Finally, positive muon beams of even lower energy ("ultra slow muons" with energy down to the eV-keV range) can be obtained by further reducing the energy of an Arizona beam by utilizing the energy-loss characteristics of large band-gap solid moderators. This technique was pioneered by researchers at the TRIUMF cyclotron facility in Vancouver, B.C., Canada. It was christened with the acronym, μSOL (muon separator on-line) and initially employed LiF as the moderating solid.  The same 1986 paper also reported the observation of negative Muonium ions (i.e., Mu− or μ+ e− e−) in vacuum.  In 1987, the slow μ+ production rate was increased 100-fold using thin-film rare-gas solid moderators, producing a usable flux of low-energy positive muons.  This production technique was subsequently adopted by the Paul Scherrer Institute (PSI) for their low-energy positive muon beam facility.