Frequently Asked Questions

Understanding Photomultipliers

The photomultiplier is an extremely sensitive light detector providing a current output proportional to light intensity. Photomultipliers are used to measure any process which directly or indirectly emits light. Large area light detection, high gain and the ability to detect single photons give the photomultiplier distinct advantages over other light detectors.

For more information see the page on Understanding Photomultipliers.

Dear Photomultiplier Doctor,

Can you please explain why photomultipliers are offered with different pin configurations. I have searched manufacturer’s published data hoping to find guidance on the merits of: flying leads, hard pin and capped, but to no avail.

Yours sincerely,

Dr James Rautenbach

Dear Dr Rautenbach,

This is a very good question.

First let me explain about the four types of pin connections that are on offer: hard pin, capped, flying leads for direct soldering to a pcb and flying leads with temporary cap for the customer to solder in place.

The majority of photomultipliers are supplied with hard pins sealed into the glass base and a matching socket. The seated height of the photomultiplier is the same as the length of the glass envelope and this may be a consideration where space is a concern.

The hard pin base photomultipliers are generally lower cost. Careful mounting on axis is important here and strain must not be exerted on the glass base of the photomultiplier, particularly when inserting and removing the tube from its socket.

Some, but not all photomultipliers, are available capped, which means we attach a cap made of a special plastic material as an extension to the base of the tube. The pins of the cap are longer and broader than hard pins and so better electrical contact is assured. In addition, the photomultiplier seats more securely in its socket.

1. The cap is opaque and makes life a lot easier when attempting to make a light-tight enclosure. Commercially available scintillation crystal assemblies are a good example of this. What you get is a photomultiplier and a NaI(Tl) crystal in a metal tube with the blue cap completing the enclosure - it keeps out the light and the atmosphere.
2. Capped photomultipliers are recommended for use in all housings because the cap removes mechanical pressure from the glass-to-metal pin seals at the base of the photomultiplier. Beware of do-it-yourself, spring-loaded housings used with hard pin photomultipliers.
3. They have lower capacitance, which is important if you have to deal with fast signals and they are usually lower cost.

Finally we turn to flying leads photomultipliers which don’t need a socket.

There are cost saving possibilities here and some reliability gains too because you can solder a voltage divider board straight on to the leads (don’t ever do this to a hard pin photomultiplier).

Photomultipliers used in satellite experiments, where the demands on reliability are paramount, invariably call for flying leads; the mechanical interface offered by the leads provides isolation against the severe shock and vibration created during launch.

We supply flying lead photomultipliers with loose fitting caps so that customer can position the cap in the assembly to achieve a length tolerance of a fraction of a mm. Note glass envelopes are hand-made and hence vary in length by a millimetre or two.

I hope this has cleared up the problem and that nothing has been overlooked.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

My problem is: I accidentally exposed the side window of a photomultiplier to ambient light during a routine maintenance. My question is:

1. Is it a guarantee that the photomultiplier will be destroyed whenever exposed to bright light?
2. What is the threshold intensity for destruction of the photomultiplier?
3. Is time of exposure a factor?
4. What is the physics behind the damage if it occurs?

yours sincerely

John Popoolade

Dear John,

High interdynode currents are generated within a photomultiplier when exposed to bright light. The electron bombardment of the dynode surfaces releases ions and if sufficient in number, breakdown will occur causing the photomultiplier to glow. Removing the bright light leaves the photomultiplier with an ion-damaged photocathode and filled with gas.

This is the worst case scenario but a photomultiplier will self-protect if the voltage divider is sufficiently resistive. If the divider current is less than 100 µA, the dynode currents will be limited to below this value and the photomultiplier is likely to survive.

A discoloured photocathode is a sure sign of damage and spiky signal traces are also indicative of an ex-photomultiplier.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

I would like to know if it was possible to extend the shelf life of photomultipliers by storing them under a gas with a low He content.

Air has typically about 5.5 ppm He. I wondered if standard bottled N2 had significantly less He than this (say less than 1 ppm). I was thinking of storing tubes before use for about 5 years.

Do you know if anyone had tried these techniques or has details of the He content of standard industrial gasses.

Regards,

Paul Flower
Rutherford Appleton Laboratories UK

Our standard glass photomultipliers can be stored in normal atmosphere even over the extended period described without significant permeation of helium. To avoid oxidation of the pins over this extended period it is however advisable to store the photomultipliers in dry air.

If the photomultipliers have a quartz window there will be sufficient helium permeation over this extended period to be measurable in the afterpulse spectrum. The photomultiplier will continue to function because helium does not react chemically, but the electron currents flowing inside the photomultiplier will cause temporary ionisation of the helium during operation. In pulsed light applications, the impact of these ions on the active surfaces produces late pulses around 0.3 µs after the initial pulse. In dc or slowly varying light applications the helium ionisation will cause a small percentage increase in the measured signal.

