Abstract
A portable BF 3 counting system is used to monitor the low intensity cosmic neutrons. The system is calibrated by isotopic neutron sources and normalized to cosmic neutron measurement using a Monte Carlo transport code for the conversion to the dose equivalent rate. The system is field-tested in air, land, and sea around Taiwan where the vertical cut-off of geomagnetic rigidity is the highest in the world. The dose equivalent rates, measured in the middle of current cycle of solar minimum, were 0.54 nSv/h on the ground at sea-level to an in-flight 41.4 nSv/h at 16 000 ft cruising altitude. The rug-proof BF 3 counting system is suitable for field measurement of extensive survey and long-term monitoring of the low intensity cosmic neutrons.