Doctor of Philosophy
Department of Physics
Whittlestone, Stewart, The energy spectrum of neutrons in a pulsed fast assembly, Doctor of Philosophy thesis, Department of Physics, University of Wollongong, 1979. http://ro.uow.edu.au/theses/1636
Time-dependent neutron energy spectra in the range of 0.6 to 6.4 MeV have been measured in a depleted uranium assembly. In order to gain a maximum amount of information about the interactions of neutrons with uranium, the detector was placed inside the stack. This necessitated development of a small yet sensitive neutron spectrometer. The liquid s c i n t i l l a t o r NE213 was chosen. An optical system was developed which collected about 80% of the light from the s c i n t i l l a t or and transmitted i t along a 450 mm quartz light guide to a high performance photomultiplier. In order to use the detector as a nanosecond timing spectrometer, several calibration measurements were required. These included measurement of the detector e f f i c i e n c y and response to monoenergetic neutrons. It was also necessary to measure and correct for timing walk of the discriminator. When the neutron flux was changing by about a factor of two per nanosecond, timing walk of about 0.1 ns could introduce large errors into time dependent pulse height spectra. An active timing walk correction system was developed to overcome this problem. The other important source of error was pulse pile-up. This led to errors of several per cent in neutron pulse height spectra, even when the probability of a count per beam pulse was as low as 0.02. A s i g n i f i c a n t contribution to pile-up in neutron pulse height spectra came from gamma ray pulses whose rise times were increased by pile-up. The pulse shape analyser interpreted such pulses as neutrons. To analyse the experimental data, a suite of three computer codes was written. The f i r s t prepared the data for submission to the second which did the unfolding of the pulse height spectra. Comparison and presentation of analysed data was performed by the third code.