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Techniques for analysis and purification of nitrogen and argon. Grzegorz Zuzel MPI-K Heidelberg. Outline. Motivation of this research Production of N 2 and Ar Radioactive noble gases in the atmosphere Methods of analysis Purification of N 2 and Ar Conclusions and planned activity.

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Techniques for analysis and purification of nitrogen and argon


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    1. Techniques for analysis and purification of nitrogen and argon Grzegorz Zuzel MPI-K Heidelberg IDEA Meeting, MPI-K Heidelberg, 21-22.October 2004

    2. Outline • Motivation of this research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    3. Motivation • Ultra-pure LN2/LAr will be used by the GERDA experiment - Cooling medium for „naked“ Ge crystals - Shield against external radiation • Developed techniques could be applied in other low-level experiments IDEA Meeting, MPI-K Heidelberg

    4. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    5. Production of N2 and Ar • N2 and Ar are produced from air by rectification • Traces of atmospheric noble gases remain in final product • Final purity depends on individual plant and handling IDEA Meeting, MPI-K Heidelberg

    6. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    7. Radioactive noble gases in the atmosphere IDEA Meeting, MPI-K Heidelberg

    8. Requirements for GERDA • 222Rn: • MC simulations [Bernhard Schwingenheuer]: • 0.3 µBq/m3 N2 (STP) = 10-4 evt/(kgykeV) • 42Ar: • MC simulations [Stefan Schönert]: • 50 µBq/m3 Ar (STP) = 410-5 events/(kgykeV) 42Ar naturally low enough IDEA Meeting, MPI-K Heidelberg

    9. Requirements for GERDA • Q-value of 39Ar and 85Kr below 700 keV • But dead-time problem when Ar scintillation is used (slow decay time:1µs) • Assume 10 m3 active volume • 39Ar rate: 17 kHz OK! • 85Kr rate not higher  ≤ 0.3 ppm krypton required • In case of LN2 and dark matter detection: – 39Ar < 2.4 µBq/m3 N2 (0.2 ppm Ar in N2) – 85Kr < 1 µBq/m3 N2 (1 ppt Kr in N2) IDEA Meeting, MPI-K Heidelberg

    10. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    11. Low-level proportional counters IDEA Meeting, MPI-K Heidelberg

    12. Low-level proportional counters • Developed for the GALLEX experiment • Can be applied for α- and β-detection • Handmade at MPI-K (ultra-pure quartz) • Background ~1 cpd for E > 0.5 keV • Active volume of about 1 cm3 • Special filling procedure is required IDEA Meeting, MPI-K Heidelberg

    13. Counter filling line IDEA Meeting, MPI-K Heidelberg

    14. Sensitivities • 222Rn: -only α-decays detected - 50 keV threshold (bkg: 0.2 – 2 cpd) - total detection efficiency ~1.5  abs. detection limit ~30 µBq (15 atoms) • 39Ar and 85Kr: - β-decays detected - 0.6 keV threshold (bkg: 1-5 cpd) - total det. efficiency ~0.5  abs. det. limit ~100 µBq (5x10485Kr atoms) IDEA Meeting, MPI-K Heidelberg

    15. Measurements of 222Rn in gases – MoREx (Mobile Radon Extraction Unit) IDEA Meeting, MPI-K Heidelberg

    16. Measurements of 222Rn in gases – MoREx (Mobile Radon Extraction Unit) 222Rn detection limit: ~0.3 μBq/m3 IDEA Meeting, MPI-K Heidelberg

    17. Ar and Kr: mass spectrometry IDEA Meeting, MPI-K Heidelberg

    18. Ar and Kr: mass spectrometry • Devoted to investigate rare gases in terrestial and extraterrestial samples • Coupled with the sample preparation and purification sections (cryo- and getter pumps) • System operated at ultra-high vacuum (10-10 mbar) • Sample size typically ~1cm3 • Detection limits: Ar: 10-9 cm3 (1 ppb; ~1.4 nBq/m3 for 39Ar in N2) Kr: 10-13 cm3 (0.1 ppt; ~0.1 µBq/m3 for 85Kr in N2) IDEA Meeting, MPI-K Heidelberg

