Charles Barkla (1877-1944) discovers that each element has a characteristic X-ray and that the degree of penetration of these X-rays is related to the atomic weight of the element. He is awarded the Nobel Prize in Physics in 1917.
Editors. "Discovery of the characteristic X-rays of elements". The Literary Encyclopedia. First published 01 May 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=6402, accessed 28 January 2015.]