Radioactive tracer application

["C.B. Clayton" <C.B.Clayton@newcastle.ac.uk>]

I'm looking for a gamma-emitting radioactive compound which can be used 
as a tracer for oil in oil-mist extraction systems. The photon emission 
energy should, for preference, be in the range 100 to 200 kev. There 
seems to be no prospect of labelling the oil itself since it is almost 
pure hydrcarbon. The tracer should mix readily with the oil and not be 
differentially extracted from it. Does anyone have any suggestions as to 
what might be suitable and practicable? Please reply to :-

C.B.Clayton@ncl.ac.uk