Thursday, 2 August 2012

Ring-shaped nuclei

Nuclei are tricksome little beasts.  Made of two types of particles that interact in a quite complex way, according to the rules of quantum mechanics, and via the residual strong force, they can exhibit all sorts of interesting behaviour.

They can come in many shapes.  Their ground states are either spherical, or squashed or stretched spheres (smartie or rugby ball-shaped respectively), or possibly pear-shaped, or even tetrahedral.  If you excite them into some of their natural exitation modes, they can start vibrating, wobbling, or deforming in to new metastable shape configurations.

Over the years, physicists have searched, both experimentally and theoretically for interesting new shape configurations.  Perhaps the most famous topic within the nuclear physics community has been so-called "bubble nuclei" which have largely empty cores, thanks to the decreasing proportion of occupied s-orbitals.  A new paper on the arXiv server appeared earlier this week which looks at toroidal nuclei; those shaped like a lifebuoy or doughnut.  Previous calculations had shown them to be unstable, spontaneously returning to a spherical-like state, but new calculations show that a large amount of angular momentum can stabilise the torus-shaped nucleus, at least for the case of 40Ca that they calculated.

This nucleons in this nucleus would be kept in the ring by the strong nuclear force, and kept out of the centre by the centrifugal barrier.  The alignment of angular momentum would mean that they nucleons would be circulating in the same direction, creating a phenomenal magnetic field.

How such an excited state could actually be made in experiment is not clear.  The predicted excitation energy is extremely high - in a region where most if not all other excited states would quickly lead to fission, but the calculations are certainly interesting.  If the predictions are correct, we are likely to see the thing in nature sooner or later.

edit: I've added a pretty picture from the preprint to the top of the post.  It show a slice through the starting configuration they used for their iteration procedure on the left, and the resulting stable smooth ring predicted by the nuclear force on the right.

T. Ichikawa, J. A. Maruhn, N. Itagaki, K. Matsuyanagi, P. -G. Reinhard, & S. Ohkubo (2012). Existence of exotic torus configuration in high-spin excited states of
  $^{40}$Ca ArXiv arXiv: 1207.6250v1


  1. Although I don't know much about Nuclear experimental procedures (and hence I'm likely to be asking a silly question here), would it be possible to make an experiment out of the very fact that at such a high excitation energy, most states lead to fission?

    By which I mean; if one were to excite the nuclei with a specified range of energies, one could potentially compute the predicted rate of decay via fission of the nucleus with and without this metastable state. You could then look for differences in the fission rate (accounting for daughter processes) between the experimental version and the predicted models.

    I'm sure it's a *little* more complicated than that, though.

    1. I should probably defer to one of my experimentalist colleagues, but since they are unlikely to be monitoring my blog posts, let me attempt.

      I think that it would be difficult to do it exactly like that, in that I don't think you could know precisely enough the number of interactions taking place, and detect with sufficient efficiency all the fissioning events. I think you'd have to look for a signature for positively measuring the ring-like state, perhaps relying on its large magnetic moment.

    2. I suspected it would be the case. The approach I outlined assumes knowledge of all other states of the nucleus at whatever energy bandwidth one is studying - something which is evidently a silly assumption unless the energy bandwidth is extremely narrow.