Measurement of the Atomic Exchange Effect in Nuclear β Decay

Precision measurements of the β-decay spectrum of ⁶³Ni with a hermetic, calorimetric low-temperature detector have yielded quantitative evidence for the atomic exchange effect in β decay. The exchange effect is due to the probability that the β electron is created into a bound state orbital of the daughter atom and is predicted to lead to a small relative enhancement in the spectrum at low energies. In the experiment reported here the energy release in each of 6×10⁷ β-decay events was measured by detecting the nonequilibrium phonons produced in two InSb crystals which enclose the ⁶³Ni source. The best-fit value of the exchange factor is 0.98±0.06 (statistical) times the theoretical value, with an experimental systematic error of ±0.06.