Atmospheric neutron measurements with Spherical Proportional Counter and the wall effect problem

Cosmic radiation interacts with molecules, atoms and nuclei in the atmosphere producing secondary radiation consisting of charged particles, neutrons, gamma and X-rays. The cosmic neutron energy ranges from 0.01eV to 10GeV, with neutrons of low energy being difficult to detect and measure. The Spherical Proportional Counter (SPC) is a large-volume gaseous detector and has been optimized for operation with pure nitrogen gas to be used for detection of thermal neutrons via the 14N(n,p)14C reaction. When the reaction happens close to the detector wall it is possible for the produced particles to hit the wall and lose energy. This is known as the wall effect, and it leads to wrong calculations of the incident particle energy. The challenge for thermal neutron detection lies in achieving sufficient gas gain since low pressure is needed which results in non-negligible wall effect [3]. In this work, a study has been done to quantify the wall effect of thermal neutrons in the SPC. We used GEANT4 simulations to produce neutron beams that cover the total volume of the sphere interacting with the gas nuclei and then analyzed the data using python code.