X-Ray Primer - Tutorial

[Richard Hull]

The discussion on pulsed fusor system x-rays by Richard Hester and Larry brought this idea to a head.

We need to create a simple numbskull's tutorial on x-rays to save lives and educate all of us.

For those looking for a great book which covers every thing in good detail from ions to radiation with a strong section on X-rays, you couldn't do much better than......Ions, Electrons, and Ionizing Radiation by J.A. Crowther. This book saw countless editions and probably nearly a million total published as a textbook of legion in the 20's and 30's. There are a lot of 'em out there and cheap too. I would suggest an edition or printing after 1930 to get some of the later poop included. On a scale of 0-10 I rate this tome as a 42!

X-rays, (for our purposes), are generated any time charged particles impact bulk matter. These can be produced in several ways. Among them are bremstraalung radiation or k shell electron-photon emission. It is not important to know more than this about the mechanisms for our purposes.

The frequency or energy associated with the x-ray is a function of the energy of the particle at time of impact. This reduces to the voltage gradient across the tube or x-ray producing device. More voltage, more energy, more penetrating x-rays. The maximum voltage impressed on a fusor determines the maximum x-ray photon energy found in the x-ray emission of the device.

It is very important to note that x-rays are spread out over a contiunuous energy spectrum from zero volts to the maximum applied voltage. Less that 0.1% of the x-rays actually produced in any device will be at maximum energy! The bulk of the x-rays will be at far lower energies. Likewise, in any x-ray device, less than 0.25% of the total energy expended in the device as electrical energy will come out as x-rays!! All x-ray tubes designed specifically to produce only X-rays are terribly inefficient. 99.75% of the energy in goes to heating the anode. With the fusor, it is probably a lower fraction, still.

I would guess, (based on simple figures pulled out of two texts on X-ray technology), that only about .00025% of the fusor input energy winds up as x-rays at or even near the acceleration potential.

To quickly review:

The penetrating power and energy of the x-rays is a function of accelerating potential.

The number of x-rays (the quantity) is strictly a matter of beam current associated with the particles actually hitting the target. At any given beam current, more x-rays will be produced if the beam impacts a high Z material (High atomic number). So, tungsten will be a more efficient target and produce more x-rays per unit current than, say, aluminum.

X-rays that burn horribly and burn fast are in the under 20kv class. Every photon WILL stop and WILL do total damage very near the surface of the skin yielding 100% total x-ray power damage to the skin. Medical x-ray machines all use a thin aluminum filter to totally elimenate this burning radiation from the beam.

Between 20 and 80kev the body will still absorb the bulk of all x-ray radiation falling upon it, but it will be more evenly distributed over the whole depth of the body. Still not good, but no real burns provided a thin aluminum filter is used.

Above 100kev, the body generally will have most of the photons zip right through, but those that do hit atoms within the body or create secondary compton radiation will be very nasty to internal tissue.

Above 500kev all the particles see us at whispy apparitions and go through smoothly. Again, however, the few that do hit do great cellular damage along the paths of the secondary particles and ions.

A pulsed system has immense peak pulse power x-ray bursts. The burst might be of fairly low energy or penetration x-rays if we use 20-40kev. Thus, the body will absorb almost the entire burst! BUT....the fusor shell, if metal, might not let one of those x-ray photons out! There is a point in applied potential where the shell will, however, begin to become more and more transparent.

Scott Little, who is an expert in X-ray systems, calculated that the standard .0625" thick SS shell might stop all x-rays up to 50kev!

Remember, that even with 50kev applied only a tiny fraction of the x-rays in a burst will even approach 45kev! So, while being very careful with pulsed machines, it is quite possible that up to 50kev, even 1 megawatt peak, pulsed power x-ray bursts would never leave the shell!!

All pulsed systems must use a nearby electroscope, quartz fiber type pen dosimeter to get the total absorbed dosage figure during operation. A geiger counter, or scintillation device is valueless here.

For those running DC, steady potentials, the geiger counter is the best way to sniff out X-ray leaks around flanges, glass ports, rubber hose line ports, etc.

To use a garden hose analogy....

The speed and punch through ability of the droplets are the voltage on the x-ray tube. The volume of water sprayed at any speed is equivalent to the current in the charged particle beam.

200kev x-rays produced by a proton beam of 2 microamps is like a water saw. ( a microscopic stream coming out at tremendous velocity and penetrating power). A 30kev x-ray beam from a 1000amp pulsed electron beam intensity is like a fire hose at full blast hitting you. Instead of razor cutting the flesh locally in a fine point, it will throw you across the room physically.

