A large percentage of my research this summer focused on central and peripheral nuclear collisions. The data used to conduct this analysis was from an experiment at the Michigan State University’s cyclotron. This experiment used an ⁸⁴Kr beam at both 35 MeV and 60 MeV per nucleon and targets of both ²³²Th and ¹⁹⁷Au nuclei.

As the name implies, a central collision occurs when the projectile nucleus hits the target nucleus approximately in its center. This collision creates a single, hot, energetically excited group of nucleons. A peripheral collision occurs when a projectile hits a target nucleus close to the target’s edge, creating a much cooler system. The method most often used to distinguish between these is called multiplicity. Since central collisions are much more energetic than peripheral, more detectable particles will be emitted as the result of a central collision. It is therefore possible to determine whether a collision was central or peripheral by the number of charged particles, called the multiplicity, observed by the detectors.

Using this information, the energies of particles emitted from these two types of collisions can be compared. One interesting effect observed is that a plot of energy versus counts for particles emitted from a peripheral collision and detected at angles close to the beam shows two distinct peaks. This indicates two different sources for the particles, meaning that at these angles it is possible to differentiate between light charged particles emitted from the projectile and the target.

We can also observe two different types of emission in the same plot. Looking at a plot of energy versus counts for particles detected at angles around 40 to 60 degrees, statistical emission is dominant at lower energies, whereas pre-equilibrium emission dominates at higher energies. Statistical emission of particles occurs after the projectile and target nuclei have fused into a single system and must emit particles to get rid of excess energy, whereas pre-equilibrium emission takes place just as the target and projectile nuclei are colliding, before they have formed a single system. Examining the slopes of the peripheral and central-originating particles at lower energies reveals that the systems resulting from peripheral collisions are cooler than those from central collisions, as is expected. However, this is not the case in regions of the graph where pre-equilibrium emission is dominant. This result is attributed to a "squeeze out" effect in pre-equilibrium emission from central collisions. In central collisions, the target nucleus is in the path of the particles that would be emitted directly from the collision and physically blocks their emission. In peripheral collisions, however, the pre-equilibrium particles are able to escape and reach the detectors more often because of the less obstructive location of the target nucleus.

These two types of emission are currently being modeled with different computer simulations. BUU, which simulates pre-equilibrium emission, shows the same "squeeze out" effect observed in our experimental data. Statistical emission is being modeled with a program called Modgan. It should soon be possible to subtract this simulated data from the real data to leave only pre-equilibrium emission data.

Having collected and analyzed data involving various angles, two beam energies, two targets and different types of particles, we hope to use this information to gain a better understanding of pre-equilibrium emission. We are also working towards a better definition of a nuclear equation of state.