he line 4--4 in FIG. 2, showing the tungsten container with a vial of iodine solution therein; andFIG. 4B is an alternative cross sectional view, also taken along the line 4--4 in FIG. 2, showing the tungsten container with iodine capsules therein.DESCRIPTION OF THE PREFERRED EMBODIMENTReferring now to the drawings, and particularly to FIGS. 1-2 thereof, there is shown by way of example only a rad