Author Topic: Photomultiplier - Cherenkov detector success! - soldering questions  (Read 4184 times)

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Offline Kleinstein

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Re: Photomultiplier - soldering directly on tube pins?
« Reply #25 on: February 09, 2020, 09:11:06 pm »
Some magnetic shielding of the PMT may be a good idea, but it depends on the tube. I got away without any magnetic shield.  I don't think one would really need Mu-metal. Something like transformer steel should be good enough for most cases. A little fixed residual field should not be a problem.
Transformer steel would be bad, as all soft iron is very susceptible to aquiring a permanent magnetization.  Mu Metal is good in that it is much less
susceptible to getting magnetized.

Jon

Mu_metal is better than soft iron, but not that much and not always.  Mu metal is limited to low fields and the tin wall may not be enough to protect a large volume from the earths field.  Ideally one has iron outside and Mu-metal at the inside with some gap in between.

Both a relatively high ยต and low coercitivity materials. There may in theory be a closed magnetic field inside a piece of soft iron, but this hardly effects the outside. A magnetic soft material can not support much external field. So the possible high remanence in the iron is not that bad. Once demagnetized there should be no permanent field building up. A constant magnetic field would also be not such a problem - the tricky part is more with a change in the field that can effect the gain factor of the PMT and thus the calibration.
One normally has to repeat the energy calibration regularly anyway.
 

Offline ChristofferBTopic starter

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Re: Photomultiplier - soldering directly on tube pins?
« Reply #26 on: February 09, 2020, 09:11:39 pm »
Huh. This Hamamatsu PMT note sais positive HV is reccomended for scintillation counting:

"For example, in scintillation counting, since the
grounded scintillator is directly coupled to the photomultiplier
tube, it is recommended that the cathode be grounded, with a
high positive voltage applied to the anode."

https://www.chem.uci.edu/~unicorn/243/handouts/pmt.pdf

As for capacitors, I was thinking 2 10 nF WIMA FKP-1  polypropylene caps rated for 2 kV in series would be safe enough.

--Christoffer //IG:Chromatogiraffery
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Offline ChristofferBTopic starter

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Re: Photomultiplier - soldering directly on tube pins?
« Reply #27 on: February 10, 2020, 11:50:36 pm »
Small update: SHV connectors arrived (ouch, my wallet!) and I have now made a back enclosure for the PMT, as well as the divider.

The divider turned out pretty solid and stable, I've tried to maximize the spacing between HV parts. Signals and +HV will be supplied between the divider and the connectors through stripped RG174 inner conductor + dielectric (I have heard rated at 1500V RMS) and all surplus spacing will be filled with soft foam matting. The rest of the PMT will be covered in self-vulcanizing tape.

The light tight gasket at the end plate is a piece of neoprene diving suit, by the way.

Fingers crossed it doesnt  let the smoke out! Or given it's a vacuum bulb, worse yet: sucks the smoke in!

Before sealing it up I'm going to soak the entire pin end of the PMT first in contact cleaner (heptane+isopropanol), then pure isopropanol to get rid of anything that might allow HV to creep.
« Last Edit: February 10, 2020, 11:52:21 pm by ChristofferB »
--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 

Offline ChristofferBTopic starter

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Re: Photomultiplier - soldering directly on tube pins?
« Reply #28 on: February 13, 2020, 11:01:04 pm »
So the peltier cooled PMT has a really nice machined house, complete with lens and padding.

It has a potted voltage divider base, and the tube is heat shrunk with a bare wire down the side soldered directly to a pmt pin - I bet it's painted with conductive paint, as is common for negative HV operation, so I guess I also have a complete -HV PMT to compare with!

I'm considering using this one for either a plastic scintillator, or a Cherenkov detector, either a tank of water or a paddle of acrylic.

Figuring out if it's -HV or +HV should be as easy as measuring the resistance between the anode and BNC output - if it's +HV it should be AC coupled.

The pmt is a Hamamatsu R268HA, which is NOT an IR sensitive PMT as I initially thought. These are usually the ones that need cooling, but this one has been used in conjunction with a heater to heat up LiF crystal dosimeters, so it's probably just to avoid radiant heating of the PMT.

It also had a very intense blue filter which is probably also to block IR.
 
--Christoffer //IG:Chromatogiraffery
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Offline ChristofferBTopic starter

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Re: Photomultiplier - soldering directly on tube pins?
« Reply #29 on: February 14, 2020, 07:59:25 pm »
It's working!

This is the afforementioned pre-wired PMT detecting (probably muons) at -1000V bias, with no preamp.

The Cherenkov element is just a stack of perspex/acrylic plates with a drop of paraffin inbetween as coupling. No optical coupling between PMT and "crystal".

I'm pretty pleased!

--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 
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Offline Kleinstein

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Re: Photomultiplier - Cherenkov detector success! - soldering questions
« Reply #30 on: February 14, 2020, 08:43:25 pm »
Congratulations :-+   :clap:

Are the observed peaks really related to the plastic block ? At enough voltage and thus very high gain the PMT can also see single Photons / dark emissions as separate peaks. So one should to the test if the signal is there even without the plastic block. It is still not sure the light pulses are  Cherenkov radiation or scintillation.
 

Offline jmelson

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Re: Photomultiplier - Cherenkov detector success! - soldering questions
« Reply #31 on: February 14, 2020, 11:04:12 pm »
Congratulations :-+   :clap:

Are the observed peaks really related to the plastic block ? At enough voltage and thus very high gain the PMT can also see single Photons / dark emissions as separate peaks. So one should to the test if the signal is there even without the plastic block. It is still not sure the light pulses are  Cherenkov radiation or scintillation.
Dark current from the PMT photocathode will be almost totally uniform (at any specific HV setting).  Pulses from various environmental radiation (mostly cosmic ray showers) will vary in energy.
So, that's one way to be pretty sure you are seeing real radiation.  Then, of course, if you can come up with some kind of source (potassium salt, a chip of an old Corelle orange plate with
U235 glaze, or whatever you might have around) you can move it closer or farther.

Jon
 

Offline ChristofferBTopic starter

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Re: Photomultiplier - Cherenkov detector success! - soldering questions
« Reply #32 on: February 14, 2020, 11:12:21 pm »
Thanks!! I'm pretty hyped about it!

I'm not completely convinced what I'm looking at yet either, and I want to build a proper current to voltage preamp before playing around with the scintillators/cherenkov detectors.

I've also read that a lot of things scintillates, albeit inefficiently, and it is not oncommon to see glass scintillation from the PMT envelope itself.

I did try putting the pmt anode signal directly into my shaping amplifier, giving some much nicer peaks, see attached.

I'm not a big fan of this solution, though, signal quality aside, i'd REALLY like having something 'expendable' inbetween my main spectroscopy rack and my detector, in case I mess something up with the HV.

« Last Edit: February 14, 2020, 11:14:23 pm by ChristofferB »
--Christoffer //IG:Chromatogiraffery
Check out my scientific instruments diy (GC, HPLC, NMR, etc) Channel: https://www.youtube.com/channel/UCZ8l6SdZuRuoSdze1dIpzAQ
 


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