Work is nearly complete on the critical first phase of what essentially will be a subway tunnel for subatomic particles at Fermi National Accelerator Laboratory.
As part of a construction project that began more than two years ago, workers have bored and blasted about 130,000 cubic yards of dolomite rock and shale to carve out 4,000 feet of tunnel and two huge caverns under a portion of Fermilab’s 6,800-acre site near Batavia.
The $171 million project will provide research facilities for an experiment designed to study neutrinos, one of the more elusive particles in the world of high-energy physics.
The earth is bombarded constantly by billions of neutrinos, but they have proved difficult to study because they zip through most objects without leaving so much as a ripple in the sea of atoms that make up matter.
“Neutrinos interact so much less than any other particle we know about,” said Catherine James, a physicist at Fermilab who is overseeing the fabrication of a giant hexagonal detector, which will be placed in a cathedral-size hall 360 feet below ground. The detector hall is at one 4end of the new tunnel.
In the tunnel is 2,200 feet of steel pipe designed to propel neutrinos past Fermilab’s detector and through the earth toward a larger detector about 450 miles away, at the bottom of a former iron mine in northeastern Minnesota.
The steel pipe, which is about 6 feet in diameter and slopes downhill, is like a subway tunnel.
The subway train starts with a beam of particles–protons generated by Fermilab’s main injector. The beam begins its journey in what is called the target hall, a series of underground rooms that are about 120 feet below the surface.
The protons will tumble down a ramp from the main injector into a 250-foot portion of the hall toward a target–a series of carbon fins slightly more than 3 feet in length, according to Dixon Bogert, Fermilab’s construction manager for what is called the Neutrinos at the Main Injector project, or NuMI.
When the beam hits the target, particles will be created and steered into the pipe. Once in the pipe, the other particles drop off the beam, leaving only muons, which are subatomic particles, and muon-type neutrinos.
Because neutrinos have neither a positive nor negative charge, their direction cannot be manipulated by electromagnets, which means that the pipe had to be aimed precisely.
Bogert said that after the 56 lengths of steel were welded together and surrounded by up to 8 feet of concrete, the line of the pipe was found to be accurate within a quarter of an inch.
Crews poured about 1,000 cubic yards of concrete a day for 25 days to build the shield around the tube, officially called a decay pipe because all particles except the muons and neutrinos decay along its length.
Crews also poured concrete to level the tunnel floor and create drainage channels. Seepage can be controlled in the tunnel and the caverns but not prevented, Bogert said, and water is expected to be pumped out of the underground facilities at a rate of 350 gallons a minute.
The decay pipe ends in solid rock, which absorbs the muons. The neutrinos are supposed to re-emerge in the detector hall and pass through Fermilab’s detector before heading through rock toward Soudan, Minn. The trip will take a tiny fraction of a second.
What scientists hope to get are “before” and “after” pictures that will show that neutrinos have changed from one type to another. A change would be evidence that the tiny particles have mass, contrary to earlier assumptions, and would add a new chapter to physicists’ efforts to explain the universe.
The tunnel construction was a first for Fermilab, which began operating in the early 1970s.
The underground blasting ended in late May, and tunnel contractor S.A. Healey Co. is expected to wrap up work this month on its phase of the project, said Kurt Riesselmann, a Fermilab spokesman. The project was delayed in 2001 when work was halted for a week while Fermilab’s parent, the U.S. Department of Energy, launched an investigation into safety issues after a series of construction accidents.
Fermilab officials also held meetings with nearby residents to explain the construction and the blasting to head off complaints.
During the next phase of the construction, expected to take 13 months, another contractor will install wiring and other infrastructure in the tunnel and caverns. Construction also is expected to begin above ground on two service buildings.
In 2004, Fermilab crews hope to begin installing equipment. The experiment is expected to start in late 2004 or early 2005.