You must know your x-ray hazards when working with fusors, either with continuously operated (DC) or pulsed high energy systems. This can only be done with a full armory of detectors and the knowledge of when to use them and how to interperet the results.

Richard Hull

[guest]

Mr. Little is correct but a sphere needs holes cut in it.
Viewports, vacuum feedthroughs, and seams need
Professional care for it to approach the ideal to stop all
emission leaks below 50 kv. Some vaccuum sealing using viton seals or apiezion waxes and indium solders that are lead free are totally transparent to x-ray. Another myth I like to shatter is this one: X-rays cannot go around corners. X rays can bounce around due to glancing diffraction with the metal surface. It won't be at full intensity but it could burn if you to grab a seam.
This also means for cheapos no pyrex bell jar pulse
fusors, no plastic descicator or plastic bell jars either.

Larry Leins
Physics Teacher

[guest]

I agree that more information on X-rays would be helpful. Particularly helpful would be the absorbtion coefficients of material like steel, lead etc. So, that it would be easy to calculate the thicknesses of material need to sheild particular energies of X-rays/gamma rays.

I have found some material on this (on the web), but I don't know how far to trust it.

thanks Bob S.

[DaveC]

Last year, we did a short "white paper" on X-Ray measurements. Here are a couple more references that we found useful, to add to those Richard gave.

1. "X-Rays in Theory and Experiment", by A.H.Compton and S.K. Allison, D. Van Nostrand Co., New York 1935.

2. "Principles and Practice of X-Ray Spectrographic Analysis" by E.P. Bertin, Plenum Press, New York 1975.

3. "The Fundamentals of X-ray and Radium Physics." by Selman J., Thomas Books, Illinois 1994.

4. " X-Ray Science and Technology.", Editors: A.G. Michette and C.J. Buckley . Institute of Physics Publishing, Philadelphia, 1993.

Found these at the Cal Tech Library. The first reference by Arthur H. Compton is also one of the "classics".

Dave Cooper

[DaveC]

Regarding scattering and X-Ray secondaries, we used a design "rule of thumb" that you need at least three reflections to bring X-Ray intensities down to background level. Obviously, you always need to perform a careful X-Ray scan while your device is operating,... AND... it needs to be done at the highest current and voltage the system can deliver.

X Ray intensities will vary approximately linearly with respect to fusor current, but increase as the square of the voltage. Given the general inefficiency of all detection systems, one needs to try for shielding to a level indistinguishable from the background.

I believe Richard Hull has commented on this next point more than once, but it does bear repeating often. X-Rays can emerge in narrow beams and travel great distances.

In what was, in retrospect, a most hare-brained experiment, I got to make measurements of X-Ray intensities from a 90 kV electron beam source operating in the open air in an isolated parking lot!! (No, the experiment was not my idea, I thought it was in poor judgement - I just observed it from a safe distance !! )

My instrumentation showed detectable levels came through concrete block building walls and windows 30 ft away. We could detect above background for over 100 feet out from the exit end of the tube, with the beam pointing vertically into the air.

So... a word to the wise... test often, be careful... very careful.

Dave Cooper

[Richard Hull]

As most fusor builders will never ever see 40kv on their fusors, the thinest thickness of lead will stop the rays DEAD! 1/4" lead plate would be spartan protection indeed! Remember, only the most microscopic amount of the X-ray flux will be at an energy level (kev) of your applied potential. Also, the fusor is a terrible example of an x-ray machine. Real X-ray machines turn the applied energy into x-radiation at an abyssmal 1-3% efficiency and they are designed to maximize production! The fusors are much, much more x-ray inefficient. I have had my machine up to 35kv and I can't detect with any device I have, any x-radiation using a 1/16" thick layer of lead over my horribly leaking view port which produces such an unshielded torrent of x-rays that it swamps every geiger counter I have into inactivity. They are just shut down as the gas is so horribly ionized that counting can't even proceed. This is scarey, but 1/16" inch of lead sheeting turns it into a non-issue.

For sniffing out leaks, a mica windowed geiger counter is the best thing to have. Start back about two feet from the suspected leak as the counter could instantly swamp out placed directly against the leak. Thin windowed ionization chambers will not swamp, but most folks don't have such an item.

X-rays are always an issue and the major issue associated with operational fusor safety right after electrocution issues. Fortunately, they are easily and simply dealt with.

Richard Hull