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Which Photomultiplier Tube do I Choose? |
With all the choices, it can become confusing deciding on the appropriate PMT for your measurement application. Below we describe the options. If you need further help, or want to validate your choice, please call a Sales Engineer for guidance. |
Side-on Photomultipliers |
These detectors are more economical than end-on models, have the fastest risetimes and are ideal for photometry and spectrophotometry applications. Their vertical configuration takes up less space than the end-on versions. They mount in the standard or pulsed housings. The main disadvantage of these tubes is their non-uniform sensitivity (refer to Fig. 3). However, the model 77360 Side-on PMT has a good uniformity of response, similar to the end-on tubes. |
End-on Photomultipliers |
These detectors have a large photocathode area, and greater uniformity. When mounted in the 70686 Pulsed or 77265 Standard Housings, they couple directly to our 1/8 m or 1/4 m monochromators without the need for a focusing lens. |
Pulsed or DC Operation |
All our PMTs can be used in a pulsed mode or DC. The housing determines the operation. Typical risetimes and electron transit times (the delay due to the finite time needed for electrons to transverse the multiplier chain) for all PMTs are listed in the specification table. These times are a function of bias voltage and/or the voltage divider network. They get shorter at higher bias voltages. You need to balance the desire for fast response with the increased noise levels at high biases. |
Which Housing Do I Choose? |
We offer two types of housings for both end-on and side-on PMTs. |
PMT Housings for Low Noise DC Operation |
For lowest noise DC operation, use the 70680 Housing for side-on tubes or the 77265 Housing for end-on models. These housings are also the best for short pulse measurements (up to 10 µs long). You can obtain 1 mA level outputs from most of the tubes, in response to short pulses, which will provide easily measurable 50 mV readings when 50 ohm terminations are used. |
PMT Housings for Long Pulse Operation |
The 70685 and 70686 Housings allow you to obtain 100 µA level pulse responses which can also be converted to 50 mV or higher readings with larger load resistors, the nanosecond response times not being critical for longer pulses. These housings also function well in DC applications. |
Tech Note |
PMTs can be damaged by exposure to excessive light levels in the presence of bias voltage. This is due to anode damage by high current densities. Opening your optical set-up while the PMT is biased can easily lead to a need for an expensive replacement tube. Please note the maximum DC anode currents in the Ordering Information Table (averaged over any 30 time interval), and make sure they are not exceeded in your standard set-up. For pulsed applications you can draw up up to 50x higher currents for short times, as long as average current ratings are not exceeded.
The photocathodes of PMTs are very light sensitive. Their noise behavior is negatively affected by exposure to strong light, even in the absence of bias voltage, and may take some time to recover once they are screened from light. Typically, more than half an hour is required for recovery. You may need several hours to return to the lowest noise levels. |
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Fig. 1 Typical Responsivity of Photomultiplier Tubes at the Bias Supply Voltages listed in the Ordering Information Table.
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Fig. 2 Typical Responsivity of Photomultiplier Tubes at the Bias Supply Voltages listed in the Ordering Information Table. Models 77343 and 77361 come with a borosilicate glass window which limits their usefulness below 400 nm.
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Fig. 3 Uniformity of Response for Side-on and End-on PMTs.
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