Measurement of the stopping power of Au and MgF2 for slow muons

The spectral flux densityn(T) of ^– emerging from a Au or MgF₂ moderator has been measured at low energiesT using time-of-flight. Fromn(T) the stopping powerS(T) of Au was determined for 2 keVT22 keV, and of MgF₂ for 2 keVT12 keV. For Au,S(T) is smaller than calculated values obtained from proton atomic data practically in the wholeT range (Barkas effect); at lowT S(T) approaches the calculated values. For MgF₂,S(T) agrees fairly with the calculated values above 5 keV and then drops below these values. We ascribe this dropping to the large energy gap of the MgF₂ insulator.We wish to thank H. Angerer, H. Plendl, G. Schmidt, and C.A. Schug for help with the data taking, J. Homolka for computational help, H. Hagn, and H. Weiss for technical assistance and P. Maier-Komor and R. Scherrer for manufacturing the windows and targets. The hospitality of PSI and financial support by the German Bundesministerium für Forschung und Technologie are acknowledged.