Alpha Meson

Alpha Meson appears to be a type of energy.

Some of its known uses are as "alpha meson phosphor", the presence of which can provide a light source throughout a building without the need for bulbs or openings to natural light. (DW: Tomb of the Cybermen).

River Song carried an Alpha Meson pistol, which was capable of disintegrating a wooden cabinet and later destroying a weakened Dalek. (DW: The Pandorica Opens/The Big Bang) It was also capable of producing a flame similar to those on welding gear that can melt metal (DW:The Time of Angels).

It should be noted that in Tomb of the Cybermen, it was pronounced "May-zon", while in the Big Bang it was pronounced "Mee-zon". This may be a personal preference, or a localisation as with other known elements within one language. For example, methane is pronounced "mee-thane" in some countries including the UK, while it is pronounced "meh-thane" in other countries such as the US.