Transcript Radioactive Noble Gases in BOREXINO Techniques for

Operation of bare Gediodes in LN2 / LAr Purification of N2/Ar
Hardy Simgen
Max-Planck-Institute for Nuclear Physics
Heidelberg
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
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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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
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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)
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Low-level proportional
counters
222Rn:
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30 Bq  0.5 Bq/m3 for
H. Simgen, MPI for Nuclear Physics / Heidelberg
222Rn
in N2
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MoREx
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(Mobile Radon Extraction Unit)
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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
constant.
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described by Henrys
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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
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Purification in the column
N equilibrium stages
CN = ½ C 0
C0
H
VRe t
RTmads
CN
VRet  H ( )
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VP
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Final results of N2 purification
tests
 Purification
of liquid N2 from Rn
 Purification of liquid N2 from Kr
 Purification of gaseous N2 from Kr
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Adsorption model for charcoals
 Influence
of pores is neglected.
 Valid for adsorbers with wide pore
size distribution.
Gas
TC [K]
PC [bar]
TC·PC-0.5
[K·bar-0.5]
Ar
151
49
21.6
2×102
6.8
N2
126
34
21.6
2×102
7
Kr
209
55
28.2
2×105
7
Rn
377
63
47.6
1014
~8
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H [mol/(kg·Pa)]
@ 77 K
Pore size
[Å]
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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.
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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.
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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 FM1250, CarboTech C38/2).
– Carbosieve SIII (Carbon molecular sieve:
Only small pores (<40 Å)).
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Results / Breakthrough curves
T=77K (liquid phase)
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Results:
Purification of liquid N2 from Kr
Adsorber
N
H
[mol/Pa/kg]
Molecular sieves
no purification effect
Zeolithes
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
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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.
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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
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Results / Breakthrough curves
T=87K (gas phase)
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Results:
Purification 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
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breakthough curves (larger N).
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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).
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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
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removal should not be a problem.
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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
<0.0005 (90% CL) *
3
Ar 5.0
200 m3
6.0 ± 0.1
0.006 ± 0.001 **
* gas phase purification
** liquid phase purification
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8.5 ± 0.1 mBq/m3 at truck filling time
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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…
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Conceptional desgin of N2/Ar
purification plant
Experiment
El. valve
(level control)
Particle
filter
LN2/LAr
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Flow/mass
meter
AC
Pump
El. valve
(level control)
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Investigation of storage tanks
 222Rn
decays away
– Final contamination given by
emanation of storage tank
 Regular
222Rn
purity N2 @ LNGS: ~50 Bq/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 Bq/m3
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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
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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.
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Radioactive noble gases in the
atmosphere
Source
222Rn
85Kr
39Ar
42Ar
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Primordial
238U
235U
fission (nuclear fuel
reprocessing plants)
Concentration (STP)
10 - ?00 Bq/m3 air
1.4 Bq/m3 air
1.2 MBq/m3 Kr
Cosmogenic
17 mBq/m3 air
1.8 Bq/m3 Ar
Cosmogenic
0.5 µBq/m3 air
50 µBq/m3 Ar
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