Underground neutron events at Tien Shan

Abstract

The events of multiple neutron production under a 2000 g/cm2 thick rock absorber were studied at the Tien Shan mountain cosmic ray station, at an altitude of 3340 m above the sea level. From comparison of the experimental and Geant4 simulated neutron multiplicity spectra it follows that the great bulk of these events can be explained by interaction of cosmic ray muons with internal material of neutron detector. In synchronous operation of the underground neutron monitor with the Tien Shan shower detector system it was found that the characteristics of the muonic component of extensive air showers which is seemingly responsible for generation of neutron events underground do change noticeably within the energy range of the knee of primary cosmic ray spectrum. Some peculiar shower events were detected when the neutron signal reveals itself only ∼100–1000 µs after the passage of the front of shower particles which probably means an existence of corresponding delay of the muon flux in such events.