The activity of a radioisotope changes with:

The order of density in the nucleus is:

The positron is

Assertion : Nuclear binding energy per nucleon is in the order $_4^9Be > \,_3^7Li > \,_2^4He$.
Reason : Binding energy per nucleon increases linearly with the difference in the number of neutrons and protons.

All the nuclei from the initial element to the final element constitute a series which is called

Radioactivity is due to