Probing the Evolution of Stars
First Moments of Big Bang
CERN Results and Colliders
Superdeformed Light Nuclei
Gamma Rays and HFIR
Isotope Signatures in Earth's Fluids
The Physics Division operates four experimental facilities: the Oak Ridge Electron Linear Accelerator, used to measure neutron cross sections to help resolve issues in nuclear astrophysics; the Holifield Radioactive Ion Beam Facility (HRIBF), which is being prepared for the first experiments using radioactive ion beams later this year; the EN-Tandem Accelerator, and the recently upgraded Electron Cyclotron Resonance Ion Source Facility. The latter two are operated in support of the atomic physics research community.
The experimental nuclear physics program emphasizes use of heavy ions to probe structure of heavy nuclei and address questions in nuclear astrophysics. Our physicists conduct their research here and at accelerator facilities around the world, including the Super Proton Synchrotron at the European Laboratory for Particle Physics (CERN).
Atomic physicists conduct channeling and energy-loss experiments, study charge change in projectile beams at ultrarelativistic energies, and perform research in support of the fusion energy program. Theoretical physicists work in support of experimental physics to predict outcomes and better understand results.
In the Solid State Division, physicists seek to understand properties and interactions of a wide range of materials for possible use in advanced energy technologies, such as thin-film batteries and semiconducting and superconducting materials. By studying interaction of particle beams with materials, they develop new materials and materials processing techniques (see "Advanced Materials Processing, Synthesis, and Characterization," for highlights on the materials research of our solid-state physicists). Physical sciences also includes chemistry. Several of our scientists are geochemists who explore phenomena within the earth while some of our nuclear physicists look beyond it.
Probing the Evolution of Stars
Neutrons produced at the Oak Ridge Electron Linear Accelerator (ORELA) are helping ORNL's star shine in the astrophysics community. ORELA, which produces energetic bursts of neutrons by bombarding a tantalum target with electrons, has already been used to measure 180 neutron cross sections of astrophysical significance. These data are important to our understanding of how elements heavier than iron are slowly synthesized in stars by a series of neutron captures known as the s-process.
|Red giant stars are responsible for producing many of the heavy elements. Measurements at ORELA play an important role in furthering our understanding of how those elements are produced.|
In four billion years, our sun will become a red gianta bright, bulging star that will have cooled to a low temperature. Similar stars are believed to have produced many of the heavy elements in the universe. Nuclear reactions in these stars can produce large numbers of neutrons that are later captured by iron "seed" nuclei to form elements as heavy as bismuth. ORELA has supplied much of the nuclear data needed to understand the s-process. However, it was recently learned that the s-process may be occurring at a temperature three times lower than previously believed. Because more nuclear data are needed to understand effects of a lower temperature on the s-process, ORELA is again supplying this information. ORNL physicists have recently measured neutron cross sections on two key elements: barium and neodymium. Measurements of other cross sections important for understanding heavy element production are also under way.
Our physicists are also using ORELA to address other puzzles in nuclear astrophysics. A recent ORELA measurement addresses how light elements were formed during the Big Bang. The standard Big Bang model, which assumes that matter was uniformly distributed in the early universe, suggests that no elements heavier than lithium were formed until after the birth of stars. However, in Big Bang models that assume that matter existed in clumps, it may be possible to synthesize elements such as carbon, nitrogen, and oxygen in very neutron-rich regions.
In a study of a crucial link in a sequence of nuclear reactions that could form heavier elements in a clumpy universe, ORNL physicists measured the probability that lithium-7 nuclei in a target would capture neutrons from ORELA, forming lithium-8 and emitting detectable gamma rays. Their results, which help resolve discrepancies among previous measurements of this reaction, suggest that carbon may have been formed before stars were born. If these isotopes thought to be present in the early universe are observed by the Hubble Space Telescope, it may be possible to determine the "clumpiness" of the universe minutes after the Big Bang.
Another puzzle that was addressed in this measurement concerns the neutrinos produced by nuclear reactions in our sun. Solar neutrinos are the only direct probe of the nuclear reactions that power the sun. The problem is that 2 to 5 times more solar neutrinos have been predicted than detected. This discrepancy has led scientists to speculate that some neutrinos change to a different type of neutrino which cannot be detected in many experiments. However, the number of neutrinos detected in most experiments is very sensitive to the rate of fusion of protons with beryllium-7 in the sun. This reaction will be studied directly in a future experiment at the Holifield Radioactive Ion Beam Facility using a radioactive beryllium-7 beam, but the ORELA lithium-7 experiment also sheds light on this reaction.
