Measurement of Neutron Flux Distribution in Critical Assembly with Position-Sensitive Helium-3 Proportional Counter

Abstract

Neutron flux distributions in the Kyoto University Critical Assembly were measured with a position-sensitive ³He proportional counter. The authors examined the results of the first attempt to apply the position-sensitive counter for such measurement in the critical assembly. Some problems to be solved were drawn from the present examination for the further development of this type of counters. The counter had a sensitive length of 118 cm and an outer diameter of 2.5 cm. The counting gas was a mixture of ³He (270 Pa) and CF₄ (3×10⁴Pa). The observed distribution was very similar to that measured with the conventional gold wire activation method. The position resolution was 2.7 cm in the experimental condition. The measurable neutron flux (at peak position) was limited to the order of 10⁴ n-cm⁻²-s⁻¹ due to the pileup of signal pulses. The counter was a useful tool to quickly measure the flux distribution in the assembly.

KEYWORDS:

• position sensitive proportional counter
• helium 3
• critical assemblies
• thermal neutrons
• neutron flux
• flux density
• distribution
• position resolution
• pileup
• neutron streaming