Use of the KL→3π0 decay as a tagged photon source to measure material thickness in a neutral kaon beam

A new method for measuring the thickness (in radiation lengths) of thin windows or gas-filled regions in a high-energy neutral kaon beam is presented. Using an electromagnetic calorimeter to reconstruct the six-photon signature of a KL→3π0 decay, we search for the presence of charged tracks to measure the probability of photon conversions in material upstream of the spectrometer. A magnetic field is used in the vacuum between the decay region and the material to be measured, allowing external photon conversions to be distinguished from decay processes with an internal photon conversion such as π0→e+e−γ.