Scientific Method / Science & Exploration

I’m in ur atom, probing ur nucleus

Recent analysis of data gathered from accelerator experiments at the Thomas …

It has been 100 years since the Geiger-Marsden experiment upended humanity's longstanding view of the atom as a nice, relatively homogeneous particle. When the alpha particles shot at gold foil targets bounced off at odd angles, it shocked everyone involved (the original paper is now open access). The realization that atoms have a high-mass center lead to the formation of the Rutherford (or planetary) model of the atom, where electrons orbit a dense nucleus. 

It was quite the most incredible event that has ever
happened to me in
a piece of tissue paper and it came back and hit you. - E. Rutherford

The Rutherford model is now recognized as a useful metaphor, but, as soon as you get into the details, it falls apart. Electrons don't travel in nice orbits; rather, their orbitals exist as a cloud around the nucleus. The nucleus is not a singular positive mass, but a complex ball of protons and neutrons that are made up of even smaller particles known as quarks and gluons. Although all this information indicates that the Rutherford model is wrong, it doesn't necessarily indicate what a replacement for the model should look like.

Progress towards an improved replacement has been erratic. In 1983, the European Muon Collaboration discovered that quarks were distributed differently in large nuclei than they are in small nuclei—a phenomenon dubbed the EMC effect. This finding has since been the fodder for any number of theoretical studies. Many theoretical models began with the assumption of an infinite amount of nuclear matter, and then used scaling laws to bring it into the realm of reality. However, this procedure will end up ignoring the effects of the surface of the nucleus, which plays a larger role the smaller a nucleus gets. 


Analytical descriptions, based on data from the Stanford Linear Accelerator, have either used fits based on the nuclear density or on the nuclear mass. Both types of analytical fits are only statistically good for nuclei larger than 12C, which still leaves the behavior of small nuclei difficult to explain. Very small nuclei—those below 4He—can be described with a high degree of precision by various quantum theories, but we haven't had a lot of experimental data to compare these predictions with. 

Using data from the Thomas Jefferson National Accelerator facility (JLab) in 2004, a cadre of researchers have examined nuclear data for 1H, 2H, 3He, 4He, 9Be, and 12C. The results have led the team to propose a new theory about the structure of the nucleus—as a bonus, it indicates why it has been so hard to create a general theory.

The results are set to be published in an upcoming edition of Physical Review Letters. The researchers examined the EMC effect in these small nuclei and found anomalies that don't fit with either class of theories. The results from high-energy lepton scattering experiments clearly showed that the nuclear effects are smaller for 3He than for 4He or 12C. This is consistent with the concept that the EMC effect is a function of the average nuclear density, since the nuclear density of 3He is about one half that of 12C. Problems arise when you move to slightly larger atoms; 9Be in particular gives results that cannot be explained. The magnitude of the effect observed in 9Be was nearly the same at that seen in 12C, even though both the mass of the beryllium nucleus and its density are lower than the carbon. 

The authors argue that their results can be explained if the EMC effect is not related to the average nucleus density, but rather to the local density of hadrons inside the nucleus itself. This hypothesis works if, as previously postulated, the beryllium nucleus is described as two alpha particles (two protons and two neutrons) and a single lone neutron. This would mean that if the majority of the nucleons (protons and neutrons) are in a dense environment similar to 4He, the average density would be much lower due to the distributed nature of the particles. As usual in these types of works, further experiments will be needed to refine the data and potentially confirm or deny the hypothesis, but the authors put forth an explanation for the EMC effect that takes into account a much wider range of atoms than previously studied.

Physical Review Papers, 2009. DOI: 10.1103/PhysRevLett.103.202301
* Rutherford gold foil experiment paper is freely available

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