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FAQ - X-ray radiation!!

Posted: Mon Oct 29, 2012 5:16 pm
by Richard Hull
Due to some recent questions and concerns I will make this FAQ about only this subject, even though it has been dealt with before.

All fusors, demo or real, that work over 20kv need to be made of 304 stainless steel for sufficient operational safety at these voltages.

All glass or bell jar type demo fusors should not be operated above 15kv and holding the voltages below 10kv would be better.

If there is any glass in the system, View port or bell jar, do not allow the port to point at any human or observer when operating over 20kv This is an ideal x-ray port and is very dangerous.

With a real fusor operating at 35kv or above, you will start to emit x-radiation from the SS shell. DO NOT sit or operate the fusor closer than 6 feet to any person for any period of more than 10 minutes at a time. Remote operation (15 feet or more) and or a small lead shield is recommended.

Above 45kv, remote operation is highly recommended and a simple lead shield would normally be a must. (1/8 to 1/4-inch thick lead should be OK up to 80kv) Any shield only needs to create a shadow cone in the direction of the operator/observers. Always make it larger than actually required.

The best advice for all operating SS fusors at 35kv or above, follows.

1. Never run a Stainless steel fusor at 35kv or higher for long, protracted periods of sustained operation without a lead shield or a good separation!
2. Always wear a 100-200 mrem pen dosimeter! Do not use the common, yellow 100r Civil defense dosimeter.
3. Do what you can to put distance between the fusor and you and, or observers. This can mean, no shield needed.
4. Avoid using very large view ports!
5. Aim or point all view ports and ceramic insulators away from personnel. Assusme they are x-ray beam ports!
6. Try to create at least a 1/8-inch thick lead shield such that a shadow cone protects the operator. For fusors up to 45kV applied only distance between the operator and fusor is needed. A 1/8 inch shield will stop 99.9% of all x-rays up to ~120kev!

I have operated my fusor at up to 43kv for short periods and have received, at most, a 5mrem total dose, over an entire 2 day period of intermittent operation. This is a very low dose for two days of operation. I operate my fusor maybe 4 times each year.

WARNING! ***** BEWARE! Any experiments using glass sections in an accelerator, large view ports or large ceramic sections and voltages beyond the common 20-40kv in all stainless steel fusors and currents in excess of 10ma will present special x-ray problems and dangers to the experimenter. You are on your own if you go "outside the box". You MUST have a direct reading electronic ionization chamber meter to sniff out x-ray hazards if you work a special system at higher voltages and currents.

Finally, if you are making an oddball, experimental device, ie, an accelerator, highly modified, non-standard, glass containing device that is not all stainless steel, it is recommended to all that a 1/8" solid lead shield encase the entire device. You MUST have a direct reading electronic ionization chamber meter to sniff out x-ray hazards if you work a special system at higher voltages and currents.

We cannot stress too much that all x-ray hazard advice given here is specific solely to all stainless steel, standard, spherical construction, amateur fusors, operated at voltages under 40kv and currents under 10ma! This advice is not necessarily correct for any other type of homemade fusion device.

Richard Hull

Re: FAQ - X-ray radiation!!

Posted: Mon Oct 29, 2012 6:33 pm
by Doug Coulter
Some further observations.
Sitting in front of my big viewport, which has 2 3/8" pieces of pyrex followed by a 1/2" by 15x24" piece of leaded glass from a hospital rad lab, my Canberra dosimeter doesn't report anything above background.

But let me say- almost the entire tank has 1/8" lead covering it, and this port is quite aways from the "action" - a little over a foot. I feel like I need better viewing that I get with any of my cameras or anything I've seen posted here, since my poisser focus is one of the things I'm working on, and no camera can resolve this so far.

HV and other feed throughs are leaks - bad ones. I've jury rigged big lead pipes and such and run HV into the otherwise sealed end via special coax.

But you have to let some out to measure. So I have a deliberate leak right back on the sidearm where the fusion happens, normally covered by my lead-shielded neutron oven. There's about 1-2 sq inches of "leak" there, and it makes a geiger counter read perhaps 100x the normal background over most of the room due to scatter off things. Even 10 feet away, and on the opposite side of the room from the "leak" and with intervening lead. In my case, it's actually lower dose right at the window than 4 feet back due to this scatter. Close in, the lead window catches it...

Sure, those scattered X rays are lower energy than they started out with - but they'll still make a geiger tube count hard, and it's a bit scary how they bounce around.

So, truth is, no simple set of rules applies. Get a portable geiger counter and check! You're probably going to get some surprises when you do. Obviously, everything gets more critical at 53kv/30ma than Richard's conditions, but it's something everyone should pay attention to for their own safety.

Don't guess. Measure. Period.

Re: FAQ - X-ray radiation!!

Posted: Fri Nov 02, 2012 10:25 am
by Dennis P Brown
Really a great post and I do not think this topic is ever over worked.

I have discovered that slate works very well as a shielding material; better still, slate is a great insulator; also, ultra cheap and so easy to obtain (Home Depot carries cheap 12 x 12 squares in boxes.) As for using it, slate is easy to SAFELY cut using a cheap abrasive blade, and can be disposed in the yard when not needed. Yes, requires a thicker slab for a given energy but can be drilled (using a standard bit or masion bit) and bolted/glued as needed. Can't think of a more perfect shielding material (the insulator part and being so cheap.)

