Experimental test of nuclear magnetization distribution and nuclear structure models

Models exist that ascribe the nuclear magnetic fields to the presence of a single nucleon whose spin is not neutralized by pairing it up with that of another nucleon; other models assume that the generation of the magnetic field is shared among some or all nucleons throughout the nucleus. All models predict the same magnetic field external to the nucleus since this is an anchor provided by experiments. The models differ, however, in their predictions of the magnetic field arrangement within the nucleus for which no data exist. The only way to distinguish which model gives the correct description of the nucleus would be to use a probe inserted into the nucleus. The goal of our project was to develop exactly such a probe and to use it to measure fundamental nuclear quantities that have eluded experimental scrutiny. The need for accurately knowing such quantities extends far beyond nuclear physics and has ramifications in parity violation experiments on atomic traps and the testing of the standard model in elementary particle physics. Unlike scattering experiments that employ streams of free particles, our technique to probe the internal magnetic field distribution of the nucleus rests on using a single bound electron. Quantum mechanics shows that an electron in the innermost orbital surrounding the nucleus constantly dives into the nucleus and thus samples the fields that exist inside. This sampling of the nucleus usually results in only minute shifts in the electron� s average orbital, which would be difficult to detect. By studying two particular energy states of the electron, we can, however, dramatically enhance the effects of the distribution of the magnetic fields in the nucleus. In fact about 2% of the energy difference between the two states, dubbed the hyperfine splitting, is determined by the effects related to the distribution of magnetic fields in the nucleus, A precise measurement of this energy difference (better than 0.01%) would then allow us to place stringent bounds on the models predicting currents and magnetic fields in the nucleus. We have implemented our method by constructing a very high-resolution spectrometer sensitive to light near 3800 A, which is the wavelength of light corresponding to the hyperfine splitting of the orbital of a single electron bound to a thallium nucleus, Tl 80+ . The spectrometer is unique in its design and consists of two independent arms separated by 90°, each with an ultra sensitive CCD camera, a large-diameter transmission grating, and various focusing elements The spectrometer was successfully tested at the Livermore EBIT facility using the lower energy device, EBIT-II, and found to produce results to an accuracy of about 1 part in 65,000 with typical line widths on the order of 5.5 channels of the CCD detector, or equivalently, 0.55 A. Despite the breakdown of SuperEBIT, 98-LW-057 was successful in creating new capabilities unique to LLNL. High-precision measurements in the near-UV domain made possible with the development of this unique spectrometer system, coupled with the diverse capabilities of an electron beam ion trap, provide a pathway for the undertaking of scientifically and programmatically interesting measurements that test our fundamental knowledge of the physical world, such as the nuclear magnetization measurements we set out to do, as well as provide necessary data for the understanding of high-temperature plasmas.