Dr. DeYoung Dr. Peaslee and Dr. Jolivette (Nuclear Group)
Supported by the NSF-REU
Previous studies only observe short lifetimes of a composite system at forward angles. This is contrary to statistical emission, in which the measured lifetime of a compound nucleus has similar lifetimes at all angles. Our hypothesis is that nucleon-nucleon scattering is occurring and therefore this preference manifests itself in the system. Our present model involves just statistical emission and does not include nucleon-nucleon scattering as a processes that occurs during fusion.
When two nuclei collide, fusion between the nuclei can occur to form a compound nucleus. This happens within 10^-22 seconds after collision. After fusion, the compound nucleus has too much energy and must emit the energy through some mechanism. This is often done by emitting light charged particles (protons, deuterons, alphas, etc.). Statistical emission states that there is no dependence of lifetime on emission angle. The typical decay lifetime is 10^-21 to 10^-18 seconds. Nucleon-nucleon scattering is when individual nucleons (e.g. protons and neutrons) are ejected from the nucleus before the compound nucleus can form. Nucleon-nucleon scattering takes place at approximately 10^-23 seconds.
At the University of Notre Dame, the reaction 16O + 27Al --> 43Sc was used in the experiment. The beam was run at 72 MeV. A low beam energy was used because the purpose of the experiment was to observe only nucleon-nucleon scattering versus statistical emission. At higher beam energies, a compound nucleus does not necessarily form. The nuclei would then fragment into smaller nuclei in which the desired processes can not be observed. The scandium created from the reaction had an excitation energy of 59 MeV and emitted particles to become more stable.
The emitted particles were detected and their momentum was measured. From this, the relative momentum could be determined between any two particles detected. The lifetime could then be deduced from a correlation function.
To demonstrate how a correlation function works, relate it to gambling. Imagine going to play craps. You should bet on seven because that number statistically occurs most often. Unless, of course, the casino loads the dice so you receive seven less often that you should. If the correlation function, i.e. the ratio of experimental values to theoretically expected ones, shows that there is a deviation from one, then the experiment did not behave as our statistical model predicted. Therefore we need a new model, or we can conclude that the dice were loaded. This conclusion is much easier to form from a correlation function as opposed to other ways of looking at the experimental results.
Our present model predicts long lifetimes for this energy and reaction. The results of the experiment show that the proton-deuteron correlation behaves as our present model predicts (no deviation from one) and thus we do not need a new mechanism to explain it. However, the proton-proton correlation deviates from one. We can conclude that the deviation from one means that shorter lifetimes are occurring. Therefore a possible explanation is that nucleon-nucleon scattering is happening before compound nucleus formation.
Future steps will involve modeling the experiment with the current nuclear model. If the model proton-proton correlation does not show a deviation from one, then we have found additional evidence that supports nucleon-nucleon scattering.