Stratospheric Flight with the Muon Detector

On May 1st, 2026, we carried out a new stratospheric balloon launch as part of the Balloon Probe project coordinated by the Agrupación Astronómica de Madrid (AAM) in preparation for the total solar eclipse of August 12th, 2026. This date was deliberately selected because the position of the Sun closely reproduced the same geometric conditions expected on the day of the eclipse, allowing tests to be performed under conditions very similar to the real event.

The payload incorporated new communication and image-recording systems intended for in-flight validation, as well as the new version of the Muon Detector, designated V3. This new version included improvements both in the acquisition software and in the onboard instrumentation, adding two new sensors capable of measuring the platform’s acceleration and rotation during the flight.

The muon detector was configured in coincidence mode, using two detectors facing each other and connected together to register only those events detected simultaneously in both sensors. This method makes it possible to efficiently discriminate muons — capable of crossing both detectors due to their high energy — from other particles or spurious events.

Image of the V3 Version of the Muon Detector

The results obtained were extraordinarily satisfactory. The following figure shows the temporal evolution of the muon rate during the flight. Two distinct peaks can be clearly observed, both associated with the Pfotzer maximum.

Graph of the measurements obtained during the flight, showing the muon rate and altitude.

The Pfotzer maximum is the region of the atmosphere, typically located between 15 and 20 km altitude, where the intensity of secondary cosmic radiation reaches its maximum value. This phenomenon occurs because primary cosmic rays arriving from space interact with nuclei in the Earth’s atmosphere, generating cascades of new particles, including muons. At very high altitudes the atmosphere is still too thin and only a small number of interactions occur; however, as altitude decreases, the number of produced particles rapidly increases. At lower altitudes, the particles begin to be absorbed, slowed down, or decay, causing the radiation intensity to decrease again. The balance between particle production and absorption gives rise to this characteristic maximum, experimentally discovered by Georg Pfotzer during his stratospheric balloon studies.

As shown in the figure legend, the probe reached an estimated maximum altitude of 33.5 km at 22:52 local time. During the ascent, a maximum muon rate of 12.56 Hz was recorded at 22:06 local time, corresponding to an altitude of approximately 17.5 km. Later, during the descent, a second maximum of 12.74 Hz was observed at 22:59 local time and at an altitude of 17.9 km, once again reproducing the Pfotzer maximum. In addition, the minimum temperature recorded inside the probe was −10.9 °C.

Muon detector installed in the balloon probe.

On the other hand, the tests carried out with the acceleration and rotation sensors were also highly satisfactory, providing detailed information about the flight dynamics. The following figure shows the results obtained with these sensors.

Evolution of acceleration and rotation during the flight. The final signals correspond to the recovery of the probe.