Daryl Hartley's Research Page

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Powerpoint of my invited talk at the Niels Bohr Institute (December 2005)

Download a copy of my latest paper on ¹⁷¹Ta which has been accepted by Physical Review C.

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 the 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 to make it spin – in other words, we give it a lot of excitation energy and angular momentum. Why would spinning the nucleus tell us anything about nuclear structure? Here's a simple experiment you can do at home – spin a raw egg and hard-boiled egg. You will see a dramatic difference which is the result of different moments of inertia. In the same way, spinning nuclei reveal the moment of inertia, and thus the shape of the nucleus. For those who want to learn more about how we make nuclei spin, click here.

My experiments are performed at various accelerator laboratories. In particular, I have primarily worked at the ATLAS superconducting linac facility (Argonne National Laboratory), the HRIBF tandem facility (Oak Ridge National Laboratory), the FSU tandem-linac facility (Florida State University), and the Wright Nuclear Structure Laboratory (Yale University). Pictures of some of these labs are shown below.

Left: A portion of the ATLAS superconducting linear accelerator. Right: That's me inside the tank of the FSU tandem during my graduate school days.

The accelerated beams of nuclei are focused onto targets (normally of a different nucleus with respect to the beam) to create nuclei which have not been seen in the universe since the beginning of time! These excited nuclei emit gamma rays to rid themselves of extra energy and angular momentum. We can observe these gamma rays with high-purity germanium (Ge) detectors. By surrounding the targets with an array of these Ge detectors, we can collect extremely useful information regarding the excited states that are present in a particular nucleus. Pictures of a few arrays are shown below:

GAMMASPHERE: Currently the largest Ge array in the world with the capability of holding 110 detectors. This array has been located at Lawrance Berkeley and Argonne National Laboratories. It also makes an appearance in the movie The Hulk. Here's a TV news story about Gammasphere, The Hulk, and my graduate advisor at FSU.

CLARION: An array of Clover detectors at Oak Ridge National Laboratory. This array was specifically built to study nuclei created with radioactive beams.

Why do these nuclei emit gamma rays? When they are spinning, the nuclei are in a excited state, similar to a spinning top. The spinning top will not stay in this state forever, it gradually loses energy and angular momentum until it stops. A spinning nucleus does the same thing, except it loses energy in bursts (or quanta) which are called gamma rays. The nucleus continues to emit gamma rays until it reaches the ground state (where it is no longer spinning and it has no excitation energy). During the time in which the nucleus slows down, it passes through many excited states. We can build an energy level diagram (or level scheme) from our gamma-ray data, which reconstructs the slowing down process. This process is shown in the following animation. An example of a very simple level scheme is shown below, where the arrows represent gamma rays emitted from the excited states. The energy of the gamma rays is shown in units of keV (kiloelectron volts). The energy of the states (in keV) and their angular momentum (in units of hbar) are also shown.

Believe it or not, by determining the properties of the states and the gamma rays emitted from them, we can actually determine the shape of a given nucleus. Many people think all nuclei are spherical (if they've even given much thought to nuclei at all), but actually nuclei come in a variety of shapes! Click here to learn more about nuclear shapes.

I am currently interested in finding nuclei with mass distributions that are different along all three body-fixed axes (these are called triaxial nuclei). There are only handfull that are known. Recently, I have been looking at hafnium (Hf) nuclei with 72 protons. You can look at some of my results from a recent talk I gave at the Nulcei at the Limits conference.