Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N 2 /Ar

Operation of bare Ge-Operation of bare Ge-diodes in liquid diodes in liquid

nitrogen / argon - nitrogen / argon - Purification of NPurification of N22/Ar/Ar

Hardy SimgenHardy SimgenMax-Planck-Institute for Nuclear Physics Max-Planck-Institute for Nuclear Physics

HeidelbergHeidelberg

19/04/2319/04/23 H. Simgen, MPI for Nuclear Physics / HeidelbergH. Simgen, MPI for Nuclear Physics / Heidelberg 22

MotivationMotivation Experimental techniquesExperimental techniques Summary of theoretical studiesSummary of theoretical studies Purification of NPurification of N22 from from 222222Rn/Rn/8585KrKr

– Gas phase versus liquid phaseGas phase versus liquid phase Measurements of Measurements of 222222Rn in ArRn in Ar Conclusion and outlookConclusion and outlook

OutlineOutline


Radioactive noble gases in the Radioactive noble gases in the atmosphereatmosphere

SourceSource Concentration (STP)Concentration (STP)

222222RnRn Primordial Primordial 238238UU 10 - ?00 Bq/m10 - ?00 Bq/m33 air air

8585KrKr235235U fission (nuclear fuel U fission (nuclear fuel

reprocessing plants)reprocessing plants)1.4 Bq/m1.4 Bq/m33 air air1.2 MBq/m1.2 MBq/m33 Kr Kr

3939ArAr CosmogenicCosmogenic 17 mBq/m17 mBq/m33 air air1.8 Bq/m1.8 Bq/m33 Ar Ar

4242ArAr CosmogenicCosmogenic0.50.5 µµBq/mBq/m33 air air50 50 µµBq/mBq/m33 Ar Ar


Ultra-pure LNUltra-pure LN22/LAr will be used in the /LAr will be used in the GERDA experiment.GERDA experiment.– Cooling medium for Ge crystals.Cooling medium for Ge crystals.– Passive shield against external radiation.Passive shield against external radiation.– Active shield (LAr).Active shield (LAr).

Removal of Rn (Ar/Kr) crucial.Removal of Rn (Ar/Kr) crucial. Developed techniques can be applied Developed techniques can be applied

in other low-level projects.in other low-level projects.

MotivationMotivation


Ar and Kr: mass spectrometryAr and Kr: mass spectrometry

Ar: 10Ar: 10-9-9 cm cm33 (1 ppb; ~1.4 nBq/m (1 ppb; ~1.4 nBq/m33 for for 3939Ar in NAr in N22) ) Kr: 10Kr: 10-13-13 cm cm33 (0.1 ppt; ~0.1 (0.1 ppt; ~0.1 Bq/mBq/m33 for for 8585Kr in NKr in N22))


Low-level proportional Low-level proportional counterscounters


getter-pum p

connections for d iffe rentgases (K r, A r, Xe, N , C H , ...)2 4

m ain line

h igh ly pureo ld xenon

Toepler pum p m ercurym anom eterto m ain pum p

flow m eter

carbon trap forhe lium -purifica tion

sam ple conta iner(d ism ountab le)

N aO H -trapagainst C O 2

silicageltrap

sm allcarbon

trapb ig

carbontrap

chrom osorbtrap

gasexit

exchangeablechrom atography co lum n

helium

port forproportionalcounter

topum p

gasexit

therm istor

Gas purification and proportional Gas purification and proportional counter filling linecounter filling line

Absolute sensitivity: ~30 Bq for 222Rn


Measurements of Measurements of 222222Rn in gases – Rn in gases – MoRExMoREx ( (MoMobile bile RRadon adon ExExtraction Unit)traction Unit)

222Rn detection limit: ~0.3 Bq/m3


MoREx MoREx ((MoMobile bile RRadon adon ExExtraction Unit)traction Unit)


Overview of theoretical studiesOverview of theoretical studies

Efficiency of adsorption process Efficiency of adsorption process depends ondepends on– Temperature.Temperature.– Pore size structure of adsorber.Pore size structure of adsorber.– Polarity of adsorber.Polarity of adsorber.– Mobility of gases (gas phase / liquid Mobility of gases (gas phase / liquid

phase).phase). Equilibrium described by Henrys Equilibrium described by Henrys

constant.constant.


Henrys law and retention volumeHenrys law and retention volume

n = number of moles adsorbed [mol/kg]n = number of moles adsorbed [mol/kg] p = partial pressure of adsorptive [Pa]p = partial pressure of adsorptive [Pa] H = Henrys constant [mol/(kgH = Henrys constant [mol/(kg··Pa)]Pa)]

H determines the retention volume:H determines the retention volume:

n = H n = H pp

VVRetRet = H = H R R T T

mmAdsAds


Purification in the columnPurification in the column

C0

CN

VPVRet H ()

CN

= ½

C0

ads

t

RTm

VH Re

n = Hp


Simple adsorption modelSimple adsorption model

GasGas TTC C [K][K] PPC C [bar][bar]TTCC··PPCC

-0.5-0.5

[K[K··barbar-0.5-0.5]]

H [mol/(kgH [mol/(kg··Pa)]Pa)]

@ 77 K@ 77 KPore size Pore size

[[ÅÅ]]

ArAr 151151 4949 21.621.6 22××101022 6.8 6.8

NN22 126126 3434 21.621.6 22××101022 77

KrKr 209209 5555 28.228.2 22××101055 77

RnRn 377377 6363 47.647.6 10101414 ~8~8

Influence of pores is neglected.Influence of pores is neglected. Valid for adsorbers with wide pore Valid for adsorbers with wide pore

size distribution.size distribution.


