In our society, the monitoring and radioactivity metrology of gaseous radionuclides is a critical topic, which is pivotal for several applications. For example, the accurate detection and monitoring of radioactive isotopes of Xe and 85Kr and other activation products such as 3H and 37Ar is strictly needed for the survey of nuclear power plants, reprocessing plants and nuclear waste treatment activities, or for the search for clandestine nuclear testing operation forbidden by the Comprehensive Nuclear-Test-Ban Treaty (CTBTO, https://www.ctbto.org). Precise measurements of the 133Xe concentration are increasingly demanded to further develop advanced techniques for diagnostic inhalation studies, evaluation of pulmonary function, imaging of the lungs and the assessment of cerebral blood flow. And, similarly, the evaluation and monitoring of natural isotopes such as 222Rn and 220Rn, which are significant cause of lung cancer, and other natural radioactive gases are becoming more and more urgent for healthcare and geosciences.
The cited radionuclides are usually probed by detecting the beta particles (electrons) emitted during their decay. The detection of b-emitters is generally not straightforward because of the short-range path of beta particles in matter. Several technologies have been developed to measure β-emitting gases depending on the radionuclide and the target values of activity that have to be detected. Today, in environmental laboratories liquid scintillation counters are the gold standard to measure radioactive liquids and gases by exploiting time coincidence detection techniques. However, the use of these detectors is significantly hampered by the maximum amount of gas that can be bubbled in the liquid, thus by a limited sensitivity. Moreover, the use of this technique at large scale generates a significant amount of organic pollution and requires to mix the gas and the liquids, hence giving dual chemical and radiological wastes that must be carefully managed.
Over the past years very sophisticated techniques have been developed to measure low concentration of Xe isotopes in air. Although the detection is effective, the system displays some drawbacks as it is limited to some isotopes, offering a specific disintegration scheme, and an extremely difficult quality control and calibration protocols which are strictly required for instruments dedicated to trace extremely low levels of radioactivity.
A research team of the Department of Materials Science led by Prof. Angelo Monguzzi, Angiolina Comotti e Anna Vedda in collaboration with the University of Lyon and the University of Paris-Saclay proposes a radically novel radioactive gas detection strategy, by introducing highly porous scintillating Metal-Organic Framework (MOFs) crystals designed to dramatically extend gas-matter interaction for effective b detection through scintillation. These materials combine an efficient, fast, and isotropic scintillation, ensuring homogeneous 3D response and high sensitivity. We demonstrate the capability of porous hafnium-based MOF crystals exploiting dicarboxy-9,10-diphenylanthracene (DPA) as organic scintillating linker. They show fast scintillation properties, appropriate for time-coincidence based detection techniques, and high fluorescence efficiency. Experiment results correlated to a detailed computational modeling demonstrate a porosity particularly suitable to host noble gases atoms and ions, such as Xe, Kr, Rn and H. The presence of a heavy element such as hafnium enables a good light intensity output that exceeds commercial plastic scintillators. The achieved excellent properties allow us to test the absorption and detection of the radionuclides 85Kr, 222Rn and 3H in a newly developed device that exploits the nanoscintillator crystalline powder as a gas harvester and concentrator. The porous MOFs show an improved sensitivity with respect to the reference scintillating powders currently tested for gas detection and an excellent linear response down to an activity value well below 1 kBq·m-3, thus surpassing the lower sensitivity limit of some commercial devices. Importantly, these results are obtained in a newly designed compact and easy-to-handle architecture for the detector that strongly supports the potential technological transfer of the proposed device by the use of scintillating porous MOF crystals as active elements to fabricate technologically appealing sensors for the detection of natural and anthropogenic radioactive gases at ultralow concentrations.
The results of the research are reported in the paper “Efficient radioactive gas detection by scintillating porous metal–organic frameworks” (DOI:10.1038/s41566-023-01211-2) published in Nature Photonics (Impact Factor 35.0, Journal Citation Report (Clarivate Analytics, 2022)).