COLD NEUTRON BEAMS AT THE INTENSE PULSED-NEUTRON SOURCE

The three moderators at the Intense Pulsed Neutron Source (IPNS) are o f cryogenic methane (CH4); 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 phys ical configuration in order to tailor their performance for the 14 ins truments and test facilities that operate on the 12 neutron beams. IPN S has started a program to enhance the effectiveness of its target/mod erator/reflector system. This program involves both Monte Carlo comput er modeling of the system and measuring the intensity distributions of the neutron beams. This paper reports the first phases of this projec t - calculations and measurements of the existing system - which will serve as points of reference for gauging the effectiveness of the prop osed improvements. The measurements provide absolute spectra using foi l activation techniques joined with time-of-flight measurements perfor med with thin beam monitor detectors installed at the neutron-scatteri ng instruments. The comparison of the experimental and computational r esults provides verification for the physical model of the system and the cross-section data used in the calculations. (C) 1998 Elsevier Sci ence B.V. All rights reserved.