On Wednesday, July 20, 2022 at 5:55:04 PM UTC-7, John Miles, KE5FX wrote:> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: > > Yup. Grounded-cathode is the usual method with scintillators. You > > couple the pulses out with a capacitor, so it's not that big a deal. > I seriously do not understand this. With a grounded cathode, the > signal you're extracting at the anode end is exposed to ripple from > the PMT supply...Photomultipliers are current sources; the current gain is 'exposed to ripple from the PMT supply', so that isn't a design feature from which to expect any difference at all. If the pulses are short, filtering against the ripple (longer duration than the pulses) might not be difficult. You'll be rejecting dark current either way. I'd think transformer-coupling would be a natural way to capture pulses while rejecting lower frequency ripple and DC. In my experience with PMTs we used good regulated DC power.
Low Level Gamma Radiation
Started by ●June 5, 2022
Reply by ●July 21, 20222022-07-21
Reply by ●July 21, 20222022-07-21
On 21/07/2022 07:36, whit3rd wrote:> On Wednesday, July 20, 2022 at 5:55:04 PM UTC-7, John Miles, KE5FX wrote: >> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >>> Yup. Grounded-cathode is the usual method with scintillators. You >>> couple the pulses out with a capacitor, so it's not that big a deal. >> I seriously do not understand this. With a grounded cathode, the >> signal you're extracting at the anode end is exposed to ripple from >> the PMT supply... > > Photomultipliers are current sources; the current gain is 'exposed to > ripple from the PMT supply', so that isn't a design feature from which to expect > any difference at all. If the pulses are short, filtering against the ripple (longer duration > than the pulses) might not be difficult. You'll be rejecting dark current either way.PMTs were mostly used in pulse counting mode for astronomy. Height of the pulse doesn't matter provided that there is one (or not). Image Photon Counting System, IPCS from Imperial College being the first new generation microchannel plate based imaging device in the 1980's. https://www.ing.iac.es/PR/wht_info/ipcs.html For the time it was incredibly sensitive (when compared to film or CCDs). CCDs improved very rapidly in the following decades.> > I'd think transformer-coupling would be a natural way to capture pulses while rejecting lower > frequency ripple and DC. In my experience with PMTs we used good regulated DC power.My own experience was mostly of ion counting mass spec systems rather than photon counting. Deconstructed PMT in a hard vacuum and ion beams rather than light. We had to do some elaborate ion optics to provide a photon stop to prevent light from the plasma reaching the detector. Hard vacuum requires stainless steel and no paint so an effective photon stop is harder to make than it sounds. ISTR that in pulse counting mode it needed dead time correction once the count rate got high and that there was a cute way to run it in analogue mode by dropping the supply voltage and monitoring the current. In both cases the supply voltages were as stable as we could reasonably make them (more so for the analogue mode). Cross calibrating the analogue to pulse counting modes as a function of mass was quite bad for the detector but essential if the results were to be meaningful. https://en.wikipedia.org/wiki/Electron_multiplier -- Regards, Martin Brown
Reply by ●July 21, 20222022-07-21
John Miles, KE5FX wrote:> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >> Yup. Grounded-cathode is the usual method with scintillators. You >> couple the pulses out with a capacitor, so it's not that big a deal. > > I seriously do not understand this. With a grounded cathode, the > signal you're extracting at the anode end is exposed to ripple from > the PMT supply, without benefit of a multi-megohm divider chain. > There's also the need to use a DC restorer of some sort to figure out > where the baseline is. Both of these problems go away with a > grounded anode. Seems like a no-brainer. > > -- john, KE5FXRight, as far as that goes. The photocathode is the high-Z end of the photomultiplier, and so most naturally goes furthest from ground. In free-space applications, we run the anode near ground and the photocathode at -1 to -2 kV, because the PC end doesn't draw any cuarrent to speak of. That makes it easy to get the (largish) anode current out into normal low-voltage circuitry. (*) The large scintillator crystal is out there where you can touch it, so for safety reasons it's good if it's kept near ground. It's important for sensitivity reasons that the scintillator crystal be connected to the PMT faceplate through a continuous path with refractive index at least as large as the glass of the PMT envelope. The reason is that a major fraction of the light from the scintillator is incident on its exit facet from angles beyond the critical angle for CsI -> air. I suspect that this is where the difficulty lies: If you have diffuse light in a medium of refractive index n, and want to couple the light efficiently into air, it turns out that your maximum efficiency is 1/n**2, even with an arbitrarily good AR coating, That's usually