Dr. DeYoung, Dr. Peaslee and Dr. Jolivette (Nuclear Group)
Supported by the NSF (Nuclear Physics Division)
Using recently developed Radioactive Nuclear Beam technology, a study of the fusion properties of 6He was conducted at the University of Notre Dame's Twinsol Facility. As a summarization of this research, I will provide a quick explanation of why scientists are interested in studying 6He, discuss two experiments studying the fusion of this nucleus with 209Bi and briefly discuss the results and conclusions drawn from these experiments.
There are several properties of the 6He nucleus which make the study of it worthwhile. One of the most basic of these is that it can be made into a radioactive nuclear beam. 6He also has the more unique property of possessing a neutron skin. Skin nuclei are nuclei which consist of a tightly bound nuclear core and a "skin" of additional nucleons, which is bound relatively loosely to the core. In the case of 6He, there is an alpha particle core (two neutrons, two protons), and a skin of two additional neutrons. At this time, it is not well understood how this neutron skin affects the likelihood of a fusion reaction with other nuclei.
Additionally, this isotope of helium is what is often called a Borromean nucleus. This name is borrowed from the medieval princes of Borromeo. As their heraldic symbol, they used a set of three rings which were linked in such a manner that the removal of one ring would cause all three to become completely disconnected. In the same way, if one of the two neutrons of the skin or the central core were removed from the 6He system, the other two components would no longer be bound to each other.
In the experiments conducted at Notre Dame, a 6He nucleus fused with a nucleus of 209Bi in a bismuth target to form 215At. The unstable astatine nucleus would then emit a number of neutrons to shed excess energy and gain greater stability. The exact number of neutrons depended on the beam energy, with a greater number of neutrons corresponding to higher beam energies. At the range with which we dealt, around 18 to 30 MeV total energy, three or four neutrons evaporated most commonly. The remaining astatine isotope was still unstable after shedding neutrons, however, and eventually decayed again by emitting an alpha particle. This alpha particle could then be detected using an array of silicon detectors.
Due to differences in beam energy and lifetime of the astatine nuclei, two different experiments were performed to study the reaction mechanism in which 4 neutrons were evaporated (referred to as the 4n experiment) and the one in which 3 neutrons were emitted (referred to as the 3n experiment). In the 4n experiment, the beam of 6He passed through a series of 16 bismuth targets, each with a mylar energy degrader behind it. This setup allowed the irradiation of targets at 16 different beam energies simultaneously. Due to time constraints and the 7.2 hour half-life of 211At (the isotope resulting from 4 neutron evaporation), the targets were placed in an offline chamber for several hours after the irradiation was completed to count alpha particles.
The setup for the 3n experiment was somewhat simpler. Since 212At has a half-life of only .3 seconds, the counts could be made online using an array of 8 detectors in a "box kite" formation. The beam itself was pulsed, with .3 seconds on alternating with .6 seconds off, so that the particles could be counted in the off period without background from the beam. In this experiment, there was only one target and one energy degrader, whose thickness was varied throughout the experiment to adjust beam energies.
In the case of the 4n experiment, the experimental data did not show any unusual effects of a neutron skin. In the lower energy 3n experiment, however, there was a significant increase in the probability of fusion near the coulomb barrier. This increase is attributed to the theory of neutron flow. Under this theory, one of the skin neutrons of 6He interacts with the 209Bi nucleus and drags the rest of its nucleus with it, overcoming the electromagnetic repulsion which would normally have kept the helium core from fusing with bismuth.