PMT operating voltage, who knew?

[Bill Sloman]

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.

There goes the neighbourhood. Side-look tubes may be fine in
projectors for talking movies, but look a bit cheap and nasty in
scientific instruments.

 

[Bill Sloman]

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.

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).

(They also look really retro, of course.)

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.

 

[Tim Williams]

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.

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

 

[Bill Sloman]

That should be a much smaller problem than in end-on tubes, though,
since you don't have the light guiding in the envelope.

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

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.

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.

Plus you can paint most of the envelope black if you need to.

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.

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.)

For retro one might read backward.As I said, they belong on the cheap
and nasty side of the tracks.

 

[Bill Sloman]

Nice data sheet,  too bad the dark current graph doesn't go to lower
voltages.
figure 9 here shows dark current 'turning on' at 600-700 volts.

www.bo.infn.it/ams/Hamamatsu-PMT.pdf




Did you ever look at the dark current at low voltages?

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 ...