Perhaps you should try some spelling variations:
medium radioactivity » cesium radioactivity, made radioactivity, medium directivity
medium radioactive » medium radiative, serum radioactive, term radioactive
beam directivity » medium directivity, array directivity, field directivity

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: ((beam directivity) OR (((medium radioactivity) OR (medium radioactive)))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((beam directivity) or (((medium radioactivity) OR (medium radioactive))))".
• Adding a wildcard symbol may retrieve word variants: ((beam directivity) or (((medium radioactivity) OR (medium radioactive))))*.