ILC Innovation: Improving the Photocathode
When built, the International Linear Collider (ILC) will generate one of the world's highest energy electron beams. And it will all start with an electron source called a cathode. This source releases the electrons when hit by laser and is the size of a nickel.
SLAC physicist Takashi Maruyama has spent twenty years studying photocathodes and is now working on one for the ILC. Using an innovative design for these crucial wafers, he has greatly increased their efficiency.
The ILC will require a high energy beam that is polarized, meaning that the electrons must all spin in the same direction.
Photocathodes are made of gallium arsenide, a semi-conductor grown as a crystal. By itself, gallium arsenide polarizes a beam at about 50 percent. In 1991, Maruyama and his collaborators increased the polarization to 80%. For certain measurements, the improved polarization even allowed SLAC detectors to outperform CERN's LEP, which had 30 times more events. For the ILC, Maruyama is challenged to reach even higher polarization.
To do this, he alternated layers of gallium arsenide with gallium arsenide phosphide in the top 100 nanometers of his cathode disk, like a stack of tightly-packed pages, one hundred thousandth the thickness of a single piece of printer paper.
Today, thanks to the new layering technique, almost 90 percent of the electrons spin in the same direction.
In addition to this polarization increase, the layering technique has also improved the gallium arsenide quality and increased efficiency in electron production.
Although his work has reached the ILC expectation, Maruyama continues to refine his design. “We are trying to reach 100 percent, with all electrons in one polarization,” he said.