As an example

we have calculated the radiation dose distribution near by a γ-ray density meter equipped in a 1600 type dredge ship

using Geant 4 （Monte Carlo simulation software）. The most serious radiation leakage is found in the region close to the detector and it’s safe for using the γ-ray density meter under normal working condition. But under some abnormal condition such as the empty pipe

the density meter will detect a huge amount of radiation. For providing a safe circumstance

additional shield should be adopted. In fact a piece of steel plate of φ 50 cm ×7 cm in thickness could prevent most of escaping radiation and ensure the safety.