Animated Solution for Physics - Atoms and Nuclei: In gamma ray emission from a nucleus,

Imagine a nucleus that has just survived a dramatic event, like an alpha or beta decay. It has transformed, but the violent process has left it agitated and vibrating with excess energy. In physics, we call this an excited state, and we denote it by placing an asterisk next to the chemical symbol, like ${}_Z^A{X}^{\ast }$.

Just like a stretched spring wants to return to its relaxed position, this excited nucleus desperately wants to settle down into its lowest possible energy state, known as the ground state. But energy cannot simply vanish; it must go somewhere.

To shed this excess energy, the nucleus emits a gamma ray ($\gamma$). But what exactly is a gamma ray? Unlike alpha particles (which are chunky helium nuclei) or beta particles (which are speedy electrons or positrons), a gamma ray is not a particle of matter at all.

It is a high-frequency, high-energy electromagnetic wave—a photon. Because it is pure energy, a gamma ray has absolutely zero rest mass ($m_{\gamma }=0$) and zero electrical charge ($q_{\gamma }=0$).

When we look at the conservation laws governing nuclear reactions, the emission of a massless, chargeless photon has a profound implication.

Since no protons are lost or gained, the atomic number $Z$ remains perfectly constant. Similarly, since no neutrons are lost or gained, the total mass number $A$ also remains unchanged. The nuclear equation looks like this:

The nucleus hasn't changed its identity; it hasn't transmuted into a different element. It has simply relaxed. Therefore, during gamma ray emission, there is absolutely no change in the proton number and the neutron number.