PMT operating voltage, who knew?

I wonder if the voltage across the first stage or three (near the photocathode) dominate the s/n. Later stages probably just amplify, so maybe you could crank them up.

What sort of dark count rates were you seeing?

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The power supply is a all in the socket thingie made by Hamamatsu. It's a Ccokroft Walton ladder so I can't really change the divider ratio. The maximum count rate with light one is ~10k/sec. The dark count is all over the map, 3k to 10's. If I pick the discriminator setting that gives the best ratio (~3-4k light counts per second.) Then dark counts are about

(Like I said I picked a noisy pmt off the shelf.. the range of reported dark currents was from 5nA to 0.05nA I haven't looked at one with a low dark current.)

The only thing I could find on the web that hinted at this is a graph of dark current vs voltage by hamamatsu... It showed a dark current 'turning on' at ~400V.

It's not really a problem or anything, I just never knew... and if someone was digging for a signal in the noise then it would be useful.

George H.

Interesting plot!

Looks like you have a lot of ion events, or maybe light emission being piped up the walls of the tube to the PC. Reducing the electron energy and the average dynode current will reduce both.

Cheers

Phil Hobbs

What that a metric "tad" or an imperial "tad"?

Dark current is supposed to be thermal electrons boiling off the photocathode, so presumably higher voltage increases the chances these get away from the photocathode and start into the multiplier chain. So, it doesn't surprise me at all that you got this result. The PMT makers do sometimes show different division ratios to optimize the tubes for linear vs. photon counting applications.

Jon

He maybe?

Are the tubes old?

Yeah maybe... looking at the 'scope and low rate dark counts there are a number of 'big' events. (maybe 1/3?) (I can see why you might want an upper window level on the discriminator.) I need some MCA...

or maybe light emission being

I'm not sure what you mean... light from the tube base/socket going up the side of the PMT? There was a lot of effort put into keeping light out of the PMT space. (work not done by me, I do know that some BNC sockets are better than others, and then we paint the ends with epoxy. And shouldn't stray light 'dark count' look like real light in terms pulse height?

I'll have to look at a low noise pmt. (tomorrow should be a quite day.) George H.

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The discriminator is set with a ten turn pot and dial. I set it at ~0.01 turns, right at zero you start hitting the noise... Well it's not really all noise, there's some ~1 MHz interference getting in too... anyway right at zero the 'dark count' doubles.

George H.

Yeah I used 'good' pmt's with coolers built in to keep down the dark count... I never looked at how the rate changed with voltage. There are certainly speed advantages that come with higher voltage, but you don't always care about speed.

George H.

(Turning down the voltage is certainly easier than a cooler.)

gy

No. Dark count data on box had a 2012 date.

George H.

Electrons that hit the glass envelope sometimes give rise to light that gets guided in the envelope until it hits the photocathode. That's what DAG coating the PMT is supposed to prevent--by keeping the tube envelope at photocathode potential it repels the electrons, and by being black it absorbs the guided light.

Cheers

Phil Hobbs

Most of it. The first dynodes are reputed to produce a bit, particularly af ter they've been busy - not every incident electron produces only prompt se condaries.

There's all sorts of interesting stuff that can go on. I used a zener to fi x the voltage from photocathode to first dynode at close to it's maximum to maximise the signal to noise ratio, and the RCa 8850 went further by using a GaP semiconductor on the first dynode and some 800V between cathode and first dynode to get about 40 seconary electrons per photo-electron.

It makes sense to use a zener diode between the last dynode and ground - th at voltage drop doesn't add to the gain of the tube, and too much anode cur rent can drop the voltage across this stage and increase the voltage across the rest of the tube (which does affect the gain) to give you a perceptibl e positive non-linearity.

If you are pulling large enough currents out of the anode, you can need to raise the voltage across the last few dynodes to avoid space charge effects , and Sauerbrey thought that the a single photo-electron in the photo-catho de to first dynode space could be shown to have created a significant space charge.

Photomultipliers are fun devices.

Sloman, A.W. "Comment on 'Computer aided simulation study of photomultiplie r tubes'", IEEE Transactions on Electron Devices, ED-38 679-680 (1991).

lists a few useful references.

Err...the 600V plot is almost two times better...

Excellent point; treat the PMT as a series of (noisy) amplifiers; that implies the first stage is the most critical,with the second stage contributing (to S/N) by about the inverse of its gain (i think).

Cambridge Instruments bought selected high gain photomultipliers so that th ey never ended up with more than 1000V across the glass face-plate - betwee n the photocathode and the external metal work.

Above 1000V you got electrical conduction through the glass, which was gene rating electro-luminescence, which showed up as extra dark current at high gain settings.

Regular insulators between the glass and the metal-work didn't help - they weren't as resistive as the glass, so the voltage drop across the glass sta yed at 1000V. We could have done better, but paying a bit extra for selecte d high-gain tubes was the cheapest option.

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Use a plain socket and put in your own resistor / regulator chain, just like many did 30-40 years ago.

after they've been busy - not every incident electron produces only prompt secondaries.

fix the voltage from photocathode to first dynode at close to it's maximum to maximise the signal to noise ratio, and the RCa 8850 went further by usi ng a GaP semiconductor on the first dynode and some 800V between cathode an d first dynode to get about 40 seconary electrons per photo-electron.

Nice data sheet, too bad the dark current graph doesn't go to lower voltages. figure 9 here shows dark current 'turning on' at 600-700 volts.

that voltage drop doesn't add to the gain of the tube, and too much anode c urrent can drop the voltage across this stage and increase the voltage acro ss the rest of the tube (which does affect the gain) to give you a percepti ble positive non-linearity.

o raise the voltage across the last few dynodes to avoid space charge effec ts, and Sauerbrey thought that the a single photo-electron in the photo-cat hode to first dynode space could be shown to have created a significant spa ce charge.

Did you ever look at the dark current at low voltages?

George H.

ier tubes'", IEEE Transactions on Electron Devices, ED-38 679-680 (1991).

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Sorry I cut the top off the graph and the single data point at 537 volts was lost. The light /dark ratio was 455 compared to ~70 at 1000V. The pulses are only a few mV high basically lost in the noise... I can't get the 'scope to trigger on them, the comparator finds them just fine though. Looking at the 'scope display I'd never have guessed that the light/dark count ratio would be best there.

George H.

One other thing that occurred to me--the number of light counts is decreasing as well, so ISTM that either (a) you're effectively losing photocathode area due to regions where the field is too weak to make the primaries hit the first dynode, (b) the first dynode gain has dropped low enough that there's a significant chance of getting zero secondaries, or (c) the field at the PC surface is too weak to extract all the primaries from the PC.

If (c) is the case, it might suppress thermionic emission more than photoelectrons, provided that the photoelectrons haven't entirely thermalized by the time they reach the PC surface, which I'm pretty sure they haven't.

Does this tube have a NEA photocathode?

Cheers

Phil Hobbs