The M67 sight is illuminated by what substance?

The M67 sight is illuminated by a self-contained, radioactive tritium gas. This substance is chosen for its properties that allow it to provide a consistent and reliable source of illumination in low-light conditions without the need for external power sources or batteries. Tritium is a hydrogen isotope that emits beta particles and can be used in luminescent paints. When integrated into the sight, it allows users to see aiming points clearly at night or in dark environments, enhancing visibility and operational effectiveness for infantry soldiers using the mortar system.

Other gases like argon, neon, and helium do not possess the same radioactive properties and would not provide the necessary self-luminescence that tritium offers, making them unsuitable for this specific application. Thus, tritium remains the optimal choice for illumination in the M67 sight.