This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
This award will support the development of a detection system for the recoil mass separator (RMS), St George, which is being constructed at the University of Notre Dame. The RMS St George will be used to separate products of (alpha, gamma) reactions in inverse kinematics from unreacted beam particles. St George is designed to suppress beam particles by a factor of 10^12. Even with that suppression, comparable numbers of reaction products and unreacted beam particles will be delivered to the end of the RMS St George. The proposed detection system will use measurements of both energy and time-of-flight to distinguish between the various reaction products and beam particles. A broad program in nuclear astrophysics will be addressed with the proposed system.
The development of the proposed detector system and its use will contribute to research training at many levels during the project duration and even more so through the experimental work by post-docs, graduate students, and undergraduates.
This award supported the development and construction of a powerful and flexible system for detection of charged particles, to be used in conjunction with the St. George recoil mass separator (RMS) and the new 5U particle accelerator at the University of Notre Dame. These devices will be used to study nuclear reactions involving the capture of a helium-4 nucleus (also known as an alpha particle) by a heavier nucleus, typically in the range of elements from carbon to calcium (atomic numbers 6 to 20). The detection system utilizes two transmission detectors to measure the time-of-flight of ions over a known distance, along with a silicon strip detector for measuring the ion's energy. The combination of energy and time-of-flight measurements allows a determination of the ion's mass, and so its isotopic identity. The St. George RMS will deliver two species of ions to the detector system -- unreacted beam particles from the 5U accelerator and ions four mass units heavier that are products of an alpha-capture reaction -- and this detector will allow experimenters to separate the reaction products from the unreacted beam. In addtion to the design and construction of the transmission detectors, which utilize magnetic and electric fields to direct electrons emitted as the ions pass through a very thin carbon foil to a microchannel plate (MCP) detector, the project involved the design and purchase of a dedicated vacuum chamber, with detector-positioning hardward and associated pumps, and a dedicated data acquisition system. Intellectual Merit -- All the elements other than hydrogen and helium (and a tiny fraction of the lithium that now exists) have been produced in stars and stellar explosions; and the goal of nuclear astrophysics is to understand in detail how the heavier elements have been built up from the primordial hydrogen and helium produced in the big bang. Among the unknown quantities needed to complete our understanding of these processes are reaction rates at very low energies for alpha-capture by certain isotopes, including isotopes of carbon, oxygen, and calcium, that are present in massive stars during their helium-burning phase (i.e. after most of their original hydrogen has been converted into helium by fusion reactions). The St. George RMS and its dedicated detector system are designed specifically to measure several of the most important of these reaction rates. Broader Impacts -- The project has contributed extensively to research training on multiple levels during the project duration, and it will continue to do so for many years to come. Seven undergraduate students at Indiana University South Bend (IUSB) have participated in the project, with three of them spending entire summers full-time on the project with support from this grant. These immersive research experiences have greatly enriched the education of these students and given them skills and experience they could not acquire in a classroom setting. The project also supported a post-doctoral researcher for two years, allowing her to develop further as an independent scientist. The finished detection system, in combination with the St. George RMS and the 5U accelerator, will provide rich experimental opportunities for many years for graduate students at Notre Dame and elsewhere, undergraduates at IUSB and elsewhere, and nuclear physicists from around the world.