The fusion rate of the proton/beryllium-7 reaction is determined using a theoretical model that predicts that fusion occurs only in "head-on collisions." A recent Duke University study of a similar reaction suggests this may not be the case. The ORELA experiment is relevant because the lithium-7 reaction with neutrons "mirrors" the beryllium-7 reaction with protons. In the ORELA experiment, no evidence of fusion was seen in collisions other than head-on ones, supporting the standard theoretical model. ORELA continues to offer shining examples of its benefits to the astrophysics community.
The research has been supported by DOE, Office of Energy Research, Office of High Energy and Nuclear Physics.
Closing in on First Moments of the Big Bang
Re-creating the first ten microseconds of the Big Bang requires a big boost in accelerator beam energy. ORNL physicists working with an international team have been searching for the primordial soup of the early universe using ultrarelativistic beam energies 10,000 times those obtained at ORNL's Holifield Heavy Ion Research Facility. Since 1986 their hunting ground has been the Super Proton Synchroton of the European Laboratory for Particle Physics (CERN). In 1999 they will begin a series of experiments at the Relativistic Heavy Ion Collider (RHIC) at DOE's Brookhaven National Laboratory.
Some 15 billion years ago, an infinitely compact, superdense particle hotter than 1500 billion kelvins began to expand and cool rapidly in what is called the Big Bang. After a few microseconds of cooling, vast amounts of energy were partially converted to particles of matterfree quarks and gluons, leptons (such as electrons), and photons. By the tenth microsecond, this quark-gluon plasma condensed into the known particles that form the nuclei of atoms. Quarks are the basic constituents of protons and neutrons in atomic nuclei, and gluons are the force-carrying particles that bind quarks together.
Quarks have never been observed in a free, unconfined state. They prefer to exist only in quark-antiquark pairs (mesons) or quark triplets (protons and neutrons). The goal of the CERN experiments has been to show that free quarks can be produced and detected by "boiling" them out of protons like a yolk bursting from the shell of a microwaved egg. To heat protons enough so that the quarks would slam together and break out of the proton "skins," nuclei of oxygen, sulfur, and lead were stripped of all electrons, accelerated, and collided in different experiments with target nuclei of carbon, copper, silver, and gold at ultrarelativistic energies of 32 trillion electron volts (99% the speed of light).
The experiments dramatically demonstrated the conversion of energy into matter; for example, a collision between nuclei of oxygen and gold involving initially only 87 charged particles (protons) sprayed out over 400 charged particles. But the experimenters were mostly interested in measuring emitted photons because they would signal the presence of free quarks. Because each quark has an electric charge, it will radiate a photon any time it changes direction after it breaks free. This experiment, however, has a complication: Any quark-antiquark pair formed in these collisions may recombine later into a neutral pi meson, which subsequently decays and emits two photons. Thus, lead-glass calorimeters containing over 10,000 pieces of glass were built by Russian researchers to detect the many photons. German scientists built instruments to calibrate the photon energies, and Oak Ridge researchers, who built calorimeters for the early CERN experiments, developed the electronics to distinguish between photon signals from quarks and those from pi mesons.
The international team increased energy densityinternal energy of nuclei at rest 20 times, generating many more decay products; saw no evidence to rule out the existence of the quark-gluon plasma, and found no proof that deconfinementthe existence of lone quarksdid not occur. They also could set limits on how hot the resulting system must have been, finding temperatures about 100,000 times hotter than those at the core of the sun.
As the experiments are continued at RHIC to achieve even higher energy densities, our understanding of the conditions right after the universe was born should receive another big boost.
The research was supported by DOE, Office of Energy Research, Division of High Energy and Nuclear Physics, Office of Nuclear Physics.
CERN Results May Affect Collider Operation
Like two race cars careening around a circular racetrack in opposing directions, two beams of gold ions will smash into each other at a velocity greater than 99% the speed of light. At least that's one plan for the RHIC, now under construction at DOE's Brookhaven National Laboratory.
|Herb Krause examines the beam line of the Super Proton Synchrotron at the European Laboratory for Particle Physics (CERN).|
There may be a problem with this plan. ORNL atomic physicists in an international collaboration have observed an interesting effect when an ultrarelativistic beam of lead ions accelerated at 33.2 trillion electron volts strikes a thin gold target. During experiments at the Super Proton Synchrotron at the European Laboratory for Particle Physics (CERN) near Geneva, Switzerland, researchers from ORNL, Denmark, Germany, and Sweden noted a change in the lead ion beam that stemmed from a dramatic conversion of energy to matter. All the lead ions had been fully stripped of electrons before they were accelerated toward the target, but at the time of impact, up to 0.1% of the lead ions each picked up an electron. Where did these electrons come from?
