Products for Research Inc.

COOLED PHOTOMULTIPLIER HOUSING WINDOWS
Figure 1: Heat Throughput of Single and Double Pane Window
Figure 2: Standard Window Dimensions
Figure 3: Window Transmission Characteristics

Products for Research cooled PMT housings are supplied with an evacuated double pane window to insure a light path free of condensation or ice particles. These window cells are assembled using proprietary technology and state of the art materials, and cryogenically evacuated to a pressure below 5*10Torr.
Single pane windows are standard only for PMT coolers where the differential in temperature across the window is 20°C or less. Single pane windows can be provided to special customer request or for the needs of specific applications such as chemiluminescent reaction detection.
In normal laboratory use, the outer surface of the input window of a photomultiplier cooler should remain at a temperature above the ambient dew point. At temperature differentials greater than 20°C, a single pane window must be heated to keep its outer surface above the dew point.
It can be seen from figure 1 that the heat throughput of a single pane window at a 20°C differential is not reached for a double pane window until the differential is almost 50°C. Standard Products for Research double pane windows operating at temperature differentials less than 50°C do not require heating in normal laboratory environments.
A portion of heat applied to the outer surface of the light input window of a cooled photomultiplier housing will pass through the window and contribute to nonuniform cooling of the photocathode. If the average temperature of the outer surface of the window is maintained at +20°C in a photomultiplier cooler with an internal temperature of -30°C, a differential of 50°C, the resulting heat throughput into the cold interior for the Products for Research double pane insulating window is less than half the heat throughput for a single pane window, as can be seen from figure 1. For larger differentials, the difference is clearly more significant.

Figure 1
Heat Throughput of Single and Double Pane Window
as a function of Temperature Differential Across the Window
Heat Throughput of Photomultiplier Cooler Window
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Since the structural cylindrical barrel of the Products for Research evacuated double pane window cell provides an additional thermal path, tests were run at Products for Research to determine optimal dimensions for these barrels. The results of these tests were used to define standard window cell dimensions. The Products for Research standard window cell dimensions are shown in figure 2.

Figure 2
Standard Products for Research Evacuated Double Pane Window Dimensions

Drawing of Large Evacuated Window for Photocool Photomultipler Coolers Drawing of Small Evacuated Window for Photocool Photomultipler Coolers

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Products for Research standard evacuated window cells are available with panes of Plexiglas *, pyrex and fused silica. Standard Fabry lens evacuated window cells are also available in pyrex and fused silica. The transmission characteristics of these standard evacuated windows are shown in figure 3. In addition, custom window cells can be fabricated using customer specified filters or lenses or combinations.

Figure 3
Standard Products for Research Double Pane Window Transmission Characteristics
Spectral Transmission of Photocool Photomultiplier Housing Evacuated Windows
* Registered trade name of Rhom and Haas Corp.
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