Standard Thermal Neutron Irradiation Laboratory

Among radiations generated from high-energy accelerator operation, neutron and gamma ray should be measured to estimate radiation dose correctly since they can penetrate a shielding wall. Neutron looses their energy in a shielding wall through scattering and then becomes "thermal neutron" the velocity of which is equivalent to thermal motion of a material, 2200 m/sec. For calibration and development of a neutron detector, thermal neutron is widely used. The thermal neutron can be generated through slowing down in a media with adequate size. Too small size gives insufficient slowing down, too large gives high neutron absorption.

KEK has the largest graphite pile in Japan, the dimension of which is 250 cm × 190 cm × 190 cm, based on joint research with National Institute Radiological Sciences. The graphite pile consists of high purity carbon. Carbon is the element having the lowest neutron absorption cross-section, therefore well stabilized thermal neutron field is available when neutron source is placed at the center of the pile. Standard thermal neutron field is established through the study for intensity and spatial distribution of the thermal neutrons. The field is used to calibrate and develop neutron detectors not only for KEK radiation monitoring but also for joint research with private company.