The Spallation Neutron Source's fundamental neutron physics program, led by Geoff Greene, will be part of a large, exciting research enterprise that addresses important questions about the cosmos.
Geoff Greene wants to know what makes the universe tick, so he follows the best neutron sources around the world. Now, he's preparing to study the fundamental characteristics of the neutron at what will be the world's best neutron science facility. Greene came to ORNL in 2002 through a joint faculty appointment with the University of Tennessee. In collaboration with colleagues from ORNL and several other institutions, he is designing two instruments for the Fundamental Neutron Physics Beam Line at the Department of Energy's Spallation Neutron Source at ORNL, which will be completed in 2006.
Greene has conducted research at neutron sources in France, at DOE's Los Alamos National Laboratory, and at the National Institute of Standards and Technology (NIST). "I came here because of the SNS," he says in his ORNL office. "There's no question that the next big thing is up the hill."
Why is he interested in studying the nature of the neutron at the SNS? "Some large questions concerning the universe can be addressed by studying the neutron," he says, giving these examples. Why does the universe discriminate between matter and anti-matter? How were elements made during the first few minutes of the Big Bang? How much energy is produced in the sun? What is the nature of the weak nuclear interaction between quarks? Why does the universe apparently show a "preference between left- and right-handedness," an effect known as parity violation?
Measurements of neutron lifetime could shed light on this last question. "Our neutron measurements will complement the findings of researchers in high-energy physics, nuclear physics, theoretical physics, cosmology, astronomy, and astrophysics," Greene says.
Neutron scattering researchers see the neutron as a point-like object that interacts simply with matter, providing an attractive probe for understanding complicated materials. To nuclear and particle physicists, the neutron is a much more complicated object, made of "spinning" quarks and filled with a host of "virtual" particles and anti-particles that are being continuously created and annihilated.
"Although the neutron exhibits all the complexity of an atomic nucleus, it is really much simpler," Greene says. "From a nuclear and particle physics standpoint, the neutron is complicated enough to be interesting but simple enough to be understandable."
Greene and his colleagues from Harvard, NIST, Los Alamos, and Germany's Hahn-Meitner Institute will try to determine more precisely how long the unstable neutron lives before decaying. "The free neutron has a 'half-life' of about 10 minutes," he says. "When a neutron decays, it releases enough energy to be observed in a sensitive detector."
Pulses of neutrons from the SNS will pass through cold liquid helium before entering a magnetic "neutron bottle" containing liquid helium. Because neutrons are tiny magnets, they can be trapped in a non-homogeneous magnetic field created by a superconducting magnet.
Each decaying neutron produces an energetic electron that disrupts the helium atoms, causing a flash of ultraviolet light that is converted to visible light and detected. These light pulses count the neutron decay rate in the bottle as a function of time. The rate of decrease of neutron radioactivity in the bottle indicates the neutron lifetime.
"We expect the SNS to give us 100 times the count rate of the current neutron lifetime experiment," Greene says. Such an increase will allow significant improvement in the measurement of the neutron lifetime.
"It is an astonishing fact that the universe seems to make a big distinction between right and left," Greene adds. "We don't really know why radioactivity is left-handed, meaning that when a radioactive nucleus decays, it emits more electrons in one direction than another. Nuclear radioactivity is really the decay of a neutron within the nucleus. The study of the decay of the free neutron may help us better understand the origin of parity violation." And it will help Greene and his colleagues better understand the universe.
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