Ultralow-background photon-detection system for collinear laser spectroscopy of high-energy highly charged ion beams

An ultralow-background fluorescence-photon detection system has been successfully developed for the collinear laser spectroscopy of highly charged ion beams having a relativistic velocity. We applied the technology of a cryopump to a photon-detection system. A charcoal panel, optical filters and four movable collimators strongly reduced the background due to scattered photons of the incident laser beam. The background due to collisions between the ion beam and residual gas was also suppressed by improving the vacuum using a cryopump technique. With this system, we clearly observed the laser-induced fluorescence spectra of the 2S1/2-2P1/2,3/2 transitions in an Li-like 11B2+ ion beam with a signal-to-noise ratio of 200. The ion-beam energy was 11.25A MeV and the intensity was 104 ions/bunch. We also demonstrated that the system has the capability of measuring the lifetimes of excited states.