Rick pointed out that there are four main frontiers for us nuclear physicist to explore: proton-rich region, neutron-rich region, super heavies, and the boundaries between them.
We explore those nuclear regions by means of a few observables, like:
1) R4/2, the ratio of excitation energy of the 4+ state to that of the 2+ state.
2) the size of nuclei - notice that the summed volume of all nucleons making up a nucleus account for only about 60% of the volume of the nucleus! and they orbit around each other around 1021 times per second, and still they exhibit a coherent collective behaviour! ... How is this possible?! ...obviously, nuclei know what they're doing!
3) the deformation of nuclei, a.k.a. B(E2) values
4) masses - they reflect nucleonic interactions
5)separation energies
In addition, we can get information from the differential observables, such as isotope shifts.
Beta indicates how ellipsoidal a nucleus is, which in turn depends on the valence proton-neutron interactions.
We can then plot the nuclear potential as a function of beta, and we see that for spherical nuclei the potential has only one minimum at zero and then it smoothly increases. As deformation sets in there appears a second minimum, and the minimum energy increases gradually for elliptical shapes. The different shapes, nonetheless, coexist at the critical point. Nuclei with "magic numbers" of neutrons and protons have no valence proton - neutron interactions, and their shapes are spherical. As we add more protons or neutrons deformation sets in and we observe it as a phase transition.
There are many interesting such phase transitions in the chart of nuclei that can be clearly observed in between closed shells (an interesting example is the region of heavies close to 206Hg, recently measured at GSI). Realistic calculations are needed in order to describe these observations. Rick showed us results of a density functional theory calculation with skyrme interactions that agree remarkably with experiments.
What do we need to measure now? everything! more masses (those are "easy"), excitation energies, gammas, etc.
A nice talk indeed. Playing with systematics subtracting masses and counting valence nucleons is somehow simple thing to explain, but the talk presented very well the elegant insight we get from it.
ReplyDeleteThe question I got was if the proton neutron interaction (Vpn), as treated in the talk, was an empirically defined thing or it referred to some precisely defined type of interaction between valence neutrons and protons. A similar thing to what happens with the odd-even staggering of nuclear masses, which is reality is due to different things: e.g. pairing, and the effect of occupying higher orbitals. In more general terms, I still struggle to identify which of the many interactions and potentials we play with in nuclear physics are treated or defined from a "fundamental" level, and which are a phenomenological description of the problem. I looked like it was just phenomenology, however the experimental Vpn agreed very well with the DFT calculations so someone must have told the mysterious DFT Fortran code what a proton-neutron interaction is.
Maybe it depends who you speak to, and you can always keep peeling the onion and find a more fundamental layer to the problem: in one end we have the people trying to predict nuclear masses using image reconstruction techniques (the group at UNAM is working on it, but I think no one is speaking here), and you can go all the way to start from quantum chromodynamics and include the quark degrees of freedom in the description of the nuclei. In any case, Jorge Piekarewicz was a few steps ahead and already knew the answer to my question, so he asked if how you could reconcile the 200keV order of magnitude of the Vpn from the observables with the much larger proton-neutron interaction 'in vacuum'... I'm just an experimentalist humbly answered Prof. Casten.