So for this extended period it is recommended that quartz photomultipliers are stored in dry nitrogen at normal atmospheric pressure in an air tight container (really helium tight). Any sealed container (usually metal, standard borosilicate glass or pyrex glass containers that can be sealed are suitable).

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

I have an old photomultiplier. How can I easily find out if it is in working order?

Apart from doing a visual test, there are no other tests other than electrical ones. Sometimes from mishandling, the vacuum within the photomultiplier is compromised, in which case it will not work. When you look at the window of the photomultiplier, it should be difficult to see the internal structure because the photocathode appears a yellow/brown colour. If this is not the case and the window looks clear then the photocathode may be lost.

If the photomultiplier passes visual inspection then apply the HV slowly (making sure you use the correct polarity) and look on a scope for negative pulses. There should be sufficient background counts for this purpose. Do not exceed the maximum HV on the test ticket (usually less than 1300V).

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

I have used EMI 9524S photomultipliers, which has given excellent performance of low and stable dark counts, even after several TLD measurements where the temperature of the photomultiplier housing is bound to increase due to repeated heating. Which photomultipliers do you currently recommend?

Dr. Ganesan Ramanathan
National Physical Laboratory

Dear Dr. Ramanathan,

The potassium bialkali photocathode has replaced the S11 type used in the 9524 photomultipliers you have used. This has even lower, more stable dark counts (as low as 10 counts per second) and is largely insensitive to temperature change up to about 40°C.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

I have some questions about Photomultiplier 9111B80.

1. What is the available operating range of the input light power? We have two laser sources: one of them is Helium-Neon laser which has 4 mW maximum output power and operates at 594 nm; another one is Argon Ion laser which has 500 mW maximum output power and operates at 457~514 nm. Can I use these lasers directly as input light?
2. If the input light power exceeds the available operating range, which causes count overflow, will this result in physical damage to the module?

Wei Wei Wang

Dear Ms Wang,

Please study the photocathode spectral responses given in our brochure. You will note that the bialkali cathode has a response of 6 mA/W at 594 nm and about 60 mA/W for Argon Ion. Note 4 mW of laser power will give a photocurrent of 6 x 10^-3 x 4 x 10^-3 = 2.4 x 10^-5 amps but for the argon ion the cathode current is 30 mA.

The answer to your question is: no you can't use these to view the lasers. Note that pmts are designed to measure nA or pA of cathode current. You are in danger of doing damage to the photomultipliers.

If these lasers are pulsed then the answers may be quite different based on the pulse duration.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

Can you please resolve a dispute I am having with a physicist at work. The problem concerned having too much signal from the photomultiplier, which I remedied by attenuating the light input with a neutral density filter. He said that I should turn the gain of the photomultiplier down rather than attenuate the light input but I think the two options should produce the same result.

Yours sincerely,

S C Jamieson

Dear Mr Jamieson,

The physicist is right - don't attenuate the light. The information in the light signal is statistical in nature and by attenuating the signal you throw away information. This is always true, regardless of the statistics that govern the behaviour of the light source, and has the effect of increasing the noise in the signal. For light sources that obey Poisson statistics, the signal-to-noise is proportional to the square root of the number of photoelectrons detected so do what ever you can to detect more light.

Whatever you do, never throw away light.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

I have a 100 µJ laser which operates at 500 nm. How many photoelectrons will this produce from a typical photomultiplier?

yours sincerely

Cedric de Lange

Dear Cedric,

Far too many. We use the Einstein relationship: E = hc/λ, where h is the Planck constant and c is the speed of light, to calculate the energy of one photon at this wavelength.

E = 6.6 x 10^-34 x 3 x 10⁸ / (500 x 10^-9 )

E = 4 x 10^-19 J

100 µJ produces n photons where

n = 10^-4 / ( 4 x 10^-19 ) = 2.5 x 10¹⁴ photons

As the quantum efficiency of the popular photocathodes is around 10% at this wavelength we have 2.5 x 10¹³ photoelectrons. Note that in 1 - 10 MeV, high light level scintillator applications we seldom get more than about 104 to 105 so your light levels are really high. You need to estimate what proportion of the laser light hits the photomultiplier. If it's anywhere near unity, then please speak to us about a replacement photomultiplier.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

The direct o/p of my 9558QB photomultiplier, without amplification, is an ac component with approx a 200mV dc offset. Is this right? It means I cannot use a dc coupled amp as it saturates.

Using a slower amp, measurements can be done with typical PL measurements giving a few mV of signal to a lock in using an amp with 103 amplification (this amp must be ac coupled).

It sounds like you have a light leak. You can check by doing any of the following and checking the output again:

1. Throw a cloak over the photomultiplier.
2. Switch off the lights.
3. Wait until dark.
4. Check you are not operating the photomultiplier above the recommended voltage as given on the test ticket.

I have come across this before because it is actually quite difficult to make a light tight enclosure. If this is not the problem then maybe the photomultiplier has developed an internal leak path.

If you can't resolve the problem let me know and we can arrange for you to send the photomultiplier back to us for retest.

