Development and Use of Neutron Scintillator Systems for Fusion Plasma Diagnostics

Doctoral thesis, 1992

This thesis reports on research and development work, made in connection with the use of organic scintillators, for neutron spectrometry. It also gives a general review on the use of organic scintillators for the detection of neutrons generated from fusion of deuterium (D) and fusion between a mixture of deuterium and tritium (T) in thermonuclear plasmas. The basic mechanisms and limitations of organic scintillators are described. A library of computer routines utilizing Monte Carlo technique for neutron transport in organic media written by the author of this thesis, is presented. The applications of organic scintillators in different neutron time-of-flight spectrometers developed at, or with, participation of the Dept. of Reactor Physics, are also described. The thesis is based on 6 papers. Paper 1 gives a method how to make a relative and absolute calibration of an array of 16 neutron scintillator detectors. The method utilizes the measurement of differential count rates for different high voltages applied on the photomultiplier tubes at different discriminator levels. If the maximum of the differential count rate is plotted as a function of high voltage, calibration coefficients for each neutron detector are obtained and these coefficients can then be used for the calibration. Papers 2 and 3 describe a Monte Carlo code for the 14.1 MeV combined proton-recoil and neutron time-of-flight neutron spectrometer TANSY. Paper 2 is a short version of paper 3. The TANSY spectrometer will be installed at the Joint European Torus (JET) before the D-T phase starts and the papers describe the calculation methods and the calculated response functions of this instrument. The response functions will be used for the unfolding of measured neutron energy distributions. Paper 4 gives a detailed description of the D-D neutron time-of-flight spectrometer which was in use at JET for several years. The hardware, the software for the random correction and some typical neutron energy spectra measured at JET are presented and described. Papers 5 and 6 describe an upgraded version of the D-D spectrometer which recently has been installed at JET. Paper 5 gives some experimental results from measurements of JET plasmas while paper 6 describes a Monte Carlo program simulating the neutron transport of this spectrometer. The calculated response functions have been used for the unfolding of the measured neutron energy spectra.