after they've been busy - not every incident electron produces only prompt secondaries.
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.
The dark current goes up by about 20 times from 700 to 1000V, which is about the same as the change in gain.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
Hmm the number of light counts is prety much constant except at the lowest voltage (537 V) With the discriminator set at zero (probably about 1mV) I see,
I get the best light to dark ratio when the discriminator is set such that the light count rate is down to 1/2 to 1/3 it's maximum value.
NEA photocathode? Here's a data sheet... javascript:openreq('
formatting link
hamamatsu/R212.pdf') (well maybe not sure that will work)
Don't spend any skull sweat on this on my account. It's pretty much a done deal.. just getting data for a manual re-write. (And scratching my head.)
It doesn't seem that photoexcited electron's from the photocathode can have that much more energy than the thermal ones... all green photons so a maximum energy of a volt or two. Would that make such a big difference?
No the anode is near ground... that's a lot easier for the pulse amp electronics. (It's a CFA run as a current to voltage converter.... I didn't quite understand how my circuit was working till I tried to get a bit more gain out of it, then I had to give myself a dope slap.)
Okay, that's a side-looking bialkali PC device, not NEA. Those are different from the usual end-looking ones, because the QE isn't uniform across the photocathode due to nonuniform extraction efficiency. (That Hamamatsu catalogue that you linked has a discussion of this effect.)
It's interesting, though, that the dark current sometimes has a different voltage dependence than the gain. That's new news, and potentially pretty useful.
The reflection coefficient at the boundary depends on the incident electron's energy and direction. Negative electron-affinity photocathodes make the boundary height at the surface negative, leaving only the image potential. They have a bunch more dark current as a result, but also higher QE since the electrons can still escape even after thermalizing.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Note my second contribution to this thread yesterday. The PMT where we started getting extra dark current when the photocathode was more than
1KV negative than the metalwork on the othere side of the glass was a EMI tube, not Hamamatsu - so the leakage current through the glass might not be generating electroluminescence or might start to generate it at different voltage difference, depending on the transition metal impurities in the glass used to make the tube envelope, the thickness of the glass and the phase of the moon.
This is a side-looking tube, so the PC isn't deposited on the glass. It shouldn't be sensitive to the voltage from the PC to ground.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Depends. The main issue with side-lookers is the nonuniform photocathode sensitivity, but that isn't always such a big deal. On the plus side, opaque photocathodes have a bit of a sensitivity advantage, and of course they're not vulnerable to photocathode corrosion or photoemission due to huge E fields in the glass.
(They also look really retro, of course.)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
But they are potentially vulnerable to photoemission due to electroluminescence due to current through the glass (I don't think that the E field matters if there isn't any actual current flow). The photoemission will be in the glass between the glass-to-metal seals in the base of the tube, which isn't quite as close to the photocathode as in an end-on tube, but it can be refracted and reflected all over the internal volume of the tube.
I should have woken up to this when I first reacted to your previous post, but it was early in the morning (my time) and I was poking around on my computer because I couldn't get back to sleep (not a frequent problem).
Except that you can't see them when the gear is in working condition. Tube-based audio amplifiers are better placed to exploit the retro image.
Pffbt, just spritz a bit of ITO on the glass, "ground" it to the negative supply, and put your insulators and real ground outside of that. Was that so hard? ;-)
I recall some tubes were coated with what looked like lead foil. Don't know why they did... they were regular 6F6 sized coke bottles as far as I know...
Tim
--
Deep Friar: a very philosophical monk.
Website: http://www.seventransistorlabs.com/
That should be a much smaller problem than in end-on tubes, though, since you don't have the light guiding in the envelope. More than half the light (~1-1/n**2) is trapped inside the envelope and rattles around until it hits something. The something is usually the photocathode unless the tube is coated with DAG.
Plus you can paint most of the envelope black if you need to.
It attracts us retro designers, though. ;)
Steampunk optics, kewl.
(I agree that end-on tubes are preferable in general, but side lookers do have their place.)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
That was probably Aquadag, which combines the electrostatic shielding you describe with optical absorption. A bit messy if it gets scraped, but oh well.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058
hobbs at electrooptical dot net
http://electrooptical.net
Light is light. Once it's generated, some of it is going to end up hitting the photocathode, and the amount will rise a good deal more rapidly than the voltage between the pins
The sharp bend at the photocathode edge lets a lot of it out before it can hit the photocathode. I got my nose rubbed in that when looking at the light reflected from beer foam spread around the sides of a standard (for the purposes of that particular test) beer glass.
Why bother? If the light gets far enough outside the envelope to be absorbed by the paint/DAG, it wasn't going to hit the photocathode anyway.
For retro one might read backward.As I said, they belong on the cheap and nasty side of the tracks.
y after they've been busy - not every incident electron produces only promp t secondaries.
o fix the voltage from photocathode to first dynode at close to it's maximu m to maximise the signal to noise ratio, and the RCa 8850 went further by u sing a GaP semiconductor on the first dynode and some 800V between cathode and first dynode to get about 40 seconary electrons per photo-electron.
- that voltage drop doesn't add to the gain of the tube, and too much anode current can drop the voltage across this stage and increase the voltage ac ross the rest of the tube (which does affect the gain) to give you a percep tible positive non-linearity.
to raise the voltage across the last few dynodes to avoid space charge eff ects, and Sauerbrey thought that the a single photo-electron in the photo-c athode to first dynode space could be shown to have created a significant s pace charge.
I never used the RCA 8850 - much too expensive for the sort of work that I ever got involved with.
I did meet people who had used it, who drew my attention to it's Achilles heel, which was that if you ever got the final dynodes hot, you could poison the GaP on the first dynode.
It didn't take prolonged over-heating to do the damage. The tube was fine for single-photon work - which is what it was designed for - but if it was ever exposed to lots of photons for period of a second or so when powered up for single-photon detection, the first dynode could be seriously and permanently degraded. The intelligent user would design the resistive parts of the voltage divider to make sure that this didn't happen, but physicists can't be relied on to be all that intelligent about electronics, as Review Scientific Instruments reminds us from time to time - you may recall my most recent contribution there ...
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