Volume 65A, number 4 PHYSICS LETFERS 20 March 1978 PRODUCTION OF ThERMAL MUONIUM IN THE VACUUM BETWEEN THE GRAINS OF FINE SILICA POWDERS G.M. MARSHALL, J.B. WARREN, D.M. GARNER, G.S. CLARK, J.H. BREWER and D.G. FLEMING TRIUMF and Departments of Physics and Chemistry, The University of British Columbia, Vancouver, BC, V6T I W5, Canada Received 29 December 1977 When spin-polarized positive muons are stopped in fine Si0 2 powder under vacuum, muonium precession is observed in- dicating 45% muonium formation. Depolarization by oxygen indicates that muonium, like positronium, emerges from the powder grains into the voids. The recent precision measurement [1] of the vacu- 0.03 g/cm 3, radius ~—35 X 10—8 cm,(Cabosil EH5, urn annihilation rate of triplet orthopositronium has made by Cabot Corporation, Boston) has been found emphasized the value of fine powder insulators, such to give maximum muonium yield. The powder was out- as MgO and Si0 2 [2] , in the production of useful gassed and dried at 10~Torr and ultimately held at a amounts of orthopositronium in a vacuum environ- pressure of 10—6 Torr. Provision was made for the in- ment. It is thought that positronium is formed in the troduction of small known concentrations of pure oxy- grains, then diffuses rapidly to the surface and out gen, a paramagnetic gas which is known to destroy the into the vacuum where it annihilates [3]. muonium spin polarization. The thin beam defining Whether positive muons behave similarly in such an counter, 90 cm 2 in area provided excellent pulse environment is of interest, since, for example, the height discrimination against beam positrons while re- muonium atom, (p+e_), may transform into antimuon- taming high efficiency (>95%) for muons. Muon stop ium, (~r-e~) [4]. Such a transformation is permitted rates were typically 2 X i04 s~ per microampere of if the muon number is a parity-like quality rather than 500 MeV protons incident on a 10 cm long Be produc- an additively conserved quantum quality [5]. Feinberg tion target. The vacuum vessel was located in an appa- and Weinberg have shown that such a conversion prob- ratus similar to that described elsewhere [9] in order ability would be greatly reduced by local fields which to monitor the formation and behaviour of muonium break the (i+e_)_(p_e+) degeneracy; such a reduc- atoms by the muonium spin rotation technique [6,9]. tion can be overcome if the (p+e_) spends a signifi- Polarized positive muons often form muonium as cant fraction of its life without collisions, they stop in a target. In a weak transverse magnetic While muonium is known to form in crystalline and field, the coherent Larmor precession of the polarized fused quartz [6] its formation has hitherto been unob- F = 1 component ~5Q%of the ensemble) is recorded served in silica powder [7] . We have investigated the via the asymmetric decay of muons into energetic posi- production mechanism further, using the stopping iT//I trons. The time interval from muon stop to positron channel at TRIUMF. A spin-polarized beam of 29.8 detection in a fixed direction is measured and a histo- MeV/c momentum j.i~,from the decay of pions stopped gram for many such events is accumulated. This spec- in the surface of the beryllium pion production target trum has the form [8], passed through thin Mylar windows and a thin, / 2 . . . N(t)~Noet1n,2{I1 +AMe t cos(wMt + 0.040 g/cm , beam defining plastic scrntillator and 1 then came to rest in a silica powder target. The finest +A cos(w t—çb))+B, grade of silica powder available to us, bulk density M 351