University of the District of Columbia
Dr. DeYoung, Dr.Peaslee and Dr. Jolivette (Nuclear Group)
Supported by the NSF-REU
FUSION OF A NEUTRON SKIN NUCLEUS:
THE 209Bi (6He,2n) REACTION
The purpose of this experiment is to determine the shape of a 6He nucleus. 6He has valence neutrons somewhat outside a 4He core and so is considered a neutron-skin nucleus. The skin nuclei are composed of a tightly bound core of inner nucleons surrounded by a less tightly bound "skin" of additional neutrons. In the case of 6He, there is a core of two protons and two neutrons and a skin of two additional neutrons.
It is a Borromean nucleus which is an example of a 3-body system in which the entire system would immediately become unstable and come apart if one of the pieces were removed. For example, if one of the neutrons in the skin of 6He were removed, the core and the remaining neutron would no longer be bound to each other. The extended size of the 6He enhances the fusion process.
The 6He beam used in this experiment was produced by Twinsol, a modified and upgraded version of a radioactive nuclear beam facility that has been in operation at the University of Notre Dame since 1987. Twinsol are superconducting solenoids used to select and focus the beam. The 6He was produce from the reaction of 73Li + 94Be => 62He + 105B. The maximum 6He energy is 35 MeV, which is sufficient to study the fusion cross section of interest from below to well above the Coulomb barrier at about 20 MeV.
The fusion of 6He + 209Bi => 215At has a 2n decay channel to 213At, a 3n decay channel 212At, and a 4n decay channel to 211At. For this experiment, we are interested in a 2n decay channel to 213At because the 2n is most sensitive at the Coulomb barrier. 213At emits a 9.02 MeV alphas and has a half life of 125 ns. The number of decay alphas is proportional to the fusion cross section.
For the experimental detail, there were 2 Si telescopes that were 75 mm, 300 mm thick and was placed at a 45 degree angle. There were also 6 Si wafers that were 150 mm thick. The detectors were placed close to the target to maximize solid angle. This way we were able to detect most of the particles. This was very important since this was a radioactive beam and the beam intensity was low. The beam energy was approximately 20 MeV. The beam was a pulsed beam that was calibrated to 100 ns burst in order to eliminate the background coming from prompt reactions. The time-of-flight, which was the time of each event relative to the beginning of a counting cycle was recorded. Each event was written to a tape in which I analyzed. I analyzed the spectrum of each detector which had coordinates of energy vs. time. By knowing the energy of the decay alphas, gates were set around energy of 8.1 MeV to 8.9 MeV. This energy range was chosen because there was an energy loss from the target. Due to contamination coming from other reactions, I subtracted the background from above and below the region of interest. For the statistical analysis, I calculated the average number of counts/channel in the region of interest from the two downstream telescopes and three upstream detectors.
These detectors were chosen because they had the lowest contamination. For the left and right downstream detectors, there were evidence of a non-zero cross section which is good. However, the average number of counts/channel was calculated to be zero for the three upstream detectors. At this point it is unclear why there wasn’t any consistency showing in each detectors.
In conclusion, there was some indication of a non-zero fusion cross section at the Coulomb barrier. In the future, more work needs to be done to provide some consistency and absolute normalization.