Why X-Rays can go through Materials

[Freespader]

Hello! I was wondering, why X-Rays can go through some materials, such as skin, whereas light from the visible spectrum cannot go through these(although holding a flashlight to your hand does create a red glow). Likewise, why are X-Rays stopped at the bone, and not the skin? Lastly, why does lead stop most electromagnetic radiation? Thanks.

 

[davenn]

Xrays have much higher energy than visible light photons
that being said, have you never held a very brioght light up to your fingers?
it does fiantly shine through and also shows the darkr areas where the bones are.

Lead is a VERY dense element and quite effectively absorb those hi energy particles

That also being said, any sealed metal box will stop a large portion of the EM spectrum
be it made of tin, copper, steel, lead, aluminium etc

Am sure others will add to my brief comments :)

Dave

 

[granpa]

low energy x-rays produce the photoelectric effect
yet high energy x-rays pass through!

the higher the atomic number of the atom the higher the frequency of x-ray can still be able to produce photoelectrons.

I wonder if isotopes have different properties

 

[daveb]

EM radiation interacts with matter by interacting with the electrons in atoms. The more atoms you have per unit volume, the larger the probability of interaction. In general, the higher the energy of the radiation, the less the chance of interaction as well. The three major types of interactions are photoelectric effect, Compton scattering, and pair production (which needs photons of greater than 1.02MeV in order to happen). PE dominates at lower energy, Compton in the 100keV to 10MeV range, and PP dominates after that. EM radiation that doesn't interact either passes through or reflects off without transferring energy to the material. The longer the wavelength (e.g., visible light) the more likely it is to just reflect off rather than pass through, though some materials allow it to pass through (e.g., glass and other transparent materials). Bone and tissue have different densities, which is why you see the contrast between them.

This makes lead an effective shield (DU, tungsten, and several others such as platinum or plutonium are even better, but more expensive or rare) against most EM, but only if it is thick enough.

 

[Freespader]

What about radio waves, then? My understanding is that they have much longer wave lengths than light, but they can pass through walls. Also, what is the Photoelectric Effect, Compton Scattering, and Pair Production? Thanks!

 

[daveb]

That's why I said in general. There are some materials radio waves don't pass through (e.g., the lined walls of a room with an MRI in it), and if the walls are thick enough, radio waves won't pass through them (try using your cell phone inside a cave).

As for the three modes of ionizing radiation, hyperphysics has a decent treatment of it. Just scroll down to the section on ionizing radiation.

 

[Freespader]

Thanks!