A nuclear density gauge has an elongated transmission chamber adjacent to an elongated source chamber so that they can both be installed through a single nozzle on a high pressure vessel, making an airtight seal with the nozzle. The shape and position of the source chamber allows the positioning of a radiant energy source inside the vessel, a distance from one end of the transmission chamber. The radiation emitted by the radiant energy source travels through contents of the high pressure vessel and then through the elongated transmission chamber to a detector. The method of use of the gauge or multiple gauges, and the adaptation of vessels for such gauges, are also disclosed.