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Scintillating Porous Architectures for RadioacTivE gas detection

Periodic Reporting for period 2 - SPARTE (Scintillating Porous Architectures for RadioacTivE gas detection)

Reporting period: 2021-10-01 to 2023-03-31

Scintillating materials are widely used in many detection systems addressing different fields, such as medical imaging, homeland security, high energy physics calorimetry, industrial control, and oil drilling exploration. These ionizing radiations are photons or particles such as gamma, X-ray, alpha, beta particles. While technology to detect gamma and X-rays is mature and commercially available, alpha and beta particles are trickier to detect due to their short mean free path in the matter including gas. It strongly depends on the energy of the particle, in the case of beta it can range from a few tens of micrometres up to over centimetres in gas. They can thus hardly be detected from far as it is done for gamma rays. When their energy is reduced, their detection becomes critical because they have to be almost in contact with the sensor. Beyond detecting the presence of radioactive elements, metrology requires to quantify the activity which involves the modelling of the matter radiation, and for most of the beta-active critical elements, reliable and widely deployable technology does not exist. In environmental laboratories liquid scintillation counting techniques (LS) is the gold standard to measure radioactive liquids and 3H in gases. In the case of gas analysis, it is bubbled in a solution containing an aromatic solvent and a fluorophore. The use of this technique at large scale generates a significant amount of organic pollution and requires to mix the gas and the liquids, and the measurement cycle is at the week scale. On-site measurements for some particular isotopes in the environment use sophisticated detection technology which cannot be widely deployed. The SPARTE projects aims to propose a breakthrough solution based on highly efficient porous scintillators enabling the detection and the quantification of several critical radioactive rare gas isotopes of primary importance. The material proposed are based on 3 different approaches: aerogels of inorganic nano-scintillator, aerogel of metal-organic-frameworks (MOF) showing an intrinsic porosity and crystal of MOF. The targeted isotopes are (from high to low energy emitters) 85Kr, 133Xe, 3H, and potentially 37Ar.
We focused on material developments as well as on the benchmarking of the developed material. The selected materials have been evaluated under various activities of 85Kr as well as 3H. Material development is based on 3 alternative strategies:
1) to develop aerogel type architectures based on inorganic scintillator nanoparticles. Their properties in the bulk state in terms of efficiency and timing allow to anticipate performances adapted to our objectives. To date, we prepared Y3Al5O12 doped with cerium aerogels based on the nanoparticles developed in the first project period. The aerogels are now functional and have demonstrated excellent detection efficiency (about 95% for Kr and 17% for tritium) and we have achieved detection sensitivity of 0.05 Bq / cm3 for tritium and 85Kr measured in 100 s. With particular care of the radioactive background, we anticipate an improvement of the sensitivity by 1 order of magnitude at least. We have developed protocols to enlarge the size of the aerogel. In parallel, we have explored alternative compositions such as LuF3:Ce, CeF3, YPO4:Ce and SiO2:Ce.
2) to elaborate MOF nanocrystals based aerogels. In order to increase de intrinsic density, Hf based MOF have been prepared. As building block, the scintillating properties have been validated. In particular, it exhibits a very fast decay (<5 ns), opening the perspective of potential use of very short time windows. It has been demonstrated the ability to adsorb noble gaz such as Ar, Xe and Kr, opening the route of concentration of radioactive elements. Monoliths have been prepared from these Hf-MOF. The balance between the porosity and the transparency is still under study, but first monolith with improved transparency have been obtained. Under radioactive gas exposure, the Hf-MOF building block have demonstrated a sensitivity down to 3 Bq/cm3 for 85Kr.
3) in using these MOF structures, but to produce single crystals in macroscopic format. Similarly, many formulations have been explored and the most performing one demonstrate as polycrystalline assembly a detection limit at 6 Bq/cm3 and show a very good response linearity in the explored activity range.
4) Dedicated experimental set-up for porous analysis have been validated (Compton TDCR experiment). It enables to established the yield as a function of the electron energy and to deduce the potential detection efficiency for various gases. A dedicated set-up has also been validated for radioactive gas measurement. Several experiments with various activities have been performed with 85Kr, 3H and 222Rn to extract linearity behaviours, detection limits and scintillation yields.
The obtained performances allow to develop a reference detection system, which does not exist so far, and thus a method of calibration for noble radioactive gases of low concentration-activity down to the level of mBq/cm3 at least. It is crucial for the Comprehensive Nuclear Test-Ban Treaty Organization and does not exist yet since current reference measurement methods are limited to about 300Bq/cm3 for only some isotopes of noble gases such as 85Kr, 133Xe, 3H and 127Xe with the Triple Compensated Length Proportional Counters. The new materials we propose already surpass this sensitivity. Since the concept is validated, at least one of the materials is secured and validated, we are starting to develop a reference measurement technique that is expected to be implemented in other national metrology institutes. SPARTE will thus target the following 3 breakthrough objectives:
• While the current calibration methods are limited to 1kBq/m3 for 85Kr and 133Xe, the first target is to reach a detection sensitivity of mBq/cm3 at least and to propose a calibration method for low activity range (down to 1 Bq/m3).
• The second target, based on the experience gain of the first one, is to achieve real time detection of 3H with a significantly improved sensitivity in an easy deployable system. The aim is to combine real time and a sub-kBq/m3 sensitivity. The concept can in addition be widely deployable for the 3H (and of course 85Kr) and tritiated water vapour detection in air which would be an important breakthrough in this field.
• The third target, is to achieve the detection of 37Ar. This isotope is strategic, because it is produced by the activation of calcium by high energy neutrons and has a relative long half-life time. It is thus a clear and good indicator of underground nuclear tests. We have already achieved a detection efficiency of almost 20 % for 3H, allowing to consider now the detection of 37Ar achievable.

The consortium involves 2 start-ups, respectively in the synthesis of MOF-aerogels and in the field of radioactivity detection. The achieved progresses allow to seriously consider the development and exploitation of detectors including 3H, 85Kr for nuclear activity survey. Note that regulation imposes the 85Kr survey, the monitoring of radio-nuclide of the territory present an important economic potential.