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The U.B.C. Hanson-McKibben type Long Counter has been modified to permit insertion of a larger diameter BF₃ proportional counter. An investigation was made to determine the effects of the modification on the neutron detection characteristics of the counter. 2.60 MeV and 3.09 MeV neutrons were obtained from the reaction D(d,n)³ He and 4.5 MeV neutrons from a standard²⁴¹AmBe source. The efficiency of the Long Counter was determined in terms of counting rate per unit neutron flux occurring at the effective centre of the counter. The count rate used was that due to the ¹ºB(n,α)⁷Li pulses from the BF₃ counter located in the centre of the Long Counter. The effective centre was defined as that point inside the counter such that the counting rate varied as the inverse square of the distance of the source from the effective centre. The following table lists the neutron energy, efficiency and distance from the front reference face of the Long Counter to the effective centre. [ Formulas omitted ] The efficiency for 4.5 MeV neutrons was found to have increased by at least an order of magnitude from that of the original Long Counter. However, the dependence of the efficiency on energy has been increased, varying by 50% over the range of energies measured. The shielding on the sides of the Long Counter was found to reduce the intrinsic efficiency for neutrons incident from the side to 0.4 of that for neutrons incident on the front face. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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Assuming an epithermal neutron flux distribution function of the form Φ'epi(E) α 1/E¹⁺ᵅ, the epithermal flux-shape parameter, alpha, and the integrated subcadmium to epicadmium flux ratio, f, were experimentally determined for two irradiation facilities in the University of Arizona TRIGA Mark I reactor. Two equivalent approaches: the Multimonitor method and the Non-linear least-squares method, based on integral (reaction rate) techniques applied to thin foils of gold, zirconium, vanadium, dysprosium, copper, manganese and molybdenum, were used. The Non-linear least-squares approach was found superior to the Multimonitor method on the basis of smaller resulting uncertainties. The resulting parameters are: α = -.0396±.0104 and f = 16.89 ± 1.08 for the Rabbit and α = -.0296 ±.0089 and f = 19.57 ± 1.26 for the Susan, where the reported uncertainties represent one sigma values.

As disc-shaped detectors are one of the primary means of measuring the neutron density, a better solution is desirable if the error due to the depression factor is to be made negligible. In this paper, an attempt is made to solve this problem in the oblate spheroidal co-ordinate system which most nearly describes the disc-shaped detector, so that solutions may be obtained that describe depression factors for detectors of varying thicknesses and radii.

by Hong Kuen Wong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1976. / Includes bibliographical references (leaves 112-121).

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