The Quark Hunters
Beneath the rolling hills just west of Stanford University is one of the longest, straightest structures in the world: a 3.2-kilometer (2-mile) linear accelerator. Founded in 1962, the SLAC National Accelerator Laboratory was built for a singular purpose: to accelerate electrons to 99.9999999% of the speed of light and smash them into targets to see what is inside.
In a series of experiments starting in 1968, a team from SLAC and MIT fired these high-energy electrons at protons and neutrons. The way the electrons scattered was unexpected. Instead of passing through or glancing off a uniform target, some ricocheted at sharp angles, revealing that the protons and neutrons were composed of hard, point-like objects. These objects were the quarks that physicists had theorized but never detected. For this direct proof of the quark model, Richard Taylor of SLAC and Jerome Friedman and Henry Kendall of MIT received the 1990 Nobel Prize in Physics. Research at SLAC has contributed to three other Nobel Prizes, including one for the 1974 discovery of a new type of quark called the "charm" quark.
An X-ray camera for atoms
Today, the original accelerator has a new mission. The final one-third of the linac is used to power the Linac Coherent Light Source (LCLS), the world's first hard X-ray free-electron laser. Rather than smashing particles, the LCLS uses the beam of accelerated electrons to generate brilliant and brief pulses of X-ray light, a billion times brighter than those from any previous source.
These X-ray pulses are measured in femtoseconds, or quadrillionths of a second. This speed allows scientists to create molecular "movies" of chemical reactions as they happen. Researchers can watch individual atoms form and break chemical bonds, observe the real-time dynamics of photosynthesis, and map the 3D structure of proteins used for developing new medicines. A major upgrade, LCLS-II, has increased the laser's firing rate from 120 pulses per second to up to a million per second. This creates an almost continuous X-ray beam that is, on average, 10,000 times brighter than its predecessor, opening new fields of atomic and molecular science.