Fluorescent sponges to detect radioactive gases
Due to their volatile nature and low-energy ionizing emissions, radioactive gases, and particularly their detection, currently represent a major public health issue and a challenge for the industries that produce them in various scenarios. Improving this detection involves developing new systems with optimised properties. Scientists at the Systems and Technology Integration Laboratory (List - Univ. Paris-Saclay, CEA), are using ultra-porous and fluorescent materials - Metal Organic Frameworks (MOFs) - to develop a more sensitive and practical radioactive gas detection system.
Naturally present on Earth, radioactive gases such as radon, tritium and krypton are volatile chemical elements whose unstable atomic nuclei emit alpha or beta radiation (a helium nucleus or an electron) as they decay. Originally present in trace amounts, some show higher concentrations due to human activities or specific phenomena and eventually lead to public health or waste management issues. Although relatively weak, the electrical charge of their radiation is in fact sufficient to penetrate one to two centimetres into living tissue. Radon (222Rn), produced by the decay of uranium and radium atoms in granite soils, is considered the second leading cause of lung cancer in France, after tobacco. Although less dangerous and initially produced by cosmic radiation acting on air atoms, tritium (3H) is produced in large quantities by industry and nuclear reactors. Krypton (85Kr), formed in the cores of stars, is found on Earth as a byproduct of radioactive waste processing and accounts for 99% of man-made radioactive gas emissions.
Although there are already detection systems for these gases on the market, some scientists are directing their research towards materials with novel properties, in order to develop new systems that redefine current performance standards. Such is the case with the work of the team at CEA-List's Sensors and Instrumentation for Measurement Laboratory (LCIM), which studies Metal Organic Frameworks or MOFs. These are crystalline networks composed of metal atoms linked by organic molecules.
MOFs, ultra-porous scintillators
In theory, a perfect radioactive gas detection material must have two properties. Firstly, it needs to be porous, since porosity increases the contact surface between the gas and the detector, thereby enhancing the system's sensitivity. Secondly, it should be capable of scintillation. Similar to fluorescence, scintillation is a phenomenon where a material emits photons following excitation by a given particle. In the case of a scintillating material, the excitation is not induced by a photon, as in fluorescence, but by an ionizing particle: the scintillator produces light in the presence of radioactivity.
Given their properties, MOFs are strong candidates for detecting radioactive gas. Depending on the nature of the organic and metallic molecules used, they can either scintillate or not. However, it is their porosity that makes them particularly intriguing, as Guillaume Bertrand, a researcher at the LCIM, points out: "These materials have the world's largest known contact surface: 2,000 m2 per gram, which is akin to folding two football fields into a teaspoon!"
The synthesis of these materials is relatively straightforward, as described by Sharvanee Mauree, a PhD candidate in the same team: "This is a solvothermal synthesis. We start with elementary organic and metallic building blocks. These initial products are dissolved in solvents under specific pressure and temperature conditions, which induces a spontaneous assembly of the molecules into a MOF." By varying the nature of the organic and/or metallic building blocks used at the outset, scientists can alter the porosity and scintillation properties of the resulting material. By slightly changing the composition of the organic component, the CEA-List team have successfully produced MOFs with enhanced scintillation. And of the approximately 100,000 MOFs that exist worldwide, they have created sixteen in the laboratory and, after rigorous characterisation of their properties, focused on the four most promising ones.
A highly sensitive detection system
To test the ability of these MOFs to detect radioactive gases, the LCIM team, in partnership with Benoît Sabot from the Henri Becquerel National Laboratory (LNHB) of CEA-List, developed a unique tool: the TDCR or triple-to-double coincidence ratio system. "This is an extremely sensitive photon detection system that eliminates all kinds of external noise, for highly accurate measurements," adds Guillaume Bertrand. The MOF to be tested and the detector are placed in a black box, into which the scientists introduce a controlled quantity of radioactive gas. The concentration of the gas, which permeates all the pores of the MOF, increases within the material, and the photons emitted by the MOFs are then detected by the TDCR system.
The team carried out an initial series of experiments on the four pre-selected MOFs. For these tests, they chose to use krypton as this radioactive gas emits relatively high-energy beta particles, which are easier to detect. Another advantage of this gas is its lack of induced contamination, which means that the tested MOF can be reused. The tests revealed the unique qualities of two MOFs, MOF-5 ADC and MOF-205, which showed gas detection rates of 188% and 264% respectively. Obtaining such detection rates, in excess of 100% is not unusual, as Guillaume Bertrand explains: "Some MOFs act like a sponge for radioactive gases, resulting in much higher activity rates than expected."
In a second series of experiments, the scientists showed that MOF-5 ADC and MOF-205 are also capable of detecting radon in good percentages, and even tritium, which is much more difficult to detect. In fact, it is the ability of these MOFs to concentrate tritiated dihydrogen within these networks that makes detection of this radioactive gas possible.
Small, fast and practical detectors
Capable of detecting low concentrations of radioactive gases, LCIM's new detector offers many advantages over existing systems, particularly in terms of practicality. Although sensitive methods for detecting krypton and tritium already exist, they often take a long time to deliver a result (24 hours) and require heavy, bulky, non-reusable devices and a laboratory analysis. In contrast, the system proposed by the LCIM team is small, easily transportable and guarantees on-site measurements in less than 20 minutes. In addition, the MOF can be "washed" at the end of the experiment and reused indefinitely.
While existing radon detectors on the market are already highly effective, the LCIM team envisages another use for its new detection system: trapping this radioactive gas in the pores of the MOF, then, after hydration, causing it to collapse on itself and successfully extracting it from a room's atmosphere. This application, still under consideration but already promising, would be particularly useful in homes with high radon exposure or in certain laboratories where the slightest presence of radioactive activity could distort measurements.
Reference:
Sharvanee Mauree, Vincent Villemot, Matthieu Hamel, Benoit Sabot, Sylvie Pierre, Christophe Dujardin, Francesca Belloni, Angiolina Comotti, Silvia Bracco, Jacopo Perego, Guillaume H. V. Bertrand. Detection of Radioactive Gas with Scintillating MOFs. Advanced Functional Materials, 33 (2023).