# Fermilab

## Inquiring Minds

You Wrote:

Magnets

To All the Hardworking Scientists,

Hi, I was wondering exactly how magnets are used at Fermilab? I understand that they are used in "detectors" and "particle accelerators", but I would like more specific information.

Student of Physics,
Ami

Dear Ami:

Yes, we use magnets both in our accelerators and in our detectors.

Dipole magnets (one north pole, one south pole) are used to bend the path of an electrically charged particle. They are essential in building ring-shaped accelerators as they steer particles in a curve. The stronger the magnets, the sharper turn the particles take.

Dipole magnets are also used in detectors to determine the electric charge of a particle. Flying through the same dipole magnet, positively charged particles curve in one direction whereas negatively charged particles turn in the opposite direction (left curve versus right curve). This helps to distinguish particles from their antiparticles. An electron, for example, has a negative charge, but its antiparticle, called positron, has a positive charge. If these two particles travel at the same speed (same amount of kinetic energy) they leave identical tracks - unless there is a magnetic field. Then one track curves to the left, the other to the right.

A more complicated magnet is the quadrupole magnet, which has four alternating poles: north-south-north-south. It is used to focus a particle beam. A particle beam consists of billions of charged particles that all travel on slightly different path, some more at the center of the accelerator beam line, some more on the outside. By just using dipole magnets, more and more of the particles on the outside tracks would be lost. Physicists use quadrupole magnets to focus the beam at certain locations along their accelerator, bringing all particles closer to the central track.