    19. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    20. Different possibilities • Distillation - high costs and energy consumption • Sparging (e.g. with He) - boiling point for contaminants must be lower than for the gas to be purified • Adsorption - successfully used for 222Rn removal from nitrogen - a lot of experience at MPI-K IDEA Meeting, MPI-K Heidelberg

    21. Gas purification by adsorption • Applied when high purities are required • Based on differences in binding energies • Strong dependence on temperature • Activated carbons and zeolites are widely used as adsorbers IDEA Meeting, MPI-K Heidelberg

    22. Henrys law • n = number of moles adsorbed [mol/kg] • p = partial pressure of adsorptive [Pa] • H = Henry constant [mol/(kg·Pa)] • H determines the retention volume: n = H  p VRet = HRTmAds IDEA Meeting, MPI-K Heidelberg

    23. Purification in the column IDEA Meeting, MPI-K Heidelberg

    24. Single component adsorption model • Prediction of Henry constant for adsorption on activated carbon • Only one parameter is involved: TC•pC-0.5 • Allows to compare adsorption of different components S. Maurer, Ph.D. thesis, TU Munich (2000) IDEA Meeting, MPI-K Heidelberg

    25. Single component adsorption model IDEA Meeting, MPI-K Heidelberg

    26. Purification of N2/LN2 from 222Rn • Strong binding to almost all adsorbers • Easy trapping with activated carbon at 77 K • Problem: 222Rn emanation due to 226Ra • Requires careful material selection • Activated carbon „CarboAct“: • 222Rn emanation rate (0.3  0.1) mBq/kg • 100 times lower than other carbons IDEA Meeting, MPI-K Heidelberg

    27. Purification of N2/LN2 from Kr • Single component adsorption model fails for binary system N2/Kr • More advanced models predict strong dependence of H on the pore size of the adsorber and its internal polarity • Henry coefficient expected to be higher for pure gas phase adsorption (at T > 77 (87) K for N2 (Ar)) • Cooling: LAr (for N2) or pressurized liquid gases • Pores, low polarity and adsorption from gas phase should lead to H ~1 mol/kg/Pa IDEA Meeting, MPI-K Heidelberg

    28. Henry constant and pore size IDEA Meeting, MPI-K Heidelberg

    29. Considered adsorbers • Hydrophobic zeolite MFI-type: low internal polarity, pores ~5.3 Å • Hydrophobic zeolite BEA-type: a bit larger polarity than for MFI, pores ~6.6 Å • “Carbo Act” F3/F4: low 222Rn emanation rate, wide pore size distribution • Charcoal Cloth FM 1-250, fabric • Activated Carbon C38/2, optimized for solvent recovery IDEA Meeting, MPI-K Heidelberg

    30. Experimental setup IDEA Meeting, MPI-K Heidelberg

    31. Purification of N2 – Summary • 222Rn removal rather easy, even from LN2 • Ar removal impossible • Kr removal requires: • Low temperature gas phase adsorption • Pore size-tuned adsorbers with low internal polarity • Low 222Rn emanation rate IDEA Meeting, MPI-K Heidelberg

    32. Single component adsorption model IDEA Meeting, MPI-K Heidelberg

    33. Purification of Ar • (Almost) no difference between Ar and N2 for adsorption on activated carbon • However higher temperatures have to be considered • 222Rn removal should not be a problem • Kr removal from Ar even more challenging than for N2 (especially for large amounts) IDEA Meeting, MPI-K Heidelberg

    34. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2 and Ar • Conclusions and planned activity IDEA Meeting, MPI-K Heidelberg

    35. Conclusions and planned activity • Techniques for measuring ultra-low radioactivity levels available @ MPI-K • Nitrogen purification intensively studied - Adsorbers selection based on the adsorption theory - Experimental tests are ongoing - Purity tests for different supply chains are planned • Argon purification seems to be a very similar problem • Purity and purification tests for Ar recently started • Although the program was slightly extended it is progressing as scheduled IDEA Meeting, MPI-K Heidelberg