Geiger Counter for X-ray Application

snipped-for-privacy@yahoo.com wrote: Since these are numbers going into FDA

Might want to look at the calibration rules a little harder. The rules could say that it has to be locally calibrated after shipment from the factory. If so, you might want to call around to some local labs to see what they say about this.

Thanks for the response. That's a good point. What confuses the heck out of me is why the two units I listed are so cheap (like many others, too). They cost like $400. An analog or digital unit from one of the well known brands (Bicron/NDS/Ludlum) are huge and and cost from $600 (analog)-$3000 (digital). Where does that price difference come from and should I care about it (the functional difference, not price difference)?

Geigers are not used for medical X-rays. The energy spectrum of geiger tubes forbids that.

You may use either a ionisation chamber, or even better a modern cadmium-zink-telluride sensor.

There are very stringent rules how to monitor medical equipment and you better contact the manufacturers of your machines.

w.

w.

Thanks for the input. This is actually not for a medical device. FDA does not just regulate medical devices but also for instance x-ray cabinet systems. They have certain requirements that need to be met for these kind of devices. One stipulation is that you cannot exceed

I'm still puzzled why there are such huge differences between different devices ($500 vs $3000).

Since we use tube voltages similar to medical I am intersted in your statement that a GM tube can't be used. Why is that?

I am going to take your word for it and assume you know what you are doing. You ask why prices differ so much. I can think of may reasons, boiled down to words such as MILSPEC, Marine and Medical. Those words can make a world of difference in prices, usually up.

Of those, I think that Milspec is legitimate in this case. A counter of that type must meet a pretty good shock.

X-rays and gamma rays are essential the same thing. X-rays are below 100keV, gamma rays range from 100 to 3000 keV with some overlapping, do not take that as a sharp distinction.

Let us look at the energy spectrum of a typical medical device: it ranges from 0 to 100 keV

Now we look at the energy response of a geiger tube:

The sensing threshold is about 60 keV, with a large overproportional peak around 100 keV. For linearity over the energy range the tube has to be linearized by wrapping it in lead and copper. Still there is no response below 60 keV.

Geiger tubes are calibratet with Co60 radiation at 1,33 MeV e.g. the LND712 which is used in the Gamma-Scout

I cannot give a recommendation about what device to buy, please join the Yahoo groups and put your question in there, they are very knowledgeable people.

w.

You may need to employe a Certified Health Physicist (CHP) to make the measurements and certify the results if FDA is involved.

Depending on your state regulations you may need to use a CHP approved by your state Nuclear Regulation division, usually part of the state health department.

GM tubes can be used at low X radiation levels but normal surveys are done with ionisation chamber or solid state detector.

Hugh Retired Nuc and not looking for work

I do not know just what the FDA regulations aare, but a true Geiger counter is not going to give a good exposure measurement. Every Geiger click will be treated the same way whether it comes from a high or low energy photon. My guess is that you want something like an ionization chamber in which high energy photons can generate a number of counts roughly proportional to the photon energy. Even an ion chamber will not tell you how tissue will react because tissue will have a different composition than will air or whatever gas fills the chamber.

The right way would be to use a proportional counter and a pulse height analyzer. That way you will get a spectrum of the various energies in the x-ray. That could be converted to an effective exposure in various ways by weighing pulse heights in proportion to how they affect tissue or whatever material is of interest.

Solid state radiation counters would probably be better to use rather than gas counters.

Bill