Ambient neutron background in the shallow-underground laboratory Felsenkeller

One important component of the ambient background in underground laboratories are neutrons, which cover a wide energy range from thermal up to 100 MeV. After a few meters rock overburden, cosmic-ray neutrons are a negligible contribution underground and the remaining flux is due to neutron production by cosmic-ray muons and by (α,n) reactions from natural radioactivity in the rock. There are only a few measurements of the full spectral neutron flux available in the literature, a fact which hampers comparisons between laboratories and negatively affects the planning of future experiments. In an effort to overcome this issue a setup consisting of six moderated and one unmoderated 3He neutron counters that has been used at a depth of 850 m in the Canfranc underground laboratory, Spain [1], was utilized to study the neutron flux in the 48 m deep Dresden Felsenkeller underground laboratory, Germany. At Felsenkeller, an additional counter with a lead liner was used in order to address also the high-energy flux up to several hundreds of MeV.