Re: FAQ - X-ray radiation!!

Posted: Fri Nov 02, 2012 3:15 pm
by Doug Coulter
I like this idea Dennis, if you don't have ready access to lead sheet as I do. It was a major pain to shield my fusor with overlapping solder joints and little cut pieces to fit around all the warts my tank has.

Re: FAQ - X-ray radiation!!

Posted: Fri Nov 02, 2012 3:29 pm
by Richard Hull
For those who can't remote operate 20 or more feet from the fusor, and don't want to coat their fusor with lead sheeting, an operator's shadow cone shield is acceptable. I think I did an old FAQ on this. Each method has its own advantages and disadvantages, but all shield adequately.

If you are regularly working over 40kv, a shield is just about imperative along with a direct reading electronic ionization meter like a "CUTIE-PIE". Hanging 200mrem doismeters in an around the fusor can be instructive as well, especially at really high operating voltages, (>80kv) where scattering might be significant.

Richard Hull

Re: FAQ - X-ray radiation!!

Posted: Fri Nov 02, 2012 4:31 pm
by Rich Feldman
Richard Hull wrote:
> ... Hanging 200mrem doismeters in an around the fusor can be instructive as well ...

Agreed. But I have found 200 mR pen dosimeters insensitive to point-blank x-rays from a Coolidge tube operating at 11 kVp, while a nearby Geiger counter registered lots of activity.
Didn't matter whether the beam was aimed at the side or either end of the pen.

With Coolidge tube at 22 kVp (full winding of a 15030 NST), a pen dosimeter responded rapidly from any angle. It was an easy exercise to locate its sensitive spot.

Around fusors, I think a quiet Geiger counter (one with a thin window) indicates that x-ray hazard is probably negligible. The one reported above easily detected x-rays from unspooling Scotch tape in a vacuum. A noisy Geiger counter implies POSSIBLE x-ray hazard, and need to get more quantitative.


Re: FAQ - X-ray radiation!!

Posted: Fri Nov 02, 2012 5:21 pm
by Jim Kovalchick
Interesting reference to the adhesive tape x-rays. My son measured his tape winder rays at 27 keV. Others showed that they range in value from about 10 keV to 50 keV with most being around where Mike measured them. As you noted, that range is easily detected with a thin window Geiger. Less than a 15 feet of tape unrolled at a few inches a second was enough to overexpose dental x-ray fill placed close to the roll.
Jim K

Re: FAQ - X-ray radiation!!

Posted: Mon Nov 05, 2012 5:50 pm
by Richard Hull
A GM counter will tell you nothing about the x-rays other than something is there.
A true SS fusor will never emit 20kv x-rays except out of the viewport window which will never be pointed at people. All SS fusors will leak radiation of an energy that all low reading pen dosimeters will always record. If pen doismeters read, then you need shielding when running a fusor.

Bell jars or any system with glass unwisely positioned or placed in the assembly can be a hazard for x-radiation at elevated voltages. Simply, never use glass! If you must, then shield with lead as a matter of course and be armed with an electronic direct reading ionization meter. The best of these have a thin aluminum window that will stop very low energy x-rays for the most part. (Below approx.10kev.)

Richard Hull

Re: FAQ - X-ray radiation!!

Posted: Wed Dec 05, 2012 3:08 am
by Andrew Haynes
2. Always wear a 100-200 mrem pen dosimeter! Do not use the common, yellow 100r Civil defense dosimeter

would this be a good geiger counter ... QgodVxcAkA

Re: FAQ - X-ray radiation!!

Posted: Wed Dec 05, 2012 4:06 am
by Carl Willis
I like the pen dosimeters. They are cheap and they respond to pulsed and high-intensity sources with no problems.

My recommendation for real-time measurement of x-ray fields near high voltage gaseous electronics is an ion chamber, essentially a larger version of the pen dosimeter that operates on the same principle but is continuously-readable and more sensitive. Geiger counters typically provide no means of interpreting count rates due to low-energy x-rays (either because they are not energy-compensated at these energies, or because they become paralyzed in high fields, or because count-to-dose conversions are not established or available for the tube). The best ion chambers for this purpose operate at atmospheric pressure and have a thin-walled chamber. Good examples are the Victoreen 470/471, the "cutie pies" made by various manufacturers, or the Ludlum Model 9. These instruments indicate in exposure units, typically roentgens per hour, and you can easily calculate dose rate from the measurement and be confident in its accuracy.

Geiger counters are less useful as x-ray survey instruments, but of course are very handy for many other jobs. My recommendation to get best performance for the value is to buy a "survey meter" and one or more separate Geiger probes. A good and popular survey meter is the Ludlum 3. The Ludlum 2 is more economical on the used market, and the Ludlum 12 is a premium instrument that can also operate proportional tubes and a wide range of scintillation probes. Eberline's ASP-1 is regularly encountered used; this has a microprocessor that performs real-time correction of detector dead time and also provides a count integration feature. A survey meter gives you the flexibility of using multiple kinds of detector probes. You can put on an energy-compensated tube that gives you accurate dose readings in a gamma field, or you can put on a pancake tube that gives you high sensitivity for beta and alpha emitters on surfaces. Most of the handheld "dosimeters" like the SOEKS strike me as being a poor value in comparison, but I have never handled the SOEKS itself.