Thursday, February 15, 2018

Research will help scientists understand how stars create elements

New research involving The Australian National University (ANU) has, for the first time, demonstrated a long-theorised nuclear effect, in a feat that will help scientists understand how stars evolve and produce elements such as gold and platinum. Physicists first predicted the effect, called Nuclear Excitation by Electron Capture (NEEC), more than 40 years ago, but this research was the first positive observation and has achieved the first quantified measurement of the phenomenon. Co-researcher Dr Greg Lane said the new research would improve scientific understanding of the nuclear reactions that occur in stars. "The abundance of the different elements in a star depends primarily on the structure and behaviour of atomic nuclei," said Dr Lane from the ANU Research School of Physics and Engineering. "The NEEC phenomenon modifies the nucleus lifetime so that it survives for a shorter amount of time in a star." The NEEC effect occurs when an ionised atom captures an electron, giving the atom's nucleus enough energy to transition to a higher excited state.

ANU and other research institutions in the United States, Poland and Russia supported the project, which was led by the U.S. Army Research Laboratory.

Dr Lane said the NEEC phenomenon could also potentially be harnessed as an energy source with 100,000 times greater energy density than chemical batteries.

"Our study demonstrated a new way to release the energy stored in a long-lived nuclear state, which the U.S. Army Research Laboratory is interested to explore further," he said.

The research team observed the NEEC effect by producing an exotic isotope, molybdenum-93, in an excited state with a half-life of about seven hours.

Dr Lane said the NEEC effect accelerated the isotope's decay through an excitation pathway with a unique set of gamma rays, different from the normal pathway, which are a signature of NEEC.

The Heavy Ion Accelerator Facility at ANU was used to confirm that the NEEC signature would be unique, in readiness for the discovery experiment that used the ATLAS Accelarator at Argonne National Laboratory in the United States.

The Heavy Ion Accelerator Facility uses electricity and magnets to guide particles and speed them up to extreme energies to study the internal make-up of atomic nuclei, and how they behave when they collide.
Understanding Conditions for Star Formation

Sapporo, Japan (SPX) Feb 06 - The mechanism by which hydrogen sulphide is released as gas in interstellar molecular clouds is described by scientists in Japan and Germany, in the journal Nature Astronomy. The process, known as chemical desorption, is more efficient than previously believed, and this has implications for our understanding of star formation in molecular clouds.

Molecular clouds are rare, but are important parts of the galaxy where molecules form and evolve. In the colder, denser areas, and under the right conditions, stars are formed. Theoretically, in molecular clouds at temperatures of 10 kelvin, all molecules except hydrogen and helium should be locked into ice on the surface of dust, not freely floating around. However, observations have shown this is not the case.

Understanding how molecules are released from dust at low temperatures is crucial to explaining how chemicals evolve in such cold clouds. The dissolution of particles from ice due to ultraviolet radiation, a process called photodesorption, has been demonstrated to play a role in some parts of the massive clouds. However, this would be inefficient in the darker, denser areas where stars are formed.

Researchers have supposed chemical desorption is at work in those areas, releasing particles using excess energy from a chemical reaction. The idea was first proposed 50 years ago, but scientists had not provided proof of the process until now.

The research team led by Yasuhiro Oba and Naoki Watanabe from Hokkaido University in Japan, in collaboration with the University of Stuttgart in Germany, set up the conditions to investigate.

Using an experimental system containing amorphous solid water at 10 kelvin and hydrogen sulphide (H2S), the team exposed the H2S to hydrogen and monitored the reaction with infrared absorption spectroscopy.

The experiment demonstrated that the desorption is caused by hydrogen interacting with H2S and the reaction is therefore a chemical one. They were able to quantify desorption after the reaction, and found it was a much more efficient process than previously estimated.

This work is the first infrared in-situ measurement of chemical desorption, and gives detailed descriptions during reactions which are key to understanding interstellar sulphur chemistry.

"Interstellar chemistry is of great importance to understanding the formation of stars, as well as water, methanol and possibly to more complex molecular species," says Watanabe. A significant step forward in the fields of astronomy and chemistry, the experimental setup can now be used to examine other molecules in the future.

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