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| Products for Research Inc. |
CONTENTS (HOME)
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TO COOL OR NOT TO COOL |
Figure 1: Photocathode Dark Current vs. Temperature Figure 2: Photocathode Responsivity vs. Temperature Figure 3: Cooler Temperature Performance Guide |
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| The question is often raised whether or not a cooled photomultiplier housing is necessary. Armed with certain information, even the casual user of photomultipliers can answer this question and in fact determine the optimum operating temperature for the tube in the user's application.
The first bit of information necessary is the spectral region being observed. This will frequently dictate the tube to be used or at least minimize the number of choices to be made. All tube manufacturers publish spectral response curves for the wide variety of tubes they make, thereby making the photocathode selection fairly straightforward. | The second bit of information required relates to signal level. Simply stated the question is "Will the expected signal level exceed the expected dark current at room temperature?" The answer may come through experiment, past experience, or any of a number of other sources. Once answered, however, this question leads to the decision to cool or not to cool.
If the choice is to cool, the next question is how much cooling is the optimum. To answer this, two items must be considered. The first is the effect of cooling on dark current. The second is the effect of cooling on spectral response in relation to the region being observed. |
| Figure 1 gives typical cathode dark current density as a function of cathode temperature for some of the more common photocathode materials. To find the approximate anode dark current to be expected, multiply the current density by the cathode area and then by the overall gain at which the tube is being run.
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| Figure 2 is from studies done at Products for Research which show the effect of cooling on spectral response and give information that will aid in determining whether a specific amount of cooling will enhance or degrade a particular region of the spectral response curve. This information is available on request, and is included in the latest Catalog.
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Figure 3 is a reference table indicating the approximate typical temperatures at the photocathode of a tube being cooled by housings in the various series' of Products for Research PMT coolers.
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| Cooler Temperature Performance Guide with links to POWER Index | ||||||||||||||||||||||||||||||
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Marked PHOTOCOOL
