Aug. 16, 2002 Building a bridge over land or water requires careful engineering. There is the weight of passing cars and trucks to consider. Will high winds or turbulent weather threaten the structure? How deep should the concrete foundations be poured? How best to affix the steel supports? What is the ideal mix of materials for strength, durability and corrosion resistance? Nature has long ago figured out how best to arrange atoms that comprise ordinary matter. The nuclei inside those atoms are systems of quarks, the particles thought by many to be matter's basic building blocks. Humans are only now beginning to unravel the engineering secrets of quarks, how they are precisely arranged and how their interactions determine the properties of the atomic nucleus.
In Hall C, in an experiment that began on January 21 and concluded on March 3, researchers took a closer look at matter's blueprints with a study of the spin-structure functions of the proton and the neutron, collectively known as nucleons. Nucleons are the smallest "everyday" objects made of quarks. Spin is a mathematical property analogous to the way objects physically spin in space, contributing to and affecting the subatomic properties within an atom's nucleus. Although seemingly limited to the realm of the infinitesimal, scaled up macroscopically those properties eventually affect all things of "normal" size.
"In the big picture, we'd like a better understanding of how quarks are bound up in nucleons," says Mark Jones, a Hall C staff scientist and co-spokesperson for the experiment. "Quarks are not free-floating particles. Because they're in the nucleus, the nucleus becomes a much more complex object. We're interested in the details of that complexity."
The Hall C experiment was sensitive to the kind and degree of spin, with a powerful detector that is able, like a kind of electron microscope, to "see" into regions otherwise hidden from view. The CEBAF beam of electrons "illuminated" the nuclear material of their target, and investigators measured the number of electrons that scattered into the detector. With these data, researchers hope to discern the distribution of the quark's spin inside the nucleons.
The genesis of the experiment was in studies at the Stanford Linear Accelerator, or SLAC and at CERN, in Geneva, which probed the quarks under conditions in which large amounts of kinetic energy are exchanged. In these "deep inelastic scattering" conditions the movement of quarks is not completely understood. The Hall C experiment was proposed in 1996 by co-spokesperson Oscar Rondon, a University of Virginia scientist, to extend the range of those measurements including all possible combinations of spin orientations for protons and neutrons. Related experiments in Halls A and B have explored complementary aspects.
The study required a specialized target that was polarized. Polarization refers to the alignment of spin of protons and neutrons within the target material: in this case, small chunks of solid ammonia that were kept near absolute zero, at one Kelvin, or minus 458 degrees Fahrenheit. A strong magnetic field created the desired polarization. A University of Virginia team developed and prepared the experiment's target.
"We can get a highly polarized beam," Jones says. "The Lab has spent a lot of time developing such a beam and has the expertise to maintain it. But as the target gets irradiated it loses polarization. Periodically, we had to stop the beam, remove the radiation damage by annealing, and then repolarize the target." In the experiment's aftermath, during data analysis, researchers are also having to adjust for the subatomic structure of nitrogen, a chemical constituent of the ammonia target, which affects the data taken.
Jones reports that the experiment went well, and that investigators obtained the amount and quality of data they expected. The study, he believes, will make a unique contribution to determining structure functions, with credit going to the Lab's accelerator for its unique capabilities.
"We've made measurements of spin in [certain directions] that can't be done elsewhere, at least easily," Jones says. "While it's too early to tell the results, we have seen preliminary indications that the data we took was pretty good. Our error bars are reason-able. I think we'll be able to extract useful information."
The spin-structure experiment involved a 50-member team of inter-national researchers from the United States, Switzerland, Armenia and Israel. Results should be published within the coming year.
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