Challenges in identifying very low-energy ions with the St. George detection system

The University of Notre Dame’s St. George recoil mass separator is used to study the process of nucleosynthesis during stellar helium burning by measuring the cross sections of alpha capture reactions.  Beams of heavy ions are directed onto a helium gas jet target, and reaction products are separated from unreacted beam particles by a series of dipole magnets and a Wien filter. The rejection of unreacted beam is not perfect, however, so it is necessary to distinguish between reaction products and unreacted beam at the end of the separator.  The St. George detection system does this by measuring the speed of detected ions with a pair of timing detectors utilizing microchannel plates and a silicon strip detector for measuring the particle’s kinetic energy.  Ions of different mass appear on different bands on a two-dimensional plot of time-of-flight (TOF) vs. Energy.  However, there are some challenges in identifying very low energy ions, where the mass bands are less well separated. This presentation will present possible explanations for an apparent discontinuity in the mass band of unreacted beam ions at very low energy, around 100 keV/nucleon. Another challenge arises when dealing with energy resolutions, this is due to the limitations of the precision with which low-energy ions can be identified. Lastly Calibrating the detection system for very low-energy ions can be challenging since there are limited well-known standards at such energies, potentially impacting the accuracy of energy measurements and the identification of specific ion species.