Fermilab – Part 2: Particle Acceleration & Detection

August 7, 2011

Perhaps the most amazing achievement – and most basic to the purpose of the laboratory – is the way in which the particles are accelerated to near the speed of light.  It all begins with a retro-futuristic device with an insatiable appetite for electricity.  Within the Cockcroft-Walton pre-accelerator, hydrogen gas is ionized to create negative ions, each consisting of two electrons and one proton. The ions are abruptly accelerated by a voltage differential to achieve an energy of 750,000 electron volts, about 30 times the energy of the electron beam in a television’s picture tube.

For more information about the accelerator chain at fermilab see: http://www.fnal.gov/pub/inquiring/physics/accelerators/chainaccel.html

Next, the negative hydrogen ions enter a linear accelerator comprised of a series of copper toroid-shaped chambers.  Each segment contains oscillating electrical fields which further accelerate the negative hydrogen ions. High powered radio waves – strong enough to overtake the entire commercial spectrum – are carefully exhausted with special “mufflers.”  Before entering the next stage of acceleration, the ions pass through a carbon foil, which removes the electrons, leaving only the positively charged protons.

About 20 feet below ground the electrons enter the Booster stage.  This circular accelerator uses magnets to bend the beam of protons in a circular path until they reach an energy level of about 8 billion electron volts (GeV).  In the next stage, the Main Injector, the beams gain further energy, up to 150 GeV.

Finally, the protons and antiprotons reach the 3.9 mile Tevatron.  Here the beam, now traveling only 200 miles per hour slower than the speed of light, is accelerated to almost 1000 GeV.  Superconducting magnets cooled to very low temperatures using liquid helium bend the path of the particles along the arc of the Tevatron.  At this point, protons and antiprotons traveling in opposite directions are collided to create a shower of new and exotic particles.

When the high energy particle beam collides with a target (or an anti-mater beam), all sorts of particles spray out.  Many of these particles are not naturally occurring.  It’s possible they are made nowhere else in the universe.  By studying these particles, physicists learn about the elementary building blocks and fundamental forces that determine the nature of matter—and the ultimate structure and evolution of the universe.

In order to study the high energy collisions, scientists have designed and built huge particle detectors. Fermilab has two huge detectors called CDF and DZero. Each detector weighs about 5,000 tons and stands three stories high.  Visitors are able to look down into the construction bay where the detectors were assembled and where periodic maintenance takes place.  A full-scale banner draped along a subterranean wall illustrates the immense scale of the detector.

Like the layers of an onion, an intricate array of tightly packed devices encircles the beam pipe of the Tevatron.  These detectors can count particles, identify their tracks, measure their energy, record their time of flight, and distinguish one particle from another.  Delicate silicon devices and gas-filled tracking chambers are closest to the beam pipe. They record the particles bursting outward from the central collision region. As particles pass through these devices, they rip electrons from the silicon or gas molecules. These electrons create an electrical impulse, which scientists use to locate the track of a particle. A particle’s path can thus be measured to a small fraction of a millimeter.

Beyond the central tracking devices, the particles enter the detectors’ calorimeters, which measure their energy. As the particles traverse the calorimeters, they lose their energy to a succession of dense, absorbent materials. While passing through layers of heavy metal plates, the particles create showers of light. Homemade fiber-optic tubes – simply foil wrapped plastic strands – carry signals to devices which record the intensity of the light flashes.

For more information about particle detection see: http://www.fnal.gov/pub/inquiring/physics/collider/index.html Also, below, see the short video I made during our tour.

The physicists at Fermilab have made some amazing discoveries including the discovery of the top quark, the last undiscovered quark of the six predicted to exist by current scientific theory.  A quark is an elementary particle and a fundamental constituent of matter. Future studies will look for new phenomena, including supersymmetry, extra dimensions and a mass-carrying particle called the Higgs boson thought to be the missing link in proving a unified theory.

Although the new larger Hadrion collider in Cern is currently receiving most of the scientific community’s attention, work at Fermilab will continue.  In the final part of this series about Fermilab, I will describe the Neutrino experiments that are unique to Fermilab and explain how these curious particles might shed light on dark matter and provide a treatment for cancer.

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