Thorium ( /ˈθɔəriəm/ THOHR-ee-əm) is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 by the Swedish chemist Jons Jakob Berzelius and named after Thor, the Norse god of thunder.

In nature, virtually all thorium is found as thorium-232, and it decays by emitting an alpha particle, and has a half-life of about 14.05 billion years (other, trace-level isotopes of thorium are short-lived intermediates of decay chains). It is estimated to be about four times more abundant than uranium in the Earth's crust and is a by-product of the extraction of rare earths from monazite sands.

Thorium was formerly used commonly as (for example) the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity. Thorium is also used as an alloying element in non consumable TIG welding electrodes.

Canada, Germany, India, Netherlands, the United Kingdom and the United States have used thorium in various experimental and power reactors as fuel. There is a growing interest in developing thorium fuel cycle for various reasons, including its safety benefits, its high absolute abundance and relative abundance compared to uranium.India's three stage nuclear power programme is possibly the most well known and well funded of such efforts.

Carlo Rubbia Knight Grand Cross (born on 31 March 1934) is an Italian particle physicist and inventor who shared the Nobel Prize in Physics in 1984 with Simon van der Meer for work leading to the discovery of the W and Z particles at CERN.

Carlo Rubbia studied at Scuola Normale in Pisa and graduated doing cosmic ray experiments at Pisa University in 1959. He then went to the United States where he spent about one and a half years at Columbia University performing experiments on the decay and the nuclear capture of muons. This was the first of a long series of experiments that Rubbia has performed in the field of weak interactions and which culminated in the Nobel Prize-winning work at CERN. He received his PhD in physics from Columbia in 1959.

In 1960 he moved back to Europe, attracted by the newly founded CERN, where he worked on experiments on the structure of weak interactions. CERN had just commissioned a new type of accelerator, the Intersecting Storage Rings, using counter-rotating beams of protons colliding against each other. Rubbia and his collaborators conducted experiments there, again studying the weak force. The main results in this field were the observation of the structure in the elastic scattering process and the first observation of the charmed baryons. These experiments were crucial in order to perfect the techniques needed later for the discovery of more exotic particles in a different type of particle collider.