There are special nuclei with "magic numbers". Such nuclei have a closed shell (all of the states in an orbital manifold are occupied by protons or neutrons) and become very stable. One of the fundamental issues of nuclear physics is the question as to which numbers become magic. Magic numbers provide important clues to the production of super-heavy elements and to the process of nuclear synthesis in the universe.
The Ni (nickel) nucleus has a proton magic number of 28. Thus, one can obtain important knowledge about magic properties far from the stability line by studying nuclei in the neutron-rich Ni region. These nuclei, however, have only rarely been studied up to now, because they have been difficult to produce and to identify as reaction products.
We have produced neutron-rich Ni nuclei using the tandem superconducting booster
accelerator. Furthermore, we have developed a new "isomer-scope" instrument (Fig.
4-1) and have succeeded in measuring the gamma-rays emitted by excited levels
in these nuclei. In the isomer-scope, "transmission" and "full-energy" type Si
detectors select a nucleus from the reaction products. A tungsten block shields
the Ge detectors from unnecessary gamma-rays. Using the isomer-scope, we have
achieved the best sensitivity in the world for measuring gamma-rays from nanosecond
isomers.
Using the isomer-scope, we have identified the excited energy levels of 68Ni (Fig. 4-2). The first excited level of this nucleus lies at a relatively high energy. At energies above that, the level spacing becomes markedly smaller. This result indicates that the neutron number 40 corresponds to a closed shell. Therefore, 68Ni has a doubly closed shell with both neutron and proton magic numbers. On the other hand, this level structure may also suggest a weakness for the 28 proton closed shell, which, in turn, shows a deeper structure in the nucleus. |
