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College Park, Maryland      June 6 - 10 , 2004

TP23: A multi-rotor T0 chopper for HYSPEC

V.J. Ghosh, I. A. Zaliznyak, S.M. Shapiro, L. Passell (Brookhaven National Laboratory)

One of the design goals for HYSPEC has been to achieve the highest possible signal-to-background ratio. For neutron spectrometers in general, when the sample environment is in the direct line of sight of the neutron source the performance of the T0 chopper becomes crucial to achieving low beam-related backgrounds and, therefore, high instrument performance. The traditional design of these T0 choppers involves a single rotor, 30 to 40 cm thick, phased so that it blocks the beam at the time of arrival of the prompt gamma-rays and the highest-energy neutrons. As the power of the source increases, so does the radiation load on the T0 choppers. Consequently, the requirements for the amount and type of material of which these choppers are made become more and more demanding. In addition, for optimum efficiency these massive rotors should rotate at rates of 60-120Hz. The design of these choppers often becomes a delicate balance between neutronic performance and mechanical safety. We propose the use of a multi-rotor setup for HYSPEC so that we do not have to compromise either neutronic efficiency or mechanical performance. We have studied a setup consisting of two counter-rotating rotor pairs, where each rotor is a 10 cm thick disk, made of tungsten, inconel or stainless steel. These counter-rotating pairs act as velocity selectors, the doubling of the effective rotation rate allows us to reduce the beam-related background. In addition this setup allows us to use a lot of heavy material for stopping the fast neutrons without compromising mechanical safety. The performance of the HYSPEC multi-rotor T0 chopper setup was studied using ray-tracing (MCSTAS) and neutron transport (MCNP-X) codes. These results will be presented and compared with the results for a traditional T0 chopper.

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