a problem. Sooooo, to avoid losing signal, you want to avoid coupling light from your scintillator into air. The most straightforward way to avoid that is to optically-couple the scintillator to the PMT faceplate, either by direct contact or using some sort of index oil or gel. OK so far? It's also good if your detector survives use, though. Putting the scintillator at ground and the photocathode at -1 kV Photocathode corrosion due to halide ions drifting through the glass faceplate has been a real issue for a long time, and the saPower supply ripple is an easily-soluble problem, especially since a scintillation event is very bright in PMT terms--you aren't trying to do photon counting. I've used cascades of PNP cap multipliers to do some pretty fun things with PMTs, including getting (to me) impressive linearity for analogue RF modulated detection. Usually it's important for safety reasons that the metal housing for the big ugly scintillator crystal is near ground. Overall, for slowish counting applications, floating the PMT anode is a win. Cheers Phil Hobbs (*) That approach is also a good match to the (IME badly overrated) Cockroft_Walton scheme, where you run each dynode off a tap of a C-W multiplier. The C-W approach naturally gives you vaguely equally-spaced taps, and the high-Z end of the C_W matches the high-Z end of the PMT. It's superficially neat, but has serious problems with linearity and noise (especially at lower gain), not to mention the super ugly supply ripple effects. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 21, 20222022-07-21
Martin Brown wrote:> On 21/07/2022 07:36, whit3rd wrote: >> On Wednesday, July 20, 2022 at 5:55:04 PM UTC-7, John Miles, KE5FX wrote: >>> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >>>> Yup. Grounded-cathode is the usual method with scintillators. You >>>> couple the pulses out with a capacitor, so it's not that big a deal. >>> I seriously do not understand this. With a grounded cathode, the >>> signal you're extracting at the anode end is exposed to ripple from >>> the PMT supply... >> >> Photomultipliers are current sources; the current gain is 'exposed to >> ripple from the PMT supply', so that isn't a design feature from which >> to expect >> any difference at all. If the pulses are short, filtering against >> the ripple (longer duration >> than the pulses) might not be difficult. You'll be rejecting dark >> current either way. > > PMTs were mostly used in pulse counting mode for astronomy. Height of > the pulse doesn't matter provided that there is one (or not). > > Image Photon Counting System, IPCS from Imperial College being the first > new generation microchannel plate based imaging device in the 1980's. > > https://www.ing.iac.es/PR/wht_info/ipcs.html > > For the time it was incredibly sensitive (when compared to film or > CCDs). CCDs improved very rapidly in the following decades. >> >> I'd think transformer-coupling would be a natural way to capture >> pulses while rejecting lower >> frequency ripple and DC. In my experience with PMTs we used good >> regulated DC power. > > My own experience was mostly of ion counting mass spec systems rather > than photon counting. Deconstructed PMT in a hard vacuum and ion beams > rather than light. We had to do some elaborate ion optics to provide a > photon stop to prevent light from the plasma reaching the detector. > > Hard vacuum requires stainless steel and no paint so an effective photon > stop is harder to make than it sounds.You can use black chrome, though, no?> > ISTR that in pulse counting mode it needed dead time correction once the > count rate got high and that there was a cute way to run it in analogue > mode by dropping the supply voltage and monitoring the current. > > In both cases the supply voltages were as stable as we could reasonably > make them (more so for the analogue mode). Cross calibrating the > analogue to pulse counting modes as a function of mass was quite bad for > the detector but essential if the results were to be meaningful. > > https://en.wikipedia.org/wiki/Electron_multiplier >Yup. The microchannel plate is basically a massively-parallel version of the Channeltron electron multiplier. I was looking at Photonis's MCP page the other day, and they were all smug about nobody using curved or chevron-shaped channels anymore, just their original version. The reason for the curved channels, of course, was to force ions to hit the walls instead of being accelerated directly at the photocathode, damaging it and causing very very bright ion events. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 21, 20222022-07-21
