Physicists have for the first time unlocked one of the key secrets of the rarest naturally occurring element on earth: astatin.
Astatin is a “halogen”, which means that it shares chemical properties with fluorine, chlorine, bromine and iodine (all elements that usually bind to metals to form salts). But with 85 protons, it is heavier than lead and is unusually rare on Earth – the rarest of the elements that occur naturally in the earth’s crust, according to chemist John Emsley’s 2011 book “Nature’s Building Blocks” (Oxford University Press). It is formed by decaying uranium and thorium, and its most stable version, or isotope, (called astatine-21
Until recently, scientists have never been able to collect enough of it to test how it interacts with electrons. This is a problem, in part because one of its radioactive isotopes has astatine-211 potential to be useful in cancer therapies. However, researchers were not sure how likely it is to attract electrons and form negative ions, which can be harmful to healthy cells. A new paper changes that.
The project to understand astatin took place at ISOLDE, the part of the EU’s flagship CERN particle physics research center that focuses on blasting proton beams against hot and heavy targets made of various chemical elements.
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For this effort, the researchers fired the protons at a target made of thorium atoms, an unstable element with 90 protons. It produced a smashing of new atoms, including astatine-211.
By filtering astatine-211 from the rest of the atoms, the researchers built a sufficiently large layer of the ever-decaying radioactive substance for use in their laser experiments. Focused light can orbit the electrons on an atom, allowing scientists to make accurate measurements of their behavior.
In an earlier paper, published in 2013 in the journal Nature Communications, the ISOLDE team measured the ionization energy of astatin: how difficult it is to remove an electron from the atom. Now, in a July 30 paper, also published in Nature Communications, they calculated its electron affinity: how easily the isotope attracts new electrons.
The ionization energy was approximately 9.31752 electron volts. The electron affinity is approximately 2.41579 electron volts. Together, the two numbers form a complete picture of how the heavy, rare element interacts with electrons. (It will take many years to understand the full practical implications of these numbers, but it is a major obstacle to monitor precisely to determine them.)
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ISOLDE trumped the result as confirmation of theoretical modeling work that had been going on simultaneously in the lab. The immediate consequence of this result could be for cancer research, ISOLDE said. Astatine-211 releases alpha particles when it decays, radioactive carbohydrates consisting of protons and neutrons that can kill cancer cells. However, determining how to use radioactive astatin salts in cancer therapies requires a deep understanding of how the element creates negative ions. When astatin draws electrons from molecules in healthy cells in the body, it turns into negative ions, which can damage healthy cells. Researchers need an accurate understanding of that process to deliver the best therapies.
Eventually, ISOLDE said, their engineers were able to unlock the mystery of the super-hungry – elements that are only briefly found in laboratories in small quantities with properties that scientists barely understand at all.
Originally published on Live Science.