There are no photons <52 kev., none >310 kev., and 0.5 of the photons are 52-kev. x-rays from a 169Yb source encapsulated in Al or stainless steel.The scatter from the air, the object, or surroundings is less with 169Yb than with conventional x-ray sources, 192Ir, or 60Co so that the problem of personnel protection is min. for the portable exposure units used for radiography of castings, welded products, and assemblies of Al, Mg, Fe, plastics, and wood.