GERDA General Meeting,

1
Nitrogen and argon radiopurity

• Grzegorz Zuzel
• for TG11
• MPI-K Heidelberg

2
Outline

• 39Ar, 42Ar and 85Kr
• - in argon
• - in nitrogen
• 222Rn in nitrogen and argon
• - concentrations in
  different qualities
• - emanation from the
  storage tanks
• - purification tests
• Purification system for LN2/LAr (?)
• Conclusions and planned activity

3
39Ar, 42Ar and 85Kr
Decay mode Source Concentration (STP)
222Rn T1/2 3.8 d ?, ?, ? Primordial 238U 1 - ?00 Bq/m3 air
85Kr T1/2 10.8 y ? (687 keV) , ? 235U fission (nuclear fuel reprocessing plants) 1.4 Bq/m3 air 1.2 MBq/m3 Kr
39Ar T1/2 269 y ? (565 keV) Cosmogenic 17 mBq/m3 air 1.8 Bq/m3 Ar
42Ar T1/2 32.9 y ? (600 keV) Cosmogenic 0.5 µBq/m3 air 50 µBq/m3 Ar

• Q-value of 39Ar and 85Kr below 700 keV relevant
  in case of dark matter detection
• Dead-time could be a problem when Ar
  scintillation is used (slow decay time 1µs)
• 42Ar is naturally low

4
39Ar and 85Kr in argon

• Dead time
• Assume 10 m3 active volume
• 39Ar rate 15 kHz ? 1.5 Fine!
• 85Kr rate not higher ? 0.3 ppm Kr required
• Results from a 2.3 kg WARP test stand 0.6
  ppm
• Other measurement possibilities
• mass spectrometry of Kr
• direct counting of 85Kr

5
39Ar in argon

• 39Ar and dark matter detection (H. Simgen)

6
39Ar, 42Ar and 85Kr in argon

• 39Ar gives a significant increase of background
  in the low energy region
• Ar scintillation used as a veto helps, however...
• background index stays at the level
• of 1 cts/keVrec/kg/y, which limits the
  ability for dark matter detection
• Kr present in Ar at the ppm level would give
  similar contribution (through 85Kr)
• Contribution from 42Ar gives 4?10-5
  events/(kg?y?keV) at Q?? and is also negligible
  for dark matter

7
39Ar and 85Kr in nitrogen
Description Ar ppm Kr ppt
MESSER RPN2 (4.0, used in HD) 200 1 680
Air Liquide RPN2 (4.0, used at GS) 10 40
Air Liquide HPN2 10 30
Air Liquide 6.0 14 13
Linde Worms (7.0) 0.015 0.05
SOL Mantova (6.0) 0.005 0.04
Westfalen AG Hörstel (6.0) 0.0005 0.06
1 ppt Kr 1 µBq/m3 (STP) 1 ppm Ar 1.2 µBq/m3
(STP)
If LN2 is used 39Ar and 85Kr should not affect
detection of dark matter
8
Outline

• 39Ar, 42Ar and 85Kr
• - in argon
• - in nitrogen
• 222Rn in nitrogen and argon
• - concentrations in
  different qualities
• - emanation from the
  storage tanks
• - purification tests
• Purification system for LN2/LAr (?)
• Conclusions and planned activity

9
222Rn in nitrogen and argon
According to the simulations
222Rn in LN2/LAr at the level of 0.3 mBq/m3
? B 10-4 cts/keV/kg/y 0.3 mBq/m3
liquid ? 0.5 µBq/m3 gas (STP)
10
222Rn concentration in Ar of different qualities
(Westfalen AG)
Quality 222Rn concentration mBq/m3 (STP) Remarks
5.0 8.4 0.2 At the filling time
6.0 0.38 0.03 At the filling time
Specification 0.5 ? 10-3 mBq/m3 gas (STP)
11
222Rn concentration in N2 of different qualities
Quality 222Rn concentration mBq/m3 (STP)
4.0 0.05 0.2
7.0 0.001

