|Gammasphere with its two hemisphere's opened. In view is the FMA's first quadrupole (blue) and first electric dipole (yellow).|
Studies at the Limits of Proton Binding
Exciting new results have been found for the structure of nuclei at the proton dripline. This is the region in the nuclear chart where a large proton-to-neutron excess allow nuclei to become unstable against spontaneous proton emission, with microseconds to seconds half-lives. It is difficult to reach these nuclei since the production cross sections are very small. As a result, the gamma rays emitted from these nuclei after they are produced in a heavy-ion fusion reaction are impossible to identify using only Gammasphere because they are completely obscured by the gamma rays from other isotopes produced with much larger cross-sections. Fortunately, the decay protons which have well defined energies can be effectively utilized to select out these rare products. This is accomplished by allowing the nuclei produced at the target position to pass through the FMA and implant in a pixilated silicon detector. If the implanted nucleus subsequently proton decays, the measured energy of the proton identifies the implanted nucleus. In this way, the gamma-rays detected when the isotope was created can be identified and separated from the gamma-ray background.
|Figure 1: (a) Gamma-ray spectrum for 141Ho produced by coorelating gamma-rays with the proton decay of 141Ho. (b) Gamma-ray spectrum for 254No produced by requiring gamma-rays to be in coincidence with nuclei with mass 254. The angular momentum of the state emitting the gamma-ray labels the peak in the spectrum. (c) Theoretical predictions for the deformation of heavy elements. A deformation of 0.0 corresponds to a spherical shape.|
Studies of the Heaviest Elements
The search for new elements is an ongoing activity in nuclear physics. Elements up to Z=112 (and possibly Z=114) have been identified up to now by measuring the energy of the alpha particle emitted when the nucleus decays. Theoretical models predict that the identified isotopes between Z=100 and Z=112 are deformed (see figure 1c) . As in the case of 141Ho, a direct measurement of the deformation can be obtained by measuring the gamma-rays emitted after the formation of the nucleus in a heavy-ion fusion reaction. For heavy nuclei, this is extremely difficult due to the fact that the production cross sections are very small and the gamma-ray spectrum is dominated by nuclei which fission. However, by selecting only those nuclei which do not fission using the FMA, a gamma-ray spectrum associated with the heavy element can be measured with Gammasphere. This has been accomplished recently for 254No (Z=102), and the measured gamma-ray spectrum is shown in figure 1b . The observed gamma-ray transitions are regularly spaced in energy and confirm that 254No is deformed as previously postulated (see figure 1c).
Summary and Outlook
Studies of excited states in nuclei at the limits of stability with
a device such as Gammasphere
are in their infancy. In many instances, only a limited number of excited states have been placed,
however, with the refinement of techniques and the development of more efficient ancillary
detectors, a more complete understanding of these nuclei should become available in the coming years.
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