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Efficiency calibration of a neutron long counter. MacFarlane, Tracy Lynn

Abstract

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.

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