The Nuclear Structure of Exotic Fission Fragments

点击翻译按钮获取中文摘要

At the heart of the atom lies the atomic nucleus, an object so minute that it would take tens of billions laid end-to-end to make up the width of a human hair. The nucleus is itself composed of even smaller particles called neutrons and protons. The neutrons, as the name suggests, carry no electrical charge while the protons have a positive charge. Thus, the total number of protons in a nucleus determines the nuclear charge and is denoted by Z, the atomic number. It is this net positive charge that enables negatively-charged electrons to 'stick' to the nucleus to make neutral atoms. The electric forces within the nucleus try to push the protons apart and it is only the presence of the the strong, short-range attractive nuclear force between neutrons and protons that enables nuclei to exist in a stable state. The total number of neutrons and protons is called the mass number, A. Atomic nuclei are stable only for particular combinations of Z and A. If we try to make a nucleus with too many neutrons for a given number of protons it becomes unstable and tries to decay to products with lower total mass by a process known as beta decay. For very high atomic numbers the long-range electric repulsion between the protons becomes competitive to the nuclear attraction. These high-Z nuclei need proportionately more neutrons to protons than low-Z nuclei in order to be stable. Some high Z nuclei can be made to fission into two lighter nuclei with a large release of energy. The fission process results in products with roughly the same Z/A ratio as the parent nucleus. Since the parent is rich in neutrons, the products are also. Fission is therefore a natural way in which to produce unstable neutron-rich nuclei. Such nuclei are of great importance since they are used as stepping stones in the astrophysical rapid neutron-capture process that is the mechanism by which elements heavier than iron are made. Fission is also important in that it is currently the only way in which w