Responses should be WRT neutron detection.
1) Which is the preferred configuration: grounded cathode or grounded anode? 2) Which is the most common configuration? 3) Is there an optimum (or common) first dynode potential? I think that too much voltage would make for too much noise, but too little voltage would tend to kill sensitivity.Thanks.
On a sunny day (Thu, 30 Dec 2010 22:48:16 -0800) it happened Robert Baer wrote in :
IIRC the answer to everything is 42.
2) On the subject of PMTs, perhaps read this:
4) Don't drink alcohol while posting to Usenet, or doing anything else.
5) Have fun.Jan Panteltje a écrit :
In that case I think it's more 43.
Sorry, no wikipedia entry to backup that.
-- Thanks, Fred.
de?
oo
You've got some sort of sintillator to 'see' the neutrons? The anode near ground is nice, less chance of HV. (Though it can still charge up to some fraction of the supply.) I was reading a nice discussion of PMT's in the back of an old SRS catalog a few weeks ago.
George H.
Thanks for the hammy guide.
Do not have anything yet..just gathering info. Am told the usual way is to use a He3 detector. Thanks foor the tip.
de?
It depends, Grounding the anode makes thsignal easy to extract, but the photo-cathode is left sitting a kilo-volt or two away from ground. Cambridge Instruments had to buy slected high gain tubes because more than 1kV across the glass carrying the photo-cathode generated a current through the glass, producing electro-luminescence.
Don't know. Physicists do seem to go for grounded photocathodes, while everybody else seems to opt for grounded anodes.
oo
The optimum first dynode voltage is as high as you can get away with. Increasing the voltage doesn't increase noise - provided that you don't cause something to break down and conduct - and does maximise the number of secondary electrons generated by each individual photo- electron, which minimises extra noise introduced by the multiplication process - which falls as the squre root of the number of secondary electroncs generated.
With fast-focussed tubes, it is important to have a constant or - at worst - slowly increasing voltage between sucessive dynodes because the electron trajectories are controlled by the shape of the electric fieeds between dynodes.
The gap betwen the photo-cathode and the first dynode is a lot bigger than the gap between subsequent dynodes, so you can vary the electric field across this gap without messing up the focussing between subsequent dynodes.
-- Bill Sloman, Nijmegen
I think He3 is not a scintillator, but an ionization detector, kind of like a Geiger-Mueller tube. A thin-walled tube (but for neutrons the wall thickness in no problem) filled with He3 (a rare isotope of He, and therefore damned expensive) with a fine wire running down the center of the tube. Apply about 1000 V to the wire, and observe for small pulses, which you capacitively couple out. Beware of ceramic "capacitor quakes", under electrical stress, the ceramic creaks, which mimics the real signal.
I have no idea what these cost, but I'd guess $3-5 K for a small detector today. We paid about half that two decades ago.
Jon
Thank you very much for that informative tutorial. Greatly appreciated.
Thanks for the info. That price puts me out of the game; was thinking of building up a system; guess i will have to go for OLDtrons in that maybe its detector will pay me...
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