A nuclear reaction rearranges nuclei and particles while conserving total energy, momentum, angular momentum and relevant charges. A reaction can be written schematically as $$a+A\rightarrow b+B.$$

The Q-value measures the change in rest-mass energy: $$Q=(m_a+m_A-m_b-m_B)c^2.$$ If $Q>0$, rest-mass energy is released into kinetic energy or radiation. If $Q<0$, the incoming particles must supply at least the required energy, with additional threshold energy often needed to conserve momentum.

For example, if the initial rest masses exceed the final rest masses by $0.005,\mathrm u$, then $$Q\approx0.005\times931.5,\mathrm{MeV}=4.66,\mathrm{MeV}.$$ That energy appears in final-state kinetic energies and any emitted radiation.

Energetic permission and reaction probability are different questions. The Q-value tells how much energy is available or required; the reaction cross section describes how likely the specified process is for an incident beam. Cross sections can vary strongly with collision energy and can show resonances when the interacting system accesses a short-lived excited configuration.

A reaction with positive $Q$ need not occur frequently, while a process with a large cross section may still require sufficient incident energy if $Q<0$ or momentum conservation creates a threshold.

Nuclear reaction analysis therefore combines relativistic energy-momentum conservation with measured or calculated cross sections. This framework is used in nuclear experiments, reactor reactions, stellar nucleosynthesis and accelerator studies.