Operation of bare Ge-diodes in LN2 / LAr - Purification of N2/Ar - PowerPoint PPT Presentation

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Operation of bare Ge-diodes in LN2 / LAr - Purification of N2/Ar

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Conceptional design of a gas purification plant for GERDA ... Krypton is nobler than radon. Binding energies are smaller Henrys constants are much smaller ... – PowerPoint PPT presentation

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Title: Operation of bare Ge-diodes in LN2 / LAr - Purification of N2/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

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

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)
5
Low-level proportional counters
222Rn 30 mBq ? 0.5 mBq/m3 for 222Rn in N2
6
MoREx (Mobile Radon Extraction Unit)
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.

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/(kgPa)
  • H determines the retention volume

VRet H ? R ? T ? mAds
9
Purification in the column
N equilibrium stages
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

11
Adsorption model for charcoals
  • Influence of pores is neglected.
  • Valid for adsorbers with wide pore size
    distribution.

Gas TC K PC bar TCPC-0.5 Kbar-0.5 H mol/(kgPa) _at_ 77 K Pore size Å
Ar 151 49 21.6 2102 6.8
N2 126 34 21.6 2102 7
Kr 209 55 28.2 2105 7
Rn 377 63 47.6 1014 8
12
Purification of LN2 from 222Rn
  • At low temperatures Strong binding of radon to
    all surfaces.
  • Easy trapping with activated carbon _at_ 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 lt0.5 ?Bq/m3 achieved.

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.

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 (lt40 Å)).

15
Results / Breakthrough curves
T77K (liquid phase)
16
ResultsPurification of liquid N2 from Kr
Adsorber N H mol/Pa/kg
Molecular sieves no purification effect no purification effect
Zeolithes poor purification ability poor purification ability
Synthetic carbon CarboAct 1 1 (6 2) ? 10-2
CarboTech C38/2 8 2 (2 1) ? 10-2
Charcoal Cloth FM1-250 13 3 (3 1) ? 10-2
Carbosieve SIII 2 1 (8 1) ? 10-2
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.

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

19
Results / Breakthrough curves
T87K (gas phase)
20
ResultsPurification of gaseous N2 from Kr
Adsorber N H mol/Pa/kg
Synthetic carbon CarboAct 15 3 0.21 0.02
CarboTech C38/2 13 3 0.19 0.01
Charcoal Cloth FM1-250 9 2 0.16 0.02
Carbosieve SIII 29 4 0.34 0.02
  • Purification ability in gas phase 4-10 times
    better than in liquid phase!
  • steeper breakthough curves (larger N).

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).

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.

23
Measurements of 222Rn in argon
LN2 class 4.0 CRn 50 µBq/m3
No. Gas Amount CRn in trap 1 mBq/m3 CRn in trap 2 mBq/m3
1 Ar 4.6 117 m3 2.9 0.2
2 Ar 4.6 141 m3 0.20 0.02 lt0.0005 (90 CL)
3 Ar 5.0 200 m3 6.0 0.1 0.006 0.001
gas phase purification liquid phase
purification
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

25
Conceptional desgin of N2/Ar purification plant
26
Investigation of storage tanks
  • 222Rn decays away
  • Final contamination given by 222Rn emanation of
    storage tank
  • Regular purity N2 _at_ 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

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

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.

29
Radioactive noble gases in the atmosphere
Source Concentration (STP)
222Rn Primordial 238U 10 - ?00 Bq/m3 air
85Kr 235U fission (nuclear fuel reprocessing plants) 1.4 Bq/m3 air 1.2 MBq/m3 Kr
39Ar Cosmogenic 17 mBq/m3 air 1.8 Bq/m3 Ar
42Ar Cosmogenic 0.5 µBq/m3 air 50 µBq/m3 Ar
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