Regards,

Photomultiplier Doctor

I ordered a 9112B together with a voltage divider C6007AFN2. I am not sure whether to regard the output signal as a current or voltage. What is the difference between having an anode load or no anode load as described in your brochure?

Regards,

Ms Libei

Dear Ms Libei,

The photomultiplier is a current generator and if you want a voltage signal then you must allow the current to flow in a resistor.

You can either connect the anode output to a resistor to ground and pick off the signal with an oscilloscope if you just want to look at the output for initial tests. Or you can connect the anode to the input of an op amp wired in the transimpedance mode. In this case you have a feedback resistor across which you can connect a capacitor to set the bandwidth you want. The op amp has a low output impedance so you can drive other electronics from it.

We have recently produced a technical paper on 'The use of amplifiers with photomultipliers' RP094 in our technical paper series. You can dowload a copy, in PDF format here.

Regards,

Photomultiplier Doctor

Dear Photomultiplier Doctor,

Whilst cooling my extended red response photomultiplier I can decrease the dark count (aka background) but how do I keep the signal from vanishing?

Ganesan Ramanathan
National Physical Laboratory

You must cool as little as possible and use the photomultplier with optimal signal/background ratio.

The change in photocathode sensitivity with temperature depends on the photocathode type and the wavelength of incident light. Note how the variation is greatest at the long wavelength limits of sensitivity. The loss in red sensitivity should be noted whenever cooling is employed to reduce dark counts. The temperature coefficient of the photomultiplier is the combination of the change in photocathode sensitivity and electron multiplier gain, e.g. +0.4%/ °C for the S20 at 900 nm.

<Temperature coefficient of photocathode sensitivty graph image>

It is never a good thing to be operating a photomultplier where the QE is a fraction of a percent - this is where the temperature coefficient is very large. You are unfortunately up against fundamentals of physics. Maybe you need to consider some light chopping.

However, we do offer photomultipliers with prisms on the inside of the window to bend the incident light to enhance the QE for this purpose. If your light is parallel and of diameter less than 4 mm then we can also offer an external optical enhancer that would work. This gives from 2 to 4 times increase.

Regards,

Photomultiplier Doctor.

Why do I get negative values when photon counting? I am using a photon counting package (P25P-06) with a CT2. I am detecting at 200 nm wavelength and getting negative values. What is the problem?

There are a few reasons for your system not working:

1. If you have an air path between light source and photomultiplier window then maybe the light is being absorbed before it reaches the photomultiplier.
2. The strength of the light might be so high that the electronics is saturated.
3. The photomultiplier has lost its vacuum. The window should have a yellow colour. If it is clear then this indicates loss of vacuum.

The customer advised that counts were measured without the light source and confirmed that it was the extremely high light levels that totally overloaded the package electronics. This resulted in spurious results.

This scenario highlights how sensitive photomultipliers are at detecting light. This is compounded by the fact that the human eye cannot even make a judgement as to the brightness of the light.

Regards,

Photomultiplier Doctor

In the choice of suitable photomultipliers for photon counting, photomultipliers with Venetian blind structures have been found to be not suitable since they do not show a good SER pulse height distribution. The reasons attributed for this is the possibility of some electrons not undergoing the full multiplication and get transmitted to the final stages. One way to improve the response is to connect a pulse squaring circuitry which will enhance the light induced pulse amplitudes while reducing the noise or dark pulse amplitudes. What would you recommend?

Dr. Ganesan Ramanathan
National Physical Laboratory

Dear Dr. Ramanathan,

Yes, Venetian Blind photomultipliers do not have good SER distributions. For this reason focused dynode structures are now commonly used instead. These have been designed with the aid of electron optical simulation software to have high collection efficiency dynodes with low losses between stages. Because multiplication spread is governed by Poisson statistics a high first dynode gain improves the single electron peak allowing better discrimination between real signal and small noise pulses. The Venetian Blind structure also suffers from space charge saturation with larger pulses and the superior linear focused structure has largely replaced it in recent years.

Regards,

Photomultiplier Doctor.

Dear Photomultiplier Doctor,

In the optimisation of discriminator bias setting, we have found that if the bias is set just beyond the knee of the integral dark pulse-height distribution, it gives better stability. Do you agree?

Dr. Ganesan Ramanathan
National Physical Laboratory

Dear Dr. Ramanathan,

The meaning of the term "knee voltage" is not universally agreed but I can tell you about ET Enterprises' approach. Taking a bent human leg as an example, where the knee corresponds to the highest rate of change of slope, should we really operate at this point on the HV vs count rate curve. Certainly not, because this is such an unstable point on the curve. A small shift in HV or a gain change will bring about a considerable change in count rate. It is preferable to operate to the right of this point where the slope of the curve is much lower.

Our recommendation is to operate at a point on the plateau where the slope first attains 0.1% per volt. This is typically 100V higher than the knee. For more information on this subject please refer to our technical paper "Optimal use of photomultipliers for chemiluminescence and bioluminescence applications" - RP090.