Definition of the Radiation Fields for the JET Gamma-Ray Spectrometer Diagnostics
The JET gamma-ray diagnostics provide information on the evolution of fast particles within the tokamak configuration. Information on the spatial distribution of the interacting fast particles is obtained from the gamma-ray cameras, while energy distribution information is provided by gammaray spectrometers. These techniques have been successfully applied so far in fast particle simulation experiments at JET. The extension of these diagnostics to high power (high neutron yield) discharges is not straightforward. One necessary condition is to have a proper definition of the radiation (neutron and gamma) fields along the diagnostics line-of-sight. Two gamma-ray diagnostics upgrade projects at JET addressed this issue by developing neutron/gamma radiation filters ("neutron attenuators") and collimators. Neutron attenuators have been designed and constructed for the JET neutron/gamma-ray cameras and a pair of neutron/gamma collimators working in a tandem configuration have been designed and constructed for the JET tangential gamma-ray spectrometer. The tandem collimators were designed to provide collimation factors of about 5¥102 for 2.45MeV neutrons and about 103 for 9MeV gamma-rays. The devices have been installed on the JET machine and the paper presents a first evaluation of the results.