6 days ago
Wednesday, May 26, 2010
How magic is your work?
If you're Dr. Kate Jones of the University of Tennessee in Knoxville, the answer is very.
Inside of a nucleus, just like for the electrons in an atom, there are discrete energy levels into which the neutrons and protons can arrange themselves. In chemistry, this behavior - in electrons - leads to the periodic table: different elements behave in different ways chemically because of the number of electrons they have. More precisely, what matters is the number of electrons outside of a closed shell. I'll explain what this means.
One can (albeit simply) view the electrons of an atom as orbiting the nucleus, and the energy of the electrons are determined by the orbital in which they reside. Electrons can change orbitals, if the precise amount of energy necessary to "jump" from one orbital to another is put into the system (or carried out of the system). And because of the intrinsic nature of electrons (and all nucleons), only so many of them can be in a given orbital at once. As soon as the orbital, or "shell," is full, it's referred to as a "closed shell." The electrons in the last shell (which is typically not full) are known as valence electrons.
Interestingly, the nucleons (protons and neutrons) inside the nucleus of an atom behave in a similar way. They also pack into shells, with discrete energy differences in between. The study of this behavior has elucidated many variations on the simple theme, but one thing which continues to be of interest is the location of the "shell closures" - these locations being indicated by magic numbers. Once a magic number of nucleons (a magic number of protons, or neutrons, or both) is attained, like in the ultra-stable nucleus 208Pb, the nucleus behaves in a different way. It's like the "noble gases" in chemistry - helium, neon, argon, etc - which are much more stable and inert than other elements, because they have closed electron shells (in other words, magic numbers of electrons).
So the search was on for other nuclei that might be just like lead-208 because of their "magicity" (that's a great term, isn't it?). In essence, nuclear physicists wanted to find other nuclei that behave as almost perfectly magic due to their internal structure. It just so happens that tin-132, an isotope of tin with 50 protons and 82 neutrons (both magic numbers, making 132Sn "doubly magic"), is one such nucleus. And Dr. Jones and her collaborators proved it (the Editor's Summary can be read for free on the Nature webpage here).
By looking one nucleon away from the doubly-magic tin isotope, at tin-133, the collaboration was able to show that the tin-133 behaved essentially like a single particle - in other words, the single nucleon outside of the closed shell of tin-132 completely dictated the behavior of the entire nucleus. That means the tin-132 "core" was as inert as it could be, precisely what you expect from a doubly-magic nucleus.
Dr. Jones and her collaborators performed the ground-breaking work at the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) in Tennessee. And it couldn't have been done anywhere else. I know, because I was there. "Magical" results like this (if you'll excuse the pun) don't come easily; it takes years of work. But the reward - finding another nucleus which demonstrates so clearly the successes of the nuclear shell model - was well worth the wait.
Jones, K., Adekola, A., Bardayan, D., Blackmon, J., Chae, K., Chipps, K., Cizewski, J., Erikson, L., Harlin, C., Hatarik, R., Kapler, R., Kozub, R., Liang, J., Livesay, R., Ma, Z., Moazen, B., Nesaraja, C., Nunes, F., Pain, S., Patterson, N., Shapira, D., Shriner, J., Smith, M., Swan, T., & Thomas, J. (2010). The magic nature of 132Sn explored through the single-particle states of 133Sn Nature, 465 (7297), 454-457 DOI: 10.1038/nature09048