  Recently, a new neutron spectroscopy for the dynamics in complex (bio-)
systems has been proposed [A. Benedetto, and G. J. Kearley, Sci. Rep. 6, 34266,
(2016)]. This spectroscopy is ideal where only the overall relaxation time in a
parameterless way is required, for example in complex systems, because only the
elastic-scattering intensity as a function of the energy resolution is
required. This has been termed "Elastic Scattering Spectroscopy" (ESS). It is
based on the inflection points in the elastic-scattering intensity at the
energy-resolution value corresponding to the overall system relaxation-time. A
Constant wavelength (CW) option, more suitable for reactor sources, and the
time-of-flight (TOF) option, more suitable for spallation sources, have already
been proposed, and here we examine the concept of a third option based on
neutron spin-echo (NSE), called ESS-NSE. In principle, this consists of simply
measuring depolarisation at the relatively-intense elastic echo-condition, as a
function of resolution, with the basic set-up being similar to standard
spin-echo. In its basic set-up, ESS spin-echo can access 5 orders of magnitude
in time from nanoseconds to tens of picoseconds, reaching slower relaxation
processes than the CW and TOF options recently presented. However, spatial
focussing is important for the small sample-sizes of biological systems, so we
also explore how the fields may be shaped to enable a small neutron beam to be
focussed on the sample.
