A measurement of the flux, angular distribution and energy spectra of cosmic ray induced neutrons at fission energies

When searching for SNM simply designing a better detector to optimize the signal S from the source is not enough. It is important to know the background B to maximize S/N, where N is the noise in B. Cosmic rays are a dominant source of neutron background. It is therefore important to know their flux, angular and energy distribution. Over the last 50 years work has been done to study cosmic ray neutrons and their variation. The full hemispherical neutron flux is usually quoted at a certain altitude (e.g. Altitude = 0 meters above sea level, pressure = 1033 g/cm²) and geomagnetic rigidity (e.g. GMR = 1.2GV). Neutron fluxes at other locations are scaled from the sea level data using a well determined prescription. However, there is a lack in knowledge of the angular dependence of the neutron flux at sea level. The angular dependence is important for two reasons; first many detectors have an efficiency that changes with the direction of the incident neutron. Second none of the measurements to date have determined how the flux changes with angle, their data must be modeled to estimate the full hemispherical flux. In this paper we present the cosmic neutron background flux measured by a neutron scatter camera in the energy range 0.2-10MeV. Our measurements are in agreement with the best fit to past data. We present for the 1^st time the neutron zenith angle dependence at fission energies which is observed to be a function of the form cos^2.7thetas.