FAQ - X-ray radiation!!

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Richard Hull
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FAQ - X-ray radiation!!

Post 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
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Doug Coulter
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Re: FAQ - X-ray radiation!!

Post 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.

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Re: FAQ - X-ray radiation!!

Post 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.)
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Re: FAQ - X-ray radiation!!

Post 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.
Why guess when you can know? Measure!
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Re: FAQ - X-ray radiation!!

Post 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
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - X-ray radiation!!

Post 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.

-Rich
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Re: FAQ - X-ray radiation!!

Post by Jim Kovalchick »

Rich,
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.
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Richard Hull
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Re: FAQ - X-ray radiation!!

Post 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
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - X-ray radiation!!

Post 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 http://www.dosimeter4you.com/geiger-cou ... QgodVxcAkA
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Re: FAQ - X-ray radiation!!

Post 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.

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Richard Hull
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Re: FAQ - X-ray radiation!!

Post by Richard Hull »

I am surprised that Andrew asked this question. In two posts in this flow, I specifically recommended an ion survey meter if you are working over 40kev and mentioned that a GM counter was only to be used as an indicator of the presence of ionizing radiation.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - X-ray radiation!!

Post by Andrew Haynes »

Thanks Carl , I found this site http://www.chpconsultants.com/index.php ... ments.html which has the Eberline's ASP-1.
Richard just was shore if that was a good meter
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Re: FAQ - X-ray radiation!!

Post by ab0032 »

A long time ago I calculated the attenuation of wax for x-rays on some online calculator and got something like 15cm of wax would absorb 99% of the x-rays. I probably would never find this attenuation calculator again, but you could choose materials and thickness and perhaps also the x-ray energy, I cant remember.

Could this be true? A wax shield with some borax in it is easy to make.

Any comments?
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Re: FAQ - X-ray radiation!!

Post by Richard Hull »

I can't imagine 3" of wax stopping hard x-rays. (>40kev). Use lead for xrays first and then backed with borated paraffin if you also want to attenuate the neutrons, too.

Sheet lead in any useful thinckness and size can be readily purchased from Roto-metals ready to rock and roll and at a reasonable price. I now use them exclusively for all my specialty metals needs. (Cast my own bullets - Pb:Sn:Sb, make my own specialty low melting alloys - Pb:Sn:Cd:Bi:In and special solders Pb:Ag:Sn:Sb:Zn) Roto has it all in experimenter sized lots and shapes, except the Ag.

I have discussed making a small "shadow cone" shield for fusors in prior posts here.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: FAQ - X-ray radiation!!

Post by Rich Feldman »

Alexander Biersack wrote:
> A long time ago I calculated the attenuation of wax for x-rays on some online calculator and got something like 15cm of wax would absorb 99% of the x-rays. I probably would never find this attenuation calculator again, but you could choose materials and thickness and perhaps also the x-ray energy, I cant remember.

I agree with Richard. Low-Z materials like wax and boron will do even less than water for x-rays.

I've used this online calculator for x-rays, and corresponded with the author:
http://www.radprocalculator.com/XRay.aspx
It lets you choose x-ray tube voltage (thus an energy spectrum, I bet assuming a tungsten anode) and shielding materials.
The following NIST website has tables (and an online calculator somewhere) of x-ray attenuation coefficients for many materials. Wax isn't on the list, but polyethylene is also just C and H at about the same densities, so looks the same to x-rays. http://physics.nist.gov/PhysRefData/Xra ... /tab4.html

[edit] just used radprocalculator for the first time in a couple years.

For x-ray tube at 40 kV (and 10 mA, which shouldn't matter) the thickness for 99% attenuation is
0.33 mm of lead
1.9 mm of iron
42 mm of aluminum
70 mm of concrete
345 mm of water

At 25 kV, Alexander's 15 cm of wax would probably do what he said.
As would 0.09 mm of lead (0.0035"), or 0.42 mm of steel (0.0165", about 28 gauge).

What attenuation factor is needed for safety? Even at 25 kV, the x-ray flux inside the fusor is enough to kill cells rapidly. But an ordinary stainless steel fusor enclosure, as touted by Richard H. et al, allows less than one millionth to escape. If not for that inherent shielding, the dose rate 1 meter away could be thousands of R/h (tens of Sv/h). Look up what that would do to you.
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Re: FAQ - X-ray radiation!!

Post by Steven Sesselmann »

Alexander,

You can set up a really simple experiment to measure attenuation, with an Americium source and a geiger counter or scintillation detector. Use two lead bricks with a small hole in, to collimate the beam set up the source bricks and detector so the 59keV x ray beam traverses a gap of say 4" , take a steady reading of the counts over a set period of time.

Now try inserting various materials across the beam and see how it attenuates the 60keV x-rays.

You will see that a thin 1/16 piece of lead will almost fully stop the counts, but what about a block of wax, glass of water or a leg of pork?

Even if we think we know the answers, there is nothing like actually doing it, and it is a lot safer to do it on a leg of pork, than using your own body as an attenuator.


