Plutonium

Plutonium was first produced in 1940 and 1941 at Berkeley by Glenn Seaborg, Edwin McMillan, Joseph Kennedy and Arthur Wahl, who made it by bombarding uranium with deuterons. It was named after the planet Pluto, then reckoned the ninth and outermost planet. Plutonium is a dense, silvery, intensely radioactive metal, notorious as a fissile material of nuclear weapons and useful as a long-lived power source for spacecraft. A few kilograms of plutonium form the core of a fission weapon, while the isotope plutonium-238, warm from its own decay, has powered spacecraft and instruments far from the Sun for decades, including the Voyager probes and Mars rovers; the metal must be handled with great care for its radioactivity and toxicity.

Plutonium exists in two chemically identical but practically very different guises: plutonium-239, bred in reactors and fissile enough to sustain the explosive chain reaction of a weapon, and plutonium-238, a different isotope that simply generates steady heat from its own decay and powers spacecraft rather than bombs. The world's first plutonium-fuelled detonation was not Nagasaki but the Trinity test in the New Mexico desert on 16 July 1945, a secret trial three weeks earlier that confirmed the implosion-type weapon design before it was used in combat.

Plutonium completes the planetary trio that runs uranium, neptunium, plutonium, taking its name from Pluto, which had been discovered only in 1930 and named for the Roman god of the underworld and the dead, the ruler of the realm of shades and, in another aspect, the giver of the buried wealth of the earth. The name proved grimly apposite, for plutonium became the fissile core of the bomb dropped on Nagasaki in 1945 and a central material of the nuclear arms race, an element of the underworld indeed.

Seaborg is said to have chosen the symbol Pu, rather than the more logical Pl, half in jest. The naming is astronomical in form, but through the god of the dead it gives this weapon-material one of the darkest and most fitting associations in the periodic table. Popular imagination often exaggerates plutonium's danger in one particular way: its alpha radiation cannot penetrate skin or even a sheet of paper, so a sealed sample can be handled with gloves relatively safely, and the real hazard comes from inhaling or ingesting even microscopic particles, which then irradiate internal tissue directly, a distinction lost in most dramatic popular depictions of the metal as something instantly lethal to the touch.