Influence of environmental parameters on secondary cosmic ray neutrons at high-altitude research stations at Jungfraujoch, Switzerland, and Zugspitze, Germany

Abstract Neutrons of secondary cosmic radiation (CR) are produced when primary cosmic radiation interacts with the nuclei in the Earth's atmosphere. The secondary neutron spectra in the atmosphere do not include many thermal and epithermal neutrons at energies below several eV. In contrast, close to the Earth's surface many more of those neutrons are present, due to albedo neutrons backscattered from the ground. The number of albedo neutrons is mainly determined by the environment, i.e. the material in the environment (characteristics of the underground and the environment (building, topology of landscape)). If the environment (buildings, underground) is constant with time then the variation of the albedo neutron fluence is dependent on the soil moisture and snow cover. To investigate snow cover effect in detail, in June 2016 and September 2018 two measurement campaigns were carried out at the High Altitude Research Station Jungfraujoch, Switzerland. During these campaigns the energy distributions of secondary CR neutrons were measured by means of a mobile extended range Bonner sphere spectrometer (ERBSS) at two different positions with different environmental conditions: under the cupola of the astronomical observatory in the Sphinx building at an altitude of 3,585 m a.s.l., and below the shelter roof of the research station at 3,466 m a.s.l. In addition, since 2004 the energy spectra of neutrons from secondary CR have been continuously measured at the Environmental Research Station (UFS Schneefernerhaus; 2,650 m a.s.l.) close to the summit of the Zugspitze mountain, Germany. To measure the neutron spectra in the energy range from a few meV up to several GeV, a stationary ERBSS has been used. The chosen measurement positions allow quantification of environmental conditions which affect the neutron spectral distribution in the whole neutron energy range up to several GeV. With this spectral information, it is also possible to derive detailed information about the neutron ambient dose equivalent (H*(10)) at these altitudes and geomagnetic latitudes. The ERBSS measurements at Jungfraujoch presented here show that snowpack in the surrounding area does not affect the fluence rate of secondary neutrons significantly. However, the influence of topography of the chosen measurement locations on secondary CR neutrons was observed.