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FAQ - X-rays, X-rays, X-rays....A tutorial

Posted: Sat Dec 09, 2017 7:16 am
by Richard Hull
I do not like starting a FAQ in an imperious, condescending manner, but not enough to not do it.

It seems we have the usual dunderheads arriving here refusing to consult the FAQs. But far more often, refusing to be pro-active and become involved in what is termed "self-directed learning" outside of these forums, like God-forbid, looking something up in a good book on the subject. I hope all who read this and feel they might fit into this category are now suitably shamed and admonished. For those who at the very least take the time to actually read this FAQ desiring a quick rinse into the ever asked "X-ray question", thank you for your attention in this matter.

Here beginneth the lesson
X-rays are classically produced when any high speed particle impacts a solid of suitable density at a suitable speed. In our case, this is electrons hitting the shell of our fusors. The fusor is an x-ray producing piece of scientific apparatus. X-ray energy production tends to follow a more or less Maxwellian distribution curve. That is a curve plotted as a function of energy applied to the electrons, versus the energy of the X-ray photons produced. The overall implication is that the X-rays produced are spread over a vast spectrum of energies from 0 eV to the maximum voltage applied to the X-ray source, i.e. 50,000 volts sent to an X-ray tube will have very few x-rays emitted at 250 eV and very few emitted at 50 keV. The highest energy attained by the most number of the X-ray photons might peak in the 30 keV range, rapidly falling off at the higher keV range. This rapid fall off in high energy is often referred to as the "maxwellian tail".

We speak of energy applied in our discussions above. This energy involves two factors; the accelerating voltage which determines the spectrum of energies of the X-rays and the current fed to the x-ray tube. The current determines the number of X-ray photons produced. The more current applied at any given voltage, the more intense the x-ray blast. (The more X-ray photons per unit area within the blast.)

The keV of the photon determines its penetrating power, (energy). The current determines the intensity of the X-ray blast. Current has nothing to do with the penetrating power at all, just how wide the X-ray flood gates are opened. (A trickle or a flood)

As an X-ray tube, the fusor is of a rather terrible construction, therefore, it is not an efficient x-ray tube.

"The X-ray Question" - God save us, henceforth from these

"Can I die operating a fusor?"..."How bad are the X-rays?"..."Do I need a lot of lead shielding against x-rays"...."My mom and dad or my professor/teacher are concerned about the x-rays from my fusor." Rather than answer these questions individually, let's look at the real life scenario regarding x-rays and the fusor.

For practical purposes, we may look at the fusor as a more or less point source of X-ray radiation. The laws of physics governing radiation says that the intensity of the radiation, (number of x-ray photons/unit area), falls off or decreases as the inverse square of the distance from the source of radiation, i.e. at twice the distance the radiation is cut to only 1/4 of what is was. At three times the distance only 1/9 remains, etc.. This is simply known as the Inverse Square Law. Thus, putting distance between a source of radiation, (the fusor), and yourself quickly reduces any radiation flux hitting you. So, operating an x-ray producing fusor at one foot is 36 times more dangerous than operating it at 6 feet.

"The Great Rule": Put distance between you and your fusor and you might not need shielding at all!
"What?! No shielding at all??!!"
"Are you nuts?"
No, it is true, no shielding is needed in most all situations regarding fusor operation. Use the inverse-square law as your first line of defense and safety regarding any source of radiation. It is worth 3,000 lbs of lead shielding any day.

Now to the nitty-gritty of the x-ray tutorial

Demo fusor - In general, demo fusors are of glass, bell-jar construction. The normal danger here is not X-radiation, but collapse and implosion of an ill-chosen glass vessel placed under vacuum. This can throw glass shards about the room. (Very nasty) Assuming you have a suitable pyrex bell-jar of suitable thickness, no x-rays will be emitted as few demo fusors ever operate over a few kilovolts, (3kv to 8KV). Most thick glass vessels will absorb x-rays that can be produced in the 10-15 keV range. Thin glass jars or vessels that survive being vacuumed to plasma producing levels, and are managed to be operated above 15 kv applied, can produce nasty skin burning x-rays if you dare to stand close to them when operating at these elevated voltages. DO NOT DO THIS! Ion beams at these voltages can cause a dangerous implosion of thin walled glass vessels.

Metal Demo fusors - Serious amateurs who want to go on to fusion might construct their first device as a formal stainless steel vacuum chamber easily moving directly from a demo fusor to a real nuclear fusion device. Most such demo fusors can easily handle 15-20 kv applied if the vacuum gear can take the chamber low enough in pressure and the power supply can supply currents needed at these advance demo fusor levels. If there is a glass window or view port you must realize that it is now a very dangerous x-ray port. Remember that x-rays will not penetrate a suitable .060 inch thick chamber at all until about 30kev applied. However, at about 15 kv or more applied, x-rays will pour out of a view port window. Point your window away from you or any observers! Nasty, burning x-ray blasts will exit this window. To observe the inner grid, use a mirror with a small binocular or telescope to look in the window. Or better still, use a cheap video camera looking directly into the window and view on a nice large monitor screen.

