Cold neutron beams at the Intense Pulsed Neutron Source

Abstract

The three moderators at the Intense Pulsed Neutron Source (IPNS) are of cryogenic methane (CH 4); one of liquid methane at 100 K, and two of solid methane at 30 K. These moderators produce intense beams of both cold and thermal neutrons. The moderators are each of a different physical configuration in order to tailor their performance for the 14 instruments and test facilities that operate on the 12 neutron beams. IPNS has started a program to enhance the effectiveness of its target/moderator/reflector system. This program involves both Monte Carlo computer modeling of the system and measuring the intensity distributions of the neutron beams. This paper reports the first phases of this project — calculations and measurements of the existing system — which will serve as points of reference for gauging the effectiveness of the proposed improvements. The measurements provide absolute spectra using foil activation techniques joined with time-of-flight measurements performed with thin beam monitor detectors installed at the neutron-scattering instruments. The comparison of the experimental and computational results provides verification for the physical model of the system and the cross-section data used in the calculations.