When a lead projectile collides with a target nucleus, the electric field of the two nuclei is so strong that a virtual photon arises, generating at least one electron-positron pair. Because the lead ion has a strong positive charge, the pair's negatively charged electron may be captured in a lead electron shell while its positively charged mate, the positron, is repelled. The captured electron may be in the excited or ground state. The probability of electron capture was determined by measuring the rate at which lead ions emerging from the target changed their charge.
What are the implications for RHIC (and CERN's Large Hadron Collider, also under construction)? If colliding gold ions in one of two beams pick up electrons, the charge-to-mass ratio for which the accelerator ring magnets are tuned will change. Thus, the beam may crash into the wall, requiring a new beam to be introduced frequently into the ring. More electron-capture experiments are needed to determine if the desired efficiency and reliability of collider operation will require use of ion beams from lighter elementsnot quite as good as gold.
Another phenomenon studied by ORNL physicists at CERN was the ability of the target material to slow down ultrarelativistic projectile ions after impactthe so-called electronic stopping power. For lead ions on a lead target, a new theory that considers the effect of finite nuclear size calculates that the stopping power should be less than what the old theory concludes. Our physicists found that lead ions lose less energy in a lead target than the old theory predicted, validating the new theory for heavier elements. Energy losses for lighter target elements, such as carbon, were closer to those predicted by the old theory. The drive to do new experiments to help refine the theory has not stopped.
The research was supported by DOE, Office of Energy Research, Basic Energy Sciences, Office of Chemical Sciences.
ORNL Discovers Superdeformed Light Nuclei
Some nuclear physicists like them exoticslender, not round. In their atom chase, they search for "superdeformed" nuclei that are twice as long as they are wide. These atomic cores are different from normal baseball-shaped nuclei because, for a split second, they look like footballs.
|This rapidly spinning nucleus has assumed the "superdeformed" shapeit is twice as long as it is wide, like a football.|
In 1986 the first experimental evidence of a superdeformed shape in a nucleus outside the actinide group was observed in dysprosium-152. In 1989 a similar shape was seen in mercury-192. But until recently, the race by many research groups to catch a glimpse of nuclear footballs in the mass 80 to 90 range went on without success. The physicists in pursuit hailed from Denmark, England, France, Germany, Japan, and the United States, including a group at Oak Ridge.
The ORNL physicists' first hunting ground was a tandem accelerator at Daresbury Laboratory in England. There, in 1993, they discovered the first light-mass superdeformed nucleus in strontium-83. Since then, they have continued their search at DOE's Lawrence Berkeley National Laboratory (LBNL), where they take advantage of the Gammasphere, the world's most sensitive gamma-ray detector system.
The nuclei of interest are typically synthesized by bombarding a nickel-58 target with beams of silicon-28 or sulfur-32 ions. Occasionally, some ions strike the target nucleus nearly head-on and fuse with it with little spin. But many ions hit the target nucleus off center and produce fused compound nuclei that rotate very rapidly. As a spinning nucleus drops down from its excited state, it releases its excess energy, first by emitting particles (neutrons, protons, alpha particles) and then gamma rays. The gamma rays are particularly effective in carrying away the excess energy of rotation.
In superdeformed nuclei, the gamma-ray energies drop with decreasing spin in a very regular fashion. The regularity of the energies of these gamma rays and the time it takes for the nucleus to emit them are used by physicists to determine how deformed these nuclei are. Their measurements of lifetimes of these superdeformed states indicate that, unlike toy gyroscopes, these fast-rotating nuclear tops last for a mere 10-15 to 10-13 seconds before they jump to their next lower state.
By now, the ORNL group and collaborators from Washington University, LBNL, the University of Pittsburgh, and Florida State University have detected more than 10 cases of superdeformed nuclei that have masses in the range of 80 to 90 and span four different elements: strontium, yttrium, zirconium, and niobium. Rotating at the rate of about 1022 turns a second, they are the fastest-spinning nuclei yet observed. The measured properties of these nuclei are in reasonable agreement with the predictions of theory, but some puzzles remain. The race is on to find the solutions.
The research is supported by DOE's Office of Energy Research, Office of High Energy and Nuclear Physics.
Gamma Rays Speed HFIR's Embrittlement
In November 1986, ORNL's High Flux Isotope Reactor (HFIR) was temporarily shut down because its vessel was found to be undergoing a higher than expected rate of embrittlement. Neutron radiation was initially blamed. ORNL scientists now believe that gamma rays, which were not previously suspected, are responsible for the unusual embrittlement.
All nuclear reactor vessels experience radiation-induced loss of ductility, or embrittlement. The main reason: fast, or high-energy, neutrons collide with atoms of the vessel material, knocking them out of their regular positions, creating tiny point defects. Some point defects crowd together into clusters that harden the metal and make it less ductile.
