Subject: x-ray absorbtion equations for wavelength vs. material density

Is there a formula that calculates the amount and density of material required 
to stop x-rays of a given wavelength? Also, is it at all possible for a single 
photon to pass this limit? (Like say, to pass through 10cm of lead if that were 
the limit?)

Thanks

Re: x-ray absorbtion equations for wavelength vs. material density