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Prof. Daryl J. Hartley
evan.
sandy.

Research

Although the atomic nucleus was discovered over 100 years ago, many mysteries concerning this complex collection of protons and neutrons remain to this day. While nuclear theorists try to perfect their models (there is NO model that accurately describes every nucleus), experimentalist (like myself) probe the nucleus to reveal new properties which may lead to a more complete picture.

There are many ways to probe the structure of the nucleus, but my favorite way is the make it spin.  Why would spinning the nucleus tell us anything about the inner structure of the nucleus?  Here's a simple example from every day life - this is a movie of the difference between spinning a hard boiled egg and a raw egg - in the same way, by observing how nuclei spin, we can figure out the nuclear inner structure. 

My experiments are performed at various accelerator laboratories.  In particular, I have primarily worked at the ATLAS superconducting linac facility (Argonne National Laboratory).  The accelerated beams of nuclei are focused onto targets (normally of a different nucleus with respect to the beam) to create nuclei that are not seen in the universe except in supernovae explosions! 

These excited nuclei emit gamma rays to rid themselves of extra energy and spin.  We can observe these gamma rays with high-purity germanium (Ge) detectors.  By surrounding the targets with an array of these detectors, we can collect extremely useful information regarding the state in the nucleus, which gives clues to the nuclear shape.  ATLAS is currently home to the largest array of these Ge detectors, which is known as Gammasphere.  Over 100 detectors can be placed in this device and it was featured in the Ang Lee movie The Hulk.  (Editorial comment: the movie stunk, and they completely made up the physics concerning Gammasphere, but they made a perfect model.) A picture of Gammasphere is shown below with one of my former research students, Eowyn Pedicini

Eowyn at Gammasphere

The nuclear structure group at ATLAS put together a very nice video about the type of physics we explore, especially regarding how we can determine the shape of the nucleus from these experiments.  You can watch this video at this link.

Most people think all nuclei are little spheres (if they think about it at all) - but it turns out only about a third of all the known nuclei are spherical.  Most nuclei are deformed and look similar to a rugby ball.  But other nuclei take on more exotic shapes, which is what I am interested in.  For the past 10 years or so, I have looked for evidence of asymmetrically-shaped (or triaxial) nuclei.  When a nucleus that has an asymmetric shape is spun, it acts similar to an asymmetric top that is spun - that is, it will have a precession and a wobbling motion.  Through an analysis of the gamma-ray decay pattern, we actually tell if the nucleus is wobbling which confirms the asymmetric shape.  To date, only 5 nuclei (out of 3000 that are known) have exhibited wobbling motion: 161Lu, 163Lu, 165Lu, 167Lu, and 167Ta (Lu = lutetium and Ta = tantalum).  I was fortunate to find the wobbling excitation in 167Ta - and you can read about in the article found at this link.

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Research Sponsored by the National Science Foundation
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