Sudden to adiabatic transition in beta decay

We discuss effects in beta decays sir very low beta energies, of the order of the kinetic energies of atomic electrons. As the beta energy is lowered the atomic response changes from sudden to adiabatic. As a consequence, the beta decay rate increases slightly and the ejection of atomic electrons (s hake-off) and subsequent production of x rays is turned off. We estimate th e transition energy, and the change in decay rate. The rate increase is lar gest in heavy atoms, which have a small Q value in their decay. The x-ray s witch-off is independent of the Q value. [S0556-2813(98)06710-7].