Electronics > Projects, Designs, and Technical Stuff
Photomultiplier - Cherenkov detector success! - soldering questions
Kleinstein:
--- Quote from: jmelson on February 09, 2020, 08:55:57 pm ---
--- Quote from: Kleinstein on February 09, 2020, 10:17:35 am ---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.
--- End quote ---
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
--- End quote ---
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.
ChristofferB:
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.
ChristofferB:
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.
ChristofferB:
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.
ChristofferB:
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!
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