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Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N 2 /Ar

Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N 2 /Ar. Hardy Simgen Max-Planck-Institute for Nuclear Physics Heidelberg. Outline. Motivation Experimental techniques Summary of theoretical studies Purification of N 2 from 222 Rn/ 85 Kr

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Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N 2 /Ar

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  1. Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N2/Ar Hardy Simgen Max-Planck-Institute for Nuclear Physics Heidelberg

  2. Outline • Motivation • Experimental techniques • Summary of theoretical studies • Purification of N2 from 222Rn/85Kr • Gas phase versus liquid phase • Measurements of 222Rn in Ar • Conclusion and outlook H. Simgen, MPI for Nuclear Physics / Heidelberg

  3. Radioactive noble gases in the atmosphere H. Simgen, MPI for Nuclear Physics / Heidelberg

  4. Motivation • Ultra-pure LN2/LAr will be used in the GERDA experiment. • Cooling medium for Ge crystals. • Passive shield against external radiation. • Active shield (LAr). • Removal of Rn (Ar/Kr) crucial. • Developed techniques can be applied in other low-level projects. H. Simgen, MPI for Nuclear Physics / Heidelberg

  5. Ar and Kr: mass spectrometry Ar: 10-9 cm3 (1 ppb; ~1.4 nBq/m3 for 39Ar in N2) Kr: 10-13 cm3 (0.1 ppt; ~0.1 Bq/m3 for 85Kr in N2) H. Simgen, MPI for Nuclear Physics / Heidelberg

  6. Low-level proportional counters H. Simgen, MPI for Nuclear Physics / Heidelberg

  7. Gas purification and proportional counter filling line Absolute sensitivity: ~30 Bq for 222Rn H. Simgen, MPI for Nuclear Physics / Heidelberg

  8. Measurements of 222Rn in gases – MoREx (Mobile Radon Extraction Unit) 222Rn detection limit: ~0.3 Bq/m3 H. Simgen, MPI for Nuclear Physics / Heidelberg

  9. MoREx (Mobile Radon Extraction Unit) H. Simgen, MPI for Nuclear Physics / Heidelberg

  10. Overview of theoretical studies • Efficiency of adsorption process depends on • Temperature. • Pore size structure of adsorber. • Polarity of adsorber. • Mobility of gases (gas phase / liquid phase). • Equilibrium described by Henrys constant. H. Simgen, MPI for Nuclear Physics / Heidelberg

  11. Henrys law and retention volume n = H  p • n = number of moles adsorbed [mol/kg] • p = partial pressure of adsorptive [Pa] • H = Henrys constant [mol/(kg·Pa)] • H determines the retention volume: VRet = H  R  T  mAds H. Simgen, MPI for Nuclear Physics / Heidelberg

  12. Purification in the column n = Hp H. Simgen, MPI for Nuclear Physics / Heidelberg

  13. Simple adsorption model • Influence of pores is neglected. • Valid for adsorbers with wide pore size distribution. H. Simgen, MPI for Nuclear Physics / Heidelberg

  14. Purification of LN2 from 222Rn • At low temperatures: Strong binding of radon to all surfaces. • Easy trapping with activated carbon @ 77 K. • Problem: 222Rn emanation due to 226Ra! • Activated carbon „CarboAct“: 222Rn emanation rate (0.3  0.1) mBq/kg. • ~100 times lower than other carbons. • N2 purity <0.3 Bq/m3 achieved. H. Simgen, MPI for Nuclear Physics / Heidelberg

  15. Purification of LN2 from Kr • Krypton is nobler than radon. • Binding energies are smaller  Henrys constants are much smaller. • Moreover: Similar size of N2/Kr. • N2 may displace adsorbed Kr. • Adsorption efficiency drops down. • N2 purification from Kr requires careful selection of adsorber/temperature etc. H. Simgen, MPI for Nuclear Physics / Heidelberg

  16. Henrys constant and pore size H. Simgen, MPI for Nuclear Physics / Heidelberg

  17. Investigated adsorbers • Molecular sieves 5 Å/10 Å (polar). • Hydrophobic zeolites: low internal polarity, but small pores (7 Å). • Carbo Act: low 222Rn emanation rate, wide pore size distribution. • Activated carbons with enhanced fraction of pores around 7 Å (Charcoal Cloth FM1-250, CarboTech C38/2). • Carbosieve SIII (Carbon molecular sieve: Only small pores (<40 Å)). H. Simgen, MPI for Nuclear Physics / Heidelberg

  18. Experimental setup H. Simgen, MPI for Nuclear Physics / Heidelberg

  19. Results / Breakthrough curves T=77K (liquid phase) H. Simgen, MPI for Nuclear Physics / Heidelberg

  20. Results H. Simgen, MPI for Nuclear Physics / Heidelberg

  21. Liquid phase versus gas phase • Liquid phase purification is preferred from economical point of view, but: • higher mobility in gas phase. • faster diffusion in gas phase. • Low temperature required (exponential dependency of adsorption process on T). • Better results are expected for low temperature gas phase purification. H. Simgen, MPI for Nuclear Physics / Heidelberg

  22. Removal of Kr from N2 in gas phaseFirst results • Two easy approaches for gas phase adsorption: • high flow rate: No time for gas to cool down. • Liquid argon cooling (TLAr = TLN2 + 10 K). • CarboAct tested • Liquid phase: H=0.06 mol/kg/Pa, N=1. • Gas phase: H=0.21 mol/kg/Pa, N=15. • Gas phase adsorption is very effective! H. Simgen, MPI for Nuclear Physics / Heidelberg

  23. Purification of N2 – Summary • 222Rn removal easy, even for liquid N2. • Low 222Rn emanation rate of the adsorber required. • Ar removal by adsorption is impossible. • Kr removal by adsorption is possible: • Gas phase adsorption is much more effective. • Further improvement by pore size tuning expected. H. Simgen, MPI for Nuclear Physics / Heidelberg

  24. Purification of Ar • Theory predicts very similar adsorption behaviour for Ar and N2. • However TLAr = TLN2 + 10 K: Adsorption at higher temperatures less efficient. • T  100 K required for gas phase adsorption. • 222Rn removal should not be a problem. H. Simgen, MPI for Nuclear Physics / Heidelberg

  25. 8.5 ± 0.1 mBq/m3 at truck filling time Measurements of 222Rn in argon LN2 class 4.0 CRn ~ 50 µBq/m3 * gas phase purification ** liquid phase purification H. Simgen, MPI for Nuclear Physics / Heidelberg

  26. Towards 222Rn-free argon • Gas phase purification works well. • No breakthrough observed! • Liquid phase purification also works. • but efficiency <100%. • Scaling of adsorber column required. • Alternative approach (also for N2): • Avoid primary 85Kr/222Rn (or let it decay). • Requires storage in low 222Rn environment. • Investigation of 222Rn emanation of storage tanks. • Tests of supply chain for N2/Ar. H. Simgen, MPI for Nuclear Physics / Heidelberg

  27. Conclusion and outlook • Selection of adsorber candidates (based on theoretical studies) finished. • All adsorber tested for LN2 adsorption. • Gas phase adsorption tests are ongoing. • CarboAct still very attractive due to low 222Rn emanation rate. • Argon purification tests started. • Alternative concept: No purification - instead avoiding contaminations! • Planned: Test of full gas supply chain. H. Simgen, MPI for Nuclear Physics / Heidelberg

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