The significance of gamma rays to the HFIR vessel embrittlement was discovered when beryllium and neptunium-237 monitors were used to verify the fluxes of fast neutronsthe number of neutrons striking a certain area per secondat vessel surveillance positions. These monitors seemed to indicate a much greater fast-neutron flux than did the regular iron and nickel monitors. This discrepancy was traced to interference from gamma rays. Beryllium and neptunium-237 are sensitive to both fast neutrons and high-energy gamma rays, whereas iron and nickel monitors are unaffected by gamma rays. The ratio of gamma flux to fast neutron flux was found to exceed 1000 to 1. It was realized that this exceptionally large ratio could explain the extra embrittlement of the vessel.
Although gamma rays have insufficient momentum to directly dislodge an atom in steel, they can energize the steel's electrons, which can displace atoms. It takes more than 1000 high-energy gamma rays to create the same number of atomic displacements as a single fast neutron. However, for a ratio greater than 1000:1, the gamma-induced atomic displacements will make a larger contribution to embrittlement than will the fast neutrons. Thus, gamma irradiation accounts for the hitherto puzzling rapid embrittlement of the HFIR vessel.
|In the HFIR, a 300-millimeter- thick beryllium reflector and a 600-millimeter- thick cooling water shield separate the corethe primary source of high-energy gamma rays and neutronsfrom the vessel. These features block the flow of most neutrons, not gamma rays.|
The HFIR case is the first known example of reactor pressure vessel embrittlement dominated by gamma irradiation. The reason lies in the design of the reactor. A 300-millimeter-thick beryllium reflector and a 600-millimeter-thick cooling water shield separate the core, the primary source of high-energy gamma rays and neutrons, from the vessel. Although these features present an effective barrier to most of the neutrons, they do little to deter gamma rays. Consequently, more than 1000 times as many gamma rays as neutrons strike the vessel wall.
Commercial nuclear power reactors have no beryllium and a much narrower water path, resulting in only a small ratio of gamma-to-neutron flux at the vessel and little or no embrittlement from gamma rays. The effects of gamma rays on the HFIR vessel are now being addressed in predictions of the vessel's lifetime.
This research was supported by DOE, Office of Energy Research, Division of Materials Sciences, and the Nuclear Regulatory Commission
Pinning Down Isotope Signatures in Geological Samples
The waters in the oceans, rivers, lakes, groundwater, and fluids deep within the earth are different; each water has its own distinctive stable isotopic signature. The H2O water molecules in a sample carry a special distribution of common light isotopes and rare heavy isotopes of hydrogen (deuterium, or D) and common oxygen-16 and rare oxygen-18 isotopes. Thus, the sample can include various minor amounts of DH16O, DH18O, and H218O molecules, as well as the common H216O.
Using sensitive mass spectrometers, geoscientists can determine the stable isotope signatures of the various water typesan extremely powerful approach in elucidating the temperature, material fluxes, fluid sources, and time scales of ancient and active fluid-rock interaction processes in the earth's crust. Information of this sort can guide geothermal energy firms seeking to drill new wells to the most productive zones of steam or hot water.
The exchange of isotopes between water and other coexisting oxygen- or hydrogen-bearing phases (such as minerals, gases, or water vapor) is controlled by temperature and by the type and amount of dissolved solids in water. Fluids on and within the earth's crust contain a variety of salts, the most common being the familiar sodium chloride. However, ions in solution modify orientations of at least the nearest-neighbor water molecules relative to water as a whole to form an inner hydration shell. This rearrangement of water molecules in the vicinity of ions and the disruption of the hydrogen bonding network in the liquid lead to quite profound and quantifiable stable isotope redistributions. Until now, however, the influence of dissolved salts on stable isotopes in water has been largely ignored by the earth science community.
Our geochemists determined the effects of different concentrations of dissolved saltssodium, potassium, calcium, magnesium chlorides, and sodium and magnesium sulfateson the redistribution of oxygen and hydrogen isotopes between liquid water and water vapor as a function of temperature. The magnitude of the isotope "salt effect" was found to increase with increasing temperature and concentration of the salt. From a practical point of view, data from this study have been used by the geothermal industry to understand the pressure-temperature-composition conditions of steam separation from brines both at the well head and in the subsurface-producing reservoir.
|An image showing the relative amount of iron in an iron-calcium zoned mineral called a garnet. The intense, yellow bands are enriched in iron. Ion microprobe analysis provides information that can be used to interpret changes in the chemical environment as the garnet grew. Illustration by Dave Cottrell.|
|A geothermal system formed over a pluton heat source and a power plant that extracts energy from underground reserves of steam and hot water.Illustration by Dave Cottrell.|
The research was sponsored by DOE's Office of Energy Research, Basic Energy Sciences, Geosciences and Chemical Sciences Divisions.