Using Laser Ionization to Study Super Heavy Elements

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While they have been identified, named and classified, not much is known about the heaviest, radioactive elements at the bottom of the periodic table.

Using an incredibly delicate process involving laser light and supersonic gas jets, a team of researchers was able to discover the atomic and nuclear structures for isotopes of the superheavy element actinium, according to a report in the journal Nature Communications.

Elements at the bottom of the periodic table cannot be found in nature as far as we know. These elements have been made and identified in a laboratory environment only through the use of extremely powerful particle accelerators. The resulting exotic specimens experience radioactive decay very quickly and persist for just fractions of a second, making the study of these elements and isotopes very challenging.

In order to perform spectroscopy of one of these elements, actinium, the study team developed a new, highly sensitive technique based on the well-known technique of laser ionization, but used it in combination with a gas jet moving at supersonic speeds, a technique used in the high-precision studies of nuclei located at the extremes of atomic stability.

Laser ionization has been used in past studies to establish nuclear qualities like spins, magnetic moments and contrasts in nuclear charge. However, the existing use of resonant laser ionization can be limited as a result of poor spectral resolution, particularly in the GHz range.

Additionally, the usefulness can be limited for short-lived nuclei, as a result of physical-chemical properties.

With 89 protons, actinium has just one persistent isotope, with a life of 21.8 years. The fact that it has just one stable isotope limits knowledge of atomic transitions, and makes laser spectroscopy tests difficult.

In selecting neutron-deficient isotopes of actinium, researchers resolved many problems for high-resolution resonance ionization spectroscopy of the heavy elements, including limited data on the atomic levels as a result of lack of stable isotopes, limited production rates of the element to be studied as a result of need for using heavy-ion-induced fusion reactions and significant background noise.

The researchers generated actinium atoms in a nuclear-fusion reaction, by flooding a thin gold foil with nuclei of the noble gas neon. The actinium atoms were then trapped in argon gas and carried by a supersonic jet streaming from a tiny version of a rocket engine towards a laser interaction zone.

Resonance laser ionization was then used to ionize the atoms and carry out spectroscopy analysis. Pure ion beams of actinium were then separated based on their mass to gain isotopic selection and were electrostatically led to a bank of detectors.

The process brought the outer electron in another orbit and removed it with a laser beam. The result was a positively-charged ionized atom, that is simple to manipulate and identify.

"By ionizing the atom we significantly increase the sensitivity of the technique,” the study team said in a press release. “The production of a few atoms per second is already enough for measurements during the experiments.”

With this new technique, which is generally applicable, the spectral resolution is improved by more than an order of magnitude without loss of efficiency, and detailed experiments now become possible on nuclei produced at a rate of only one atom every ten seconds.

Rafael Ferrer, Scientist, KU Leuven Institute for Nuclear and Radiation Physics

The researchers said their work represents tantalizing new possibilities for research on the heaviest elements. They added that their work could make it possible to test and adjust the foundational theoretical models in nuclear physics and chemistry.

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