  Inelastic neutron scattering instruments require very low background;
therefore the proper shielding for suppressing the scattered neutron
background, both from elastic and inelastic scattering is essential. The
detailed understanding of the background scattering sources is required for
effective suppression. The Multi-Grid thermal neutron detector is an
Ar/CO$_{2}$ gas filled detector with a $^{10}$B$_{4}$C neutron converter coated
on aluminium substrates. It is a large-area detector design that will equip
inelastic neutron spectrometers at the European Spallation Source (ESS). To
this end a parameterised Geant4 model is built for the Multi-Grid detector.
This is the first time thermal neutron scattering background sources have been
modelled in a detailed simulation of detector response. The model is validated
via comparison with measured data of prototypes installed on the IN6 instrument
at ILL and on the CNCS instrument at SNS. The effect of scattering originating
in detector components is smaller than effects originating elsewhere.
