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<\/p>\nBand-gap moderated slow positive muon beams \u2013 <\/strong>While positive muons (\u03bc+<\/sup>) typically behave as light protons, they can also act like heavy positrons (e+<\/sup>). Bearing this in mind, Dr. Harshman and coworkers, considering results detailing e+<\/sup> moderation in large band-gap solids, proposed the possibility of similar behavior for positive muons. The mechanism relies on energy loss per unit length (dE\/dx), which decreases as the stopping particle energy falls below the band-gap energy of the moderating solid, resulting in a moderated particle beam of a few eV. Since the only relevant difference between muons and electrons in this context is mass, such a mechanism was deemed applicable for producing a slow \u03bc+<\/sup> beam. Conducted at the TRIUMF facility in Vancouver, B.C., Canada, this research resulted in the discovery of the first practical technique for producing a low-energy (<10 eV) positive muon beam. It was christened with the acronym \u03bcSOL (i.e., muon separator online). The figure to the right shows the time-of-flight spectra for low-energy \u03bc+<\/sup> from the surface of (a) LiF, (b) SiO2<\/sub>, and (c) Cu. The data are normalized to 10 incident \u03bc+<\/sup>, and the errors shown are statistical.\u00a0[Physical Review Letters 56<\/strong>, 2850 (1986)<\/a>].<\/p>\n The same 1986 paper reported the first observation of (<10 eV) negative muonium (i.e., Mu\u2013<\/sup> or \u03bc+<\/sup>e\u2013<\/sup>e\u2013<\/sup>) ions in vacuum. This was accomplished by again using a Cu moderator, and reversing the polarity of the spectrometer to focus and transport the Mu\u2013<\/sup> ions.<\/p>\n The development of the band-gap moderated slow \u03bc+<\/sup> beam is documented in Discover Magazine<\/a> and Wikipedia<\/a>.<\/p>\n Below is a slideshow of the slow muon spectrometer as configured in 1986 for non-cryogenic moderators, mounted on the TRIUMF M20 surface muon channel.<\/p>\n The slow muon production technique pioneered by Harshman and coworkers was adopted by the Paul Sherrer Institute (PSI) for their slow positive muon beam facility.<\/p>\n Ultra-Low Energy Muon Source, \u03bcSOL Band-gap moderated slow positive muon beams \u2013 <\/strong>While positive muons (\u03bc+) typically behave as light protons, they can also act \u2026 Continue reading ![\"\"](\"http:\/\/physikon.net\/wp-content\/uploads\/slowmu2.jpg\")
In 1987, the slow muon production rate was increased by a factor of ~100 using solid rare-gas moderators, producing a usable flux of slow positive muons. This was accomplished with the same spectrometer as employed in the 1986 paper, incorporating a specially-designed 4<\/sup>He cryogenic target. The figure to the left shows the slow \u03bc+<\/sup> emission from solid (a) Argon and (b) Krypton at two different acceleration grid biases [Physical Review B 36<\/strong>, 8850 (1987)<\/a>].<\/p>\n
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