photoelectric experiment idea holding frequency steady and varying intensity

Hi,

Is there a way to use the PMT as a type of "photoelectric spectrometer"? Or at least a threshold spectrometer to reject photoelectrons below a threshold energy. I guess that might be an neat invention to have a fully functioning photoelectric spectrometer that is sensitive to single photons :)

cheers, Jamie

An electron deflector that deflects each electron based on the momentum of the electron (same idea as a mass spectrometer) into one of 1024 paralleled micro-machined dynode stacks , for a nice spectrometer :)

They would have to be micromachined dynode stacks to fit that many of them in one small area and voltage insulation might be an issue.

cheers, Jamie

Geez, any university physics lab ought to have the gear to do your experiment. What you need is a collimated light source, a photomultiplier tube and a way to move the two pieces a controlled distance. And, of course, a really dark room. Oh, and an oscilloscope to look at the pulses. Up close, you get a continuum. As you get the source farther away, you see individual pulses. You will see singles, as well as double and triple hits, clearly quantized. As you get farther away, you will pretty much only see the singles, and they will have the same amplitude.

Jon

PMTs, especially in photon counting mode, are linear. The electrical pulse height is proportional to incident photon energy. That's why, on an oscilloscope, you can easily see doubles and triples. So, a fast comparator makes it easy to select particular photon energies. However, when photon counting, the electrical pulses are REALLY short, often just a few ns wide, so you do need a very fast comparator.

Jon

Hi,

Thanks, I would like to use a window comparator just to make sure too though since it is possible two or more lower energy photoelectrons could add up past the comparator threshold.

cheers, Jamie

Hi,

So the spectrally filtered photoelectron detection rate should be proportional to the light intensity and negating background noise, the photoelectrons should not disappear until the light intensity reaches zero, although the emission rate should decrease linearly to zero. I am skeptical that an experiment like this could be done since the noise floor (or dark count rate) of the PMT has to be very low I think, that is why I was thinking it might be good to use a photoelectric emission material that has a high electron binding energy, maybe that would decrease the dark count rate - just speculating.

cheers, Jamie

Hi,

Apparently the comparator method isn't sufficient to differentiate between photocathode emissions from noise and from the light being measured, so it is not a good enough experiment I think:

from the page:

"Due to the statistical nature of the secondary emission process, there is a distribution of signal pulse heights coming from the PMT. There is another distribution of noise pulse heights. Noise which results from thermionic emission from the photocathode can not be distinguished from signal. However, noise pulses from dynode thermionic emission will have a lower mean pulse-height. "

cheers, Jamie

Actually maybe an off the shelf microchannel plate would work easier than 1024 parallel dynode stacks, just scan the photoelectrons onto the microchannel plate to bin them based on their energy.

I think a streak camera would work for this if it had a DC voltage put on the sweep circuit to deflect the electrons into an energy spectrum:

I am assuming electrons will change path more or less based on their energy, but I'm not sure if that is valid.

Also this may not be as sensitive as a PMT, since the microchannel plate could have less gain than the dynode stacks.

cheers, Jamie

Nope, it's proportional to the number of photoevents.

Typical first dynode yield is about 5, so a single primary photoelectron gets you 5 +- sqrt(5) secondaries. Thus you can see doubles at a SNR of a bit less than 1.0. You can distinguish triples from singles better than that, but triples vs doubles is harder.

