Chopper cascades: An analytic treatment of the contamination problem

Abstract

Disk chopper cascades are used to produce pulsed beams of monochromatized neutrons. The optimization problems involved in the design of a new cascade are complex due to the interrelation of neutron guide and chopper system. We will present here a novel approach towards analyzing the part of the optimization concerned with the contamination of the spectrum by parasitic neutrons. To this end the cascade is decomposed into a suite of band pass filters. By folding the Boolean transmission function of the single filters we obtain an analytic solution for the contamination problem. In contrast to Monte-Carlo simulations our approach offers a better insight into the parameters governing the transmission of side bands.

Key Words:

• Time-of-fight
• Choppers
• Velocity selectors
• Band pass filters
• Neutron instrumentations