Experiment explores elusive properties of symmetry energy

After years of preparation, an NSCL-based experiment is poised to help nuclear scientists peer at nucleus-nucleus collisions with unprecedented accuracy. The results of the experiment, which was run at NSCL at the end of 2006 and included collaborators from several institutions around the world, may shed new light on long-standing questions about both basic nuclear properties and neutron stars.

In a quest to better describe symmetry energy, nuclear scientists at NSCL, Indiana University in Bloomington, and Washington University in St. Louis built a new device – the High Resolution Array Detector, or HiRA – capable of closely measuring the mini-explosions that take place when atomic nuclei collide at high speeds, become compressed, and then expand. HiRA is tuned to gather data on how pairs of protons – all two-particle systems are linked by the strong nuclear force – behave in the aftermath of a collision.

By measuring proton-proton pairs, researchers, in effect, capture an image of nuclear systems. The technique is made possible by the work of NSCL theorist Pawel Danielewicz, who refined a model commonly used to measure vast distances in astronomy to help experimenters describe the size and shape of a nuclear region that emits two protons. The experimental technique can resolve details down to the femtometer level, one million times smaller than the dimensions common in the high-tech field of nanotechnology.

HiRA is composed of removable modules, each of which contains several detectors. (The module shown here contains 17 detectors.) Collectively, these detectors are capable of making fine-grained measurements of careening proton-proton pairs traveling at thousands of different angles, detail that will help nuclear scientists better understand elusive qualities of symmetry energy.

Here, in a moment of levity after the conclusion of the experiment, the faces of several NSCL researchers and students are reflected in the aluminum-coated surfaces of the detectors in a single module, which had been removed for calibration and maintenance.