College Park, Maryland      June 6 - 10 , 2004

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T1-D4 (9:15 AM): Experimental test of the cold coupled liquid H₂ moderator at LANSCE

M. Russina (Los Alamos Neutron Science Center, Los Alamos National Laboratory), F. Mezei (Los Alamos Neutron Science Center, Los Alamos National Laboratory; Hahn-Meitner-Institut Berlin), P. Lewis, T. Kozlowski (Los Alamos Neutron Science Center, Los Alamos National Laboratory), S. Penttila (Los Alamos National Laboratory)

We present the results of a performance test of the coupled cold liquid H₂ moderator installed at Manuel Lujan Jr. Neutron Scattering Center (Lujan Center) at LANSCE. This moderator is the first of its kind ever installed on a spallation source. To measure the moderator performance we have implemented a novel method based on two-dimensional time-of-flight (TOF) analysis to determine both the pulse shape and the absolute value of the flux. The key of the setup was a mechanical chopper with a small diaphragm placed close to half way between the moderator and a detector in the beam axis. The chopper run asynchronously to the source and neutron counts were stored in a two dimensional histogram I(t_o,t_ch), where t_o and t_ch are, respectively, the times elapsed since the last firing of the source and the last chopper opening. The method functions with full efficiency without phasing the chopper to the source and it is in this sense a precursor example for the event recording data collection approach, which will enable us to fully eliminate the effects of the jitter of chopper phasing on pulsed source spectra.

Using this approach we determined the moderator time average- and the peak flux, the time structure of the pulse as a function of neutron wavelength as well as observed the influence of radiation on the performance of the supermirror neutron optics placed close to the source inside the bulk shield. In addition the temperature dependence of the moderator characteristics have been checked in the H₂ temperature range of 20 K to 25 K. For this purpose we used an improved experimental setup with higher resolution. The data show that the coupled moderator at LANSCE provides the highest cold neutron flux (both average and peak) at pulsed spallation sources in the world and that the peak intensity of the pulse exceeds the intensity of the ILL.

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