The "Real Nuclear Fusor" - Few are the amateur scientists and would-be fusioneers who ever enter this realm. As noted, 100% of of all successful fusioneers use a stainless steel fusor vessel of suitable thickness. View ports become, literally, a deadly source of intense x-radiation in fusors operating at or above 40 kV and the needed fusion currents demanded to do easily detectable fusion. The shell starts to go "transparent" to only the highest energy x-radiation photons at about 35kv applied. These are not deadly or dangerous as they tend to bore right through you and are of minimal intensity. This is due to most of the X-ray energy produced by the fusor is still absorbed by the fusor chamber's shell. (Remember from above that most of the x-ray energy is far short of the maximum energy imagined or achieved by the highest applied voltage!! <Maxwellian-tail>)

Once applied voltages soar above 50 kV applied, the fusor, as a device,becomes a dangerous source of x-rays as the "shine-through" x-rays are getting very intense. Lead shielding is a must above 50 kV. Such shielding recommendations are found in other FAQs in this radiation sub-forum.

Sniffing out x-ray leaks around a fusor

A simple geiger counter can be used to locate and, hopefully, help eliminate or shield out x-ray hazards around a fusor. GM counters are unusually sensitive to x-rays. Test the inverse square law if you find an x-ray leak. Close-in to the fusor, (under 1 foot), a counter might go totally nuts or saturate to total silence. Move back 6 times the current distance and the counter counts less rapidly. Double this distance perhaps to your operating station and the counts are way down. All of this is great for locating nasty blasts of x-rays, but a GM counter is worthless in quantifying just how much radiation is actually there. The GM will only tell you there is radiation present.

To determine just how much radiation is there and get a handle on the real danger or lack thereof, you must have a calibrated ion-chamber type detector. The average neophyte, would-be fusioneer rarely has this sort of instrument. If you ever get to a serious, functional fusing fusor, you really must obtain an ion chamber type of instrument. This will indicate the dose rate of x-ray radiation in milli-rem per hour that penetrates its often thin, yet solid, window at the opening of the chamber.

Here endith the lesson...and in summary

As a parting shot, I offer a typical example of how I put on my big-boy pants and run my fusor that does produce real neutrons and x-rays.

I am fully instrumented and can operate my fusor in the 43kv applied range producing over 1.4 million neutrons per second emitted isotropically. The fusion running currents rarely exceed 15 milliamperes. I have no shielding around my fusor. My view port points down into the concrete floor of my lab and has a small video camera jammed against the glass. I view the grid and plasma star on a large video monitor. I operate from a distance of 7 feet from the fusor body. Yes, real x-rays are emitted due to shine through at 43kV, but the intensity is marginal and acceptable at my operator's station. A professional Victoreen ion-chamber instrument can read as high as 50 milli-rem/hour at the shell, but only reads about 1.5 milli-rem/hour at the operator's station. I rarely run the fusor at full tilt for more than 10 minutes. Next, I do a data reduction and then take it back up for another 10 minutes.

On a busy day with the fusor, (only happens about 5 days per year now), I might be exposed to full run operation for a total of 30-40 minutes over a total 5 hour session from start up of the vacuum pumps after lunch to shut down for dinner. I also wear a 0-200mr/hr dosimeter on such days and it always reads about zero due to "scale crowding" at the end of the day. However, based on the Victoreen's reading, I really probably absorbed about a 1 or 2 mrem dose. This is nothing, of course. Most people on earth get about 350-400 mrem per year total dose just by living in a NORM and cosmic ray environment and consuming barely radioactive food and drink. So I get about 1 full day's normal external background dose in the 40 minutes that I run the fusor. This means I get an additional 5-6 mrem per year because of my fusor. Not worth honorable mention. I could lay out on a nude beach for an afternoon in the thorium rich sand of Brazil or Sri-Lanka and get 8 times that dose, plus some disgusted looks from good looking gals which would hurt me far worse.

99.99% of the people in the plasma club need not consider x-rays at all. Only 1-3% of the real fusing fusioneers here ever need to be seriously aware of their X-ray operational environment and provide for the blessed relief of the inverse-square law. In reality, far less than 1% of we "big operators" need any form of lead shielding.

My last X-ray FAQ of 2012... viewtopic.php?f=31&t=6313
can give a long 6 page diatribe of responses and questions including some nit-picks near the end of the responses to those who wish to read more

Richard Hull