Regarding alpha decay, the answer is obvious: strong force and coulomb force result in a potential barrier where particles can tunnel through with some probability. But what about beta decay? Is there tunneling as well?
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This is just the usual randomness associated with quantum mechanics.
The transition rate between two excited states is described by Fermi's golden rule, which calculates the probability of transition per unit time:
$$ \Gamma_{if} = \frac{2\pi}{\hbar} |M_{if}|^2 \rho $$
where $M_{if}$ is in effect the strength of the interaction between the initial and final states:
$$ M_{if} = \langle \psi_f | H | \psi_i \rangle $$
where $H$ is the Hamiltonian for the interaction. This applies to all decays. For example it tells us the decay probability for excited states of atoms, the hydrogen 21 cm line and beta decay in nuclei.
The decay probability is dominated by the matrix element $M_{if}$. For electronic transitions in atoms this element is large because the electric dipole interaction is strong. For the hydrogen 21cm decay the element is small because that is a magnetic dipole transition and is a lot weaker. For beta decay the element is very small because the decay precedes by the weak interaction.
There is no tunnelling involved in any of the decays. Their statistical nature is inherent to quantum mechanics. I'm not sure there is a good intuitive description of why this is. You could think of the initial state evolving into a superposition of the initial and final states at a rate determined by $M_{if}$ and that superposition collapses into the final state in a random way.
John Rennie
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