• Conclusions
• initial 222Rn content in nitrogen is lower than
  in argon
• of a similar class
• however 222Rn decays (T1/2 3.8 d) emanation
  from the
• storage tank limits the final purity

12
222Rn emanation from storage tanks
Tank description Volume m3 222Rn emanation rate mBq
Storage tank for argon 5.0 (Westfalen) 2.9 177 7
Storage tank for argon 6.0 (Westfalen) 0.66 42 2
Special storage tank for ultra-pure gases (Linde) 3 2.7 0.3
CRn ? 1µBq/m3 Ar (STP)
It should be possible to have argon with low
enough 222Rn concentration without purification
13
Outline

• 39Ar, 42Ar and 85Kr
• - in argon
• - in nitrogen
• 222Rn in nitrogen and argon
• - concentrations in
  different qualities
• - emanation from the
  storage tanks
• - purification tests
• Purification system for LN2/LAr (?)
• Conclusions and planned activity

14
Nitrogen purification from 222Rn
Liquid nitrogen purification plant works at GS
for Borexino
222Rn in N2 before purif. 50 µBq/m3
222Rn in N2 after purif. lt 0.3 µBq/m3
Red. factor gt 100 Ads. mass 2 kg
15
Argon purification from 222Rn
MoREx Mobile Radon Extraction Unit
222Rn in Ar before purif. 200 µBq/m3
222Rn in Ar after purif. lt 0.5 µBq/m3
Red. factor gt 400 Ads. mass 0.15 kg
16
Ar purification from 222Rn
Purification in gas phase
Init. CRn mBq/m3 Final CRn mBq/m3 Red. factor R R/MAds 1/kg
0.20 0.02 lt 0.0005 (90 CL) gt 400 gt 2700
Purification in liquid phase
Init. CRn mBq/m3 Final CRn mBq/m3 Red. factor R R/MAds 1/kg
5.4 0.1 0.60 0.02 9 300
0.10 0.01 0.006 0.001 17 570
Average Average 13 400
17
LN2/LAr purification from 222Rn

• It is possible to effectively remove radon from
  argon / nitrogen
• Removal of 222Rn from liquid phase is somewhat
  less effective than from gas phase

18
Outline

• 39Ar, 42Ar and 85Kr
• - in argon
• - in nitrogen
• 222Rn in nitrogen and argon
• - concentrations in
  different qualities
• - emanation from the
  storage tanks
• - purification tests
• Purification system for LN2/LAr (?)
• Conclusions and planned activity

19
Purification system for LN2/LAr (?)

• Having clean storage tanks we do not need a
  purification system, but it has several
  advantages
• Operational conditions for the cryo-plant are
  more relaxed (safety, shorter access time)
• The same system can be used for LN2 and LAr
  purification
• We know how to build such a plant (Borexino
  plant)
• Optimal column size 200 g (LArGe) / 1 kg
  (GERDA) of CarboAct (need some further tests)

20
Purification system for LAr
21
Conclusions

• LN2 allows investigation of 0?2?? (and dark
  matter)
• 39Ar limits ability to detect dark matter if LAr
  is used
• It is possible to buy 222Rn-free LN2 (proven in
  frame of Borexino)
• 222Rn content in fresh argon is higher than in
  nitrogen of similar quality
• It should be also possible to have 222Rn-free
  LAr without purification still to be confirmed
• 222Rn removal from argon as effective as from
  nitrogen
• From the radiopurity point of view for GERDA
  0?2?? experiment LAr and LN2 can be used

22
Planned activity

• Measure 222Rn content in argon after long storage
  in the tank of known emanation
• 85Kr/Kr measurement in argon
• Test of a purification system for LArGe (design
  for GERDA)
• Full supply chain test of ultra-pure nitrogen for
  Borexino results interesting also for GERDA