If the purpose of the nuclear accelerating machines were to produce cheap energy, they would not be of much value. A nuclear reaction may typically release five to 20 million electron-volts of energy. But to obtain this reaction, a million particles had to be accelerated to energies of several million electron-volts. The recoverable and useable energy will be only a minute fraction of the total invested. On the other hand, as a tool for scientific discovery, the atom-smashers have been of great importance. That one event in a million has given us much of our knowledge of nuclear physics. The achievement of nuclear reactions by particle bombardment did not actually wait on the invention of man-made accelerating machines. Energetic alpha particles are available from the radioactive decay of heavy elements. In 1919 Ernest Rutherford used such radioactive elements as a source of alpha particles. The alpha particles were made to bombard ordinary nitrogen, causing the reaction: He⁴ + N¹⁴ → O¹⁷ + proton (2 protons) (7 protons) (8 protons) (2 neutrons) (7 neutrons) (9 neutrons) That is, an alpha particle plus a nitrogen¹⁴ nucleus react to produce a nucleus of (stable) oxygen¹⁷ plus a proton. Oxygen¹⁷ is a nucleus with 8 protons and 9 neutrons. The ordinary abundant form of oxygen has 8 protons and 8 neutrons. Natural oxygen contains a small amount of oxygen¹⁷. Later, in 1934, Irene Curie Joliot (the daughter of the discoverer of radium, Madame Curie) and her husband, Frederic Joliot, used naturally available alpha particles to make artificial radioactive nuclei for the first time. The reaction was: He⁴ + aluminum²⁷ → phosphorus³⁰ + neutron (2 protons) (13 protons) (15 protons) (2 neutrons) (14 neutrons) (15 neutrons) Phosphorus³⁰ is an unstable nucleus and emits a beta ray (a positron) to become silicon³⁰ (which is stable). The half-life for this decay is about 2.5 minutes. The Joliots’ reaction was the first instance in which man had produced radioactivity and known it. Actually cyclotrons had been producing radioactivity in good abundance for the preceding two years—but physicists had been unaware of this fact. It is amusing that nature has also provided us with an atom-smashing machine and indeed one that produces far greater energies than any apparatus yet devised by man. This machine operates on the principle of fluctuating, turbulent magnetic fields in interstellar space. Cosmic particles—mainly protons, but also some alpha particles and even heavier nuclei—are accelerated by these changing magnetic fields and hurled occasionally into the earth’s atmosphere. The energies of these cosmic particles are enormous, ranging from billions of electron-volts to values a million times higher.