In the nuclear process ${ }_Z^{A}X{+}_0^{1}n{\to }_{Z-2}^{A-3}Y+a,\text{'}a\text{'}$ represents

Mass number of $\text{'}a\text{'}=(A+1)-(A-3)$

$\Rightarrow$Mass number of ‘a’ = 4

Atomic number of $\text{'}a\text{'}=Z-(Z-2)=2$

So ‘a’ represents $\alpha -$particle