Nuclear Astrophysics
Nuclear astrophysics entails studying some of the most dramatic events in the cosmos such as stellar explosions. Such events hold clues to the mystery of how the elements were formed.
Accreting Neutron Stars
Neutron stars are among the most fascinating and elusive astronomical objects. They are the densest form of matter naturally occuring in the cosmos - a tablespoon of neutron star matter has a weight comparable to about 3,000 aircraft carriers.
The best natural laboratories to study neutron stars are X-ray binaries. X-ray binaries are stellar systems where a neutron star orbits a regular companion star, sucks matter from its companion onto its surface, and fuses this matter into a heavy element in huge thermonuclear explosions. The explosion creates a strong X-ray radiation that can be observed by space-based observatories (Fortunately, it is absorbed in the Earth's atmosphere).
Huge amounts of observational data are collected by new observatories like Rossi X-ray Timing Explorer or Chandra X-ray Observatory. This data might provide new clues about the nuclear reactions occuring on the neutron star surface or deeper in the neutron star crust where elements are transformed into neutrons. These nuclear processes generate the energy that can be observed as X-ray radiation. They might create a few Ruthenium and Molybdenum isotopes that might escape from the neutron star and, therefore, could explain the mysterious enrichment of these isotopes in our solar system and on Earth.
However, to a large extent we don't know what these nuclear processes are like. The main problem in understanding the nuclear processes on and in neutron stars in X-ray binaries is that the nuclei involved are very exotic species. These exotic nuclei decay within fractions of seconds, so we know very little about them.
NSCL is able to produce many of the critical nuclei in larger quantities than can any other facility. This allows us to measure many of the properties that are needed to correctly model the nuclear processes, such as the decay rate, weight, and size.
NSCL scientists also are involved in running computer models of the thermonuclear explosions occuring on the surface of neutron stars. This allows us to determine the critical nuclei that need to be studied and to explore the consequences of our new experimental results.
The Equation of State of Dense Matter
The attractive forces that bind nucleons together to make nuclei become repulsive when the nucleons get very close to each other. That is why all nuclei have about the same density (mass per unit volume).
In neutron stars or during the collapse of very heavy stars that have burned their nuclear fuel, much higher densities can be achieved. That is because the gravitational force of such massive objects compresses the nuclear material. Nuclear collisions are the only way that one can compress nuclear material in the laboratory and learn what the relationship is between the density of nuclear material and the pressure needed to compress it. The information obtained from collision experiments can help us understand why neutrons stars don't collapse into black holes and help us predict some of the properties of the interiors of neutron stars, the densest objects in the universe.
At NSCL, we can compress nuclear material above its normal density. We also can decompress nuclear material to low densities where it transforms to a gas of nucleons. From these collisions we can learn about how the pressure depends on density for densities of up to twice the normal density inside nuclei. More importantly, we can learn about how the pressure depends on the concentration of neutrons and protons within the nuclear material.
Neutron stars are made of nuclear material that is probably more than 90 percent neutrons. To determine how the pressure in nearly pure neutron matter differs from the pressures measured in nuclear collisions, we need to vary the neutron and proton concentrations in the nuclei we collide and measure the differences in the pressures achieved. Then we can extrapolate our measurements to neutron stars.
The rare isotope beams at NSCL are ideal for these studies because they provide a wider range of proton and neutron concentration than one can obtain by using only the normal stable nuclear beams. This gives a much greater sensitivity to the dependence on neutron and proton concentration and a more accurate extrapolation to neutron stars.
The Rapid Neutron-Capture Process
The origin of the chemical elements that make up our world is one of the oldest most fundamental scientific questions. The universe after the Big Bang consisted only of hydrogen and helium with traces of lithium. All the other elements, including the carbon in our bodies, the iron, silicon, and oxygen that makes up most of our earth, have been created later by nuclear reactions in stars. When stars explode they eject their freshly made nuclei into space. This stardust can then form new stars that continue the process of the synthesis of elements and planets like the earth.
However, the origin of many elements beyond iron, including gold and uranium, is still a mystery.
These elements are attributed to a process called the r process: rapid neutron capture process. In this process iron nuclei are bombarded with neutrons, which they rapidly capture until a very exotic nucleus is formed. Such an exotic nucleus decays quickly (within 10-100 milliseconds) before another neutron can be captured. In this decay a neutron is converted into a proton and a new element is created. Then the process continues with more neutron captures and decays creating heavier and heavier elements. Eventually large amounts of iron are converted into gold, uranium, and many other elements.
The mystery: While we have a rough idea about what the r-process might be like, we don't understand how this sequence of reactions really works, and we don't know where in the universe this process occurs.
While the site of the r-process is not known, there are prime suspects such as supernova explosions or colliding neutron stars. However, none of these models can produce r-process elements in the correct proportions as we find them, for example, in the solar system or in certain very old stars.
Does this mean we have to discard those models and find something different? We cannot tell, because we don't know whether our description of the nuclear reactions during the process is correct.
The overwhelming majority of the nuclei involved in the r-process have never been observed in a laboratory. These nuclei might even have very unusual properties, for example skins of neutrons, that still need to be discovered.
This is where NSCL will make a huge impact. At NSCL, we can produce many of the nuclei in the r-process for the first time, and we can measure their properties with sophisticated detector systems. NSCL scientists also run astrophysical models to identify the most important measurements needed and to determine the impact of new results. This is done in close collaboration with MSU astronomers, who are among the leading observers of r-process elements in stars.
Supernovae
Supernovae are star explosions creating the greatest fireworks in the universe with a typical brightness that easily outshines whole galaxies. The last supernova in our galaxy occurred in 1604 (Kepler's star) and could be observed with the naked eye even during daylight.
However, supernovae are frequently observed in other galaxies using large telescopes. Supernovae are key to understanding the origin of the elements in the universe — the majority of atoms on earth were at some point ejected in a supernova explosion.
Supernovae occur when a star runs out of fuel and cannot maintain the pressure needed to prevent a collapse due to gravity. However, we still don't understand how a collapsing star can result in an explosion ejecting matter outward. The basic idea is that the collapsing core forms a neutron star, which suddenly stops the collapse. The resulting bounce triggers the explosion of the remaining stellar material. Neutron stars have been observed in the remnants of ancient supernova explosions. But, while scientists run a lot of interesting supernova models, these models have one common problem: they do not explode.
NSCL contributes towards the understanding of supernova explosions by providing data on the nuclear reactions that initiate the collapse of the stellar core and determine its speed. For example, nuclear theorists predicted that the nuclei present in the stellar core capture surrounding electrons at significantly different rates than anticipated before. These calculations have to be verified by experiments. Many of the critical nuclei are unstable and cannot be studied by conventional experimental methods. However, these nuclei can, for a short time, be recreated at NSCL and studied in-flight using collisions with well chosen target nuclei. This provides the necessary data to test theories and to improve model calculations of supernovae.
Operation of NSCL as a national user facility is supported by the Experimental Nuclear Physics Program of the U.S. National Science Foundation.