MadSci Network: Physics

Re: How does lead stop X-rays ?

Date: Tue May 11 11:25:52 1999
Posted By: William Lorenzen, Staff, Radiation Safety, Children's Hospital
Area of science: Physics
ID: 925549427.Ph

Well this is a complicated question and I would refer you to any college 
level physics book, but here is the short of it...

Gamma and x-rays are part of the electromagnetic spectrum and as such have 
no mass or charge.  The interaction between the rays and other materials is 
at the atomic level, by that I mean that as these rays pass near the 
electron clouds of other atoms they may (or may not) give (tranfer) some 
(or all) of their energy to a lucky electron of that atom.  The electron is 
then offered more energy that it would otherwise have and is sent off in 
motion and thus "stripped" from the parent atom (this is called 

The interaction of an x or gamma ray is by indirectly transfering some of 
it's energy to these electrons which is a function of the probability that 
the ray will come close enough to tranfer the energy.  So it is a 
probability relationship.  The higher  the probability the higher the 
chance of transfer of energy.

So why lead??  Well it's not just lead that can stop (shield) x or gamma 
rays.  It is a function of how many targets (atoms) you give it a chance to 
interact with.  So the higher the target materials density (atoms/area) the 
higher the probability the tranfer of energy will take place.  The most 
common high density material is...  Lead!!  We also use Uranium and 
Tungstun.  Both have densities greater that lead and thus shield better 
because they create a higher probability the ray will indirecty interact.

Hope this helps

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