what is Q value in a nuclear reaction? plz explain.

In writing down the reaction equation, in a way analogous to a chemical equation, one may in addition give the reaction energy on the right side:

Target nucleus + projectile -> Final nucleus + ejectile + Q.

For the particular case discussed above, the reaction energy has already been calculated as Q = 22.4 MeV. Hence:

63Li

+

21H

→

2 42He

+

22.4 MeV

The reaction energy (the "Q-value") is positive for exothermal reactions and negative for endothermal reactions. On the one hand, it is the difference between the sums of kinetic energies on the final side and on the initial side. But on the other hand, it is also the difference between the nuclear rest masses on the initial side and on the final side (in this way, we have calculated the Q-value above).

Q value is just the reaction energy of any reaction.To have a deeper knowledge and question based approach to the concepts of neuclear reaction and energy related issues follow the link  http://www.egglescliffe.org.uk/physics/particles/nucreac/nucreac.html  .

Q value of a nuclear reaction is diffence between the final and initial kinetic energy.

Q = Tf -Ti

the importance of the Q-c alue can be understood from following two statements .

1)If Q>0 then  the reaction proceeds even if  Ti =0 (exoergic reaction)

2)If Q <0 the reaction proceeds only if Ti >= |Q| (endoergic reaction)

 so |Q| is the threshold energy of the reaction.