Low Level Gamma Radiation...

[John Miles, KE5FX]

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

 

[whit3rd]

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.

 

[Martin Brown]

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

 

[Phil Hobbs]

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.

--
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

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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

 

[server]

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.

 

[Phil Hobbs]

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

 

[server]

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.

 

[Phil Hobbs]

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

 

[Martin Brown]

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

 

[Phil Hobbs]

Martin Brown wrote:

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.

Yeah, I used to own a Denton 502 bell-jar evaporator that had an oil
diffusion pump. Getting that sucker cleaned up after a roughing-pump
incident was a huge pain.

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

 

[John Miles, KE5FX]

On Thursday, July 21, 2022 at 7:31:07 AM UTC-7, Phil Hobbs wrote:

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.

Yep, I think it\'s clear enough that allowing the faceplate to act as an
HV dielectric is a Bad Idea. But it also seems easier to avoid doing
that by tying the exterior frame to the cathode as the manufacturer has
done here, than to swap the polarity and deal with the HV at the
other end where the signal is extracted.

As far as the need to insulate the crystal is concerned, you\'ll need to
do that anyway to avoid exposing the user to thallium, I\'d imagine.

The question of whether it makes a difference in terms of ripple
is worth thinking through a little further. I see whit3rd\'s point, but
the effect of stray C seems worth considering before concluding that
the polarity doesn\'t make a difference.

-- john, KE5FX

 

[Mike Monett]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

Mike Monett wrote:
Mike Monett <spamme@not.com> wrote:

Mike Monett <spamme@not.com> wrote:

[...]

It should be easy to add a simple inverter and ground the cathode.

Oops - it is already grounded.

And the positive anode should eliminate any ion migration through the
glass.


The anode isn\'t vulnerable to corrosion because it\'s not deposited on
the glass, and it\'s a nice beefy piece of metal, compared with the very
thin transparent photocathode.

Cheers

Phil Hobbs

The anode pin goes through the glass. Metal ions are positively charged, so
they are repelled by the positive anode. This eliminates any metal ion
migration through the glass.




--
MRM

 

[Mike Monett]

\"John Miles, KE5FX\" <jmiles@gmail.com> 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

The capacitor will have high voltage on it. This could destroy any electroncs
that inadvertently got connected badly.

You don\'t need to couple to the anode. Just take the signal off the cathode
like the old cathode followers of yesteryear.

--
MRM

 

[Phil Hobbs]

Mike Monett wrote:

\"John Miles, KE5FX\" <jmiles@gmail.com> 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

The capacitor will have high voltage on it. This could destroy any electroncs
that inadvertently got connected badly.

You don\'t need to couple to the anode. Just take the signal off the cathode
like the old cathode followers of yesteryear.

There\'s no signal at the cathode to speak of--almost all the anode
current comes in via the dynodes.

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

 

[whit3rd]

On Thursday, July 21, 2022 at 3:50:40 PM UTC-7, Mike Monett wrote:

\"John Miles, KE5FX\" <jmi...@gmail.com> 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...

The capacitor will have high voltage on it. This could destroy any electroncs
that inadvertently got connected badly.

You don\'t need to couple to the anode. Just take the signal off the cathode
like the old cathode followers of yesteryear.

A PMT cathode is typically a multialkali evaporated coating on the end of the tube, has
significant stray capacitance, and the signal there is weak (un-multiplied, no benefit
from the PMT gain).

 

[Mike Monett]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]

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

The CCFL inverter I got from Amazon runs at 40KHz.

A simple half-wave rectifier running at 40 KHz with a 1nF cap and 9e6 Ohm
load will produce about 2V sawtooth ripple.

This is about 2/800 = 0.0025 * 100 = 0.25% ripple.

That should be good enough.





--
MRM

 

[Phil Hobbs]

Mike Monett wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]

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

The CCFL inverter I got from Amazon runs at 40KHz.

A simple half-wave rectifier running at 40 KHz with a 1nF cap and 9e6 Ohm
load will produce about 2V sawtooth ripple.

This is about 2/800 = 0.0025 * 100 = 0.25% ripple.

That should be good enough.

Totally. You can probably use one of the HV caps that came with it to
couple the anode pulses to the outside world.

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

 

[Mike Monett]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

Mike Monett wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

[...]

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

The CCFL inverter I got from Amazon runs at 40KHz.

A simple half-wave rectifier running at 40 KHz with a 1nF cap and 9e6
Ohm load will produce about 2V sawtooth ripple.

This is about 2/800 = 0.0025 * 100 = 0.25% ripple.

That should be good enough.

Totally. You can probably use one of the HV caps that came with it to
couple the anode pulses to the outside world.

Cheers

Phil Hobbs

That is a problem. The RH Electronics MCA expects positive pulses. These
won\'t come from the anode.

https://static.wixstatic.com/media/e43988_a63520f4ed07436a843ddc1be0fda46a~
mv2.jpg

https://video.wixstatic.com/video/e43988_8006efb319884c72b9d7f7418abdfa97/7
20p/mp4/file.mp4

I shall write them and ask where they get those positive pulses.



--
MRM