MEASUREMENT OF THE NEGATIVE MUON SPECTRUM BETWEEN 0.3 AND 40 GEV C INTHE ATMOSPHERE/

A measurement of the negative muon flux in the atmosphere has been mad e using a superconducting magnet spectrometer during the ascent part o f a balloon flight experiment performed on September 5, 1989 from Prin ce Albert, Saskatchewan (Canada). The negative muon component has been measured over the momentum range 0.3-40 GeV/c with an altitude increa sing from 0.6 to 36 km above sea level. This is the first time that re sults from such wide intervals in momentum and atmospheric depth have been obtained using a single apparatus. The flux growth curve with atm ospheric depth is momentum dependent; the low energy muon flux peaks a round 150 g/cm(2) and higher energy muons penetrate to larger depths i n the atmosphere. The flux decreases exponentially with increasing dep th above 200 g/cm(2). The attenuation length Lambda(e) increases almos t linearly with the muon momentum at a rate of about 90 (g/cm(2))/(GeV /c) in the 0.3-8 GeV/c range. The momentum spectra at different altitu des can be described by power-laws, provided that the spectral index i s left free to change with altitude. We found that an index value of - 2.5 +/- 0.2 can give a good description of the data for momenta betwee n 2 and 40 GeV/c in the depth range 20-400 g/cm(2). Below 1 GeV/c, the spectrum gradually steepens as the atmospheric depth decreases. Above 600 g/cm(2) a peak around 0.5 GeV/c arises.