Laboratory Nova aims to find out explosions

Supernovas are a common occurrence in space, but their inner workings cannot be observed on Earth. Using special equipment, researchers including some from Mines created model supernovas that gave insight into the inner workings of stars.

Dr. D. W. Bardayan, in collaboration with Dr. Uwe Griefe, and CSM graduates, works at Oak Ridge National Laboratory studying what happens when stars explode . He uses gases in the laboratory to recreate supernova style explosions.

The astrophysical explosions he is referring to are nuclear reactions inside of stars that are responsible for the creation of most elements. While hydrogen, helium, lithium and beryllium were present shortly (astrophysically speaking) after the big bang, all other naturally observable elements are created through the process of nucleosynthesis inside of stars, and the matter is ejected into the universe when the star goes supernova. Nova explosions, which are distinct from the concept of a supernova, are caused by the accretion of matter on the surface of a white dwarf or neutron star. This occurs primarily in binary star systems, when the white dwarf can “leech” hydrogen from another nearby star. Because of the thermodynamic conditions caused by this leeching, pressure inside the star is dominated by density (by the exclusion principle) instead of temperature. Thus, as density increases, temperature and pressure both increase, allowing high energy levels that push sub-atomic particles together and cause thermonuclear reactions. These reactions release energy, and the temperature is then raised even higher, spurring the reactions on further still. It is this process that provides the energy nucleosynthesis needs.

Since scientists cannot go to a nova explosion to study it in detail, they instead look at the nuclear physics they can do on earth in an effort to study at least one portion of what is going on when a nova occurs. One worthwhile reaction involves the formation of 18Ne, which decays into 18F, an isotope whose abundance is readily observable in stars. The reaction involves a 17F smashing into a 1H and creating the 18Ne isotope.

Bardayan uses the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory to create conditions that are similar to the reaction occurring inside stars. This facility makes use of a 13- story-high concrete bunker that houses a cyclotron, accelerator, and other important equipment. The process uses the cyclotron to create 17F and strip off all the electrons. The beam is then accelerated to stellar energy levels and bombed into a hydrogen target prepared elsewhere at the lab.

Since hydrogen is highly explosive at concentrations above 4% in air, the lab is equipped with safety alarms and procedural precautions to prevent disasters of Hindenburg proportions.

Once the hydrogen has interacted with the fluoride, the product is sent through multiple separators before arriving at a detector which measures the energy and charge-to-mass ratio to identify its atomic number. Incidents of 18Ne are recorded, and differing initial energies are used to identify a connection between energy (of the incident 17F) and fusion rate, or the number of 18Ne recorded at the other end. This allows the researchers to find a resonance energy for these fusion reactions, and that information leads to clues of the conditions that must be present inside of stars to make these reactions occur. Putting all the clues together could lead to a greater understanding of exactly how elements are made inside of stars.

The colloquium concluded that the resonance energy (or energy which caused the highest fusion reaction rate) of the 18Ne production reaction, reported as a temperature, was below 0.3 GK.

The work presented was in collaboration with Griefe, a professor in the Physics department at CSM, and Kelly Chipps, a CSM student who graduated in 2008 with a doctorate degree and current postdoc, in addition to various other organizations affiliated with Oak Ridge.



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