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Making a Geiger-MÃ¼ller Counter

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LAN_403

Dr. Who

Sat Dec 15 2007, 12:38AM

Registered Member #326 Joined: Sat Mar 18 2006, 01:12PM
Location: Glasgow, Scotland
Posts: 66

A household smoke alarm is the most commonly available radioactive source - many use a sensor containing an alpha emitter.

plazmatron

Sun Dec 16 2007, 10:04PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Yep, Am-241 from smoke alarms (though you have to place it close to the tube to get a good reading), Th-232 from the older gas mantles, an theres always e-bay, some sellers occasionally sell old surplus luminous watch hands, containing Radium ( I managed to buy about 50 in one go!)

As c4r0 says, x-rays, will easily overload geiger counters, since they produce millions of x-ray photons/sec, so you may be as well getting an ionisation chamber detector, like the Radiac`s on e-bay, for high dose rates. Of course, because of their sensitivity, geiger counters, are great for checking the integrity of your shielding.

Leslie

MOT_man

Mon Dec 17 2007, 03:16PM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

Red/ Yellow Fiestaware from 1960's and below are colored with Uranium/ Thorium based glazes - Vasoline glass (neon tinge to it) glows under UV light contains Uranium Nitrate. Both types can be strongly radioactive, but nowhere near hazardous.... I still use them to serve off of... :) They are sealed so contamination risk is nil.

Proud Mary

Thu Jan 17 2008, 06:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

General purpose beta/gamma GM tubes are not a good way to detect and measure X-rays, and some will not react at all to even high levels of hazardous low-energy X-rays. There are specially produced X-ray sensitive GM tubes which have a much higher gas pressure, have a mica end window like a longer version of an alpha tube. Cold War era GM tubes were usually designed to be very insensitive so as to have greater linearity at high count rates of hard radiation.

Tubes with metal jackets, or where the cathode forms the tube case, will block almost all of the low energy X-rays likely to be produced by a small scale experimental set up, and so detect little or nothing. Think of the thin aluminium filters used to block low energy X-rays in dental machines. And remember that low energy X-rays are the most hazardous of all!

An ionization chamber is almost always to be preferred for the measurement of X-rays since chamber current is proportional to X-ray dose rate. One can be readily built around a diecast metal box and can be operated at atmospheric pressure so as to have only the thinnest polythene window - or no window at all. One of the plates should be charged, and the other taken to a guarded ultra high impedance amplifier which forms the first stage of a pico-ammeter. And away you go!

c4r0

Thu Jan 17 2008, 09:27PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

My counters based on SBM-20 g-m tube easily detects x-rays generated by x-ray tube powered from 50kV. I'm sure that measurement is much lower than it should be, however it detects this radiation. SBM-20 is designed for measuring radiation >100keV.

Medical x-ray machines usually uses 1-2mm Alluminium for filtering, but it's not so necessary until you're not going to x-ray yourself :)

plazmatron

Mon Jan 28 2008, 12:46AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I have a cd-v700 geiger counter and even my smallest (12W, 40kV) tube will peg the meter on the highest scale! (with Beta shield open)

I also have one of those Russian counters that uses one of those horrible mini steel can geiger tubes, the SBM21, and again, my x-ray sources will easily drive it to saturation.

The problem with these is accuracy of the reading. Normally you would use an ionization detector, or a dosimeter, from which you would be able to calculate your dose rate. However even a relatively insensitive tube, like the SBM21 should be able to verify the presence of radiation between 30 and 100keV (just assume the reading to be much higher than it is, and take precautions accordingly. Remember, you are aiming for NO detectable radiation, in the area you are in)

Leslie

Proud Mary

Sat Feb 02 2008, 03:28PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I believe that mica end-window GM-tubes - like those found in the legendary Mullard ZP1400 series - can generally detect X-rays down to about 10keV along the length of the tube's major axis, where the rays must pass through the maximum amount of gas. Even so, counting efficiency will be very low, with few X-ray photons being able to create sufficient ion pairs to start a Townsend avalanche.

The energy response curve of typical GM tubes is such that a peak in sensitivity spread across 10 - 100keV results in significant over-counting of low energy X-rays - as much as 500% for an uncompensated tube when compared with a standard 137-Cs 662 keV calibration standard.

plazmatron

Thu Feb 07 2008, 12:59PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Harry wrote ...

The energy response curve of typical GM tubes is such that a peak in sensitivity spread across 10 - 100keV results in significant over-counting of low energy X-rays - as much as 500% for an uncompensated tube when compared with a standard 137-Cs 662 keV calibration standard.

That would explain the frighteningly high readings of the CD V700 when compared with a proper dosimeter ! ( I have several, and they all agree with each other).

Proud Mary

Thu Feb 07 2008, 04:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Leslie, here is a fairly typical Photon Energy Response Curve for a small uncompensated G-M tube normalized to unity at 662 keV with beam perpendicular to detector wall.

When you are operating around the low-energy response peak then a relatively small increase - or decrease - of photon energy [X-ray tube anode voltage] can cause a very large apparent count rate change. Also, it is as well to remember that a typical GM tube may show roughly the same count rate for 20keV photons as for 120keV on the other side of the peak - an ambiguous state of affairs that can easily mislead.

Conundrum

Fri Feb 08 2008, 09:46PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Hmm. MacGyver would be impressed :)

I still want to know why someone hasn't devised a servo-like arrangement that simply moves a variable thickness lead/copper alloy on a pivot across the sensor. As the radiation flux increases it rotates to keep the sensor from saturating then applies a scaling factor.

/me scuttles off to try this.

-A

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