Radiation source

These are used for what their nuclei emit rather than for any chemical or mechanical property, which makes them the one group here where the isotope matters more than the element. Americium in a smoke detector, cobalt-60 in a sterilisation plant and technetium-99m in a scanner are chosen for a half-life and a particle type.

Half-life is the design parameter. Too short and the source is useless before it is installed; too long and the waste outlives the institution that made it — which is why a diagnostic tracer is measured in hours and a smoke detector's source in centuries.

• Element

  Americium

  an alpha source in ionisation smoke detectors and thickness gauges

• Element

  Californium

  a portable neutron source for well logging and reactor start-up

• Element

  Curium

  the alpha source in planetary X-ray spectrometers

• Element

  Polonium

  an alpha source used to bleed away static charge

• Element

  Promethium

  a beta source for luminous paint and thickness gauges

• Element

  Radium

  historically, in luminous paint and early radiotherapy; no current use

• Element

  Thulium

  a portable X-ray source needing no power supply

Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.