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Re: FAQ - X-ray radiation!!

Post by ab0032 »

Richard Hull wrote:
> I can't imagine 3" of wax stopping hard x-rays. (>40kev). Use lead for xrays first and then backed with borated paraffin if you also want to attenuate the neutrons, too.
>
You are probably right for hard x-rays. I was surprised myself. But 15cm is 6". Maybe this is an option for people who are building a borated paraffin block to attenuate the neutrons. If they use low energies they might only need to make the paraffin a little thicker to handle the x-rays with the same shield and save some hassle with the lead.

I would definitely recommend checking the attenuation by measurements as you turn up the voltage and not trust some online calculator.
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Re: FAQ - X-ray radiation!!

Post by ab0032 »

Thanks Rich Feldman for these extra words of warning and the online calculators.

These comparisons for materials and energies are quit interesting.

So one is stuck in a dilemma, from the radiation side you would like to stay at low energies around 25kV if possible and from the neutron count side, one would like to go to high voltages to get as many neutrons as possible.

Nobody should expose himself to Si/h, but as you show, this can easily happen with a fusor, this is an important warning that should be heeded.
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Re: FAQ - X-ray radiation!!

Post by ab0032 »

Steven Sesselmann wrote:
> You can set up a really simple experiment to measure attenuation, with an Americium source and a geiger counter or scintillation detector. Use two lead bricks with a small hole in, to collimate the beam set up the source bricks and detector so the 59keV x ray beam traverses a gap of say 4" , take a steady reading of the counts over a set period of time.
> Now try inserting various materials across the beam and see how it attenuates the 60keV x-rays.

This may be scientifically interesting, but in the case of a fusor you dont have a narrow beam.
The attenuation is affected not only by the thickness of the material, but also the width of the material and the spread of the x-rays.

Hence I would always recommend to turn up the voltage slowly on the fusor and check your radiation exposure behind your shield for the real dose you are getting to be on the safe side.
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Re: FAQ - X-ray radiation!!

Post by George Dowell »

Unfortunately the housing on pen dosimeters limit their low end response. Even the latest technology using conductive plastic instead of metal housings is pretty puny below 16 keV. Those low energy rays are the very ones absorbed by the skin.

Energy response chart encloed.

Geo
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Re: FAQ - X-ray radiation!!

Post by George Dowell »

RadPro Calculator- Fooling around with dose at 10 cm from a 30 kVp 10 Ma
X-Ray source, shielded by iron or lead.

Dose at 10 cm with 2mm Iron shielding (fusor chamber? = 5 R/H

Dose at 10 cm with 2mm Pb shielding = 1 10E-21

It takes little effort to shield 30 kV rays.

George Dowell
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Richard Hull
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Re: FAQ - X-ray radiation!!

Post by Richard Hull »

The assumption here in these calcs is that you are operating a formal, tungsten target "x-ray tube". All the energy is in a focused beam blasting "naked" into air. The fusor is never this kind of device, of course, so all the unshielded values given by this program are super wrong to start with, when referencing a fusor, as the levels will be 2-3 orders of magnitude lower due to the isotropic emission and SS lower Z target-shell shield.

A better test is to let the x-ray tube filter and target be 2mm of iron which is closer to the natural fusor. Unfortunately, we aren't allowed to choose the target, but can choose Cu for our filter. The program, though, forces a minimum of 85kv for that filter and will not accept lower KV numbers. Thus, it is valueless, on many levels, for use with a fusor.

We have actually, physically measured at the SS shell of a fusor working around 35kv @10ma. We record about 100 mr/hr at the shell ~3 - 5cm distant and at 1 meter, less than one mr/hr. This is not worth consideration as a rad hazard if your operating station is over a meter distant from such a fusor and operated for infrequent, short periods.

For the paranoid, hyper-ventilator's, a millimeter or two of lead will leave you in, hopefully, a "feel good" operational environment.

For me, 50-60 kv applied is when I might put up a thin Pb shield, assuming I am 1.5 meters from the fusor.... as it is in my case.

Richard Hull
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Re: FAQ - X-ray radiation!!

Post by George Dowell »

Richard, may I ask what you measured the 100 mR/H with? A low energy intergrating ion chamber I assume? George Dowell
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Re: FAQ - X-ray radiation!!

Post by George Dowell »

By the way a tungsten target does not turn on until above 59 keV (tungsten K edge) . A real X-Ray tube designed to work at lower HV would have an MO or copper target. Or in the case of a fusor, a very large iron target. A peak of about 6 keV would result, with a background of Bremsstrahlung from 0-35 keV (average of around 1/3 that).

Geo
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Re: FAQ - X-ray radiation!!

Post by Chris Bradley »

George Dowell wrote:
> By the way a tungsten target does not turn on until above 59 keV (tungsten K edge) .
There is an X-ray continuum too, related directly to the electrons' energies. It is not just the atomic signature from which one gets X-rays.
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