PMT Questions & Answers

When looking at your PMTs, I see that there are some “P” types and regular types. What’s the difference?

Well, I’m glad you asked. The P (like in R1924P) stands for “photon counting” type. When producing PMTs, you can get a typical production distribution for a number of different PMT characteristics. Two characteristics that are specifically important for photon counting applications (where you use a photon counting circuit and a counter) are gain and dark counts. The “P” type PMTs are selected out of the distribution for higher than average gain and lower than average dark counts. The high gain helps you set your threshold level for the lower level discriminator in the photon counting circuit. The low dark counts help you achieve good signal-to-noise ratio (SNR) even at low photon levels (see the equation in a previous Q&A). So really, the difference is that “P” types are selected out of the distribution for good photon counting characteristics.

Thanks for the info. What about the PMT modules? I heard that there’s a P type module and even a PA type module. What’s the deal?

We have a few different types of PMT modules.

  • Current Output Type: This is a PMT and a voltage divider circuit with a high voltage transformer. So it only needs low voltage (5-15 V) for operation. These can come in “P” type where the PMT inside is selected for photon counting as mentioned above. These can be used with a photon counting circuit.
  • Voltage Output Type: These modules are the same as above and also have a current-to-voltage amplifier. Because there’s an amplifier already built-in and you need specific amplifiers for photon counting, we typically do not have “P” designations for them.
  • Photon Counting Type: These PMT modules are the whole kit and caboodle! They have the PMT (P Type) with the voltage divider socket and the photon counting circuit. The output of these modules are logic pulses which can go right into the counter! The added bonus is that the gain/voltage of the PMT is already set for perfect photon counting. No adjustment is needed; you can just start counting!

Wait a minute, what about the PA type I hear so much about?

Man, you’re sharp! For some modules, specifically modules with a GaAsP/GaAs photocathode, we have what’s called “PA” type. The P still stands for photon counting as above. The extra A stands for AWESOME. Just kidding, the extra A is a designation that the protection circuit in the module has been changed. The protection circuit monitors the average signal current from the PMT. The GaAsP/GaAs photocathodes are particularly sensitive to high light levels. The protection circuit is put in place so that when the signal output current reaches a specific value (which relates to the amount of light on the photocathode), the PMT voltage shuts down or an alarm goes off.


More specifically, once the PMT current reaches 5-10 µA for a time period of ~100 µs, the protection circuit is triggered. It takes ~150 ms for the protection circuit to turn off the power to the PMT. Once this has occurred, it takes ~5 seconds for the module to recover. Turn the module off and back on to reset the protection circuit and use the PMT again.


We find for some applications (like multiphoton microscopy), it is OK for there to be sudden (and few) high peaks of current. In these cases, we increase the protection circuit current value of the PMT module from 5-10 µA to ~50 µA. We do this because while some sudden pulses may be high, the average current is still low, and therefore the PMT is still safe. So, the H7422PA-40, for example, is a PMT module with an increased protection circuit from 5-10 µA to ~50 µA.


Also, these modules with protection circuits can be modified to not include the protection circuit at all. Please inquire with our engineers to learn more about these options!

I see Hamamatsu offers associated components for photomultiplier tubes (PMTs). In particular, what is the difference between D-type and DP-type socket assemblies for PMTs?

D-type sockets contain a resistive divider network needed to distribute the applied high voltage across the PMT dynodes. DP-type sockets also contain this same resistive divider network, in addition to a high voltage power supply built into the socket that transforms low voltage to the high voltage required for the PMT. This allows users to operate the PMT using 5 or 15 volts depending on the model.

Can I use any socket assembly with my PMT?

Not every socket is the same! Each PMT has corresponding compatible socket assemblies. The matching socket assembly, if available, for any PMT can be found listed in our photomultiplier tubes catalog. Also, each PMT may have multiple socket assemblies available for a desired characteristic: for example, the divider may be different for high pulse linearity vs. DC linearity, etc. If you need any help selecting, please feel free to give us a call.

OK, so Hamamatsu offers PMTs and also corresponding sockets to pair with them. Are there any offerings that include a PMT and these associated components all in one package?

Why yes! Hamamatsu also offers PMT assemblies and modules. A PMT assembly integrates both a Hamamatsu PMT and resistive divider network (D-type socket assembly) into one package. They can often come with HA coating, magnetic shielding, or cases or connectors which house the PMT and socket assembly together.


All PMT modules will contain, at least, a PMT, resistive divider network, and high voltage power supply in one package. In addition, there are some module offerings that may also include amplifiers, thermoelectric cooling, and PC interfacing.

If you have a technical question you’d like to see answered on this page, email us.

Meet the engineers

Eric Mesa is an Applications Engineer out of NJ, who understands the intricacies of signal-to-noise comparisons. He likes baseball and most other sports, and when it comes to detector selection, he always hits a home run!

Kate Pritchard has gone where no applications engineer has gone managing the newly formed University Support Group. The group is dedicated to supporting our research and academic customers through technical support, collaborations, and face time. Traveling at the speed of light to explore strange and exciting new worlds for Hamamatsu! (See what I did there...)