John Miles, KE5FX wrote:> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >> Yup. Grounded-cathode is the usual method with scintillators. You >> couple the pulses out with a capacitor, so it's not that big a deal. > > I seriously do not understand this. With a grounded cathode, the > signal you're extracting at the anode end is exposed to ripple from > the PMT supply, without benefit of a multi-megohm divider chain. > There's also the need to use a DC restorer of some sort to figure out > where the baseline is. Both of these problems go away with a > grounded anode. Seems like a no-brainer. > > -- john, KE5FX[fixed a few editing scars] Right, as far as that goes. The photocathode is the high-Z end of the photomultiplier, and so most naturally goes furthest from ground. In free-space applications, we run the anode near ground and the photocathode at -1 to -2 kV, because the PC end doesn't draw any cuarrent to speak of. That makes it easy to get the (largish) anode current out into normal low-voltage circuitry. (*) The large scintillator crystal is out there where you can touch it, so for safety reasons it's good if it's kept near ground. It's important for sensitivity reasons that the scintillator crystal be connected to the PMT faceplate through a continuous path with refractive index at least as large as the glass of the PMT envelope. The reason is that a major fraction of the light from the scintillator is incident on its exit facet from angles beyond the critical angle for CsI -> air. I suspect that this is where the difficulty lies: If you have diffuse light in a medium of refractive index n, and want to couple the light efficiently into air, it turns out that your maximum efficiency is 1/n**2, even with an arbitrarily good AR coating, That's usually a problem. Sooooo, to avoid losing signal, you want to avoid coupling light from your scintillator into air. The most straightforward way to avoid that is to optically-couple the scintillator to the PMT faceplate, either by direct contact or using some sort of index oil or gel. OK so far? It's also good if your detector survives use, though. Putting the scintillator at ground and the photocathode at -1 kV |doesn't work well, it turns out. Photocathode corrosion due to halide ions drifting through the glass faceplate has been a real issue for a long time, and | I don't know whether improved faceplate materials have helped that much. Power supply ripple is an easily-soluble problem, especially since a scintillation event is very bright in PMT terms--you aren't trying to do photon counting. I've used cascades of PNP cap multipliers to do some pretty fun things with PMTs, including getting (to me) impressive linearity for analogue RF modulated detection. Usually it's important for safety reasons that the metal housing for the big ugly scintillator crystal is near ground. Overall, for slowish counting applications, floating the PMT anode is a win. Cheers Phil Hobbs (*) That approach is also a good match to the (IME badly overrated) Cockroft_Walton scheme, where you run each dynode off a tap of a C-W multiplier. The C-W approach naturally gives you vaguely equally-spaced taps, and the high-Z end of the C_W matches the high-Z end of the PMT. It's superficially neat, but has serious problems with linearity and noise (especially at lower gain), not to mention the super ugly supply ripple effects. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 21, 20222022-07-21
On Thu, 21 Jul 2022 12:36:00 +0100, Martin Brown <'''newspam'''@nonad.co.uk> wrote:>On 21/07/2022 07:36, whit3rd wrote: >> On Wednesday, July 20, 2022 at 5:55:04 PM UTC-7, John Miles, KE5FX wrote: >>> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >>>> Yup. Grounded-cathode is the usual method with scintillators. You >>>> couple the pulses out with a capacitor, so it's not that big a deal. >>> I seriously do not understand this. With a grounded cathode, the >>> signal you're extracting at the anode end is exposed to ripple from >>> the PMT supply... >> >> Photomultipliers are current sources; the current gain is 'exposed to >> ripple from the PMT supply', so that isn't a design feature from which to expect >> any difference at all. If the pulses are short, filtering against the ripple (longer duration >> than the pulses) might not be difficult. You'll be rejecting dark current either way. > >PMTs were mostly used in pulse counting mode for astronomy. Height of >the pulse doesn't matter provided that there is one (or not). > >Image Photon Counting System, IPCS from Imperial College being the first >new generation microchannel plate based imaging device in the 1980's. > >https://www.ing.iac.es/PR/wht_info/ipcs.html > >For the time it was incredibly sensitive (when compared to film or >CCDs). CCDs improved very rapidly in the following decades. >> >> I'd think transformer-coupling would be a natural way to capture pulses while rejecting lower >> frequency ripple and DC. In my experience with PMTs we used good regulated DC power. > >My own experience was mostly of ion counting mass spec systems rather >than photon counting. Deconstructed PMT in a hard vacuum and ion beams >rather than light. We had to do some elaborate ion optics to provide a >photon stop to prevent light from the plasma reaching the detector. > >Hard vacuum requires stainless steel and no paint so an effective photon >stop is harder to make than it sounds. >We had the opposite problem: detecting the light from a plasma but blocking the ions that would darken our window. I wanted to do a thing with angled slats to bounce the light forward but that the ions would impact, but the semi company stuck with a plate with tiny holes, to let a little light and a few ions through. Bad idea, but then they stole our detector circuit so I hope their windows all darken.
