Time-of-flight energy compensation to improve energy resolution in low-energy radioactive beam experiments at the TwinSol facility

Improved time-of-flight (ToF) capability has been added to the University of Michigan (UM)–University of Notre Dame (UND) TwinSol low-energy dual 6T solenoid-based radioactive nuclear-beam (RNB) facility at the UND FN tandem Van de Graaff accelerator. An extended low-background beam line has been combined with a newly-constructed ca.1 meter diameter ISO-250 based scattering chamber capable of sub-ns ToF of RNBs. These together with the intrinsic, low ToF spread in the TwinSol ion-optical system yields improved ToF information for low-energy RNB nuclear reaction studies. Using either the intrinsic accelerator bunched and pulsed-selected beams with RF timing, or with the addition of an MCP+foil timing system (or similar) detector at the dual-solenoid cross-over region allows for reduction of the energy spread inherent in these low-energy RNB reactions. As an example, using the ToF information reduces the FWHM energy spread in a high-intensity 28 MeV 8Li beam from about 1 MeV to <ca. 600 keV or more without excessive collimation (which reduces the beam). This has been utilized in a recent experiment measuring the Coulomb excitation of a short-lived 8Li RNB on gold targets. The use of ToF permitted a much more efficient measurement of this reaction as certain background measurements were no longer needed and the full 8Li beam intensity could be utilized. A number of other improvements are planned to further improve the ToF resolution and count-rate limits of the various ToF systems and we hope to utilize these in the future.