Tuesday, July 31, 2012

Ion chamber

Ion chamber is an ionizing radiation detector that detects the radiation by measuring electrical conductivity of air (or other gas). I’ve built one:

In that video, I used old radium clock to test it.

 I wrapped the clock in plastic to ensure that the radioactive dust would not get out in the event that I accidentally drop it and break the glass - cleaning up radioactive dust can spoil your entire day.

For those into electronics, I am using LMP7721 amplifier and 100 TOhm resistor (for which I thank anonymous contributor – it is very hard to find those resistors!).

The circuit is basically the one from LMP7721 example datasheet, other resistors are for input protection, etc. I’ve soldered stuff onto prototype-ish board, which I soldered to flaps bent from the can that used to contain peanuts.

The chamber is formed by the can and the wire inside. The other can that used to contain peas is used as shielding (I opened it up for the picture). I simply measure the output with digital voltmeter. Besides radium clocks, this thing can detect 1kg bag of potassium salt.

The advantage of ion chamber over Geiger counter is that it works over the entire ionizing radiation range, with the response more closely matching that of the body. (Geiger counters are very bad at counting soft x-rays.) Other advantage is that it is just a ton simpler, requires no vacuum, can test for radon directly, etc. I’m planning to build another one using a switch to keep the measurement circuitry disconnected from the ion chamber electrode except for the times when measurements are taken. That should allow higher accuracy, and permit me to use more ordinary amplifier, like TL072 .

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