these measurements on nucleobase molecules, this project will demonstrate improvements to the magnetic beamline technique, by demonstrating methods for generating high energy-resolution electron beams in strong magnetic fields. In addition, this work aims to use fast pulsing techniques to address the loss of information that is currently inherent in the technique due to reflection of electrons scattered in the backwards scattering direction.