  In an effort to characterize the fast neutron radiation background, 16 EJ-309
liquid scintillator cells were installed in the Radiological Multi-sensor
Analysis Platform (RadMAP) to collect data in the San Francisco Bay Area. Each
fast neutron event was associated with specific weather metrics (pressure,
temperature, absolute humidity) and GPS coordinates. The expected exponential
dependence of the fast neutron count rate on atmospheric pressure was
demonstrated and event rates were subsequently adjusted given the measured
pressure at the time of detection. Pressure adjusted data was also used to
investigate the influence of other environmental conditions on the neutron
background rate. Using National Oceanic and Atmospheric Administration (NOAA)
coastal area lidar data, an algorithm was implemented to approximate sky-view
factors (the total fraction of visible sky) for points along RadMAPs route.
Three areas analyzed in San Francisco, Downtown Oakland, and Berkeley all
demonstrated a suppression in the background rate of over 50% for the range of
sky-view factors measured. This effect, which is due to the shielding of
cosmic-ray produced neutrons by surrounding buildings, was comparable to the
pressure influence which yielded a 32% suppression in the count rate over the
range of pressures measured.
