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Archive for June 13th, 2015

Victoreen 710-104 Ionization Chamber

Using radiation to generate random numbers reminded me of some Victoreen 710-104 ionization chambers that have been in the pile basically forever:

Victoreen 710-104 Ionization Chamber

Victoreen 710-104 Ionization Chamber

The central contact seems to be double-insulated from the chamber with glass (?) seals in a soldered-in-place assembly:

Victoreen 710-104 Ionization Chamber - terminal detail

Victoreen 710-104 Ionization Chamber – terminal detail

That might be rosin left over from soldering, but you’d think they would have rinsed it off to reduce the leakage. Some cleaning will be in order.

A picture in The Fine Manual for the CD-V-710 Model 5 Radiation Survey Meter showed that the circuit board used point-to-point wiring, with the range switch soldered directly to that bent metal contact:

Victoreen CD-V-710 Model 5 Manual - Page 10 - circuit layout

Victoreen CD-V-710 Model 5 Manual – Page 10 – circuit layout

Another page gave some useful values and a simplified schematic:

Victoreen CD-V-710 Model 5 Manual - Page 5

Victoreen CD-V-710 Model 5 Manual – Page 5

Never fear, the manual also has the full schematic; they don’t write manuals like that any more.

The chamber bias voltage was +22.5, from one carbon-zinc battery available back in the 1950s. You can still get 22.5 V batteries at about ten bucks a pop, but 24 V from a pair of cheap & readily available 12 V A23 alkaline batteries should be close enough. There’s no current drain, so the batteries should last their entire shelf life.

The “HI-MEG” resistor represents a trio of glass-body resistors selected by the range switch:

  • R5 = 100 GΩ → 0.5 R/h
  • R6 = 10 GΩ→ 5 R/h
  • R7 = 1 GΩ→ 50 R/h

As the saying goes, if you must select R7 in an actual emergency, you should sit down, put your head between your legs, and kiss your ass goodbye.

The steel-wall chamber responds only to gamma radiation, with a nominal current of 5 pA at 0.5 R/h. However, given an op amp like the LMC6081 with 10 fA bias current, maybe building an electrometer-style amplifier that can respond to background gamma radiation or maybe secondary gamma rays from cosmic ray air showers would be feasible; I haven’t done anything like that in a while and even a faceplant would be interesting.

Alas, radium-226 and its progeny, including radon-222 decay through alpha and beta emission that’s specifically excluded by the can.

This is not a new idea, by any means, as shown by some extensive discussion and well-done circuitry. Any amplifier that works with the Victoreen can will certainly work with a homebrew ionization chamber.

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