Stopping rate of negative cosmic‐ray muons near sea level

A production rate of 0.065 ± 0.003 37 Ar atom/kg min of 39 K at a 2‐mwe (meters water equivalent) depth below sea level was measured both by sweeping argon from potassium solutions and by counting argon that diffused from dry potassium acetate powder. This rate is unaffected by surrounding the solution with paraffin or by the presence of solution water, and it is attributed to negative‐muon captures and the electromagnetic interaction of fast muons and not to a nucleonic cosmic‐ray component. The 37 Ar yield from 39 K by the stopping of negative muons in a muon beam of the SREL synchrocyclotron was measured to be 8.5 ± 1.7%. The stopping rate of negative cosmic‐ray muons at a 2‐mwe depth below sea level from these measurements and from an estimated 17% electromagnetic production is 0.63 ± 0.13 μ − /kg min. Previous measurements on the muon stopping rate vary by a factor of 5. Our value is slightly higher but is consistent with two previous high values. The sensitivity of the 37 Ar radiochemical method for the detection of muons is considerably higher than that of the previous radiochemical methods, and the 37 Ar radiochemical method could be used to measure the negative‐muon capture rates at great depths.