Q value (nuclear science)
In nuclear physics and nuclear chemistry, the Q value for a nuclear reaction is the amount of energy absorbed or released during that reaction. The value relates to the difference between the sum of the masses of the initial reactants and the sum of the masses of the final products, in energy units (usually MeV).
A positive Q value indicates that energy is released in the reaction, and thus the reaction is exothermic. The energy released can take the form of kinetic energy of the products, or excitation energy of the products which subsequently decays by emitting photons (gamma rays) or other particles. Exothermic reactions are often self-sustaining or require less energy input to initiate. Nuclear fission and some nuclear fusion reactions are examples of reactions with positive Q values.
A negative Q value indicates that energy is required for the reaction to proceed, and thus the reaction is endothermic. This means that the kinetic energy of the reactants must be sufficient to overcome the energy deficit. The absolute value of the Q value represents the minimum kinetic energy that the reactants must have for the reaction to occur (threshold energy). Nuclear reactions induced by bombarding a target with particles often require the particles to have sufficient energy to overcome a negative Q value.
The Q value can be calculated using the following formula, based on mass-energy equivalence (E=mc²):
Q = (mreactants - mproducts) * c²
where:
- mreactants is the sum of the masses of the reactants.
- mproducts is the sum of the masses of the products.
- c is the speed of light.
The masses used in this calculation are typically the atomic masses of the neutral atoms involved, rather than the nuclear masses. Using atomic masses automatically accounts for the binding energy of the electrons, assuming the number of electrons is conserved in the reaction. This is a valid approximation for most nuclear reactions.
The Q value is a crucial parameter in determining the feasibility and characteristics of nuclear reactions. It is essential for understanding the energetics of nuclear processes and for predicting the products and energy released or absorbed in these reactions.