Q: Calibrating a 1962 CD Radiation Meter

I certainly am no expert in radiation meters, but I seem to recall radioactive marbles on Ebay mentioned in conjunction with calibration. Maybe you could check that out?

Mike

Your best bet would seem to go to a university "nearby" after calling to be sure they have sources for the sensitivity check. As long as the GM tube is still OK, the instrument should be fairly close. This could be considered a type of collectors item; keep it!

Your best bet would seem to go to a university "nearby" after calling to be sure they have sources for the sensitivity check. As long as the GM tube is still OK, the instrument should be fairly close. This could be considered a type of collectors item; keep it!

You can buy some items from ebay to test with. If you are getting a click or two per min, it is picking up background radiation, which means it is working good. Many meters back then were for post necular war, which means they only regester very high levels of radiation, which you don't want to be around anyway. Do you have a make and model # on it?

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I'm guessing, from the description, that this is an ionization chamber, not a Geiger-Muller tube detector. There were dinner plates made in the 70's with a VERY bright orange glaze made from Uranium. They were recalled, but some people have kept them around. If you can find one of these at a flea market, even a small chip will easily trigger a Geiger counter. A whole plate should be able to give a reading on even a poor ionization chamber detector.

Another common radiation source is a gas mantle, as in yard lamps and Coleman lanterns. They have thorium in them. Thoriated Tungsten TIG electrodes are also radioactive. (I can't remember if this is a Gamma source or not, though.)

Even household bricks are weak radiation sources, but I suspect way too weak for such a detector to pick up.

The Geiger-tube detectors usually have a small label on the side marked "test detector here", which has a suitable source for checking the detector. The ion chamber units are usually one-piece, so a test source would be hard to provide, but you might look around and see if there is such a label present.

Jon

Your meter is not the low level training type. It is the actual usage kind.

Calibration is expensive. A few hundred dollars at a commercial laboratory I would guess.

If you live near a nuclar power plant you could run after a radioactive waste hauling truck and try to get it calibrated to the trucks postings???? Stay cool looking when you do this though, beccause you just might be mistaken for a bad guy.

Anyways, nothing in common use in the consumer realm is radioactive enough to give an indication. You are just out of luck.

The radioactive substance in smoke detectors is Am-241, but it is tiny. I am not sure if your detector would even be able to detect that. But the most important thing for calibration is having a know quantity of disinigrations per sec or, 1 bequerel.

You also mentioned the Roentgen.

The Roentgen (R) is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. One roentgen is equal to depositing in dry air enough energy to cause 2.58E-4 coulombs per kg. It is a measure of the ionizations of the molecules in a mass of air. The main advantage of this unit is that it is easy to measure directly, but it is limited because it is only for deposition in air, and only for gamma and x rays.

dan

No need to buy your own radioactive source. Pay your local fire department a friendly visit. They have meters and a source to check them. If they are not out on a job, they'll probably welcome the distraction from daily duties and take the oportunity to restudy the subject. Well, that's what happened to me when I visited the guys here... Had a very interesting lecture on the subject from the Chief! And yes, my meter worked almost perfectly.

BartW. Bart.

There may be a 'NOSPAM.' (mind the dot!) in front of my E-mail address. It is there to fool machines, not people. Please remove it when you want to reply to me personally.

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It's a Victoreen Model 1A [also called a CD V-715] "designed to be used by radiological Civil Defense personnel in determining radioactive contamination levels that may result from an enemy attack or other nulear disasters."

The range of sensitivity is roughly .03 to 500 roentgens per hour and is supposed to be sensitive to only gamma radiation. It contains a "hermetically sealed ionization chamber" and the case is fairly thick metal so I'd believe that it would stop [not to energetic] alpha and beta radiation.

Exactly what did you have in mind that I could find on eBay that would be a decent gamma emitter?

