Products for Research Inc.
About PMT Housings
THERMOELECTRIC COOLING OF PHOTOMULTIPLIER HOUSINGS
|Cooler Temperature Performance Guide|
|Simply stated, thermoelectric cooling or Peltier cooling is based on the phenomenon of cooling or absorption of heat at the junction of two rods of metal or semiconductor when a current is made to pass through them. In order for thermoelectric cooling devices to be effective, the absorbed heat as well as the heat generated as a result of IR losses must be removed from the hot side of the device. Air or liquid heat exchangers are used in Products for Research housings to accomplish this.|
Liquid heat-exchanged thermoelectrically cooled PMT housings typically use water, either by means of a recirculating liquid heat exchanger, or tap water flowing into an open drain to remove heat from the PMT cooler. Other non-hazardous liquids may also be used.
Under ideal conditions with no heat load and in vacuum, single stage thermoelectric devices are capable of producing temperature differentials as great as 70°C. In real situations, however, there is a heat load. In Products for Research housings, this load is principally the result of dynode chain dissipation and heat transferred through the housing case and insulation. Accounting for this load and the gradients that exist at the various interfaces between the photomultiplier and the heat exchange medium, all Products for Research thermoelectric housing specifications list the guaranteed temperature differential between the cooling medium (air or liquid) and the photocathode of the photomultiplier.
Using identical systems, it is possible to create an apparent improvement in this temperature differential by cooling only the region immediately adjacent to the photocathode instead of the entire tube. This technique reduces the heat load because of the poor thermal conductivity of the glass. Products for Research uses this technique only when the temperature differential over the length of the photomultiplier will not exceed 20°C. A greater differential could result in significant migration of the chemicals
|employed in the photoemitting or secondary emitting surfaces and thereby disrupt the extremely delicate balance necessary for high cathode efficiency and low dark current. Extreme temperature differentials can eventually lead to total deterioration of tube characteristics.
Multi-stage Peltier cooling further extends the capabilities of Products for Research PMT Coolers. By combining greater insulation with appropriate multi-stage thermoelectric devices, the Products for Research Multi-Stage series includes Photomultiplier Coolers with temperature differentials of 50°C, 60°C and 70°C below the temperature of the coolant medium.
All Products for Research thermoelectrically cooled housings are supplied complete with the necessary low voltage power supply. Thermal overload protection is provided in those housings in which the heat sink temperature could rise above that sufficient to damage the thermoelectric module if the liquid or air flow should be turned off.
The tube socket assembly supplied with thermoelectrically cooled housings is directly interchangeable with tube socket assemblies supplied with any of the housings of the same class or for any tube which fits into those housings. Products for Research socket assemblies are "O" ring sealed to insure that no moisture is fed into the housing through them.
Products for Research cooled housings, including thermoelectrics, are not simply coolers of ambient temperature housings but rather are unitized housings designed for the sole purpose of cooling the tubes installed in them. This insures the integrity of the hermetic seal and provides long term reliable service even under the most severe conditions of usage. In addition, when the cooling mechanism is not used, cooled housings function excellently as ambient temperature housings.
|Cooler Temperature Performance Guide with links to POWER Index|
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