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Five Nobel Laureates have been intimately involved with the use of radioactive tracers in medicine.Over 10,000 hospitals worldwide use radioisotopes in medicine, and about 90% of the procedures are for diagnosis.Thorium is a chemical element with symbol Th and atomic number 90.A radioactive actinide metal, thorium is one of only two significantly radioactive elements that still occur naturally in large quantities as a primordial element (the other being uranium).Thorium-232 (Th), which has 142 neutrons, is the most stable isotope of thorium and accounts for nearly all natural thorium, with six other natural isotopes occurring only as trace radioisotopes.

A thorium atom has 90 protons and therefore 90 electrons, of which four are valence electrons.

This Buzzle article has a list of radioactive elements that abound in nature, arranged in the order of increasing atomic number, along with their decay modes. Radioactivity arrived on the scene of world physics in the 19th century, just when people thought they knew everything in physics.

With its discovery in 1896, radioactivity opened up a Pandora's box of questions and revealed a new world, waiting to be explored in the microcosm of the atomic nucleus. Radioactivity is a very interesting phenomenon in nature.

Thorium is estimated to be about three to four times more abundant than uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare earth metals.

Thorium was once commonly used as the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity.

For most human activities involving minerals and raw materials, the levels of exposure to these radionuclides are not significantly greater than normal background levels and are not of concern for radiation protection.

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