Low energy nuclear physics experiments with radioactive nuclei often require pure ion beams free of isobaric contamination. Typical magnetic separators, with resolving powers of R < 5000, can easily separate species by mass number but lack the resolving power to remove isobaric contaminants. Multi-reflection time-of-flight mass spectrographs (MR-TOF-MS) have become a popular choice to purify radioactive ion beams (RIB), with devices installed or under construction at several RIB facilities. A MR-TOF traps ions between two electrostatic mirrors, folding the flight path into an extremely compact device, while achieving resolving powers on the order of 105. In addition to a small physical footprint, the MR-TOF is relatively simple and inexpensive to construct. With typical ion flight times of less than 10 ms, the method is compatible with short half-life and low-yield nuclei, which are of great interest at RIB facilities. The MR-TOF can also fill the role of a mass spectrograph, permitting mass measurements on short-lived nuclei with precisions comparable to those obtained using Penning traps.
An MR-TOF is currently under construction at the University of Notre Dame Nuclear Science Lab. All the electrodes have been manufactured and one of the mirrors have been cleanly assembled. The device will be tested off-line at the NSL, with a mixed K+ and Ca+ ion beam produced by a surface ion source as the separation of the 40Ca/40K mass doublet requires a resolving power of ≈ 28000, making it an ideal case for initial testing. While a radio-frequency quadrupole cooler and buncher is being developed, the ion bunches will be created using a Bradbury-Neilson gate. Simulations have indicated that due the small energy spread of the ions created by the surface source and the fast switching of the gate, bunches with comparable energy spread as provided by an RFQ can be created using this method. Mirror optimization simulation of the MR-ToF were also performed and indicate that a resolving power comparable to similar existing devices can be achieved. In the future the MR-TOF is planned to be installed in the N = 126 factory currently under development at Argonne National Laboratory.