FAQs | Photomultiplier tubes (PMTs)

1. How is a P-type PMT different from a normal PMT?

There is no difference in the design and manufacturing between a P-type and non-P-type PMT. P-type is a selection that will have a low dark count rate and sometimes higher gain. This selection makes a P-type PMT ideal for photon counting applications.

2. What do the different module types and assemblies include?

Current output type: These modules consist of a PMT, a voltage divider circuit, and a high voltage power supply. Due to this, they only need low voltage (±5 V to ±15 V) for operation. These can come in a P-type, where the PMT inside is selected for photon counting, and can be used with a photon counting circuit but requires additional electronics.
Voltage output type: These modules are the same as above but include a current-to-voltage amplifier. Because the amplifier is already built-in and the fact that you need specific amplifiers for photon counting, we typically do not have “P” designations for these modules.
Photon counting type (Photon counting head): They have a P-type PMT with a voltage divider socket, a high voltage power supply, and photon counting electronics. The output of these modules are logic pulses, which can go right into a counter.

3. What is the difference between +HV and -HV divider circuits and when do I use them?

The most common technique used for voltage divider circuits is to ground the anode and apply a large negative voltage (-HV) to the cathode. This scheme eliminates the potential voltage difference between the photomultiplier tube anode and the external circuit, making it easier to connect to an ammeter or current-to-voltage conversion amplifier. One concern is if a grounded metal holder, housing, or magnetic shield case is brought near to or touches the glass bulb of the photomultiplier tube. This could cause electrons in the photomultiplier tube to be attracted toward the ground potential near the bulb and strike the inner wall of the bulb. This may produce glass scintillation, resulting in a significant increase in noise.

This ground cathode scheme uses a coupling capacitor to separate the positive high voltage applied to the anode from the signal, and it cannot extract a DC signal, so it is only used for pulse output.

4. What are the different socket types and what do they include?

D-type socket assembly: Built-in voltage divider circuit.
DA-type socket assembly: Built-in voltage divider circuit and amplifier.
DP-type socket assembly: Built-in voltage divider circuit and high voltage power supply.
DAP-type socket assembly: Built-in voltage divider circuit, amplifier, and high voltage power supply.

5. How do I calculate bandwidth from rise time? And vice versa?

The required frequency bandwidth can be calculated from the rise time of the anode output, as follows:

\[ \mathrm{Frequency (MHz) = 350 / Rise time (ns)} \]

The frequency bandwidth calculated here is the frequency at which the amplifier gain decreases by 3 dB, which is a commonly used definition.

6. What is the difference between dark current and dark count?

Dark current is the analog current output in a dark environment from a PMT caused by thermionic emission from the photocathode and dynode surface; leakage current (ohmic leakage) between the anode and other electrodes inside the tube and/or between the anode pin and other pins on the bulb stem; scintillation from the glass envelope or electrode supports; field emission; ionization of residual gases (ion feedback); and scintillation from glass caused by cosmic rays, radiation from radioisotopes contained in the glass envelope, and environmental gamma rays. Dark counts are dark events that are measured when a PMT is operated in photon counting mode, which would typically only be caused by thermionic emission, field emission, and glass scintillation.

Depending on the discriminator that is set, ionization of residual gases (ion feedback) may also be counted as a dark count component (afterpulse component).

7. How do I convert between QE and sensitivity?

Quantum efficiency (QE) is the ratio of the number of output electrons generated to the number of incident photons. Radiant sensitivity is the ratio of photoelectric current to incident radiant flux. We can convert between the two using the factor of photon energy in electron volts, which can be estimated using the equation shown below.

\[ QE = \left( \frac{S \times 1240}{\lambda} \right) \times 100 \]

QE: Quantum efficiency (%)

S: Radiant sensitivity (A/W)

λ: Wavelength (nm)

8. What are some common customizations we can do?

Possible customizations are adding connectors to power inputs and signal outputs for sockets and modules. Additionally, we can offer custom divider circuits for bare PMTs and amplifier modifications for modules.

9. What are some common selections Hamamatsu can make?

Common selections that can be made are for dark counts, dark current, gain, and sensitivity. The feasibility is determined by the specification for selection and is evaluated on a case-by-case basis.

10. What power supplies should I use (switching vs. non-switching)?

When selecting a power supply, the ripple noise of the supply is an important characteristic to consider. Ripple noise from a power supply can be seen at the output of detectors, leading to lower SNR. Due to this, we recommend selecting power supplies with low ripple noise.

In view of the need for an ample margin of current carrying capacity, select a high voltage power supply whose current carrying capacity allows outputting a current 1.5 times or more than the current flowing through the voltage-divider circuit. The high voltage output might drop if there is no extra margin in the output current.

11. What is a HA treatment?

HA treatment is a technique of applying conductive paint around the outside of the PMT bulb and connecting it to the cathode potential. HA treatment is effective in reducing a noise increase caused by the surrounding electric potential.

Since a negative high voltage is applied to the outside of the bulb, the whole bulb is covered with an insulating cover (heat-shrinkable tube) for safety.

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FAQs | Photomultiplier tubes (PMTs)

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1. How is a P-type PMT different from a normal PMT?

2. What do the different module types and assemblies include?

3. What is the difference between +HV and -HV divider circuits and when do I use them?

4. What are the different socket types and what do they include?

5. How do I calculate bandwidth from rise time? And vice versa?

6. What is the difference between dark current and dark count?

7. How do I convert between QE and sensitivity?

8. What are some common customizations we can do?

9. What are some common selections Hamamatsu can make?

10. What power supplies should I use (switching vs. non-switching)?

11. What is a HA treatment?

Contact us for more information.

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