In a real material the surrounding atoms relax towards a vacancy owing to the removal of the repulsive core of the interatomic potential of the missing atom. This systematic disturbance of the interatomic spacing in real space must have a diffraction signature in reciprocal space. A first-principles calculation of the (100) peak from a simple-cubic nanoparticle of 100,000 atoms shows that, as the defect density increases, the peak broadens and shifts to larger d-spacing. This proposal is to investigate two related phenomena in the model hydrogen storage alloy LaNi5: (i) the reported formation of a massive vacancy density; (ii) the possible trapping of hydrogen at vacancies. The experiment will help us to understand the origins of pressure hysteresis, which is significantly affected by aging at temperatures where vacancies are mobile but dislocations are stable.