A life-sized photo of the Tevatron particle accelerator's detector hangs from a beam near a wall-sized door bearing the signatures of those who built it at the Fermi National Accelerator Laboratory. Fermi will shut down the Tevatron on September 30, marking an end to the era of big physics in the United States. (MIRA OBERMAN/AFP/GETTY IMAGES)

One scientist called it a 25-year adrenaline rush.

On Friday, though, the buzz will end. After a remarkable run as the most successful atom smasher in the world, the Tevatron — a four-mile underground ring about 50 miles west of Chicago — will smash no more.

At 2 p.m., Pier Oddone, director of Fermilab, the Energy Department facility that operates the Tevatron, will command the shutdown of the mammoth machine. Operators will switch off dual beams of particles that have been colliding since 1985, sprouting terrific sprays of fleeting particles that offered a glimpse of the subatomic world.

“That will be it,” said Gregorio Bernardi, a Fermilab physicist. “Then we’ll have a big party.”

Conceived in the 1970s as an audacious effort to probe the subatomic realm, the Tevatron discovered three of the 17 particles thought fundamental to the universe. It became a prime training ground for two generations of young physicists. And in 1995, it bagged its biggest success, finding a subatomic particle called the top quark, the last of six fundamental building blocks of matter to be discovered.

Technology developed for the Tevatron — namely superconducting, super-cooled magnets — also primed the explosion in hospital MRI machines.

Fermilab’s technicians and engineers invented what they needed on the fly, said Christopher Quigg, a theoretical physicist who has worked at Fermilab since 1974. “There were all these wizards walking around, which was exciting for someone who didn’t get to get his hands on anything,” he said.

The Tevatron’s magnet technology also forms the backbone of its European successor, the Large Hadron Collider. “There’s no way the LHC exists without the Tevatron,” Quigg said.

In 2009, the 17-mile LHC, which straddles the French-Swiss border, took from the Tevatron the title of world’s most powerful atom smasher. The LHC is closing in on a theoretical particle, called the Higgs boson, which is supposed to imbue all the other particles in the universe with mass. As the final piece of the Standard Model of physics, the Higgs is the biggest prize in physics today; a Nobel Prize surely awaits its discoverers.

The Tevatron — as powerful as it was — was too weak to see it.

“The machine has discovered what it could discover within its reach,” Bernardi said, adding that the Tevatron has, in fact, helped narrow the search for the Higgs.

But some physicists say the Tevatron could have made more discoveries. “They were in the realm where they might have seen the Higgs if they had kept running for a few years,” said Lisa Randall, a prominent theoretical physicist at Harvard University and author of the new book “Knocking on Heaven’s Door.” This year, a group of influential physicists pleaded to keep the Tevatron running through 2014. But the Energy Department deemed the $25 million annual outlay too high and instead will spend those funds on two new experiments at Fermilab, said William F. Brinkman, director of the agency’s Office of Science.

“The LHC is very rapidly outpacing what we could do with the Tevatron,” Brinkman said in an e-mail.

Tona Kunz, a Fermilab spokeswoman, said that 42 Fermilab employees took “voluntary separations” prompted by the closure but that the rest of the lab’s 1,800 full-time workers will remain employed.

The closure offers a bitter endnote for American scientists, who have long warned of a shift in physics power. European scientists once traveled to Fermilab in bunches. Now, droves of American physicists fly to Geneva, home of the European Organization for Nuclear Research, or CERN, which operates the LHC.

Of the 6,361 physicists registered to work on the LHC, the largest single contingent — 1,684 — comes from the United States, said CERN spokesman Jim Gillies. The Energy Department and the National Science Foundation contributed $531 million of the estimated $10 billion in LHC construction costs. And in 2008, when the LHC malfunctioned during an early run, Fermilab workers helped fix it.

But some American physics veterans would prefer to be operating on U.S. soil. Congress squashed that opportunity in 1993 when it canceled Tevatron’s successor, the Superconducting Super Collider, after spending $2 billion and digging 14 miles of what was supposed to be a 54-mile underground ring in north Texas.

Roy Schwitters, now a professor at the University of Texas, headed the project for four years. “High energy physics in the states has never fully recovered from the loss of the SSC,” he said. Asked whether the United States might contemplate another huge particle collider, he said: “I don’t see it right now. Those hopes were pretty well dashed and thrown to the winds.”

Over the coming days, the Tevatron’s 1,000 liquid-helium cooled magnets will slowly warm. Eventually, Oddone said, a section of the Tevatron’s tunnel and one of its massive collision detectors will be converted from atom-smashing into a more quotidian task: hosting visitors as a museum.