Dr. DeYoung and Dr.Peaslee (Nuclear Group)
Supported by the NSF (Nuclear Physics Division)
This summer was the third year I had the opportunity of doing research with the Nuclear Physics Group at Hope College.
My research consisted of the analysis of an experiment conducted at the University of Notre Dame Tandem VandeGraaf accelerator facility in 1996. An 16O beam was accelerated to 72MeV and incident on a 700mg 27Al target. Two arrays of eight, 9cm2 Si telescopes, detected intermediate mass fragments (IMFs, Z=3,4,5), evaporation residues (ERs), deeply inelastic, target-like, and a particles. Single particle events as well as multiple particle events including a and stable IMF emissions in coincidence with ERs were recorded. Each detector array spanned 10o . One array was fixed at 25o and the other placed at 35o, 40o , 45o, and 60o. Well-documented strong a-a and a-d correlation measurements, for the same system at energies ranging from 80MeV to 250MeV imply significant statistical emission of 8Be and 6Li* and suggest the emission of other IMFs (Phys rev C46, R1 1992).
The experiment was designed to more thoroughly examine the mechanisms of de-excitation of equilibrated sources at low energies. The compound nucleus, 43Sc, with 59MeV of excitation energy was formed in the fusion reaction 16O(27Al,43Sc). The total fusion cross-section is 1157mb (ref). My work involved the complete analysis of the raw experimental data including energy calibrations, gating particle types, computing cross-section and modelling the experiment with a Montecarlo statistical model (MODGAN, (ref)).
Results for a-particle single events are displayed in Figure 1. The experimental cross-section as a function of labratory angle mirrors the trends predicted by MODGAN. This implies that the mechanisms for a-emission are well understood. A dominate channel for compound nucleus de-excitation is the statistical emission of a-particles. Statistical emission of other light charged particles also occurs but protons, deuterons and tritons fell outside of the energy threshold of the Si telescopes.
Figure 1 Alpha Particles Crossection vs. Labratory Detection Angle Experimental data (red crosses) shares the same trends as MODGAN,
montecarlo statistical model (blue diamonds) within a relative normalization factor.
Examination of the cross-section for heavy residual particle single events as a function of labratory angle, as seen in Figure 2, diverges from the theoretical prediction. At more backward angles, greater than 45o , it is clear that the model predicts no ERs. In the experiment however, heavy nuclei were detected even at 65o .
A kinematical analysis of deeply inelastic scattering and particle transfer events (including a,p,d,t transfers) suggests that there exists a small cross-section for target and target-like nuclei to be detected at 40o -65o in the Lab. Since the detectors lack mass resolution for these heavy nuclei it is difficult to differentiate between target-like nuclei and ERs.
Heavy particles detected at backwards angles, between 55o and 65o come in coincidence with a-particles as well as inelastic 16O and possibly 12C (mass resolution is possible at these masses, see Figure 3). This strongly suggests that at least some fraction of the heavy particles detected are infact ERs. This would require some new mechanism of compound nuclear de-excitation that is as of yet not known.
Further analysis is currently being carried out to seperate target-like particles from true ERs. This will allow better comparison to MODGAN. Cross-sections as a function of labratory angle for ER-a and ER-IMF coincidences will then also be attainable. The first measurements ever of IMF emission cross-sections, at such low energies, will also be computed.
Figure 2 Residues Crossection vs. Labratory Detection Angle MODGAN, montecarlo statistical model (green boxes),
clearly diverges from the experimental (red crosses) at backward angles.
Figure 3 Sample detector spectrum Energy deposited in Si wafer vs Time of Flight(TOF). TOF increases towards the y-axis.
Several particle groupings are clearly visable.Publications and Presentations: