Q.Which of the following is used as the source of gamma rays ?

The noble gases — helium, neon, argon, krypton, xenon, radon — all occur naturally in the atmosphere (argon, at ~0.93% by volume, is by far the most abundant); radon also arises continuously from radioactive decay in the crust. Their completely filled outer shell (ns2 np6, or 1s2 for helium) gives them the highest ionisation energy of any element in their respective period, and only a very small tendency to gain or lose electrons — the root of their near-total chemical inertness, non-metallic character, and monoatomic, colourless, odourless, tasteless, non-flammable physical behaviour. Down the group, atomic radius and boiling point rise while ionisation energy falls, xenon and (much more weakly) krypton being the only members reactive enough to form authentic compounds (see the dedicated Compounds of Xenon concept). …