Ancient Lead Can Help Experimental Physics

Scientists from the National University of Science and Technology MISIS measured the number of trace impurities in the ingots of ancient lead from the sunken ship of the ancient Romans, using a novel technology. It was shown that lead, which stayed underwater for 1500 years, contains such a small amount of radioactive elements — Uranium and Thorim, that it can be used without any purification in one of the most “demanding” areas — Nuclear physics — in the study of elementary particles. The experiment on the selection and determination of micro-impurities was carried out in cooperation with colleagues from the Joint Institute for Nuclear Research and the National Center for Scientific Research (France). An article about the development published in the journal Talanta.

The more precise and efficient modern devices become, the more pure materials are required to construct them. These are so-called high-purity materials, which contain impurities in so small amounts that they cannot influence the specific properties of the studied objects. One of the most “demanding” spheres where high-purity materials can be used is Nuclear physics. For instance, particle accelerators, which are already built underground to protect the experiments from cosmic rays, still require additional radiation protection from special shields made of ultra-pure lead. An example of such an accelerator is the Large Hadron Collider in CERN.

High-purity lead is produced in several stages, such as the dissolution of ore, smelting, separation of the alloy into component parts, alkali purification, and separation of each impurity. When many stages of purification are performed, it is needed to analyze a probe of the high-purity lead. The permissible maximum of radioactive impurities should not exceed 0,0000000001% (one ten-billion %) of the total mass. With this amount of radioactive impurities (or lower), lead can be used to protect high-precision equipment. However, even the most modern methods of direct elemental analysis do not allow to determine such small amounts of impurities in the main component — lead.

Scientists of NUST MISIS laboratory for separation and concentration in chemical diagnostics of functional materials and objects of the environment, led by Professor Pyotr Fedotov, Sc.D. in Chemistry, suggested a new technology to separate impurities for further analysis. The experiment was conducted on the probes of ancient lead — more than 1500 years old ingots. Centuries ago, this lead was mined by the ancient Romans in the mines, located on the territory of modern England. When transporting lead ore ship sank, and was discovered only at the end of the 20th century off the coast of France.

When the experiment was conducted, this ancient lead had already been in use by the National Center for Scientific Research (France) as a high-purity material, because for 1500 years a significant part of uranium and thorium naturally decayed, and the water protected the lead ingots from “sticking” of new radioactive impurities. Uranium and thorium were not detected in this lead, but it was assumed that the amount of impurities could simply be below the “visibility” of direct instrumental analysis, that is, below 0.00000001% (one hundred-million %).

Then, NUST MISIS scientists suggested a new technique of separating impurities for further analysis. With the help of the so-called planetary centrifuge and a system of two immiscible liquids (water and chloroform) containing special reagents, the lead sample was first dissolved in extra-pure nitric acid, and then the impurities were separated and concentrated.

This combined approach, based on the separation of impurities and their subsequent determination, allowed NUST MISIS scientists to determine uranium and thorium impurities with the required accuracy of 0.0000000001% (one ten-billion %). However, the content of uranium and thorium was below this level. Thus, it turns out that the ancient lead, mined by the ancient Romans and drawn from the sea only 1500 years later, is so pure that even with the most accurate methods of separation and analysis of impurities, they cannot be seen and measured.

NUST MISIS scientists will continue using this new technique of separating impurities in a planetary centrifuge — mainly, to analyze high-purity materials.

“One of the main advantages of our technique is its flexibility: depending on the materials to be separated, we can use various reagents and immiscible liquids. Hence, we can separate, concentrate and analyze the tiniest ultratrace impurities to estimate the high-purity of the materials”, — Pyotr Fedotov, Head of the research team explains.

Moreover, if the volume of the separation container of the planetary centrifuge is bigger, this technique can also be used to purify materials by dissolving them and separating the impurities.