Cosmic-ray-induced neutron spectra and effective dose rates near air/ground and air/water interfaces in Taiwan.

The ground-level cosmic-ray neutrons were measured at various elevations from sea level to 4,000 m in Taiwan, where the vertical cutoff of geomagnetic rigidity is high. High-efficiency Bonner cylinders were used in the measurements. The measured results were used to normalize and confirm one-dimensional transport calculations for cosmic-ray neutrons near interfaces. According to these measurements and calculations, the ground-level neutron fluence rates, effective dose rates, and their altitude dependence in Taiwan were determined. As compared with that reported elsewhere, the appreciable differences both in their absolute values and associated dependence on altitude could be attributed to the substantial latitude effect. In addition, the energy spectra of cosmic-ray neutrons near air/ground and air/water interfaces were measured. The neutron fluence rate near the air/ground interface is greater than that near the air/water interface; however, the spectral shape is harder at the air/water interface than at the air/ground interface. The air/ground and in-flight spectra differ somewhat at low energies, especially in the thermal energy region, but the general shapes of the spectra are similar to each other. The influence of the difference in spectral shape on the evaluation of effective dose rate was investigated.