Reply by ●July 21, 20222022-07-21
On Thu, 21 Jul 2022 09:14:23 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Miles, KE5FX wrote: >> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >>> Yup. Grounded-cathode is the usual method with scintillators. You >>> couple the pulses out with a capacitor, so it's not that big a deal. >> >> I seriously do not understand this. With a grounded cathode, the >> signal you're extracting at the anode end is exposed to ripple from >> the PMT supply, without benefit of a multi-megohm divider chain. >> There's also the need to use a DC restorer of some sort to figure out >> where the baseline is. Both of these problems go away with a >> grounded anode. Seems like a no-brainer. >> >> -- john, KE5FX > >Right, as far as that goes. > >The photocathode is the high-Z end of the photomultiplier, and so most >naturally goes furthest from ground. In free-space applications, we run >the anode near ground and the photocathode at -1 to -2 kV, because the >PC end doesn't draw any cuarrent to speak of. > >That makes it easy to get the (largish) anode current out into normal >low-voltage circuitry. (*) > >The large scintillator crystal is out there where you can touch it, so >for safety reasons it's good if it's kept near ground. > >It's important for sensitivity reasons that the scintillator crystal be >connected to the PMT faceplate through a continuous path with refractive >index at least as large as the glass of the PMT envelope. > >The reason is that a major fraction of the light from the scintillator >is incident on its exit facet from angles beyond the critical angle for >CsI -> air. > >I suspect that this is where the difficulty lies: If you have diffuse >light in a medium of refractive index n, and want to couple the light >efficiently into air, it turns out that your maximum efficiency is >1/n**2, even with an arbitrarily good AR coating, That's usually a problem. > >Sooooo, to avoid losing signal, you want to avoid coupling light from >your scintillator into air. > >The most straightforward way to avoid that is to optically-couple the >scintillator to the PMT faceplate, either by direct contact or using >some sort of index oil or gel. OK so far? >It's also good if your detector survives use, though. Putting the >scintillator at ground and the photocathode at -1 kV Photocathode >corrosion due to halide ions drifting through the glass faceplate has >been a real issue for a long time, and the saPower supply ripple is an >easily-soluble problem, especially since a scintillation event is very >bright in PMT terms--you aren't trying to do photon counting. > >I've used cascades of PNP cap multipliers to do some pretty fun things >with PMTs, including getting (to me) impressive linearity for analogue >RF modulated detection. > >Usually it's important for safety reasons that the metal housing for the >big ugly scintillator crystal is near ground. > >Overall, for slowish counting applications, floating the PMT anode is a win. > >Cheers > >Phil Hobbs > >(*) That approach is also a good match to the (IME badly overrated) >Cockroft_Walton scheme, where you run each dynode off a tap of a C-W >multiplier. The C-W approach naturally gives you vaguely equally-spaced >taps, and the high-Z end of the C_W matches the high-Z end of the PMT. >It's superficially neat, but has serious problems with linearity and >noise (especially at lower gain), not to mention the super ugly supply >ripple effects.There is the idea of an active capacitor for an RC lowpass filter, nanely a modest-sized cap whose low end is intelligently driven by an amp, so it looks like a much bigger cap on the top end.
