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.
We have a few different types of PMT modules.
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. However, 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 ~6uA to ~50uA. 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 ~6uA to ~50uA. It takes ~150 ms for the protection circuit to respond; that is turn off the power to the PMT due to excess light input that caused the output current to rise above the threshold (~6uA for the standard type and ~50uA for the A type).
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!
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.
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.
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.
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 before...in 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...)
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