Questions about photocathodes

[tim.hbn]

Hi Everyone

I have the following questions about photocathodes:

1. Am I correct in guessing that if the PMT has no voltage applied to it, the photocathode is not charged and cannot therefore emit electrons when photons hit them?

2. Am I correct in guessing that the charge of the photocathode when the PMT is supplied with a voltage is evenly spread throughout the photocathode and that the higher the quality of the photocathode, the more evenly spread the charge is?

3. Does the quality of a photocathode always gradually degrade over its lifetime?

4. When a single photon hits a charged photocathode, is the number of electrons emitted proportional to the voltage supplied to the PMT?

5. If a very low voltage is supplied to the PMT and as a result, the charge is very thinly spread over the photocathode, can a photon hit the photocathode and completely fail to cause an electron to be emitted? (I am guessing this would happen if the photon hits a part of the photocathode where there is not an electron ready to be emitted.)

Thank you very much.

[Sesselmann]

Hi Everyone

I have the following questions about photocathodes:

1. Am I correct in guessing that if the PMT has no voltage applied to it, the photocathode is not charged and cannot therefore emit electrons when photons hit them?

Yes that's correct...

2. Am I correct in guessing that the charge of the photocathode when the PMT is supplied with a voltage is evenly spread throughout the photocathode and that the higher the quality of the photocathode, the more evenly spread the charge is?

Have not read that before, seems unlikely because at 700V any conducting surface ought to be pretty evenly spread out.

3. Does the quality of a photocathode always gradually degrade over its lifetime?

Yes it degrades over time with use, I have read that it will last longer when run at lower voltages. I once powered up a PMT in daylight by accident and it stripped the brown photocathode completely off in a few seconds, leaving a clear glass window.

4. When a single photon hits a charged photocathode, is the number of electrons emitted proportional to the voltage supplied to the PMT?

Certainly the number of electrons hitting the anode is. I'm not sure what happens at the first stage, but as the whole thing cascades down the multiplier more voltage means more gain.

5. If a very low voltage is supplied to the PMT and as a result, the charge is very thinly spread over the photocathode, can a photon hit the photocathode and completely fail to cause an electron to be emitted? (I am guessing this would happen if the photon hits a part of the photocathode where there is not an electron ready to be emitted.)

Don't know the answer to this question, not even sure how you would test it.

Steven

[tim.hbn]

Hi Steven

Thank you very much indeed for your very informative and helpful reply.

I have another question.

I am guessing that a highly degraded photocathode would count photons less accurately than a new photocathode because it would metaphorically be like photons hitting Swiss cheese. Am I correct? If yes then does even a slight amount of degradation reduce the photon counting accuracy?

Thank you very much.

Kind regards

Tim

[NuclearPhoenix]

I have some additional thoughts, maybe some of you can comment on that ;)

1. Am I correct in guessing that if the PMT has no voltage applied to it, the photocathode is not charged and cannot therefore emit electrons when photons hit them?

Well, technically since the working principle of a PMT relies on the photoelectric effect you should still be able to see electrons being knocked out of the coating by light. It's just that you don't get any big currents or gain through that because there is not electric field at the dynodes. Right? But maybe the work function of the coating depends on the external electric field to emit electrons, I'm not sure.

2. Am I correct in guessing that the charge of the photocathode when the PMT is supplied with a voltage is evenly spread throughout the photocathode and that the higher the quality of the photocathode, the more evenly spread the charge is?

That probably depends heavily on the exact electric field orientation of the PMT especially near the border region, but since the photocathode coatings are metallic, you're probably right. Not sure if quality changes this exact property, though.

3. Does the quality of a photocathode always gradually degrade over its lifetime?

Not only the photocathode but the dynodes as well, everything that's coated will wear down eventually. That's also why you don't want to use it in day light or something, you'll either burn through the tube or completely wear down all the coatings quickly and make it blind basically.

5. If a very low voltage is supplied to the PMT and as a result, the charge is very thinly spread over the photocathode, can a photon hit the photocathode and completely fail to cause an electron to be emitted? (I am guessing this would happen if the photon hits a part of the photocathode where there is not an electron ready to be emitted.)

You might not accelerate the electrons as much as needed and therefore not get any secondary emissions, i.e. no current at the output. If you turn up the voltage to the lowest possible value just for secondary emission to work, you might still not be able to measure anything because the current is so low. My best guess.

I am guessing that a highly degraded photocathode would count photons less accurately than a new photocathode because it would metaphorically be like photons hitting Swiss cheese. Am I correct? If yes then does even a slight amount of degradation reduce the photon counting accuracy?

I'd imagine that's exactly how it works. There will be regions with no or less coating that won't be able to emit as many electrons, if any. That would probably significantly degrade the efficiency more so than the accuracy. You really don't care much about accuracy, because you will never be able to detect all of the ionizing radiation in the first place (in this case probably gamma). What's important for radiation detection is the efficiency of a detector and the precision/reproducability. Everything else has to be dealt with by calibrating your device anyways, so it's not that of a big deal.

E.g., if your detector only has an efficiency of a couple %, you'll have a linear offset of all your measured values vs. the true value (e.g. 5 cps measured vs 100 Bq true activity). You can just calibrate that away with known sources and it's totally fine. Uniform degradation of your PMT will probably only affect this %, which will go down, also reducing the current that you will see at the output. As a result, your spectrum will move to the left (lower energies) as a whole. It'll also be less responsive at the low end (energy and activity), because the efficiency is just way too bad.

[tim.hbn]

Hi NuclearPhoenix

Thank you very much indeed for your very informative additional answers.

Does anyone know the answer to my question about photon counting accuracy?

Thank you very much.

Kind regards

Tim

[NuclearPhoenix]

Oops, you replied way too fast, Tim :))

I updated my original post, because I didn't see your reply in time.

[tim.hbn]

Hi NuclearPhoenix

I have just now re-read your original post.

Thank you very much for the update.

The reason I am thinking that the quality of the photocathode matters (whether or not it is like Swiss cheese) is that if the visible light photons from the crystal are landing on areas of the photocathode of more than one efficiency then it logically seems to me that this will widen FWHMs. Do you agree?

Thank you very much.

Kind regards

Tim

[NuclearPhoenix]

The reason I am thinking that the quality of the photocathode matters (whether or not it is like Swiss cheese) is that if the visible light photons from the crystal are landing on areas of the photocathode of more than one efficiency then it logically seems to me that this will widen FWHMs. Do you agree?

You mean if the cathode does not degrade uniformly with regions of more wear than others? Yeah probably. But I'm not sure about that and how large the resulting errors would be.

[tim.hbn]

Hi NuclearPhoenix

Thank you very much for your reply.

You mean if the cathode does not degrade uniformly with regions of more wear than others?

Yes, that is exactly what I mean. I am guessing that this would make the photon counting less reliable because it would depend on where the photons land.