  We report a measurement of the spin polarisation of the recoiling neutron in
deuterium photodisintegration, utilising a new large acceptance polarimeter
within the Crystal Ball at MAMI. The measured photon energy range of
300~--~700~MeV provides the first measurement of recoil neutron polarisation at
photon energies where the quark substructure of the deuteron plays a role,
thereby providing important new constraints on photodisintegration mechanisms.
A very high neutron polarisation in a narrow structure centred around
$E_{\gamma}\sim$~570~MeV is observed, which is inconsistent with current
theoretical predictions employing nucleon resonance degrees of freedom. A
Legendre polynomial decomposition suggests this behaviour could be related to
the excitation of the $d^*(2380)$ hexaquark.
