What is Branching decay?

A parent undergoes branching decay when its nuclei decay by two or more routes to different daughters. Each route $i$ has its own partial decay constant $\lambda_i$; the parent decays with the total constant $\lambda = \sum_i \lambda_i$, and a fraction $\lambda_i/\lambda$ of the decays take route $i$. Potassium-40 decays to calcium-40 with $\lambda_\beta = 4.962 \times 10^{-10}$ per year and to argon-40 with $\lambda_e = 0.581 \times 10^{-10}$ per year.