An atomic nucleus is like a crowded bus. To free up space, a group of two protons and two neutrons — a helium nucleus, an alpha particle — exits through the front door: this is alpha decay, described by George Gamow as tunneling under an energy barrier. In other cases, a neutron turns into a proton and throws an electron out the back door — beta decay, described by Enrico Fermi. Usually, the doors work separately. But researchers have proven that a simultaneous exit is possible — both doors swing open at once, and the nucleus emits both particles. This combination is the only way out for nuclei for which individual decays are forbidden. Theory pointed to several candidate isotopes, and six of them have been found in experiments. Studying their radiation spectra revealed that these nuclei had been recorded in experiments for decades, but were mistaken for a chain of ordinary events. Nature has been showing off double decay all along, and we've only now realized it. The process turned out to be an ultrasensitive probe for the Standard Model of particle physics. The weak and strong interactions work in tandem, revealing subtle effects that are invisible individually.
An alpha source in a sealed flask will produce helium on its own — just wait until the ejected nuclei capture free electrons, and you'll have gas for party balloons.