Reply by ●July 21, 20222022-07-21
Phil Hobbs wrote:> John Miles, KE5FX wrote:<snip long explanation> I should add, of course, that having a huge E field inside the glass is bound to reduce the photocathode quantum yield despite the shielding effect of the PC itself. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 21, 20222022-07-21
jlarkin@highlandsniptechnology.com wrote:> On Thu, 21 Jul 2022 09:14:23 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> John Miles, KE5FX wrote: >>> On Wednesday, July 20, 2022 at 12:51:46 PM UTC-7, Phil Hobbs wrote: >>>> Yup. Grounded-cathode is the usual method with scintillators. You >>>> couple the pulses out with a capacitor, so it's not that big a deal. >>> >>> I seriously do not understand this. With a grounded cathode, the >>> signal you're extracting at the anode end is exposed to ripple from >>> the PMT supply, without benefit of a multi-megohm divider chain. >>> There's also the need to use a DC restorer of some sort to figure out >>> where the baseline is. Both of these problems go away with a >>> grounded anode. Seems like a no-brainer. >>> >>> -- john, KE5FX >> >> Right, as far as that goes. >> >> The photocathode is the high-Z end of the photomultiplier, and so most >> naturally goes furthest from ground. In free-space applications, we run >> the anode near ground and the photocathode at -1 to -2 kV, because the >> PC end doesn't draw any cuarrent to speak of. >> >> That makes it easy to get the (largish) anode current out into normal >> low-voltage circuitry. (*) >> >> The large scintillator crystal is out there where you can touch it, so >> for safety reasons it's good if it's kept near ground. >> >> It's important for sensitivity reasons that the scintillator crystal be >> connected to the PMT faceplate through a continuous path with refractive >> index at least as large as the glass of the PMT envelope. >> >> The reason is that a major fraction of the light from the scintillator >> is incident on its exit facet from angles beyond the critical angle for >> CsI -> air. >> >> I suspect that this is where the difficulty lies: If you have diffuse >> light in a medium of refractive index n, and want to couple the light >> efficiently into air, it turns out that your maximum efficiency is >> 1/n**2, even with an arbitrarily good AR coating, That's usually a problem. >> >> Sooooo, to avoid losing signal, you want to avoid coupling light from >> your scintillator into air. >> >> The most straightforward way to avoid that is to optically-couple the >> scintillator to the PMT faceplate, either by direct contact or using >> some sort of index oil or gel. OK so far? >> It's also good if your detector survives use, though. Putting the >> scintillator at ground and the photocathode at -1 kV Photocathode >> corrosion due to halide ions drifting through the glass faceplate has >> been a real issue for a long time, and the saPower supply ripple is an >> easily-soluble problem, especially since a scintillation event is very >> bright in PMT terms--you aren't trying to do photon counting. >> >> I've used cascades of PNP cap multipliers to do some pretty fun things >> with PMTs, including getting (to me) impressive linearity for analogue >> RF modulated detection. >> >> Usually it's important for safety reasons that the metal housing for the >> big ugly scintillator crystal is near ground. >> >> Overall, for slowish counting applications, floating the PMT anode is a win. >> >> Cheers >> >> Phil Hobbs >> >> (*) That approach is also a good match to the (IME badly overrated) >> Cockroft_Walton scheme, where you run each dynode off a tap of a C-W >> multiplier. The C-W approach naturally gives you vaguely equally-spaced >> taps, and the high-Z end of the C_W matches the high-Z end of the PMT. >> It's superficially neat, but has serious problems with linearity and >> noise (especially at lower gain), not to mention the super ugly supply >> ripple effects. > > There is the idea of an active capacitor for an RC lowpass filter, > nanely a modest-sized cap whose low end is intelligently driven by an > amp, so it looks like a much bigger cap on the top end.Yup. Back in the long ago (1984ish), I built a power supply filter that got rid of 10 Vpp of 120 Hz ripple on a 2 kV supply for some piezo stacks. It worked by lifting the cold end, sensing the hot end via a 100 nF, 3 kV film cap, and wiggling the cold end to keep it still. (It had some TVS protection too, obviously.) With an LF356 op amp, I got 100 dB of ripple rejection, which came in pretty handy. I could probably have done considerably better with that feedforward trick of Woodward's, where you use another op amp to measure the error voltage of the main one, and add in its output. Another approach like that is the Kanner Kap, which wasn't first invented by Kanner, but he now owns it on account of the cute name. That's the trick where you use an RC lowpass, and drive the cold end of the cap with some super beefy amplifier to make the top stay still. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●July 21, 20222022-07-21
On 21/07/2022 15:25, jlarkin@highlandsniptechnology.com wrote:> We had the opposite problem: detecting the light from a plasma but > blocking the ions that would darken our window. I wanted to do a thing > with angled slats to bounce the light forward but that the ions would > impact, but the semi company stuck with a plate with tiny holes, to > let a little light and a few ions through.Simplest solution to stop ions going in straight lines (but not neutrals) is a local magnetic field and some drift length.> Bad idea, but then they stole our detector circuit so I hope their > windows all darken.There is usually plenty of light (and heat) from a 8000K plasma (at least one at 1 bar like ours). There is a pinhole sampling cone into it. Protecting the instrument from coolant failure and the hard vacuum from plasma flame out was a major part of the safety critical side of things. It was worse in the early days when the vacuum pumps were diffusion. Any sort of cock up and you had hot silicone oil all over the place. -- Regards, Martin Brown
