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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Hi, this is intriguing. Took apart a ZP1313, 1314 and some similar looking tube to see if the tube sensitivity was poor due to the lead shield. Turns out the problem may be fixable on one of these, they use either a metal shield (ZP1313/14) or glass mantle (unidentified tube) and once this is removed the tube responds to even low level radiation, much more sensitive than an SBM21.
For drone use these tubes are very lightweight, certainly under a gram and are compact enough to put in a wristwatch.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Without the compensating filter, which you have removed, the energy response of the tubes will be wildly non-linear below about 150 keV.
All uncompensated GM tubes have fairly sharp response peaks at 70 - 80 keV, giving up to 10 times the count rate that would be recorded at, typically, ~50 keV or ~150 keV.
Without knowledge of the energy response characteristics of a tube, dose rate estimates at these lower energies will be grossly in error.
Above ~150 keV, the energy response of GM tubes mellows out and tends to flatline out to 1MeV and beyond.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Yeah, figured as much. I am looking at detecting very low level radiation here, every particle lost in the filter is one less detected. In fact, something interesting I did notice with the SBM20 tubes is that they consistently detect the weak radiation from 40K (20 grams recrystallised from solution) whereas none of my other tubes will.
An interesting note here is that putting a meter in series with the ground connection on low volts range (ie 200mV) shows that the tube is detecting particles even though the counter does not record a count. These signals range from just above the start point of 1.2mV to about 90mV with many in the 30mV range with lo-salt near the tube.
This effect suggests that the sense amplifier needs redesigning and may be responsible for missed counts on some designs.
I recall reading somewhere that a high power magnetic field near Geiger tubes can increase sensitivity, perhaps by raising the "noise floor" to a point that the sense amplifier can detect the ionization ?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The γ sensitivity of GM tubes is very low - often less than 0.1% - so you'll be doing well if you detect 1 in 1000 of the γ-photons that pass through it. The γ-photons hardly interact at all with the counting gas, so γ counting depends on electrons knocked out of the tube wall to initiate the Townsend Avalanche.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
I did have a thought about using a graphite bismuth layered composite as a monochromatic filter as both are relatively easy to get and mixing bismuth and indium allows low temperature printing.
EDIT: Would also work as a collimator if put side-on in layers.
Wonder if Bi:Mg or Bi:Na would work for the same reason, a light metal sandwiched between two heavier ones ?
EDIT: PM, would you be able to proof read/comment on a physics paper if I mailed you a draft please? Or for that matter, anyone else who wants to have a look.
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Geiger tube hacks
