Neutron and gamma impact on devices with iron shields

The beam transport line between the rapid cycling synchrotron and the slow cycling one is called the 3-50BT line in J-PARC. The slow cycling synchrotron is called the Main Ring (MR). Both the 3-50BT line and MR have a beam collimator section each in order to reduce the beam loss in MR during the beam injection period. The beam collimator becomes a strong radiation source of neutrons, gammas, and the other charged particles. The collimator units are used to 3 GeV proton beam. Their structure is similar in both, the 3-50BT line and MR. However, the impact on the surrounding devices is not the same. All the fluorescent lights are broken beside the 3-50BT collimators. The beam loss in the 3-50 BT collimator section is a fraction of that in MR. The residual radiation dose is small, but the damage to the surrounding equipment is extremely large. The origin of this difference was examined from the perspective of the distribution of neutrons and gamma-rays. (author)