The CDM30 is a temperature stabilised light detector subassembly consisting of a 30 mm diameter photomultiplier (see 8, ordering information) and a housing. By maintaining the temperature to within 0.2 °C, stable photomultiplier gain and quantum efficiency are assured. Reduction of dark current, particularly with the S20 photocathode types, by up to two orders of magnitude, can be achieved. The CDM30 uses a thermoelectric (Peltier) cooler to transfer heat from the photomultiplier to a fan cooled heatsink. The photomultiplier is surrounded by thermal insulation, including an evacuated double-walled Pyrex glass optical window. The cooler and fans operate from a single +12 V input. An integral control board allows the photomultiplier temperature to be set by an on-board potentiometer and monitored using an external DVM (not supplied). Automatic over-temperature cutout is included to protect the unit in the event of fan failure or extreme restriction of the airflow ,to or from, the fans.