Techniques for analysis and purification of nitrogen and argon

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Techniques for analysis and purification of
nitrogen and argon

• Grzegorz Zuzel
• MPI-K Heidelberg

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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

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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

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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

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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

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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

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Radioactive noble gases in the atmosphere
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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

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Requirements for GERDA

• Q-value of 39Ar and 85Kr below 700 keV
• But dead-time problem when Ar scintillation is
  used (slow decay time1µs)
• 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 lt 2.4 µBq/m3 N2 (0.2 ppm Ar in N2)
  
  85Kr lt 1 µBq/m3 N2 (1 ppt Kr in
  N2)

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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

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Low-level proportional counters
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Low-level proportional counters

• Developed for the GALLEX experiment
• Can be applied for a- and ß-detection
• Handmade at MPI-K (ultra-pure quartz)
• Background 1 cpd for E gt 0.5 keV
• Active volume of about 1 cm3
• Special filling procedure is required

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Counter filling line
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Sensitivities

• 222Rn - only a-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 - ß-decays detected
  - 0.6 keV threshold (bkg 1-5
  cpd) - total det. efficiency 0.5 ?
  abs. det. limit 100 µBq (5x104
  85Kr atoms)

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Measurements of 222Rn in gases MoREx (Mobile
Radon Extraction Unit)
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Measurements of 222Rn in gases MoREx (Mobile
Radon Extraction Unit)
222Rn detection limit 0.3 µBq/m3
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Ar and Kr mass spectrometry
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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)

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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

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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

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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

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Henrys law
n H ? p

• n number of moles adsorbed mol/kg
• p partial pressure of adsorptive Pa
• H Henry constant mol/(kgPa)
• H determines the retention volume

VRet H?R?T?mAds
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Purification in the column
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Single component adsorption model

• Prediction of Henry constant for adsorption on
  activated carbon

• Only one parameter is involved TCpC-0.5
• Allows to compare adsorption of different
  components
• S. Maurer, Ph.D. thesis, TU Munich (2000)

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Single component adsorption model
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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

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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 gt 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

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Henry constant and pore size
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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

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Experimental setup
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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

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Single component adsorption model
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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)

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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

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Conclusions and planned activity

• Techniques for measuring ultra-low radioactivity
  levels available _at_ 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