Neutron fluence measurements on the JET tokamak by means of super-heated fluid detectors

Summary form only given. An experiment with the aim of determining the neutron field characteristics for high performance discharges is being carried out on the JET tokamak. The neutron field parameters (yield, fluence, energy distribution) at a specific location outside the JET Torus Hall are to be measured simultaneously by three independent techniques (super-heated fluid detectors or "bubble detectors", activation, and time-of-flight). Eventually the results are to be compared with neutron transport calculations. The bubble detector and activation measurement location is situated at the end of a vertical collimated line of sight, behind the TOFOR time-of-flight spectrometer. The field-of-view of the neutron detectors can be varied by means of a pre-collimator. The paper will report on neutron fluence measurements carried out by means of bubble detectors and their comparison with those done using the TOFOR spectrometer. Spatial (radial and toroidal) distributions of the neutron fluence have been obtained with a two-dimensional array of 30 bubble detectors. The operation of the bubble detector array as a neutron pinhole camera having a radial resolution at the JET vacuum chamber mid-plane of about 55 mm was demonstrated in measurements using various openings of the pre-collimator. The comparison of the area integrated fluence determined by the bubble detector array with the TOFOR neutron flux has shown a good correlation factor of about 0.99.