Tuesday - June 29, 2010
SLAC Today is
available online at:
http://today.slac.stanford.edu
In this issue:
Crafting the World's Smallest Beam
Symmetry Explains It in 60 Seconds: Shielding
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Tuesday - June 29, 2010 |
Crafting the World's Smallest Beam Seven images have been stitched together to create this 180-degree panoramic view of the ATF2 beamline. (Photo by N. Toge, KEK.)
The Accelerator Test Facility 2 in Japan reached a milestone last month when an international team of researchers—including several SLACers—successfully narrowed the beam down to a height of 310 nanometers, about 1 percent the diameter of a human hair. ATF2 began operation in 2008 as a major test bed for future linear colliders. It sends a beam of electrons along a 139-meter racetrack before funneling them into the final focus test region, which simulates the collision point in a linear collider. The ultimate goal is to narrow the beam to the record-breaking tiny size needed in a future collider: about 37 nanometers—about 0.1 percent the diameter of a human hair. For comparison, colliding beams were on the order of 650 nanometers in the smallest direction in the SLAC Linear Collider. As a pulse of the ATF2 beam heads toward the final focus test region, it looks a bit like a short needle, similarly tiny in the horizontal and vertical, and a little longer in the longitudinal direction. The goal of last month's tests was to narrow the beam's height even further so that it resembles a miniscule ribbon more than a needle. The tests were a good starting point, said SLAC Physicist Glen White, who traveled to the KEK physics lab in Tsukuba, Japan to join in the week-long R&D session devoted solely to tuning the beam. "This was our first real attempt at putting everything together. Really for the first time we started using some of our beam tuning technology" in situ, he said. "It was very exciting." Read more... Symmetry Explains It in 60 Seconds: Shielding (Image: Sandbox Studios.)
Shielding refers to layers of material that block radiation: that lead apron we wear during dental X-rays, the thick walls around a nuclear reactor, and even those cool, UV-blocking sunglasses all shield us from biologically damaging forms of radiation. We don't need shielding from cosmic rays—high-energy particles that continually rain in from outer space. But this steady background radiation does bedevil scientists and their experiments because it can drown out the nearly imperceptible signals of rare subatomic processes. Cosmic rays crashing into the atmosphere spawn secondary particles such as muons, which pass through normal layers of shielding with ease. High-energy muons can penetrate more than a kilometer of solid rock. To search for the dark-matter particles thought to fill the universe, explore the mysterious properties of neutrino interactions, or look for neutrinos from the sun or a supernova, scientists need shelter from cosmic rays and the muons they produce. So they're taking their experiments underground, to special-purpose laboratories such as SNOLAB in Ontario, Canada, which is under about two kilometers of rock and the deepest lab operating today. Each 300 meters of overlying rock reduces the rate of incoming muons by a factor of ten. In these subterranean refuges, sensitive instruments await the first faint whispers of new physics. |
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