Operation of bare Ge-diodes in LN 2 / LAr - Purification of N 2 /Ar

1. Operation of bare Ge-diodes in LN2 / LAr - Purification of N2/Ar Hardy Simgen Max-Planck-Institute for Nuclear Physics Heidelberg

2. Outline • Introduction / Motivation • Experimental techniques • Final results of N2 purification tests • Ar purification tests • Conceptional design of a gas purification plant for GERDA • Future plans (GERDA without gas purification?) • Conclusion H. Simgen, MPI for Nuclear Physics / Heidelberg

3. 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

4. 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

5. Low-level proportional counters 222Rn: 30 mBq  0.5 mBq/m3 for 222Rn in N2 H. Simgen, MPI for Nuclear Physics / Heidelberg

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

7. Gas purification by the gas adsorption process • Simple (cheap) process to obtain highest purities • Efficiency 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

8. 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

9. Purification in the column N equilibrium stages H. Simgen, MPI for Nuclear Physics / Heidelberg

10. Final results of N2 purification tests • Purification of liquid N2 from Rn • Purification of liquid N2 from Kr • Purification of gaseous N2 from Kr H. Simgen, MPI for Nuclear Physics / Heidelberg

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

12. 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 <0.5 Bq/m3 achieved. H. Simgen, MPI for Nuclear Physics / Heidelberg

13. Purification of liquid N2 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

14. Investigated adsorbers • Molecular sieves and zeolithes • not favorable • Carbon based adsorbers: • 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

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

16. Results:Purification of liquid N2 from Kr H. Simgen, MPI for Nuclear Physics / Heidelberg

17. 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 T required! • Better results are expected for low temperature gas phase purification. H. Simgen, MPI for Nuclear Physics / Heidelberg

18. Purification of gaseous N2 from Kr • Two ways to guarantee gas phase: • high flow rate: No time for N2 to cool down. • Liquid argon cooling (TLAr = TLN2 + 10 K). • Ultrapure LN2 for tests procured from “Westfalen AG” • doped with 400 ppt Kr • All carbon based adsorbers were tested H. Simgen, MPI for Nuclear Physics / Heidelberg

19. Results / Breakthrough curves T=87K (gas phase) H. Simgen, MPI for Nuclear Physics / Heidelberg

20. Results:Purification of gaseous N2 from Kr • Purification ability in gas phase 4-10 times better than in liquid phase! • steeper breakthough curves (larger N). H. Simgen, MPI for Nuclear Physics / Heidelberg

21. Purification of N2 – Summary • Ar removal by adsorption is impossible. • 222Rn removal easy, even for liquid N2. • Low 222Rn emanation rate of the adsorber required. • Kr removal by adsorption is possible: • But only in gas phase sufficiently effective. • Gas phase is technically more challenging. • still more difficult than Rn removal (much larger adsorption column required). H. Simgen, MPI for Nuclear Physics / Heidelberg

22. 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

23. 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

24. Towards the realization of a gas purification plant • Questions: • Purpose: Rn only or also Kr? • Selection of adsorber (How much?) • Selection of operating conditions (liquid phase / gas phase) • Frequency of regeneration (2 columns ?) • Degree of automation (refilling) • Knowledge is available • Decisions have to be taken now… H. Simgen, MPI for Nuclear Physics / Heidelberg

25. Conceptional desgin of N2/Ar purification plant H. Simgen, MPI for Nuclear Physics / Heidelberg

26. Investigation of storage tanks • 222Rn decays away • Final contamination given by 222Rn emanation of storage tank • Regular purity N2 @ LNGS: ~50 mBq/m3 • from 3 x 6m3 tanks • 222Rn emanation rate can be calculated  200 mBq per tank • Special 3 m3 storage tank for highest purities (LINDE): • 222Rn emanation rate: 2.7 mBq (!)  expected gas purity: 1.3 mBq/m3 H. Simgen, MPI for Nuclear Physics / Heidelberg

27. Future activities • Clean storage tanks are available (in terms of 222Rn) • But Ar/Kr contamination? • Companies can produce low Ar/Kr nitrogen • But problems in delivery (Contamination during refilling / storage) • Complete delivery chain must be carefully checked! • If result OK: No purification plant necessary for GERDA H. Simgen, MPI for Nuclear Physics / Heidelberg

28. Conclusions • Selected adsorbers were tested for gas and liquid phase purification of N2. • CarboAct was chosen (low 222Rn emanation rate) • Argon purification tests have been performed (Results similar as for N2) • Conceptional design of purification plant done: Final decisions to be taken • Tests of storage tanks and delivery chain will clarify if purification can be avoided. H. Simgen, MPI for Nuclear Physics / Heidelberg

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