On The Ways Of Krypton (Planetary Science)

In the comics of DC Comics it is the planet that gave birth to Superman. In nature it is an element synthesized by the stars through the so-called “s process”. Now a team led by Moshe Tessler – and which also includes Sergio Cristallo, Marco Limongi and Lorenzo Roberti of INAF – has discovered why the models predicted less of it than actually observed in the universe.

With the exception of hydrogen, stars are capable of producing all the chemical elements encountered in nature. Among these is also the noble gas krypton (chemical symbol Kr): known to the general public for being the birthplace of Superman – as well as for the unwanted effects caused to the superhero – this element is used in the industry for the production of fluorescent lights and in medicine as a blood flow tracer. In the universe, krypton is mainly produced by large-mass stars (at least 10 times larger than our Sun, so to speak), thanks to the slow neutron capture process: the slow process , or s process . Onestudy aimed at measuring neutron capture cross sections of some krypton isotopes , led by Moshe Tessler of the Israeli Soreq Nuclear Research Center, has now been published in Physical Review C. 

Tessler and colleagues ‘work was also selected as Editors’ suggestion . “The detailed knowledge of the isotope ratios of the s process often requires the detailed measurement of the cross sections for neutron capture of all the isotopes of the same element. This work “, we read in the summary that presents it among the Highlighted articles ,” applies a relatively new technology, the Atom-trap-trace-analysis ( Atta), where single neutrons are captured and counted through the phenomenon of fluorescence using cyclic atomic transitions excited by the fine-tuning of precise lasers. The absolute production of an isotope is then determined by comparison with reference samples. In this work, the neutron capture cross sections of four krypton isotopes were redetermined and subsequently used to calculate the relative abundances due to the s process in stars of large and intermediate mass ”.

Three researchers from the National Institute of Astrophysics also contributed to the study, providing theoretical support for the interpretation of the new experimental data: Sergio Cristallo of INAF of Abruzzo, Marco Limongi and Lorenzo Roberti of INAF of Rome. The new experimental data – obtained from the collaboration of scientists from three different laboratories: the Soreq Applied Research Accelerator Facility in Israel, the Argonne National Laboratory in the USA and the University of Bern in Switzerland – involve significant changes in the isotopic abundances of krypton and also of the later elements in the periodic table, such as rubidium. In particular, the production of krypton-80 is increased.

«In the universe this particular isotope – due to the presence of other stable isotopes that ‘shield’ a possible contribution from the rapid neutron capture process, the r process  is produced exclusively by the s process », explains Cristallo to Media Inaf . “Consequently, it is an excellent tracer for verifying the robustness of theoretical models. The calculations made with the new cross section of krypton-80 (lower than before) heal a discrepancy that we ourselves had highlighted by comparing our models with calculations of the chemical evolution of the Milky Way. In particular, star models were found to produce too little krypton-80 compared to the abundance observed in the Solar System. On the contrary, now the numbers add up. Krypton-80, however, is not very abundant in nature (about 2 percent of all krypton). Our favorite superhero can therefore sleep peacefully ».

Featured image credit: M. Tessler  et al., Phys Rev C, 2021

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Provided by INAF

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