Norm

What's the difference between the two? I thought the G-M tube WAS an ionization chamber.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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If you search on Google you'll find pictures and details of that instrument. As you say its based on an ionization chamber. I'm guessing that the electronics was some sort of electrometer circuit - it converts the ionization in the chamber to a DC current displayed on the meter. So, unlike the pulse processing used with a geiger tube, there won't be any clicks to listen for.

Your best choices for radioactive material to demonstrate that the unit is working are gas lamp mantles (Welsbach mantles) containing Thorium and old luminous watches (or luminous instrument dials) containing radium. Then there are geological samples like pitchblende (uranium) and monazite (thorium). There are ceramics around with uranium in the glaze and thoria crucibles have been used for high temperature stuff. Another approach is to take a domestic smoke detector and extract the source from it. Its normally about

40 kilobecquerels of Americium-241 which besides being an alpha emitter gives 60keV gammas. When handling this (or any other) source, treat it with respect and above all avoid ingesting the material. My last suggestion for an easily available source is a few hundred grams of "low-sodium" salt - which is mostly potassium chloride. All potassium salts contain Potassium-40 which is a gamma emitter (1.4MeV). With a geiger, 200g "low sodium" close to the tube doubles the background rate. However in your case I suspect that the ionization chamber will not be sensitive enough to detect it. Having demonstrated that the unit works, you might still want to calibrate it. I can't imagine that it was ever particularly accurate - more a go/no-go device. Accurate calibration really will require the resources of a decent radiological lab and this will take time and effort.

regards

John McMillan

Well, yes, ionization works in both. But, in a G-M tube, the high voltage gradient causes an avalanche that drains the charge stored on the capacitance of the tube and wiring until the discharge quenches, and the circuit resets. This causes a very large voltage spike which is easily detected, and you "count" the pulses to get dose rate. (Most G-M counters use some kind of gated integrator to convert the pulses to an analog voltage to be displayed on a DeArsonval meter.)

An ionization chamber radiation meter does NOT work in the avalanche mode, but uses a very sensitive current sensing circuit (usually using Victoreen electrometer tubes) to sense the current flowing across a large gas-filled chamber and polarized with a very LOW electric field. Instead of fields like 900 V/cm in the G-M tube, you'd see 20 V/10 cm in an ionization chamber. The ions drift slowly across the chamber, and picoampere currents flow. The design of the chamber and window often allow much lower particle energies to be detected with an ion chamber. Also, extremely high dose rates which would saturate a GM tube can be accurately measured with an ion chamber. Of course, if you ever see these sorts of rates, you'd better run like hell!

Jon

What's the probability that X-Rays from a dentist's equipment would possess enough energy to register in the detector?

Norm

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Yes. The circuit contains exactly one transistor used as an oscillator to drive the primary side of a transformer to get the voltages needed to run the ionization chamber and the Ion Current Detector Tube (VX86 manufactured by Victoreen) which contains a cathode, an anode, and two grids. Based on the resistors in the circuit (they range from 220 Megohms up to 220 gigohms), we're talking about picoamperes of ionization current being detected. The circuit itself is a marvel of economy. with just the two active elemetns and three diodes along with the resistors and capacitors. I'm going to try to find a luminous watch first and, failing that, use one of the two smoke detectors we have.

Thanks for the suggestions Norm

Norm Dresner: >

Depends upon what the counter was originally designed to count. If the window is thin enough, x-rays should be fine, unless you want to check the calibration rather than just find out if it detects gammas. The best way to find out is try it. If it doesn't respond, then either it doesn't or the window is too thick, in which case, you'll have to find a higher energy source. If you have a smoke detector which uses 241Am, that will also provide a source low energy (approx 50 keV) X-rays. (Your counter won't see any of the alpha's from the 241Am).

You might also try to find a physician or veterinarian who does radiation therapy. These days, even veterinarians have linacs and use radionuclides for treating cancers in animals. Decay energies for those isotopes will be nominally in the MeV range. If your counter doesn't see those, it doesn't work.