MAPS Guide and Disk Chopper Upgrade

Tuesday 07 October 2014

MAPS will be upgraded with the addition of a neutron guide in the primary spectrometer, and a disk chopper upstream of the Fermi

The absence of a guide on MAPS limits the effective exploitation of the thermal and sub-thermal energies compared to the latest chopper instruments at ISIS and other facilities. To improve this we plan to install an m=3 guide that would enhance the flux by up to an order of magnitude at sub-thermal energies. We expect a gain in flux of approximately 50% even at the highest energies due to a wider view of the moderator. The guide project is currently in the design phase, with installation provisionally in 2016.

 

Concurrent with the installation of the guide, we also plan to install a disk chopper upstream of the existing Fermi chopper. The good energy resolution of MAPS arises from the long secondary flight-path – 6 m compared to 2.5 m on MERLIN. However, it also has the effect of bringing the elastic scattering from later (hence lower incident energy) transmission by the Fermi chopper further into the useful energy transfer window of the selected incident energy. At temperatures significantly above 0 K, energy gain processes from later pulses can further impinge on the energy window of interest. A broad band-pass disk chopper before the Fermi would solve this problem by suppressing later monochromatic pulses. An additional benefit is that it would allow a straight-slitted gadolinium Fermi chopper to be used, since neutrons with energies above 200 meV, to which the gadolinium chopper is transparent, can be suppressed. This would give greater flexibility for flux-resolution compromises compared to the existing Fermi chopper set. It would also allow more extensive use of the rep-rate multiplication technique on MAPS than is currently possible.

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