The Gammasphere Detector: Probing Nuclei at the Limits of Stability
The study of nuclei far from stability has received much attention recently. As in any other physical system, measurements at the limits can provide important results which when contrasted with more stable systems can shed light on the underlying symmetries and lead to new insights and understanding.The availability of new state-of-the-art detector systems, such as Gammsphere 1 the world's premier gamma-ray detector for nuclear spectroscopy, is allowing us to perform detailed spectroscopy of nuclei at the very limits of binding: at the 'proton dripline' and in the very heavy elements.
Gammasphere consists of up to 110 Compton-Suppressed Ge detectors, and it was built by a collaboration of physicists from Lawrence Berkeley National Laboratory (LBNL), Argonne National Laboratory (ANL), Oak Ridge National Laboratory (ORNL), and a number of U.S. universities. The device offers excellent gamma-ray energy resolution (2.3 keV at 1 MeV) and an order of magnitude increase in photopeak efficiency over previous Ge-arrays (~10% at 1 MeV). Between 1993-1997, the device was sited at the 88" Cyclotron Laboratory at LBNL. A booklet summarizing the highlights of this first campaign at Gammasphere can be found here. In the fall of 1997, Gammasphere was moved to the ATLAS accelerator at ANL and placed in front of the Fragment Mass Analyzer (FMA). The FMA is a high resolution mass spectrometer which measures the properties of the residual nuclei produced in a heavy-fusion reaction, i.e. mass, energy, time of flight and/or decay products 2.
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 1a shows a gamma-ray spectrum correlated with the proton decay of 141Ho 3. Nearly all of the known proton emitters decay from nuclei which are spherical. The proton decay of 141Ho is thought to be one of only a few examples of proton decay from a deformed (football shaped) nucleus 4. The gamma rays marked in figure 1a are regularly spaced in energy and are the fingerprint of a rotatioanl band, thus confirming that 141Ho is indeed deformed.
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 di"RECT" 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 5. 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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