At low temperatures: Strong binding of At low temperatures: Strong binding of radon to all surfaces.radon to all surfaces.

Easy trapping with activated carbon @ Easy trapping with activated carbon @ 77 K.77 K.

Problem: Problem: 222222Rn emanation due to Rn emanation due to 226226Ra!Ra!– Activated carbon „CarboAct“: Activated carbon „CarboAct“: 222222Rn Rn

emanation rate (0.3 emanation rate (0.3 0.1) mBq/kg. 0.1) mBq/kg.– ~100 times lower than other carbons.~100 times lower than other carbons.

NN22 purity purity <0.3 <0.3 Bq/mBq/m33 achieved. achieved.

Purification of LNPurification of LN2 2 from from 222222RnRn


Krypton is nobler than radon.Krypton is nobler than radon. Binding energies are smaller Binding energies are smaller Henrys Henrys

constants are much smaller.constants are much smaller. Moreover: Similar size of NMoreover: Similar size of N22/Kr./Kr.

– NN22 may displace adsorbed Kr. may displace adsorbed Kr.

– Adsorption efficiency drops down.Adsorption efficiency drops down. NN22 purification from Kr requires careful purification from Kr requires careful

selection of adsorber/temperature etc.selection of adsorber/temperature etc.

Purification of LNPurification of LN22 from Kr from Kr


Henrys constant and pore sizeHenrys constant and pore size

0 .01

0.1

1

10

100

1000

10000

100000

1e+06

1e+07

1e+08

1e+09

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Hen

ry c

oeffi

zien

t H [

mol

/(kg

*Pa)

]

Pore size b [ Angstroem ]

N itrogen

Krypton


Molecular sieves 5 Molecular sieves 5 ÅÅ/10 /10 Å Å (polar).(polar). Hydrophobic zeolites: low internal Hydrophobic zeolites: low internal

polarity, but small pores (polarity, but small pores (7 7 ÅÅ).). Carbo Act: low Carbo Act: low 222222Rn emanation rate, Rn emanation rate,

wide pore size distribution.wide pore size distribution. Activated carbons with enhanced Activated carbons with enhanced

fraction of pores around 7 fraction of pores around 7 Å Å (Charcoal (Charcoal Cloth FM1-250, CarboTech C38/2).Cloth FM1-250, CarboTech C38/2).

Carbosieve SIII (Carbon molecular Carbosieve SIII (Carbon molecular sieve: Only small pores (<40 sieve: Only small pores (<40 ÅÅ)).)).

Investigated adsorbersInvestigated adsorbers


Experimental setupExperimental setup

gas

liquid

600-L dewar with Kr-enriched (100 - 400 ppt)

liquid nitrogen

Mass spectr.

N2

6.0

Samplepurif ication

LN2, LAr

100/300-cm3 column filled w ith adsorber

bubbler

VRet H ()VP

C0

CN


Results / Breakthrough curvesResults / Breakthrough curves

T=77K (liquid phase)


ResultsResultsAdsorberAdsorber NN

H H [mol/Pa/kg][mol/Pa/kg]

Molecular sieves 5Molecular sieves 5Å/Å/1010Å (Roth)Å (Roth) only very small purifi-only very small purifi-cation effect observedcation effect observed

Hydrophobic zeolite, BEA-type, Hydrophobic zeolite, BEA-type, 5.3Å, Si/Al 5.3Å, Si/Al 200 200 (Südchemie(Südchemie))

4 4 ± 1± 1 (6 (6 ± 1± 1) ) 10 10-3-3

Hydrophobic zeolite, MFI-type, Hydrophobic zeolite, MFI-type, 6.6Å, Si/Al 6.6Å, Si/Al 75 (Zeochem)75 (Zeochem) 1 1 ± 1± 1 (6 (6 ± 2± 2) ) 10 10-3-3

Synthetic carbon CarboActSynthetic carbon CarboAct 1 1 ± 1± 1 (6 (6 ± 2± 2) ) 10 10-2-2

Synth. carbon CarboTech C38/2Synth. carbon CarboTech C38/2 8 8 ± 2± 2 (2 (2 ± 1± 1) ) 10 10-2-2

Charcoal Cloth FM1-250Charcoal Cloth FM1-250 13 13 ± ± 33

(3 (3 ± 1± 1) ) 10 10-2-2

Carbosieve SIIICarbosieve SIII 2 2 ± 1± 1 (8 (8 ± 1± 1) ) 10 10-2-2


Liquid phase purification is preferred Liquid phase purification is preferred from economical point of view, but:from economical point of view, but:– higher mobility in gas phase.higher mobility in gas phase.– faster diffusion in gas phase.faster diffusion in gas phase.

