MRI: Development of an Advanced Cryogenic Gas Stopper for Energetic Rare Isotope Beams

地球上的所有物质主要由稳定的或长寿的同位素组成，这些同位素是在数十亿年前的一次恒星爆炸中产生的。然而，我们在地球上观察到的同位素只占已知存在的同位素的一小部分。对短寿命或稀有同位素的研究是理解宇宙中物质的起源、中子和质子如何集体形成原子核以及探索自然界基本对称性的关键。由于这些稀有同位素不是自然存在于地球上，它们必须在强大的粒子加速器中产生，例如位于密歇根州立大学(MSU)校园内的国家超导回旋实验室(NSCL)的加速器。稀有同位素的光束以光速的一半产生。然而，一些用于探测稀有同位素性质的最精确的实验技术要求束流的能量比NSCL产生的束流能量低10亿倍。这些高能光束可以用固体降解器减速，然后放在充满缓冲气体的小室中休息，这项技术目前正在NSCL使用。下一代先进低温气体堵塞器(ACGS)的开发将提高稀有同位素输送到实验的效率和速度，将几个实验计划的覆盖范围扩大到更稀有的同位素。它将造福于NSF资助的NSCL的用户社区，并通过它所带来的科学进步，造福于整个社会。该项目将遵循NSCL和密歇根州立大学的政策，以确保拥有一支多样化的科学队伍。像ACGS这样的最先进的开发项目，与一所积极鼓励和支持多元文化研究机构的领先研究型大学有着密切的联系，将为代表不足的学生和少数族裔学生培养成科学家创造一个最佳环境。ACGS是一种新一代高性能的线性气体阻止装置，用于将快速稀有同位素束流转换为可直接使用或再加速的阻挡束流。为了最大限度地增加可以使用的实验技术的数量，在广泛的能量范围内提供稀有同位素束流是必要的。能量从0.01到100keV的束流被用于离子陷阱和激光的高精度实验，用于测量核结合能(质量)，确定核半径和力矩，并在水平上测试基本对称性，以补充更昂贵的高能实验。能量为0.1至20 MeV/u的稀有同位素的精密束流被用来测量关键反应的截面，这些截面对于理解宇宙中的核合成和有助于确定进一步发展核理论所需的详细核性质的核反应研究至关重要。在NSCL安装后，ACGS将提供高纯度的束流，效率高，提取时间短，可以获得短寿命的同位素，并具有高束流速率的能力，以实现世界级的科学，特别是再加速的束流。由NSCL的耦合回旋加速器提供的快速光束将在固体降解器中减速，然后在ACGS内的氦气中停止。然后，利用射频离子传输技术将加热后的离子引导到引出孔处，形成所谓的低能“停止”光束。该设计允许ACGS接受超过10^8/S的入射射束速率。与现有的线性气体阻塞器相比，新的电极设计将提供高出数量级的射束速率能力，并且低温操作保证了干净的稀有同位素束流的输送。使用新的射频地毯离子传输技术将提供短至10毫秒的提取时间。ACGS最适合于阻止氦气体中具有相对窄范围离散分布的中质量到重质量同位素，并将构成对NSCL已经建造的轻离子回旋加速器阻塞器的高性能补充。高强度停止和再加速的短寿命同位素束的可用性将在NSCL未来的研究计划中发挥关键作用，并将是密歇根州立大学正在建设的稀有同位素束设施(FRIB)研究的一个关键特征。

All of the matter on Earth is composed primarily of stable or long-lived isotopes created in a stellar explosion billions of years ago. However, the isotopes we observe here on Earth comprise only a small fraction of those that are known to exist. Study of short-lived, or rare, isotopes is key to understanding the origin of matter in the universe, how neutrons and protons collectively form into atomic nuclei, and to explore fundamental symmetries of nature. As these rare isotopes are not naturally occurring on Earth, they must be created in powerful particle accelerators, such those at the National Superconducting Cyclotron Laboratory (NSCL), located on the campus of Michigan State University (MSU). There beams of rare isotopes are created at half the speed of light. However, some of the most precise experimental techniques for probing the properties of rare isotopes require beams with energies up to a billion times less than what they are created with at NSCL. These high-energy beams can be slowed using solid degraders and brought to rest in a chamber filled with a buffer gas, a technique currently in use at the NSCL. The development of a next-generation Advanced Cryogenic Gas Stopper (ACGS) will improve the efficiency and speed at which rare isotopes are delivered to experiments, expanding the reach of several experimental programs to even more exotic isotopes. It will benefit the user community of the NSF-funded NSCL and, through the scientific progress it enables, society at large. The project will follow NSCL and MSU policies for ensuring a diverse scientific workforce. A state-of-the-art development project like ACGS, with close ties to a leading research university that actively encourages and supports a multicultural research body, will create an optimal environment for under-represented and minority students to be trained as scientists.The ACGS is a next-generation high-performance linear gas-stopping device for converting fast rare isotope beams into stopped beams that can be used directly or reaccelerated. The delivery of rare-isotope beams over a wide range of energies is necessary to maximize the number of experimental techniques that can be employed. Beams with energies of 0.01 to 100 keV are used in high-precision experiments with ion traps and lasers, to measure nuclear binding energies (masses), to determine nuclear radii and moments, and to test fundamental symmetries at levels complementing much more expensive high-energy experiments. Precision beams of rare isotopes with energies of 0.1 to 20 MeV/u are used to measure cross sections of key reactions that are critical for understanding nuclear synthesis in the cosmos and for nuclear reaction studies that help determine detailed nuclear properties needed for further advances in nuclear theory. After installation at the NSCL, the ACGS will provide high-purity beams with high efficiency, with short extraction times to provide access to short-lived isotopes, and with a high beam rate capability to enable world-class science, in particular with reaccelerated beams. Fast beams provided by NSCL's Coupled Cyclotron Facility will be slowed down in solid degraders prior to being stopped in helium gas inside the ACGS. The thermalized ions are then guided to an extraction orifice using RF ion transport techniques and a low-energy, so called "stopped" beam is formed. The design is tailored to allow the ACGS to accept incident beam rates exceeding 10^8/s. A novel electrode design, compared to existing linear gas stoppers, will provide an order of magnitude higher beam rate capability and cryogenic operation guarantees delivery of clean rare isotope beams. The use of novel RF-carpet ion-transport techniques will provide extraction times as short as ten milliseconds. The ACGS is best suited for the stopping of medium-to-heavy-mass isotopes that have relatively narrow range straggling distributions in the helium gas and will constitute a high-performance complement to the light-ion Cyclotron Stopper already being built at the NSCL. The availability of intense stopped and reaccelerated beams of short-lived isotopes will play a critical role in the future research program at the NSCL and will be a key feature of research at the Facility for Rare Isotope Beams (FRIB) under construction at MSU.