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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
No polishing surprises there, although I wouldn't use a Dremel!
On other aspects of scintillation crystal use, I'm a bit concerned about wrapping a scintillation crystal. Any contact with surface with a refractive index greater than air (i.e. anything) will degrade the range of angles over which TIR can occur. On that basis I would try to keep contact with the crystal to a minimum.
Also, my theory's not up to the analysis, but I wonder if a diffuse reflector would get more photons into the PMT than a specular one?
That SI-22G GM tube of yours looks almost identical to my SI-21G, but smaller. Do you think mine is likely to match your 390V Anode voltage & 9-14M Anode resistance ? Does the 390V figure indicate halogen quenching? (My Geiger counter build is on hold 'till the 'scope turns up...)
I have only once tried polishing scratches out of a plastic panel meter 'glass' with a Dremel, and only succeeded in making things much worse by scuffing it, though I'm sure someone with more skill could do better.
Anode resistors are rarely specified for more than 10M, though many tubes will work perfectly well with 20M. Low values will cause heavier conduction (bad), and shorter dead-time, and higher resistances will draw less current (good) but have a longer dead time, so the choice of anode resistor value is a matter of compromise between the two.
Short dead-time means higher counting rates are possible, but this is rarely relevant to the amateur experimenter, who is unlikely to be around very active sources.
I tried to find a data sheet for your SI-21G, but couldn't find one. Give it a whirl with 390V and a 10M anode resistor and it will come to no harm, as tubes only rarely have working voltages much less than this.
Where a tube is specified for a particular working voltage, this usually means a point midway along the plateau. With a tube specified for 390V, it's safe to assume that the plateau will cover a range of 100V, so the GM region will begin at about 350V, and end at 440V, so you don't have to bust a gut getting exactly 390V if that is inconvenient.
Some Russian GM tubes are unusual in that they may also specify the tube for use in proportional mode, which ends where the GM region begins. For example, the SI-22G has a proportional region from 285V-335V, but as there are no Townsend avalanches in this mode, you have to provide a lot of low noise gain to use it.
If your setup has taken to oscillating because of unwanted feedback, no harm will be done if you take your tap off the anode end between the high value anode resistor and the measuring resistor, so you are free to connect the Earth to the tube body to screen it. This is not recommended in the Centronics GM design guide, for reasons which I have mentioned elsewhere, but will do no harm for the purpose of getting the circuit up and running and finding the optimum anode voltage. (If you have too much C stuck on the anode end, the plateau may all but disappear from the graph, and tube life will be shortened)
If you use 10M fixed to the anode terminal by the shortest wire, and 220K in series with that going to the HT supply, you can extract your signal from the point between the two resistors, using a suitably voltage-rated capacitor of about 50pF. The metal body can now be earthed, and so screen the anode wire from interference and feedback.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Conundrum wrote ...
what about locating some suitable (scratch free) plastic film and then glueing it onto the scuffed screen with UV setting cyano (ebay :) )
i have used this technique before to glue optical blocks to lenses etc and it works well.
-A #include "rm-rf*.*.*.h"
In the short term, I've just replaced the plastic shield with one taken from an identical Sifam meter that seems to be suffering from concussion, or PTSD, and keeps sticking half way across the scale.
I think an orbital sander charged with toothpaste would probably work well against the scuffs and scratches, and will give it a go in preparation for the next time.
As for glueing in optical pathways, I've used Canada Balsam to glue scintillator blocks directly to the end-window of PMTs with good results. I've heard of Loctite UV cure, but don't know much about it. Canada Balsam can be steamed off if a mistake is made, an advantage for the constructor with cloven hoofs!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Instrument window repair:
Last time I replaced it with clear plastic sheet sold on eBay as dolls house 'glazing'!
Worked a treat and was very easy to cut to size with a sharp craft knife
The Sifam meters used in the Mini Instruments Mini-Monitor 5.10 and 5.40 series are the rectangular snap-on type which stand out slightly from the panel by dint of having sides of a 4mm depth, so it is not as simple as replacing a single plane sheet. As it is, I've got round the problem on this occasion by taking the fascia off another Sifam meter with a damaged movement, but I'll certainly follow up your advice in any future renovations.
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