Higher Neutron Brightness from Innovative 'Pancake Moderator'

Technical highlight.
The brightness of ESS neutron beams will be increased by a factor of two or more, due to the development of the novel "Pancake moderator" concept. This break-through will enhance instrument performance and scientific output significantly.

ESS neutrons are generated by accelerating protons and directing them at a target made of tungsten, which then releases high-energy neutrons. Moderators adjacent to the tungsten target wheel slow the neutrons down to the cold and thermal energies required for experiments. Moderation inevitably leads to loss of neutron intensity, and moderator design work includes optimizing geometries to minimize losses. By removing restraints on the initial optimization, the ESS Target division found that smaller moderator dimensions tend to increase neutron brightness (see graph).

It was discovered [1,2] that the brightness of cold neutron moderators can be very significantly enhanced, compared to the established practice used at current reactor and spallation neutron sources. The new concept implies quasi-low-dimensional moderator geometries, i.e. flat 2D moderators (thus “pancake”) or rod-like 1D moderators with the smallest surfaces in properly designed shapes used for neutron emission.

An expert panel with members from ANSTO, TUM, SNS, J-Parc, ISIS and ILL convened in May to review the options and challenges that these developments offer. Based on the work presented by instrument scientists at ESS and our partner labs and the ESS neutronics and monolith and handling groups they concluded that the new moderator design can be of great benefit to ESS, and should be pursued. In concurrence with this recommendation further engineering work towards the implementation of the pancake moderator is now underway.

The gains in instrument performance are expected to land at a factor of 2 to 3, compared to the already impressive performance expectations in the ESS baseline of 2012. This break-through will expand the scientific possibilities of ESS even further.

[1] K. Batkov et al, Unperturbed moderator brightness in pulsed neutron sources,  Nuclear Instruments and Methods in Physics Research A729 (2013) 500–505
[2] F. Mezei et al, Low dimensional neutron moderators for enhanced source brightness, Journal of Neutron Research 17 (2014) 101–105.