Depth profiles of environmental neutron fluxes in water and lead

摘要

Depth profiles of environmental neutron fluxes were measured by the activation method of gold targets in 0-200 g cm~(-2) of seawater, fresh water and lead, using ten ultra-low-background Ge detectors in Ogoya Underground Laboratory. The intensity of thermal neutrons in gold has been decreasing exponentially, the mass attenuation coefficient μ in gold was determined to be 0.70 ±0.17 cm~(-2) g~(-1). The neutron profiles in seawater and fresh water were almost the same. A maximum at ～5 g cm~(-2), a saddle between 20 and 30 g cm~(-2), and an exponential decrease after 50 for fresh water and 100 g cm~(-2) for seawater were observed. The mass attenuation coefficient μ of fresh water was determined to be 0.0090 ± 0.0011 cm~(-2) g~(-1). The depth profile of neutrons in lead after having a maximum at 5-10 g cm~(-2) was almost constant.