The tritium in a gaseous tritium light source undergoes beta decay, releasing electrons which cause the phosphor layer to fluoresce. During manufacture, a length of borosilicate glass tube which has had the inside surface coated with a phosphor-containing compound is filled with the radioactive tritium. The tube is then fused with a CO2laser at the desired length. Borosilicate glass is used for its strength and resistance to breakage. In the tube, the tritium gives off a steady stream of electrons due to beta decay. These particles excite the phosphor, causing it to emit a low, steady glow.