Preparation of very small point sources for high resolution radiography

The need for very small point sources of high specific activity 192Ir, 169Yb, 170Tm, and 60Co in non-destructive testing has motivated the development of techniques for the fabrication of these sources. To prepare 192Ir point sources for use in examination of tube sheet welds in LMFBR heat exchangers, 191Ir enriched to greater than 90 percent was melted in a helium blanketed arc to form spheres as small as 0.38 mm in diameter. Methods were developed to form the roughly spherical shaped arc product into nearly symmetrical spheres that could be used for high resolution radiography. Similar methods were used for spherical shaped sources of 169Yb and 170Tm. The oxides were arc melted to form rough spheres followed by grinding to precise dimensions, neutron irradiation of the spheres at a flux of 2 to 3 x 1015 nv, and use of enriched 168Yb to provide the maximum specific activity. Cobalt-60 with a specific activity of greater than 1100 Ci/g was prepared by processing 59Co that had been neutron irradiated to nearly complete burnup of the 59Co target to produce 60Co, 61Ni, and 62Ni. Ion exchange methods were used to separate the cobalt from the nickel. The cobalt was reduced to metal by plating either onto aluminum foil which was dissolved away from the cobalt plate, or by plating onto mercury to prepare amalgam that could be easily formed into a pellet of cobalt with exclusion of the mercury. Both methods are discussed