{TAIL ~ The improvement} 9, wherein said at least one element capable of attenuating x rays is selected from {HEAD ~ the group} consisting of lead oxide, bismuth oxide, tantalum oxide, and uranium oxide 
{ TAIL ~ The improvement } 9 , wherein said at least one element capable of attenuating x rays is selected from { HEAD ~ the group } consisting of lead oxide , bismuth oxide , tantalum oxide , and uranium oxide
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