Image caption The Tevatron dominated the energy physics frontier until the advent of the LHC

One of the world's most powerful "atom smashers", at the leading edge of scientific discovery for a quarter of a century, has been shut down.

The Tevatron facility near Chicago fired its last particle beams on Friday after federal funding ran out.

Housed in a 6km-long circular tunnel under the Illinois prairie, the Tevatron leaves behind a rich scientific legacy.

This includes finding nature's heaviest elementary particle: the top quark.

Since 1985, engineers have been accelerating bunches of proton and antiproton particles around the Tevatron's main ring at close to the speed of light, then smashing them together in a bid to unlock the secrets of the Universe.

TEVATRON TIMELINE 1983 - Tevatron switched on

1985 - Collides its first proton-antiproton beams

1995 - Discovers the top quark particle

1996 - Detects simplest form of antimatter atom

1998 - Discovers last undetected meson particle

2000 - First direct evidence for the tau neutrino

2008 - Starts narrowing search area for Higgs

2011 - Shut down after funding runs out Interactive Timeline (Fermilab)

But the Tevatron has been superseded by the Large Hadron Collider (LHC) - located on the French-Swiss border - which is capable of getting to much higher energies than the US machine.

Shortly after 1400 local time on Friday, the Tevatron's designer Dr Helen Edwards was due to push a button in the control room that diverts the last beam of particles into a solid metal block, closing the book on an era in American big physics.

The particle accelerator is run by the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, which is now likely to shift its emphasis to projects that - for example - rely on particles at high intensities, rather than high energies.

"People like me, who have been here for many years, are very attached to the Tevatron. The Tevatron has really defined this laboratory over the last 30 years," said Dr Roger Dixon, head of the laboratory's accelerator division, who nevertheless said he was enthusiastic about future projects at Fermilab.

For many, the closure will be a solemn occasion, at a time when US budgets for science are increasingly being squeezed.

"If you ask me whether I'm confident the country can keep doing things, it's hard to read the newspaper everyday and believe it's going to work out, but we have to trust that it will," Dr Dixon explained.

'So close'

Earlier this year, Fermilab announced plans to lose 100 jobs from the lab, 50 of which will come from voluntary redundancy, a spokesman said.

The decision to shut down the Tevatron came as a bitter pill for physicists who had been pushing the machine's limits in an effort to smoke out the elusive Higgs boson particle.

I think a lot of people feel we could have continued for a bit longer Dr Roger Dixon, Head of accelerators, Fermilab

Researchers have repeatedly stated that they are closing in on the Higgs, which - if found - would explain the origins of mass and is the last missing jigsaw piece in the most widely accepted theory of particle physics - the Standard Model.

A bid to extend the Tevatron's lifetime by three years was denied in January 2011 because the US Department of Energy could not come up with the extra $35m per year required to keep the machine running. An expert panel recommended the extension but its advice was not followed, turning the quest for the Higgs into a one-horse race.

"In the Higgs game, we are still competitive and we hope to have our final results next year," said Professor Stefan Soldner-Rembold, spokesperson for the Tevatron's DZero experiment.

He told BBC News: "There are always reasons for and against an extension. I still think especially in terms of the Higgs it would have been nice to have another three years. We can see now that we are so close."

The data collected up until the shut down will continue to be analysed and could still produce a surprise, he explained, but extra time would have helped bolster the statistical significance of any potential discovery.

We tested the Standard Model very, very aggressively and it came out alive and well Dan Hooper, Fermilab

"I think a lot of people feel we could have continued for a bit longer, but the mood is not that bad," Dr Dixon explained.

Pier Oddone, director of Fermilab, told the AFP news agency: "In our field we don't keep beating our heads if we have been outdone by another machine."

The origins of the Tevatron stretch back into the 1970s, when the idea of building a superconducting proton synchrotron was hatched. Originally known as the Energy Doubler, the machine was switched on in 1983 and smashed together its first particle beams two years later.

In 1995, physicists at the Tevatron announced the discovery of the top quark, a new addition to the "zoo" of particles described in the Standard Model. This is the most celebrated of the Tevatron's many discoveries, but the machine has continued to push the boundaries of knowledge right up to its shutdown.

Image caption Operators in the Tevatron control room keep the matter-antimatter beams circulating

Fermilab physicist Dr Dan Hooper said of the Tevatron's legacy: "The thing it really did is took us from a point back in 25 years ago where we had every reason to think the Standard Model was not going to survive the onslaught of the Tevatron... the amazing thing about the Tevatron is that didn't happen."

He told BBC News: "We tested the Standard Model very, very aggressively and it came out alive and well. That's really the legacy in my mind."

After the Tevatron closes its doors, Fermilab will concentrate on smaller-scale experiments such as Project X, an effort to develop ever more intense particle beams.

"The idea is we shift to those areas where we can make the greatest contributions to understanding," Dr Oddone said, "sometimes the biggest discoveries come from smaller projects."

The Tevatron's main ring is likely to be used in other experiments, and components may be transferred to other accelerators.

Paul.Rincon-INTERNET@bbc.co.uk