Mass Measurements on Trapped Isomers

The ability of the proposed new generation of ion trap to determine masses, with exceptional sensitivity and precision, opens possibilities for the detection and exploitation of nuclear isomers. Isomers in the A~180 mass region are able to combine long half-lives with high excitation energies, on account of their deformed, axially symmetric nuclear shape and the associated angular momentum quantum numbers. The usage of the Penning trap mass measurement technique to determine excitation energies for isomeric states in the Hg-region [23] has been successfully demonstrated at ISOLTRAP. Such isomers can have significant astrophysical implications for nucleosynthesis [24], for example.

However, predictions [25] for nuclei on the neutron-rich side of stability, i.e. on the r-process path, indicate that there are expected to be long-lived, highly-excited isomers which beta decay directly to other isomers, severely constraining the experimental techniques available for their verification. An exciting prospect is to produce neutron-rich A~180 nuclei, and their predicted isomers, by proton spallation at ISAC, and to obtain the isomer excitation energies from the differences between the isomer and ground-state masses. At the same time, it would be possible to measure half-lives that lie in the range of tens of milliseconds to minutes.

These basic physical properties would be obtained independently of the isomer decay mode, and thus circumvent the experimental difficulties that can arise from isomer-to-isomer beta decay. In the extreme case of quasi-stable isomers (with half-lives of, say, years) their existence and excitation energy can be determined even in the absence of decay radiations. The trap method may prove to be the only way to identify such exotic nuclear states, and to learn about the stabilizing effects of their angular momentum quantum numbers.

The predicted most favored region of isomers [25], in the A~180 region, is at the neutron-rich limit of accessibility. Therefore, the high yields available at ISAC are a vital factor. There is already an approved TRIUMF/ISAC experiment which aims to study such isomers by gamma-ray spectroscopy. The use of an ion trap will provide complementary information and, at least in some cases, will be the preferred method to identify basic isomer properties.

A theoretically interesting case, that should be accessible by this method, is the neutron-rich isotope of lutetium, ¹⁸⁷Lu. EPAX [26] calculations indicate a cross section of about 3 nb (from protons incident on uranium or thorium targets) giving an ion-beam rate of about 100 per second (for a 10 uA proton current, and one percent ion-source efficiency) with perhaps 5 percent in an isomeric state. This will be sufficient for clearly identifying a 1 MeV isomer, well separated from its ground state. Furthermore, when conveniently prepared in a trap, the ideal environment may be found to study laser-induced atomic effects, and perhaps even laser-induced isomer de-excitation. This would open up new possibilities to explore the atomic/nuclear interface [27] with wide potential application.