Page 2 of 5

Re: FAQ - X-ray radiation!!

Posted: Wed Dec 05, 2012 3:37 pm
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

Re: FAQ - X-ray radiation!!

Posted: Wed Dec 05, 2012 6:59 pm
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

Re: FAQ - X-ray radiation!!

Posted: Fri Dec 07, 2012 9:46 am
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?

Re: FAQ - X-ray radiation!!

Posted: Fri Dec 07, 2012 3:21 pm
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

Re: FAQ - X-ray radiation!!

Posted: Fri Dec 07, 2012 8:35 pm
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.

Re: FAQ - X-ray radiation!!

Posted: Sat Dec 08, 2012 12:09 am
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.


Steven

Re: FAQ - X-ray radiation!!

Posted: Sat Dec 08, 2012 6:36 am
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.

Re: FAQ - X-ray radiation!!

Posted: Sat Dec 08, 2012 6:48 am
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.

Re: FAQ - X-ray radiation!!

Posted: Sat Dec 08, 2012 6:54 am
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.

Re: FAQ - X-ray radiation!!

Posted: Sun Dec 16, 2012 4:28 am
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