Wikipedia:Reference desk/Archives/Science/2019 February 14

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February 14

Aluminum luggage and laptops through x-ray

A radioactive decay chain that ends with Thallium-205.

Aluminum is supposed to dissipate X-rays, at least to a degree. How do aluminum luggage and laptops block x-rays at airport scanners? --Doroletho (talk) 02:35, 14 February 2019 (UTC)

Aluminium is a fairly light metal and blocks X-rays a lot less than iron. Also the cases are not so thick, so sufficient X-rays pass through to image contents. Graeme Bartlett (talk) 02:59, 14 February 2019 (UTC)

Walls build of bricks of Lead (see here) are used in Industry and Science whenever direct Radiation needs to be contained. These Blocks are very heavy but they really completely block any X-rays. Lead has the highest Mass attenuation coefficient and is the stable endproduct of all radioactive decay btw. Most common construction metals are sometimes actually X-rayd or radiographied with the results documented by default, for example parts for Airplanes and Helicopters. --Kharon (talk) 06:13, 14 February 2019 (UTC)

Actually, we frequently use depleted uranium as a radiation shield because it has a higher mass attenuation than lead. The trend of increasing mass attenuation coefficient with atomic mass continues well past lead into the transuranics. 202.155.85.18 (talk) 06:44, 14 February 2019 (UTC)

Lead is not "the stable end product of all radioactive decay".[1] --Guy Macon (talk) 06:56, 14 February 2019 (UTC)

Yes, as others have indicated, blocks or dissipates a bit means it just depends on how strong your X-ray is. Besides x-raying parts, x-rays are sometimes used as part of Cargo scanning where a whole container or truck may be scanned. While our page has no photos of this, you can easily find them online. E.g. this may be of interest [2] (at least I think it's x-ray), as you can see the x-rays can penetrate not only the container, but the vehicle inside the container. Here's other images which definitely say they're x-ray [3] [4]. And a video [5]. BTW, if you're worried about the driver of the car, such systems are likely to have their power adjusted depending on what's being scanned (for reasons beyond protection of things inside) see e.g. [6], although personally I'd probably leave the vehicle if allowed, especially if a regular traveler. Of course that doesn't help stowaways (like several of these depict) as our article mentions. Note that I'm not suggesting that luggage scanners at airports are anywhere near as strong. However this suggests some are 140kV which can penetrate 31mm of steel [7]. (140 kV also isn't that far off the 200 kV used for the cabin or whole small truck in that other system.) People in this discussion claim dual energy scanners (similar to Dual-energy X-ray absorptiometry) are common [8], possibly 140 kV and 60 kV although I have to admit when I looked I did find evidence for dual energy systems [9] [10], but am less certain that the other energy is necessarily 60 kV but it may just be it's not common to refer to it in that way. (This does mean penetration by 60 kV or whatever is it would be ideal.) Nil Einne (talk) 08:04, 14 February 2019 (UTC)

The good news is that the detectors are getting more and more sensitive and thus the intensity of the X-Rays used is getting lower and lower. The same thing is happening with dental/medical X-Rays, X-Rays used for nondestructive testing of welds, etc. --Guy Macon (talk) 18:11, 14 February 2019 (UTC)

Matter anti-matter annihilation

As I understand it, when anti-matter and matter annihilate, it is pairs of particles and their anti-particles that actually annihilate. If matter and anti-matter particles that are not pairs come together, they cannot annihilate. For example, a proton and an anti-proton will annihilate, but a proton and a positron will not. Now, what if the particles are composite particles? Like for example, will a proton and an anti-neutron annihilate? If they cannot, then can a nucleus composed of say 1 proton and 1 anti-neutron form? If they do annihilate, then I would assume that one up quark/anti-up quark pair can annihilate as well as one down quark/anti-down quark pair. That still leaves one up quark from the proton and one anti-down quark from the anti-neutron. Since free quarks aren't observed, what happens to them? Or can up quarks annihilate with anti-down quarks? Do the colors of the quarks need to match for them to pair for annihilation? 202.155.85.18 (talk) 03:31, 14 February 2019 (UTC)

A pion, specifically the π⁺, is an unstable particle made from an up quark and an anti-down quark. The π⁺ then typically decays via the weak interaction into a μ⁺ + ν_μ. Dragons flight (talk) 08:54, 14 February 2019 (UTC)

First, there is no anti-photons. Second, in strict sense, a particle can annihilate only with it own antiparticle, as you think. However anti-neutron (due to isotopic invariance) is close in characteristic to anti-proton. So, an annihilation-like process between a proton and anti-neutron takes place and $p{\overline {n}}$ nucleus is unstable. Ruslik_Zero 20:19, 14 February 2019 (UTC)