The Equation of State of Dense Matter

Background

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.

The Role of NSCL

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.