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

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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.