Thursday, April 19, 2012

The age of the Universe, Uranium and Thorium

Thor's battle with the Ettins (1872), painting by Mårten Eskil Winge
We have had a look at the production of iron in the majestic nuclear fusion reactions in stars spreading iron into deep space in supernova explosions. Other products of these massive explosions of elements into star dust are uranium (Uranus named by German M.H. Klaproth in1789) and thorium (Thor named by Swedish J.J. Berzelius in 1828).

In the Periodic Table of the elements iron (Fe) has atomic number 26. The nucleus is very stable and most iron in nature is found in combinations. In comparison to iron, uranium (U) and thorium (Th) are much heavier metals - Thorium with atomic number 90 and weight 232, Uranium 92 and 238.

The heavy nuclei are less stable than iron. Nuclear decay of thorium and uranium produces bluish grey lead. Lead (Pb) has atomic number 82 and normal atomic weight 207. Thorium-232 decay series ends with lead-208 and when uranium-238 nucleus looses an alpha-particle the element turns into lead-206.

Very slow decay of thorium with half-life14 gy - the age of cosmos - and considerably faster uranium decay into lead with half-life 4.5 gy - the age of Sun - is going on all the time in the Universe. Thus uranium gives better atomic clock than thorium. [Recently a more accurate figure has been suggested for uranium.]

We humans have learned to hasten the decay of uranium and use nuclear fission to produce energy and also, of course, weapons of mass destruction the Iranian's are accused of today by those, who already have the Bomb. Thorium would be even more powerful - and destructive - source of energy but the technology is not yet there.

Age of the Universe
CS31082-001European Southern Observatory, Chile 

If we can date an old star we will know that the minimum possible age of the entire universe must be equal or more than that. Sun is a relatively young star, near the half-life of uranium at 4.5 gy. A much older star, CS31082-001, has been studied to measure the decay of uranium that has taken place there.

In 2001 interesting results were reported from the atomic clock - or rather calendar - using uranium. I quote from a clearly written article in  The original article by R. Crayel in ESO has more details.

Astronomers have spotted for the first time the fingerprint of uranium-238 in an ancient star - and have used it to make the most reliable guess yet of the age of the universe. Roger Cayrel of the Observatoire de Paris-Meudon, France, and colleagues have used a kind of 'stellar carbon dating' to estimate the age of the star - and therefore the minimum age of the universe. The new estimate makes the universe 12.5 billion years old - give or take 3 billion years (R Cayrel et a l2001 Nature 409 691).

Cayrel and colleagues used the Very Large Telescope at the European Southern Observatory in Chile to measure the spectra of a very old star - known as CS31082-001 - near the edge of the Milky Way.

Astronomers know that the star formed in the very early universe because it contains so little metal.

Metals were scarce at this stage in the evolution of the universe because very few supernova, which create metals, had yet exploded.

Indeed, the traces of uranium-238 in the star's atmosphere could have come from just one supernova. The uranium-238 absorption lines are relatively easy to detect in metal-poor stars because they are not obscured by the strong absorption lines of other metals.

Taking into account uncertainties about the initial abundance of elements, Cayrel's and co-workers have obtained an estimate of 12.5 billion - or 12.5 x 109 - years old, plus or minus just 3 billion years.

This is three times more accurate than the previous best estimate, which was based on absorption lines of thorium-232. Thorium-232 has a half-life of 14 billion years - similar to our current best guess of the age of the universe, and this means it can only have decayed by about half.

Other methods of dating the universe - such as measuring how quickly galaxies recede from us - are less reliable because they are based on untested assumptions about the evolution of the universe.
Physics world 2001

A Note on Throrium
There is probably more untapped energy available for use from thorium in the minerals of the earth's crust than from combined uranium and fossil fuel sources. Much of the internal heat the earth has been attributed to thorium and uranium.

When pure, thorium is a silvery white metal which is air-stable and retains its lustre for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black. Thorium oxide has a melting point of 3300°C, the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points.

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be carefully handled.When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Thorium is named for Thor, the Scandinavian god of war. It is found in thorite and thorianite in New England (USA) and other sites.

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