Title: Operation of bare Ge-diodes in LN2 / LAr - Purification of N2/Ar
1Operation of bare Ge-diodes in LN2 / LAr -
Purification of N2/Ar
- Hardy Simgen
- Max-Planck-Institute for Nuclear Physics
Heidelberg
2Outline
- 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
3Motivation
- 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
4Ar 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)
5Low-level proportional counters
222Rn 30 mBq ? 0.5 mBq/m3 for 222Rn in N2
6MoREx (Mobile Radon Extraction Unit)
7Gas 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.
8Henrys 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
9Purification in the column
N equilibrium stages
10Final results of N2 purification tests
- Purification of liquid N2 from Rn
- Purification of liquid N2 from Kr
- Purification of gaseous N2 from Kr
11Adsorption 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
12Purification 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.
13Purification 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.
14Investigated 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)).
15Results / Breakthrough curves
T77K (liquid phase)
16ResultsPurification 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
17Liquid 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.
18Purification 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
19Results / Breakthrough curves
T87K (gas phase)
20ResultsPurification 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).
21Purification 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).
22Purification 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.
23Measurements 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
24Towards 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
25Conceptional desgin of N2/Ar purification plant
26Investigation 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
27Future 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
28Conclusions
- 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.
29Radioactive 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