1. Adsorption of radioactive noble gases in microporous materials Jose Busto CPPM/ Universite d’Aix-Marseille GDR - neutrino Marseille 26 – 27 Novembre

2. Background from radioactivity is one of the most hazardous problems and the main limitation for many experiments at low energy and low counting rate.

3. Borexino Solar Neutrino experiment 85 Kr  < 0.2  Bq/m 3 39 Ar  < 0.5  Bq/m 3 Gerda  experiment 42 Ar  < 40  Bq/L SuperNEMO  experiment 222 Rn  < 150  Bq/m 3 LUX dark matter experiment 85 Kr  < 2 ppt Background from radioactive noble gases

4. Radioactive noble gases origin Cosmogenic Anthropogenic : Fission producs La Hague Nuclear Reprocesing Plant

5. 85 Kr evolution Increase 0.025 Bq/m 3 /y

6. Uranium and Thorium Radioactive Chain

9. 222 Rn Prompt and delayed background  Surface contamination

10. Capture of noble gases Noble gases Full outer shell ( 8 e  ) => no valence electrons => very small reactivity Possible compounds for heavier noble elements Kr, Xe Rn must interact with matter better than Xe However radon is a pure radioactive short period gas = > poor knowledge of chemical properties Nevertheless Chemistry of noble gases is possible but very poor Very difficult capture by chemisorption

11. N Potential well N N N Nobel gases adsorption Capture mainly by physisorption (adsorption) Weak Van der Waals  Capture on surface - > If  is much smaller than thermal energy, kT, the molecules are regarded as “mobile”. - > If  is much larger than kT, the molecules are “localized”. In general, continuous Adsorption and Desorption Equilibrium between Adsorption and Desorption => Not permanent capture => Slow down the movement of the atoms

12. To increase the capture capacity we need : - large surface area => high porosity - reduced the temperature Typical porosity of active charcoals : The effective surface area is distributed between different porous size Enormous surface area of active charcoal

13. Enhancement of the adsorption energy in a slit shaped pore of various widths The size and the shape of the pores are also important parameters Potential energy Narrow pore Large pore DaDa DaDa DpDp DpDp

14. Adsorption competition In Xe experiments, competition between carrier gas and radon Atomic radious Gas Covalent (single bond) He0.032 nm N0.075 - Ar0.097 - Kr0.110 - Xe0.130 - Rn0.133 - CF 4 0.107 nm He 0.032 nm N 2 0.075 nm Rn 0.133 nm Rn adsorption on active charcoal in CF 4, He and N 2

15. Radon capture measurements at CPPM Characterization of radon adsorption on microporous materials

16. Input Rn Output Rn Adsorbant Trap Trap in equilibrium Rn + gaz carrier Equilibrium Adsorption – desorption time  Rn detector Rn detector Radon capture measurement by “dynamic” technique C g : Rn concentration in gas

17. Radon capture measurement by “dynamic” technique C m : Rn concentration in the trap Radon in filter material by  -Ge spectrometry Radon concentration

18. Residence time v. s. adsorption capacity If K is big enough =>  >> T 1/2 (Rn) therefore, Rn concentration after the radon trap became negligible From gas chromatography Mass Flow Filter Column ( mass : M) Flow : F Residence time :  chr

19. Experimental setup N2N2 Rn T = 12 o C Pressure Temperature Rn concentration (diffusion Rn detector) Particle filter Trap  Flowmeter 10 L / H Buffer, 10L RAD7  To exhaust Continuous Rn detector Carrier gas : Nitrogen, Helium Flow : 10 L / H Rn concentration : ~ 900 Bq / m 3 Trap temperature : + 20 o C to – 50 o C Pressure : 1 bar Dynamic Rn adsorption

20. Materials tested T = 20 o C, -30 o C, -50 o C, P = 1 bar Gas = Nitrogen, He Classical activated carbon : very wide pore size distribution Synthetic carbon - Carboact : synthetic very low activity charcoal Commercial Carbon molecular sieve : well defined pore size Metal Organic Framework : very large surface area

21. Molecular Cages : well defined structue Curcubiturils Porous organic cages CC3 (Liverpool Univ.) Carbon Aerogel : very low density large surface areas Sucrose carbon micro spheres : very high microporosity

22. m 3 /kg K_factor for several materials in N 2 Standard active charcoal Carbon Molecular Sieve

23. m 3 /kg K_factor for several materials in N 2 Standard active charcoal Carbon Molecular Sieve Standard active charcoal Carbon Molecular Sieve

24. m 3 /kg K_factor for several materials in N 2

25. m 3 /kg Aerogel

26. K_factor for several materials in N 2 m 3 /kg Aerogel New adsorbents are much better are much better than standard active charcoal

27. K_factor for several materials in He / N 2 N2N2 He Very low competition between Rn and He

28. Randon capture v.s. porosity Volume microporeux totalVolume ultramicro % ultraVolume supermicroAext EchantillonsA BET Volume totalt-plot  s -plot DR  s -plot m 2.g -1 cm 3.g -1 K6109400,3810,3700,4100,3480,174420,23640 Carboxen 5692900,4840,0800,0640,1170,0641000,000164 K48 special12680,5120,5050,5070,5200,274540,2335 Carboact11820,5340,4700,4620,4760,290630,17226 Carboxen 10009310,8420,3570,3590,3660,253700,10620 Carbosieve SIII13620,5650,510 0,5350,355700,1558 Ultramicropore : diamètre<1,062 nm

29. Adsorption v. s. microscopic properties carboxen 569 K 610 K48 S Carboact Carbosieve III Carboxene 1000 cm 3 /g m 3 /kg Adsorption ( m 3 /kg) v.s volume of ultramicropores ( < 1.62 nm) (@ - 50 o C)  Modelization of radon adsorption ??? Very nice correlation

30. carboxen 569 K 610 K48 S Carboact Carbosieve III Carboxene 1000 cm 3 /g m 3 /kg Adsorption ( m 3 /kg) v.s volume of ultramicropores ( < 1.62 nm) (@ - 50 o C) Not only, surface area, and porous distributions are important Carboxen 1000 : pore size ~ 10 – 12 Å = > high adsorption Carbosieve III S : pore size ~ 4 – 10 Å => high adsorption Zeolite 13 X : pore size ~ 10 Å = > NO adsorption Carbon Molecular Sieve Aluminium Molecular Sieve Need further measurements and a better knowledge of the structure

31. Last but not least : Emanation Radioactivity of some active charcoals Radon emanation from the trap it self  Very pure adsorbent ( synthetic materials )  Mass of adsorbent as low as possible  Self adsorption can help

32. Conclusion  Radioactivity from noble gases is a very important background source in many low energy and low count rate experiments.  Increase Rn adsorption in new microporous materials is possible. We need nevertheless a better understanding of the porosity and structure of the adsorbent.  High radio-purity is mandatory ( synthetic materials)  Optimization of production process (price of adsorbent) is also required.

35. Modelization is possible in “symmetric” carbon cages Predicted adsorption isotherme for Kr, Xe and Rn CC3 cage – University of Liverpool