RUI: Low-Energy Electron Scattering From Uracil and Thymine

自21世纪初以来，人们已经认识到，生物环境中的自由电子能够与DNA（脱氧核糖核酸）相互作用并对其造成损害。尽管这些电子的速度通常很慢。这一发现对生物医学的各个领域，特别是那些使用辐射作为治疗措施的领域具有重大意义。这是因为这种辐射在体内产生自由电子，这实际上构成了二次辐射剂量。然而，直到今天，这些电子的影响在很大程度上被排除在大多数剂量计算之外，部分原因是缺乏描述自由电子如何与构成DNA的分子相互作用的基本数据。测量电子相互作用“横截面”的实验物理技术，通过表征其物理大小和形状来描述分子的化学性质，已经为许多目标建立了良好的基础。然而，这些技术要求分子处于气相，而大多数生物分子的商业样品在室温下是固体（粉末）。虽然可以通过简单的加热将物质从固体变为气体，但这种方法与大多数标准实验技术的其他关键方面是不相容的。该项目支持的研究将委托使用一种新的仪器，该仪器将采用新的测量技术，与加热样品兼容，以测量电子与DNA分子的相互作用。提高我们对电子与生物分子相互作用的基本理解，将使辐射对生物材料的影响更准确地包括在辐射剂量计算中，这可能最终使辐射能够更好地瞄准最需要的地方。本项目将委托一台新的电子散射光谱仪，用于测量核碱基的总、总弹性和弹性微分截面，以期在当前项目的时间框架内测量尿嘧啶和胸腺嘧啶。在这个项目中，一束电子束被发送通过一个大体积的加热气体池，其中含有所考虑的目标的蒸气。使用大体积电池，而不是传统的交叉光束技术，允许足够数量的电子-目标相互作用，以在信号中实现合理的统计，并且消除了标准化测量的复杂性，因为目标体积和密度很容易表征。然而，由于散射位置不能很好地局部化，该方法无法利用探测器位置来确定散射粒子的最终状态动量。相反，这项工作将采用改良版的“磁束线”技术，该技术是由加州大学圣地亚哥分校首创的，用于研究正电子与原子和分子的相互作用，通过强磁场引导所有散射电子沿着光谱仪轴到达探测器，并使用减速电场来确定它们平行和垂直于光谱仪轴的动量分量，从而确定它们的最终状态动量。除了对核碱基分子进行这些测量外，该项目还将演示磁束线技术的改进，通过演示在强磁场中产生高能量分辨率电子束的方法。此外，这项工作旨在使用快速脉冲技术来解决由于电子在反向散射方向上的反射而导致的当前技术固有的信息损失。

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.