CPM of X Ray Machine

Thread Starter

ben sorenson

Joined Feb 28, 2022
90
I was just curious what the CPM reading would be (approx) if you took a geiger counter and placed it at/around/near the strongest point on an xray machine or tube. Do X ray machines emit extreme CPMs ? What would the strongest reading look like?
 

bertus

Joined Apr 5, 2008
22,096
Hello,

A good x-ray machine does not radiate x-rays outside.
The x-rays should be shielded for safety reasons.
Most times there are safety switches that cut the power to the x-ray source when the machine is opened.

Bertus
 

Thread Starter

ben sorenson

Joined Feb 28, 2022
90
Hello,

A good x-ray machine does not radiate x-rays outside.
The x-rays should be shielded for safety reasons.
Most times there are safety switches that cut the power to the x-ray source when the machine is opened.

Bertus
Yes, I get that they do not radiate them outside the machine. My question is if a geiger counter was placed at, in, near the most point of collision. What would be the CPM range one could expect to see on a geiger counter.? I probably didn't word the question properly, I'm sorry.
 

DickCappels

Joined Aug 21, 2008
9,320
A medical X-Ray machine makes enough particles to make a high resolution moving image on the monitor. That takes a lot of particles per second.

Actual count? A LOT!
 

Ya’akov

Joined Jan 27, 2019
6,575
Actually, a geiger counter isn’t a very good way to measure the ionizing radiation produced by X-rays. The nature of the instrument makes it slow. There is a dead time of 10-100ms between counts because of how it operates which makes it good only on the order of hundreds of counts per second.

The number of high eV electrons (eV is Electronvolts, a measure of the kinetic energy of an electron, equivalent to a Joule) will diminish with the distance squared. This means that a lot more high eV electrons will be encountered next to the tube than even a short distance away.

It is very likely the counter would be overwhelmed by this flux and simply max out, giving no reliable count at all. In the case of narrow beams used in X-ray therapies for cancer, it can even read almost nothing, the opposite problem.

A geiger counter is based on a Geiger-Meuller tube which is a metallic cylinder with a central electrode and filled with gas. The cylinder acts as a cathode and the central electrode as anode. There is a high voltage between the two.

When a particle from a radioactive decay or a high energy photon from X- or γ-rays penetrates the metallic wall it causes a Geiger Discharge which is an amplification caused by the ionization of the gas in the tube leading to a Townsend avalanche.

The avalanche happens because the the liberated electron produced by the radiation is accelerated towards the anode by the potential gradient that is a result of the high voltage between cathode and anode. This acceleration creates yet another source of ionizing radiation within the tube and so many more free electrons. The conductive path created by the ionization that results causes the click that is heard in a speaker on the counter as current flows—the Geiger discharge—from cathode to anode for a brief time.

After the discharge, the gas in the tube may be highly ionized causing additional discharge events which are not the result of a high energy electron but of the device itself. ”Quenching“ is used to eliminate this. It is done by either reducing the voltage applied to the tube for a fixed time after a discharge, or using a mixture of gases that are harder to ionize than a simple noble gas.

Because of the time it takes for the discharge to occur, and the added overhead of external (electrical) quenching if it is used, the geiger counter is a very limited instrument in terms of high electron flux. It also provides no information about particle energy, and requires calibration to a particular spectrum.

On the other hand it is very sensitive, cheap, and well suited to general radiation measurements like leak testing and total presence of radiation for surveys. As @DickCappels said, it would almost certainly count A LOT of counts, which is as scientific as it is likely to get.
 

ThePanMan

Joined Mar 13, 2020
496
If you had a 1CPM at 1 meter, at 2 meters you'd have 1/4CPM.

A 1 foot square had 1 square foot. A 2 foot square had 4 square feet. Inverse square law. Double the distance is four times lower. Vice versa, if you reduce the distance by half you increase exposure by four times. To answer your question with facts - facts are needed. In, near or around are not facts. No more than the commercial that says "I lowered my A1C from 'Here' to 'Here'."
 

MrSalts

Joined Apr 2, 2020
2,376
If you had a 1CPM at 1 meter, at 2 meters you'd have 1/4CPM.
That is a really bad example because you forgot to say you were doing the experiment in a vacuum. X-rays are strongly absorbed by air. Your 2meter example is way, way, way overestimated if you are in air.
 

Martin_R

Joined Aug 28, 2019
132
That is a really bad example because you forgot to say you were doing the experiment in a vacuum. X-rays are strongly absorbed by air. Your 2meter example is way, way, way overestimated if you are in air.
As an X-ray service engineer we always use the inverse square law to determine the dose regarding distance. Works fine in air! Most X-ray examinations are taken with a 115cm distance between tube and film( or digital plates). Sometimes a source image distance of 180cm is used, this is done to reduce the magnification on the film.
 

MrSalts

Joined Apr 2, 2020
2,376
As an X-ray service engineer we always use the inverse square law to determine the dose regarding distance. Works fine in air! Most X-ray examinations are taken with a 115cm distance between tube and film( or digital plates). Sometimes a source image distance of 180cm is used, this is done to reduce the magnification on the film.
What energy are the x-rays?
 

Martin_R

Joined Aug 28, 2019
132
Like the photon wave/pariticle duality. Under a low dosage rate you can see the "twinkling" and "crawling" on a fluoroscope screen.
The particles are quanta, and are exactly as you describe on a fluoroscopy procedure. Fluoroscopy is when the image is 'live' and displayed on a monitor screen. The X-ray tube will be operating between 40 to 110kv depending on patient object thickness, at tube currents upto 5mA.
Radiographic exposures in a typical A+E room would be between 40-125kv and the exposure is measured in milliamp seconds (MAS). A typical tube has 2 focal spot sizes, 0.6mm and 1 mm squared to obtain the sharp image required, at a power of 30 and 50kW respectively. So yes, a LOT of radiation.
 
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