Predicted Over 60 Years Ago, New Matter Found is Missing Link in Creating Ultra-Powerful Gamma-Ray Lasers

Delicate particles rarely seen in our Universe have been merged with ordinary electrons to form a new exotic form of matter. Di-positronium, as the new molecule is known, was predicted to exist in 1946 but it has taken scientists over 60 years to find it.

A US team has now created thousands of the molecules by merging electrons with their antimatter equivalent: positrons. The discovery, reported in the journal Nature, is a critical step towards the creation of ultra-powerful lasers known as gamma-ray annihilation lasers.

"The difference in the power available from a gamma-ray laser compared to a normal laser is the same as the difference between a nuclear explosion and a chemical explosion," said Dr David Cassidy of the University of California, Riverside, and one of the authors of the paper.

"It would have an incredibly high power density."

As a result, there is a huge interest in the technology from the military as well as energy researchers who believe the lasers could be used to kick-start nuclear fusion in a reactor.

Theoretical physicist John Wheeler predicted Di-positronium existence, and was able to isolate its component "atoms", or positronium, in 1951. The short-lived, hydrogen-like atoms consist of an electron and a positron. A positron is a positively charged antiparticle, which are the mirror image of ordinary particles.

According to the laws of physics, there is an antiparticle for each type of particle in the Universe. For example, a positively charged proton has a corresponding negatively charged antiproton.

Therefore, according to conventional thinking, both states of antimatter and matter should have been created in equal quantities at the birth of the Universe. The dominance of matter in our world is one of science's most enduring mysteries.

Antimatter only makes fleeting appearances in our Universe when high-energy particle collisions take place, such as when cosmic rays impact the Earth's atmosphere. They are also made in the lab in particle accelerators in advanced nuclear research facilities.

These appearances are always short lived because antiparticles are destroyed when they collide with normal matter. This meeting leave a trace, often as high energy x-rays or gamma-rays, which is the type of emission used in PET (positron emission tomography) scanners to study activity in the brain. The transient nature of antiparticles has made creating and studying di-positronium very difficult.

To make the molecule, Dr Cassidy and his team used a specially designed trap to store millions of the positrons. A burst of 20 million were then focused and blasted at a porous silica "sponge".

Dr Cassidy believes that increasing the density of the positronium in the silicon would create an exotic state of matter known as a Bose-Einstein condensate (BEC). BECs are produced by supercooling atoms to the point where they merge and begin to behave like one giant atom. They have been used in previous experiments such as the 2003 Harvard study in which scientists were able to trap light.

"At even higher densities, one might expect the material to become a regular, crystalline solid," wrote Professor Clifford Surko, of the University of Californian, San Diego, in an accompanying article.

Taking it one step further, scientists could use the spontaneous annihilation of the BEC, and the subsequent outburst of gamma-rays, to make a powerful laser.

"A gamma-ray laser is the kind of thing that if it existed people would find new uses for it everyday," said Dr Cassidy.

Posted by Rebecca Sato

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Related Blog Post:
http://dougsamu.wordpress.com/2007/09/12/di-positronium/

Links:

http://www.nature.com/nature/journal/v449/n7159/abs/nature06094.html

http://newsvote.bbc.co.uk/mpapps/pagetools/print/news.bbc.co.uk/2/hi/science/nature/6991030.stm