So, a fast comparator makes it easy to select particular

Cheers

Phil Hobbs

Hi,

I think the simplest solution to get photoelectron energy is to use a conventional PMT, but have an adjustable DC deflection plate before an off-axis dynode stage, so there is only one frequency detectable, but the DC deflection plate voltage can be adjusted to a calibrated voltage for detecting specific photoelectron energy ranges.

cheers, Jamie

I think something like that must already exist it would be useful to reduce the effective dark current in a PMT if the light frequency of interest is known. If it can distinguish 100 frequencies then the effective dark current of the photocathode would be 1% assuming an even spectral dark current emission.

cheers, Jamie

Possibly the biggest problem is the photoelectron emission angles from the photocathode are not all in the same direction, which is a problem for making a spectrometer sweep of the electron energies:

from the page:

"The spectrometer can use entrance and exit slits or use a small source, which only emits into specific angle and a small detector. Photoelectron spectra from single crystals exhibit a dependency on the emission angle, so that the entrance slit is needed. All the electrons from an isotopic source may be sucked off and focused into a directed beam (much like in an electron gun), which can then be analyzed. A position sensitive detector can detect the energy along one direction and depending on the additional optics lateral resolution or one angle along the other direction."

The dark current should be constant, so all it means is you have to run your experiment longer as the distance is increased to get a statistically meaningful result.

Place a cover over the light source and count pulses for a while, then open the cover and count for the same time, then subtract the difference.

Jon

Hi,

The dark count will be the issue still, it is random and will fluctuate enough to drown out the signal I think. It would require temperature stabilization, and electromagnetic shielding etc to make it stable enough to detect the tiny difference in photons per second of the two experiments. I think the light will have to be so dim that it adds so few photon detections over the control experiment that the difference will be drowned out in the noise..

If the photoelectron frequencies that are detected can be binned then the signal to noise ratio is higher but still there are dark count noise photons in the binned frequency so it probably still can't be a conclusive experiment unless the detector has no noise, or some noise cancellation method is used..

Also this is why I was thinking maybe it is good to use a photoelectric emitter with a high work function, as if the light is classical then it is possible that the high work function materials will give a noticeable effect (ie unexpected cutoff in photo-emissions) at higher light intensities. So counterintuitively, for this experiment it might be good to not use a high sensitivity detector like a PMT, but instead to use one with a high work function and also a low dark count, and possibly a dark count of zero (at least from the photoelectron emitter, the dynode section would still have a dark current which could hopefully be filtered out based on its lower voltage).

cheers, Jamie

How do you think the detectors work that handle the measurement of photon by photon arrival times and positions in the two slit experiment with monochromatic light intensities so low that only one photon can be inside the apparatus at a time?

There is an effect in conventional sliver halide film that requires two trigger photons in a grain within a particular timescale leading to what is called reciprocity failure on faint light (or fast pulses). The great advantage of electronic detectors is that they remain linear no matter how slowly the photons are arriving from the source. It was this kit that made high dispersion spectroscopy of faint galaxies possible.

It is always probabilistic. Not every incident photon will generate a photoelectron some will just cause a recoil in the crystal lattice.

The quantum efficiency of a given detector or surface takes into account the proportion of incident particles that convert to a pulse.

To within the experimental error of real chemical surfaces with thermal noise at finite temperatures. Yes.

Sure, or magnetic field. Deflection sensitivity of either CRT type is inversely proportional to 2nd anode voltage.

If you're talking that much energy though, you're already fine with a phosphor and PMT or something like that. Or an NaI crystal for gammas, or...

For low energy spectra, one could make a microchannel plate with extremely low external fields (so as not to disturb the

Does "count" mean nothing to you...?!

Tim

intensity must be non-zero

Prior to that baking it in vacuum and/or a forming gas immediately prior to use (nitrogen/hydrogen mixture) and keeping the plate at -44C or lower temperature during exposure. Basically you could trade all the shelf life for ultimate sensitivity in the low flux regime.

There were even schemes to deliberately prefog the film. All of this was out evolved first by Bockesnebrgs IPCS and then by CCDs.

Black flock wallpaper and a knife edge is the other way. The light traps on film canisters were a variant on this - fine for visible light but useless for IR which required darkroom handling.

Amazingly you could get away with opening the back of a camera briefly in subdued light because most normal films would have a fairly dense antihalation backing layer and you would only lose the first few frames and any unlucky bits near the perforations.

I did have a camera that due to impacts developed a light leak and left a minor flash across the first shot taken weeks after the previous one.