The observations from the balloon flight can be understood in terms of how muons are both produced and lost as cosmic radiation passes through the atmosphere. When high-energy cosmic rays strike atoms high above the Earth, they initiate cascades of secondary particles—known as cosmic ray air showers—in which unstable particles such as pions are created and quickly decay into muons.
Near the ground, many of the muons detected have already travelled a considerable distance and a significant fraction will have decayed, since muons have a short lifetime of only a few microseconds in their rest frame. As the balloon ascends, the detector moves closer to the region where these muons are formed, so fewer have had time to decay before being measured. This effect tends to increase the detected count rate. At the same time, however, the atmosphere becomes thinner with altitude, meaning there are fewer interactions between incoming cosmic rays and atmospheric nuclei, and thus fewer new muons are produced. Over a substantial range of altitudes, these two competing effects—reduced decay and reduced production—largely balance each other, resulting in a surprisingly steady detection rate.
At higher altitudes, close to the point where the balloon burst, the detector reaches the region where muon production is near its maximum while decay losses are still minimal. This leads to a noticeable increase in the measured count rate, producing a peak. This peak corresponds to the so-called Pfotzer maximum, the altitude at which secondary cosmic radiation reaches its greatest intensity. Had the balloon ascended further, the count rate would eventually have decreased again, as there would be insufficient atmosphere above to sustain the cascade processes that generate muons.