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Does aluminum foil block x-rays?


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#1 greree

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Posted 2 June 2009 - 05:42 AM

I'm having a discussion with my roommate. His opinion is that only lead will block x-rays. I think anything will block x-rays, if it's dense enough or thick enough. So I'm sure that aluminum foil will block the x-rays used to x-ray packages going through the Post Office. So I guess the question is "is it possible to wrap unexposed film in enough layers of aluminum foil to prevent exposure of the film by the x-rays, and if so how many layers?" Thanks.
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#2 swansont

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Posted 2 June 2009 - 10:28 AM

Ah, the Superman theory. "I have trouble seeing through lead."

Technically, nothing "blocks" x-rays. What you have is an attenuation which is an exponential. The advantage of lead is that has a very large attenuation coefficient due to the large number of electrons and dense structure. Aluminum has fewer electrons, and is a much less favorable target for interaction.

So you are correct anything will eventually attenuate x-rays. Lead is used because it is more efficient at doing so than most other materials and is readily available, but a thick block of aluminum could be more effective than a thin sheet of lead, depending in the actual numbers.

This shows the mass attenuation coefficient for various elements
http://en.wikipedia....oefficients.png

There's also a table here
http://physics.nist....sCoef/tab3.html

For example, at 1 MeV the mass attenuation coefficients are comparable, but lead is much more dense and is a much better attenuator.
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#3 Mokele

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Posted 2 June 2009 - 12:01 PM

So I guess the question is "is it possible to wrap unexposed film in enough layers of aluminum foil to prevent exposure of the film by the x-rays, and if so how many layers?" Thanks.


If they use anything like the fluoroscopes we use in lab, maybe 8-9 layers or a thick baking sheet style foil should prevent most of the damage.

But film? Who uses film anymore?
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#4 Ndi

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Posted 2 June 2009 - 12:58 PM

The simplest way to get film through is to mark it as film. In theory, there are lower-powered machines that they push film through undamaged. I'd not bet money on it in real life.

However, "how many layers" is a bit of complex a subject, since it depends not only on layer thickness, but also on the strength of the machine, the sensitivity of the film, etc.

Also, some materials reflect x-rays, thus being able to locally concentrate it. Since it's film, even a small amount might leave a nice stripe or some arrangement over it.

Also, I'm pretty sure that those scanners can see through electronic boards (traces included), equipment, etc. If you Google x ray luggage and go to images/very large you'll see several samples. Notably, it passes through large luggage studs, those being quite thick, through metal springs. The only thing that seems quite opaque is a slanted phone.

I think you need a whole lot of foil. A Whole lot. Possibly enough to not let you go through with a sizable ball of unscannable metal.
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#5 Phi for All

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Posted 2 June 2009 - 01:55 PM

The simplest way to get film through is to mark it as film.

I agree. Aren't you risking delay if the scanners can see that you're trying to hide something?
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#6 McCrunchy

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Posted 2 June 2009 - 03:40 PM

Hi,

You might find this link useful to calculate attenuation length (the length after which ~63 % of the photons are absorbed) of X-rays in different materials :

http://henke.lbl.gov...nts/atten2.html

For Al, it gave me, at 10 000 eV (copper emission is at 8500 eV, and I heard airport scanners use harder X-rays - I wasn't able to find any info on the exact wavelength used though), 90 incidence, 120 um. That's typically the thickness of your foil. Put ten wraps, and you'll end up with 0.63^10 = 1% radiation left.

With lead, the attenutation length is 7 um.


Yours,

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#7 John Cuthber

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Posted 2 June 2009 - 05:25 PM

If the person running the sacanner sees something on the screen that the Xrays don't get through he may chose to up the energy and exposure until he gets an image.
Is this what you want to happen to your film?
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#8 Mokele

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Posted 2 June 2009 - 08:59 PM

There's a simple solution:

Flash memory is unaffected by X-rays. Buy a digital camera. You'll save the cost of the camera in film within 2 months.
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#9 greree

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Posted 3 June 2009 - 02:53 AM

But film? Who uses film anymore?

Artsy fartsy people do. This film is a special type, and it's also almost 40 years old. Apparently photographs taken with this film have a certain look that artsy fartsy people like, so they're willing to pay a lot of money for it. My roommate is selling it to some guy in the UK, and he doesn't want to take a chance that someone will accidentally x-ray it. He's already got "Film: Do Not X-Ray" stickers all over it. He just wants a little bit more assurance that it won't get ruined.

Thanks for the info, guys.
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#10 Ndi

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Posted 4 June 2009 - 10:48 AM

Hi,
Put ten wraps, and you'll end up with 0.63^10 = 1% radiation left.


Enlightening post, but I'd like to add that a normal everyday poor kid's camera has at least 1/256 resolution equivalent (for display). Better cams have 12 bits per channel (4096 levels). Black and white goes up to at least three times that (plus no more filter attenuation). With high dynamic range, way, way more.

As a result, you get an bad image at maybe 1/8000 (conservative) of what they use to get an image, more if they use a cheap/poor sensor (as they probably do) and crank up the emitter.

Therefore, you need a lot more than attenuation than below 1% if it's to spoil a film, versus getting a nice clear image. Besides, at 10 layers it probably stands on its own anyway.
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#11 uranium238

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Posted 25 May 2010 - 12:43 AM

it needs to be very very thick, because x rays are very penatrating:)
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