Theoretical study of 238Uranium/235Uranium fractionation in nature

铀（U）在环境中的行为对地球科学的许多分支都至关重要。放射性铀同位素238 U和235 U的衰变构成了适用于研究深时间事件的最精确测年技术的基础。铀234是一种稀有的、寿命较短的同位素，也被用来测量过去100万年内发生的事件的年龄。铀也是一种重要的环境污染物，是研究地球化学过程的有用微量元素。最近，地球化学家开始研究自然化学反应（与放射性和其他核反应相反）使铀同位素相互分离的方式。这些分离是微妙的，改变了大约千分之一的238 U/235 U比率，但它们可能提供了一种方法来研究环境氧，生命，岩石熔化和其他自然过程的历史，通过它们对古代样品的铀同位素组成的影响。这项研究的目的是使用理论化学的方法，试图预测自然界和实验室中的化学反应如何优先影响不同的铀同位素。这种类型的理论研究对其他同位素技术的发展很重要，例如使用碳酸盐外壳中氧18至16的比例测量来推断古代环境温度。然而，铀的原子序数非常高，使其同位素比大多数其他元素更容易受到不同类型的化学分离，因为同位素分离几乎总是由不同同位素之间的质量差异驱动。对于铀，由物理尺寸的差异驱动的同位素分离（即，为了使理论计算准确，还必须考虑铀核的体积。包括这种核体积效应将是技术工作的重点。拟议的研究将支持培养一名女博士生、本科生研究、用于研究和课堂教学的科学计算设施，以及通过创建和改进关于地球化学和环境科学主题的维基百科文章进行宣传。该项目将利用电子结构理论研究各种晶体和水相物质中铀的平衡同位素分馏，利用最近开发的用于预测由晶体材料中的核场位移效应的体积分量驱动的与质量无关的同位素分馏的方法。目前对铀同位素分馏的理论理解仅限于一些简单的气相分子和溶解物质的类似物，尽管对238 U/235 U测量的兴趣迅速增长，研究人员希望大大扩展所研究的化合物。 - 铀是地表环境中对氧化还原敏感的金属，是最精确的地质年代计的基础。因此，它的同位素行为引起了地球科学家的广泛兴趣，但对其了解甚少。铀是一个强有力的候选人的理论研究质量无关分馏，因为它具有最高的原子序数的任何元素与一个以上的长寿命，非放射性同位素。因此，预计它将显示出任何元素中最大的核场位移效应。这一假说得到了理论和经验证据的支持，即氧化U（VI）和还原U（IV）物种之间的同位素分馏的场移效应在相反方向上引起了质量相关的分馏，导致高238 U/235 U化学还原相，如黑色页岩。铀也是一个很好的研究目标，因为它的元素邻居镎（Np）是锕系元素中研究最好的穆斯堡尔同位素，237 Np。穆斯堡尔谱测量的同分异构体位移直接探测控制核场位移同位素分馏的化学和地球化学参数，因此对类似的含Np和U物质的研究提供了对理论方法准确性的可靠检查。该项目旨在建立理论方法，以预测所有怀疑显示质量无关核场位移同位素分馏的元素的同位素特征，而不限于对小气相分子进行非常耗时的计算。电子结构建模与穆斯堡尔谱的集成可以使研究人员利用现有的文献，并可能通过有针对性的穆斯堡尔谱或近共振同步加速器X射线研究目前缺乏测量的相位来刺激跨学科研究。

The behavior of uranium (U) in the environment is of central importance to many branches of Earth Science. Decay of the radioactive uranium isotopes 238U and 235U forms the basis for the most accurate dating techniques suitable for studying events in deep time. The rarer, shorter-lived 234U isotope is also used for measuring the ages of events within the past million years. Uranium is also a significant environmental pollutant, and a useful trace element for studying chemical processes in the Earth. Recently, geochemists have begun to study the ways that natural chemical reactions (as opposed to radioactivity and other nuclear reactions) act to separate uranium isotopes from each other. These separations are subtle, changing 238U/235U ratios by roughly one part in a thousand, but they may provide a way to study the history of environmental oxygen, life, rock melting, and other natural processes through their effects on the uranium isotope compositions of ancient samples. The goal of the study is to use methods from theoretical chemistry to try to predict how chemical reactions in nature and in the laboratory might preferentially affect different uranium isotopes. Theoretical studies of this type have been important in the development of other isotopic techniques, such as the use of oxygen 18 to 16 ratio measurements in carbonate shells to infer ancient environmental temperatures. However, the very high atomic number of uranium makes its isotopes susceptible to a different type of chemical separation than most other elements, for which isotope separation is almost always driven by differences in mass between different isotopes. For uranium, isotope separations driven by differences in the physical size (i.e., the volume) of the uranium nuclei must also be accounted for, in order for theoretical calculations to be accurate. Including this nuclear volume effect will be a focus of the technical effort. The proposed research will support the training of a female doctoral student, undergraduate research, a scientific computing facility used for both research and classroom instruction, and outreach through the creation and improvement of Wikipedia articles on topics in geochemistry and environmental science.The project will investigate equilibrium isotope fractionation of uranium in a variety of crystalline and aqueous species using electronic structure theory, taking advantage of recently developed methods for predicting mass-independent isotope fractionations driven by the volume component of the nuclear field shift effect in crystalline materials. The present theoretical understanding of uranium isotope fractionation is limited to a few simple gas-phase molecules and analogues of dissolved species, despite a rapidly growing interest in 238U/235U measurements, and researchers hope to greatly expand the set of studied compounds. -Uranium is a redox-sensitive metal in the surface environment, and the basis of the most precise geochronometers. Its isotopic behavior is therefore of broad interest to Earth Scientists, but is poorly understood. Uranium is a strong candidate for theoretical study of mass-independent fractionation because it has the highest atomic number of any element with more than one long-lived, nonradiogenic isotope. It is thus expected to show the largest nuclear field shift effect of any element. This hypothesis is supported by theoretical and empirical evidence that the field shift effect on isotope fractionation between oxidized U(VI) and reduced U(IV) species overwhelms a mass-dependent fractionation in the opposite direction, leading to high 238U/235U in chemically reduced phases such as black shales. Uranium is also a good target for investigation because its elemental neighbor neptunium (Np)has the best-studied Mössbauer isotope among the actinide elements, 237Np. Isomer shifts measured by Mössbauer spectroscopy directly probe the chemical and geochemical parameters that control nuclear field shift isotope fractionation, so study of analogous Np- and U-bearing species provides a robust check on the accuracy of the theoretical method. -This project aims to establish theoretical methods to predict isotope signatures in all elements suspected to display mass-independent nuclear field shift isotope fractionations, without being restricted to very time consuming calculations on small gas-phase molecules. The integration of electronic structure modeling with Mössbauer spectroscopy could allow investigators to leverage the existing literature, and may stimulate interdisciplinary studies via targeted Mössbauer or near-resonant synchotron X-ray studies of phases where measurements are presently lacking.