# Why don't X-rays travel through water?

Why don't X-rays travel through water?

I read that X-Rays don't travel through water, but what is the main reason? See this link:http://henke.lbl.gov/optical_constants/ it shows X-ray transmission through solid & gas, but there is no mention of water here.

• where did you read this? Dec 5 '14 at 2:18
• @user23873 i edited the question to more clear content Dec 5 '14 at 2:22
• water may be absorbing some of the energy of the photons, a bit like it does with infra-red.
– theo
Dec 5 '14 at 2:27
• If x-rays don't go through water, then there wouldn't be any x-rays of bones.
– LDC3
Dec 5 '14 at 2:34
• All the information you need - including a plot of attenuation versus energy is at en.wikipedia.org/wiki/X-ray Dec 5 '14 at 9:23

X-rays very much do travel through water. I think your quote may be out of context. For example, being deep in the ocean would protect against X-rays because there is so much water above you. However, concrete or lead are two more common materials which provide more protection against X-rays.

Using the link you provided, I generated the following plot of the transmission of X-rays through water. I used the formula H$_2$O and the density 1 gram/cubic centimeter:

"Atten Length (microns)" means that after one attenuation length, about 2/3 of the X-rays are absorbed. So after two attenuation lengths, only about 10% of the X-rays survive. If you look at the top of the graph, which is a photon energy of about 30,000 eV, this is still somewhat less than commonly used for medical X-rays. (For example, the most recent medical X-ray I saw was using 80,000 eV). At that distance, the attenuation length for 30,000 eV photons is about 30,000 microns, which is about 30 mm. Hence you need the better part of one meter of water to stop 90% of the photons from penetrating. At 80 keV, you may need more like a several meters.

[The algorithm used to generate the plot is described by: B.L. Henke, E.M. Gullikson, and J.C. Davis. X-ray interactions: photoabsorption, scattering, transmission, and reflection at E=50-30000 eV, Z=1-92, Atomic Data and Nuclear Data Tables Vol. 54 (no.2), 181-342 (July 1993).]

• Two e-folding lengths is basically a third of a third, or a sixth (roughly). Quantitatively: $e^{-1}$ = 0.368, and $e^{-2}$ = 0.135 -- which is close enough to 10% transmission that I used the round number.
– ZSG
Dec 5 '14 at 9:20
• I don;t understand your numbers. If 3cm is an e-folding length, then 6cm will stop 90% of the X-rays. In astrophysics 30keV would be classed as "hard X-rays" and the X-ray band would start at around 0.1keV. SO it looks like the statement that X-rays don't travel well through water is a pretty good one. Dec 5 '14 at 9:22
• Yes, if we're talking about soft X-rays (like 0.1 keV), then a little water would stop them. But so would paper, and air... The OP wasn't very specific about which X-rays are being stopped. Hopefully this answer gives him enough info to evaluate the context from which he obtained his quote.
– ZSG
Dec 5 '14 at 9:27
• Very nice answer. Just FYI, NIST also has a calculator physics.nist.gov/PhysRefData/FFast/html/form.html which allows you to plot Linear Attenuation Coefficient over a wider range (2.00 - 433 keV) of x-rays. Dec 5 '14 at 13:57
• I didn't know about the NIST calculator, thanks!
– ZSG
Dec 5 '14 at 19:03

The main reason that X-rays don't travel well through water - see http://en.wikipedia.org/wiki/X-ray or one of the other answers for a plot of attenuation versus energy - is that the X-rays photons can be totally absorbed by the photoelectric effect. That is photons interacting with inner shell electrons of oxygen atoms.

The evidence of this can be seen with the characteristic rise in attenuation leading up to a sharp edge in the attenuation versus energy plot which corresponds to a photoelectric absorption edge for oxygen atoms at 0.54 keV.

Other contibuting effects seen at higher energies are inelastic Compton scattering. At lower energies you also get elastic Rayleigh scattering.