  Thermal neutron detector based on films of $^{10}$B$_4$C have been developed
as an alternative to $^3$He detectors. In particular, The Multi-Grid detector
concept is considered for future large area detectors for ESS and ILL
instruments. An excellent signal-to-background ratio is essential to attain
expected scientific results. Aluminium is the most natural material for the
mechanical structure of of the Multi-Grid detector and other similar concepts
due to its mechanical and neutronic properties. Due to natural concentration of
$\alpha$ emitters, however, the background from $\alpha$ particles
misidentified as neutrons can be unacceptably high. We present our experience
operating a detector prototype affected by this issue. Monte Carlo simulations
have been used to confirm the background as $\alpha$ particles. The issues have
been addressed in the more recent implementations of the Multi-Grid detector by
the use of purified aluminium as well as Ni-plating of standard aluminium. The
result is the reduction in background by two orders of magnitude. A new
large-area prototype has been built incorporating these modifications.
