By Jonathan Fildes

Science and technology reporter, BBC News



Antiparticles are the mirror image of ordinary particles

Di-positronium, as the new molecule is known, was predicted to exist in 1946 but has remained elusive to science.

Now, a US team has created thousands of the molecules by merging electrons with their antimatter equivalent: positrons.

The discovery, reported in the journal Nature, is a key step in 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.

Mirror world

Di-positronium was first predicted to exist by theoretical physicist John Wheeler and its component "atoms" - positronium - were first isolated in 1951.

These short-lived, hydrogen-like atoms consist of an electron and a positron, a positively charged antiparticle.

Positron Emission Tomography makes use of antiparticles

Antiparticles are the mirror image of ordinary particles.

There is an antiparticle for each type of particle in the Universe. For example, a positively charged proton has a corresponding negatively charged antiproton.

Conventional thinking states that both 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 such as Europe's nuclear research facility, Cern.

These appearances are always short lived because antiparticles are destroyed when they collide with normal matter. The meeting leaves a trace, often as high energy x-rays or gamma-rays.

These emissions are used today in PET (positron emission tomography) scanners to study activity in the brain.

Short lives

The transient nature of antiparticles has made creating and studying di-positronium problematic.

"We've known about this molecule; we're not surprised that it exists but it's taken us more than 50 years to create it in the lab," said Dr Cassidy.

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".

"It's like having a trickle of water filling up a bath and then you empty it out and you get a big flush," said Dr Cassidy.

As the positrons rushed into the voids they were able to capture electrons to form atoms. Where atoms met, they formed molecules.

"All we are really doing is implanting lot of positrons into the smallest spot we can, in the shortest time, and hoping that some of them can see each other," said Dr Cassidy.

By measuring the gamma-rays that signalled their annihilation, the team estimated that up to 100,000 of the molecules formed, albeit for just a quarter of a nanosecond (billionth of a second).

Laser beam

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).

Bose-Einstein condensate are like a super-atom

BECs are usually produced by supercooling atoms so that they merge and begin to behave like one giant atom.

They have been used in many 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.

He highlighted an experiment at the National Ignition Facility (NIF) in the US where scientists envisage using 192 lasers to heat a fuel target to try to kick-start nuclear fusion.

"Imagine doing that but you no longer need hundreds of lasers," he said.