Cosmic ray energy spectrum in the atmosphere measured with a novel balloon-carried detector

Evaluating the effects of galactic cosmic rays (GCR) and space weather throughout the atmosphere has motivated development of new instruments. A 1 x 1 x 0.8 cm³ and 30g microscintillator detector was flown on a meteorological radiosonde over the UK, reaching an altitude of 32 km. The flight was intended as a technology demonstrator for an improved version of the microscintillator that interfaces with the industry standard Vaisala RS41 radiosonde. GCR neutrons are regularly measured at the surface and assumed to be an indicator of ionisation above. However, neutrons are not ionising, and there are known discrepancies between surface neutrons and ionising radiation aloft. Our microscintillator is sensitive to ionising radiation with energies from 25keV-10MeV. Each pulse is recorded and pre-processed on the balloon into 17 energy channels for real-time radio transmission to a ground station.

The flight, on the afternoon of 9^th July 2024, occurred during minimal solar and space weather activity, therefore the measurements are almost entirely from the cosmic ray background. The system also recorded count rates from two Geiger counters, both independently and as “coincidences” from simultaneous triggering from higher energy particles. As anticipated, the background count rate in the microscintillator and Geigers increased as the balloon ascended, reaching the Regener-Pfotzer maximum, in this case at 22 km. Peaks in the energy spectrum occurred at 1.8 MeV, likely to be due to the gamma rays produced through de-excitation of atmospheric nitrogen nuclei excited by secondary GCR neutrons. Detection of gamma rays from neutron interactions offers the possibility of a direct comparison to neutron monitors. There were also peaks at 300keV which may be from secondary electrons created by GCR. Unlike previous flights of this detector during space weather activity, no bremsstrahlung X rays at ~100keV were observed. The Geiger and coincidence counter results were consistent with the medium and high- energy channels from the microscintillator, respectively. This combination of altitude and energy resolution is highly unusual for such a small and light weight detector.