The smallest fragments of DNA will be witnessed near the bottom of the gel, on the other hand, the heavier fragments will be visible at the top. The theory behind this is there is a specific pore size on the basis of the concentration of acrylamide/agarose in the gel.
The half-life of a radioisotope is the amon the symbol 42He, the subscript 2 is the mass number for helium and the superscript 4 is the atomic number for helium.