Energy change:

As in a chemical reaction, nuclear reaction also involves energy change, represented by the symbol, Q.  If Q is negative, the reaction is called endoergic i.e. energy is absorbed which is supplied in the form of acceleration to the bombarding particle.  On the contrary if Q is positive, a reaction is known as exoergic i.e. energy is released in which case the reaction may be carried out without acceleration or at thermal temperature.  The value of Q can be determined from the difference in the total masses of reactants and products of the reaction as;

where  m_ri  = mass of reactants r_i in amu,

m_pi  = mass of products p_i in amu, and

n & k represent number of reactants and products. This is illustrated by considering the following example.

Example: Let us calculate Q value of the reaction ²⁷Al (d, α) ²⁵Mg

Given that masses of

²⁷Al = 26.981541        ²H        = 2.014102

⁴He = 4.002603          ²⁵Mg    = 24.985839

Sum of masses of Reactants           = 26.981541 + 2.014102

= 28.995643 amu

Sum of masses of Products            = 24.985839 + 4.002603

= 28.988442 amu

.^..       Q = (28.995643 - 28.988442) × 931.5

= 6.701 MeV

Since mass of the reactants is more than that of the products, it will be an exoergic reaction. Such a reaction can take place without accelerating the bombarding particle (neutron only). Therefore, if the mass of reactants is less than which of the products, it will be an endoergic reaction.  In many cases, threshold energy is required.  Its calculation is illustrated in following example.