CERN Measured The Activity Of Atoms Of The Rarest Element That Exists On Earth, Astatine
Physicists from the ISOLDE project have accurately measured how much energy is released by the atoms of a very rare chemical element, astatine when an electron is attached to them. The results of the measurements are available in the journal Nature Communications.
"Electron affinity is the most important property of all atoms. It determines how actively they interact with other substances. 211At has long been considered one of the most promising materials for radiotherapy, but its use was hindered by the fact that we did not know how and how often negatively charged ions of this halogen are formed," the researchers write.
Among chemical elements, both stable elements cannot spontaneously decay, and unstable ones, whose life is often measured in fractions of a nanosecond. These include both very heavy elements, such as plutonium, and relatively light substances, such as technetium.
Radioactive elements are particularly interesting to physicists because they can be used to study the internal structure of atomic shells and verify various postulates of the Standard model. Besides, such substances are often used in medicine – for example, as radioactive tags or an active component of chemotherapy.
Working on the ISOLDE accelerator, physicists learned the chemical properties of the rarest element on Earth, astatine. This facility was created at CERN in 1964 to study the properties of radioactive isotopes and elements that do not exist in nature.
Using the ability of this accelerator to "sort" the produced isotopes, physicists used it to synthesize large enough numbers of atoms of 211At – a short-lived isotope with which experts in the field of nuclear medicine have high hopes.
A half-century-old mystery
Even though American physicists synthesized astatine in 1940, and found it in nature a few decades later, many of the physical and chemical properties of this element remain a mystery to scientists. This is since only a few tens of grams of all the isotopes of this element are present in the entire Earth's crust. Artificial synthesis increased the amount of astatine available for study only slightly.
In total, the development of methods of synthesis of astatine and simultaneous study of its most important chemical properties of the ISOLDE physics spent nearly ten years. One of the most important properties is electron affinity, that is, the ability to attach a free carrier of negative charge to itself and release energy at the same time.
Halogens usually have relatively high electron affinity. Because of this, they are strong oxidizers and very aggressive from a chemical point of view. In the past, scientists have tried to predict this parameter for ASTAT using quantum physics methods. However, until recently, it was impossible to verify these predictions because the electron affinity changes non-linearly during the transition from one cell of the periodic table to another.
ISOLDE participants received the first data of this kind, having learned to produce sufficiently large amounts of 211At. To do this, they bombarded the target with high-energy thorium protons. At the same time, they have together formed atoms with electrons, gaining negative ions of astatine. After that, they were passed through a laser beam.
Physicists selected the properties of this beam of light particles in such a way that they "detached" an extra electron from the 211At ion, causing it to absorb energy. By observing how many atoms passed into the neutral state, as well as tracking changes in the properties of the laser beam itself, scientists were able to accurately calculate its affinity for the electron.
It was very close to the theoretically predicted value, 2.41 electron volts. This is significantly less than that of iodine – the least active halogen and astatine's neighbor in group VII of the periodic table. However, this is higher than the electron affinity for all other elements, including oxygen. This should be taken into account when using astatine in medicine, scientists conclude.