In-vessel calibration of the JET neutron monitors using a 252Cf neutron source: Difficulties experienced

The most direct method of obtaining an absolute calibration of the set of fission chambers used at JET for the measurement of the time-resolved neutron yield is by means of a standardized 252Cf radio-isotope neutron source which is moved around the vacuum vessel to map out the volume occupied by the plasma. The accuracy of this procedure can approach 10%. However, its applicability depends on the geometry of the tokamak and the location of the fission chambers. At JET, most of the neutrons reach the detectors by passing through the main diagnostic ports. Now that these ports have become surrounded by massive items of hardware, it is found that this equipment moderates and absorbs the 252Cf neutrons more strongly than 2.5 MeV neutrons. This paper examines the magnitude of the errors on the absolute calibration of the fission chambers introduced by this effect.