Frederic Joliot-Curie quotes:

I have always attached great importance to the manner in which an experiment is set up and conducted ... the experiment should be set up to open as many windows as possible on the unforeseen.

Some months ago we discovered that certain light elements emit positrons under the action of alpha particles. Our latest experiments have shown a very striking fact: when an aluminium foil is irradiated on a polonium preparation [alpha ray emitter], the emission of positrons does not cease immediately when the active preparation is removed: the foil remains radioactive and the emission of radiation decays exponentially as for an ordinary radio-element. We observed the same phenomenon with boron and magnesium.