When the nucleus in the excited state passes to the ground state or to the lower excited state from higher excited state, it emits a high energy photon. This photon emitted from a nucleus in an excited state is known as $\gamma$ rays.

$\ {z} X^A \rightarrow \ {z}X^{A} + \gamma$

The $\gamma$ emission must satisfy following conservation laws

(a) Conservation of charge
(b) Conservation of mass energy : If transition takes place from an excited state $E_2$ to lower energy state $E_1$ then

$E_2 - E_1 = h \mu$

where v is frequency of $\gamma$ ray photon

(c) Conservation of linear and angular momentum : Emission of $\gamma$ -ray must make the daughter nucleus recoil with the same momentum in opposite direction. If parent nucleus is at rest. If M is mass and V is velocity of daughter nucleus

$\dfrac{h \nu}{c} + MV = 0$

Angular momentum must also be conserved.

Intrinsic angular momentum or spin of photon = $\dfrac{h}{2 \pi} = \hbar$

Internal Conversion

It is the one step process of transition of nucleus from higher excited state to ground state by direct transfer of energy through the electromagnetic interaction from a nucleus in excited state to one of its orbital electrons so that it is ejected from atom with kinetic energy E given by

$E = E_e - W$

Where E is available excitation energy (ie. energy given out by excited nucleus). W is binding energy of the ejected electron in its shell of origin. The emitted electron is called conversion electron. Here $\gamma$ ray is not produced and it is one step process.
The internal conversion does not complete with $\gamma$ rays emission in the sense that one process inhibits the other. The processes are independent alternatives.

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