Low temperature required (exponential Low temperature required (exponential dependency of adsorption process on T). dependency of adsorption process on T).

Better results are expected for low Better results are expected for low temperature gas phase purification.temperature gas phase purification.

Liquid phase versus gas phaseLiquid phase versus gas phase


Two easy approaches for gas phase Two easy approaches for gas phase adsorption:adsorption:– high flow rate: No time for gas to cool down.high flow rate: No time for gas to cool down.

– Liquid argon cooling (TLiquid argon cooling (TLAr LAr = T= TLN2LN2 + 10 K). + 10 K).

CarboAct testedCarboAct tested– Liquid phase: Liquid phase: HH=0.06 mol/kg/Pa, =0.06 mol/kg/Pa, NN=1.=1.– Gas phase: Gas phase: HH=0.21 mol/kg/Pa, =0.21 mol/kg/Pa, NN=15=15..

Gas phase adsorption is very effective!Gas phase adsorption is very effective!

Removal of Kr from NRemoval of Kr from N22 in gas phase in gas phase

First resultsFirst results


222222Rn removal easy, even for liquid NRn removal easy, even for liquid N22..– Low Low 222222Rn emanation rate of the adsorber Rn emanation rate of the adsorber

required.required.

Ar removal by adsorption is impossible.Ar removal by adsorption is impossible.

Kr removal by adsorption is possible:Kr removal by adsorption is possible:– Gas phase adsorption is much more Gas phase adsorption is much more

effective.effective.– Further improvement by pore size tuning Further improvement by pore size tuning

expected. expected.

Purification of NPurification of N22 – Summary – Summary


Purification of ArPurification of Ar

Theory predicts very similar adsorption Theory predicts very similar adsorption behaviour for Ar and Nbehaviour for Ar and N22..

However THowever TLAr LAr = T= TLN2LN2 + 10 K: Adsorption + 10 K: Adsorption at higher temperatures less efficient.at higher temperatures less efficient.

T T 100 K required for gas phase 100 K required for gas phase adsorption.adsorption.

222222Rn removal should not be a problem.Rn removal should not be a problem.


Measurements ofMeasurements of 222 222Rn in argonRn in argon

LNLN22 class 4.0 class 4.0CCRnRn ~ 50 µBq/m ~ 50 µBq/m33

No.No. GasGas AmountAmount CCRnRn in trap 1 [mBq/m in trap 1 [mBq/m33]] CCRnRn in trap 2 [mBq/m in trap 2 [mBq/m33]]

11 Ar 4.6Ar 4.6 117 m117 m33 2.9 2.9 ± 0.2± 0.2 ––

22 Ar 4.6Ar 4.6 141 m141 m33 0.20 0.20 ± 0.02± 0.02 <0.0005 (90% CL) *<0.0005 (90% CL) *

33 Ar 5.0Ar 5.0 200 m200 m33 6.0 6.0 ± 0.1± 0.1 0.60 0.60 ± 0.02 **± 0.02 **

* gas phase purification ** * gas phase purification ** liquid phase purificationliquid phase purification 8.5 8.5 ± 0.1 mBq/m± 0.1 mBq/m3 3 at truck filling timeat truck filling time


Towards Towards 222222Rn-free argonRn-free argon Gas phase purification works well.Gas phase purification works well.

– No breakthrough observed!No breakthrough observed! Liquid phase purification also works.Liquid phase purification also works.

– but efficiency <100%.but efficiency <100%.– Scaling of adsorber column required.Scaling of adsorber column required.

Alternative approach (also for NAlternative approach (also for N22):):– Avoid primary Avoid primary 8585Kr/Kr/222222Rn (or let it decay).Rn (or let it decay).– Requires storage in low Requires storage in low 222222Rn environment.Rn environment.– Investigation of Investigation of 222222Rn emanation of storage Rn emanation of storage

tanks.tanks.– Tests of supply chain for NTests of supply chain for N22/Ar./Ar.


Conclusion and outlookConclusion and outlook Selection of adsorber candidates (based Selection of adsorber candidates (based

on theoretical studies) finished.on theoretical studies) finished. All adsorber tested for LNAll adsorber tested for LN22 adsorption. adsorption. Gas phase adsorption tests are ongoing.Gas phase adsorption tests are ongoing. CarboAct still very attractive due to low CarboAct still very attractive due to low

222222Rn emanation rate.Rn emanation rate. Argon purification tests started.Argon purification tests started. Alternative concept: No purification - Alternative concept: No purification -

instead avoiding contaminations!instead avoiding contaminations! Planned: Test of full gas supply chain.Planned: Test of full gas supply chain.

Operation of bare Ge-diodes in liquid nitrogen / argon - Purification of N 2 /Ar

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