Monday, 30 March 2020

Li-6 as a probe of giant monopole resonances

A new paper appeared on the arXiv this morning (in nucl-ex, cross-posted to nucl-th) titled Reexamination of 6Li scattering as a Probe to Investigate the Isoscalar Giant Resonances in Nuclei.  The Isoscalar Giant Resonances are excitations of nuclei in which the protons and neutrons move together en masse, in phase.  

The two main isoscalar giant resonances, and the two examined in the paper, are the monopole and quadrupole versions.  The Isoscalar Giant Monopole Resonance (ISGMR) is sometimes called the breathing mode, since it can be loosely imagined as a kind of inflating and deflating of the nucleus, though there is no air being pushed inside or breathed out to cause the motion.  The breathing mode is set off when particular kind of particle is sent scattering of the nucleus in such a way that no angular momentum is imparted.  In this case, a nucleus which started off as spherical will retain its spherical shape as it breathes in and out.

The other one of interest here is the Isoscalar Giant Quadrupole Resonance (ISGQR) which gets excited when two units of angular momentum are  transferred to the nucleus by the excitation.  This gives a different shape for the excitation in which the nucleus is squeezed in one direction while expanding in the orthogonal plane -- imagine the Earth squeezing down along the poles and expanding along the equatorial plane as a result, and then bouncing back as it tries to restore its equilibrium shape.

A key interest in making these excitations is that they probe rather generic properties of nuclear matter.  By squeezing a nucleus you can immediately probe the matter all the way through it and try to understand how "stiff" it is - how hard it is to compress.  The answer here has to come from the underlying forces between the nucleons, and we can learn all about that force by exciting these resonances.  

The traditional particle of choice for striking on nuclei to form the ISGMR is the alpha particle.  This works well because alphas only excite isoscalar excitations (those in which the protons and neutrons behave in the same way, unlike isovector excitations in which they act out of phase, and which can strongly mask isoscalar excitations).  A difficulty, though, is that the scattered alpha particles, which have to be observed to deduce how they interacted with the nucleus, have to be measured at very small scattering angles.  In other words, these measured alpha particles are very close to the beam line which is full of alpha particles which did not interact, or even worse, with stray alphas that scatter out of the beam line but not due to the interaction being targetted.   

The paper, then, uses lithium-6 (3 protons, 3 neutrons) to scatter off the target nucleus and cause the excitation.  The benefit here is that Li-6 breaks up easily into an alpha particle and a deuteron, and the interaction with the target nucleus will often cause this to happen.  One can then look for the alpha particles coming off, which are no longer contaminated by beam alphas (since the beam is now lithium).  The cost is a more complicated analysis of the scattering process and the possibility of different excitations of the target nucleus.  From the paper, though, the results seem very nice.  Here is a plot of the excitation of carbon-12, showing the tiny error bars:

 There is some structure to the peak.  Even ignoring what is happening below ~13 Mev, there appears to be one main peak and at further peaks in the shoulders - at least one prominent one in the higher-energy side.  This could be due to all sorts of things, but with carbon-12 not being spherical (it is oblate deformed, like the Earth) then there should a strong coupling to the ISGQR when the ISGMR is excited, and that is likely what dominates the structure.

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