A radioactive isotope can decay to produce $c\alpha -$  particles if it has lower binding energy of  $\beta -$ particles . If it has lower $\frac{P}{n}$  ratio than requires for the best of stability on $P$ ( y – axis ) Vs  $n$ ( x – axis ) curves. The resultant daughter nucleus if still possesses higher energy level then only  $\gamma -$ rays are given out. Also if $\frac{P}{n}$  ratio lies above the belt of stability on  p – n curves, then position emission or $k–$ electron capture is possible.