Neutron Radiography | Hacker News Reader

Neutron Radiography

phoenixwi.com

First thing I thought was: wow, that's a quite liberal use of the term "nondestructive".

I dug around the specs, and the neutron flux is 108 n/cm2/sec max.

To approach a 600ppi resolution (and I say approach because I assume the actual vector of each neutron is random and not distributed uniformly), you'd need roungly 60k n/cm2, so the picture will take 10 minutes at max power.

If we assume half get absorbed by the object (will depend on the material and its thickness), 30k atoms are potentially becoming radioactive.

This figure, for some materials it won't matter much, for others materials will become radioactive, and for others it'll release alpha particles straight away to become stable (and for others it may result in nuclear fission, teehee).

Still, 30k atoms/cm2 doesn't sound like a lot of radioactivity added to an object, but its safety will HEAVILY depend on the material it's made of. In general, I'd say objects will remain probably safe enough to handle without protection, but probably not safe to eat; for a few weeks, at least, until the most active isotopes are cleared out.

A banana for scale[0], has 3.4mg of potassium per cm3, 0.012% of them are radioactive, leaving 0.4µg of radioactive potassium, which amounts to 6.161e15 atoms [1], so 3e5 more is nothing to write Marie Curie about.

[0] https://www.wolframalpha.com/input/?i=potassium+in+a+cubic+c...

[1] https://www.wolframalpha.com/input/?i=atoms+in+0.4%C2%B5g+of...*

The disadvantage of neutron imaging is that you may have to wait two weeks until you can pick up your imaged objects.. Many objects turn radio-active due to the bombardment with neutrons.

Right.

The article says "a powerful nondestructive testing method"

Which I don't think "nondestructive" is 100.0.0% accurate, if neutron capture will be involved in producing the dark spots.

https://en.wikipedia.org/wiki/Neutron_capture

it's basically nuclear transmutation.

Neutron scatter is the main thing producing the dark spots, though there will certainly be some neutron capture too. In some materials, e.g. boron, neutron capture can be significant.

Still, it's always important to compare the induced radioactivity with the background radioactivity as a comment above did. It's easy to forget that we live in a field of natural low-dose ionizing radiation from cosmic rays and from the bowels of the Earth.

I recall reading about using neutron beams to alter the chemistry of phonograph needles. I think the idea was to make them into a radioactive isotope that continually ionized the needle, so that it repelled dust.

But maybe I'm remembering wrong. The closest article I can find is from Theodore Gray [0]. It talks about old phonograph needles made from osmium, which is very toxic in dust form.

[0] https://theodoregray.com/periodictable/Elements/076/index.s1...

You're probably thinking of polonium record cleaning brushes sold by staticmaster.

Its an interesting case study in planned obsolescence, the radioactive half life being only a hundred days the anti-static effectiveness of the brush would completely disappear a couple years after manufacture, requiring the purchase of yet another brush.

Electronic / ham radio people would sometimes use staticmaster brushes in the winter when soldering RF preamp FET transistors to try and eliminate transistor-destroying static charge; I was never personally a believer in that technique at least at the amateur level.

Thank you! That was driving me crazy. And now I see why I got confused: it's another phonograph-related dangerous-element fact that I learned from reading Theodore Gray's book, The Elements [0] [1].

Now if I could only remember why I thought neutron beams were involved...

[0] https://theodoregray.com/PeriodicTable/Elements/084/index.s7...

[1] https://www.amazon.com/dp/B011IZNWZU

Marie Curie, call your office.

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