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. Comment: 14 pages, 8 figures. Submission to Journal of Instrumentation new version uploaded addressing referee comments (minor revision). Added discussion of radon contamination and more details on simulated spectra
