Low-level techniques applied in experiments looking for rare events

1. Low-level techniques applied in experiments looking for rare events Introduction Germanium spectroscopy Radon detection Grzegorz Zuzel Max Planck Institute for Nuclear Physics, Heidelberg, Germany Mass spectrometry Conclusions

2. 1. Introduction • Low-level techniques: experimental techniques which allow to investigate very low activities of natural and artificially produced radio-isotopes. • material screening (Ge spectroscopy, ICPMS, NA) • surface screening (,, spectroscopy) • study of radioactive noble gases (emanation, diffusion) • purification techniques (gases, liquids) • background events rejection techniques • modeling of background in experiments (Monte Carlo) • Low-level techniques are “naturally” coupled with the experiments looking for rare events (detection of neutrinos, search for dark matter, search for 0ν2 decay, search for proton decay, ...), where the backgrounds identification and reduction plays a key role. Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions

3. 2. Germanium spectroscopy • Germanium spectroscopy is one of the most powerful techniques to identify γ-emmiters (U/Th chain, 40K, 60Co,...). • excellent energy resolution (~ 2 keV) • high purity detectors (low intrinsic background) Introduction Germanium spectroscopy • In order to reach high sensitivity it is necessary: • reduce backgrounds originating from external sources • - active/passive shielding (underground localizations) • - reduction of radon in the sample chamber • assure (reasonably) large volumes of samples • assure precise calculations/measurements of detection efficiencies Radon detection Mass spectrometry Conclusions Highly sensitive Ge spectroscopy is a perfect tool for material screening

4. 2. Germanium spectroscopy GeMPIs at GS (3800 m w.e.) • GeMPI I operational since 1997 (MPIK) • GeMPI II built in 2004 (MCavern) • GeMPI III constructed in 2007 (MPIK/LNGS) • Worlds most sensitive spectrometers GeMPI I: • Crystall: 2.2 kg, r = 102 % • Bcg. Index (0.1-2.7 MeV): 6840 cts/kg/year • Sample chamber: 15 l Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Sensitivity: ~10 Bq/kg

5. 2. Germanium spectroscopy Detectors at MPI-K: Dario, Bruno and Corrado Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions MPI-K LLL: 15 m w.e. Sensitivity: ~1 mBq/kg

6. 2. Germanium spectroscopy Selected results: different materials Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Specific activities in [mBq/kg] G. Heusser et al.

7. 2. Germanium spectroscopy Selected results: steel for the GERDA cryostat (MPIK/LNGS) Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions

8. 3. Radon detection • Radon 222Rn and its daughters form one of the most dangerous source of background in many experiments • inert noble gas • belongs to the 238U chain (present in any material) • high diffusion and permeability • wide range of energy of emitted radiation (with the daughters) • surface contaminations with radon daughters (heavy metals) • broken equilibrium in the chain at 210Pb level Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions

9. 3. Radon detection Proportional counters Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions • Developed for the GALLEX/GNO experiment • Hand-made at MPI-K (~ 1 cm3 active volume) • In case of 222Rn only α-decays are detected • 50 keV threshold • - bcg: 0.1 – 2 cpd • - total detection efficiency of ~ 1.5 • Absolute detection limit ~ 30 µBq (15 atoms)

10. 3. Radon detection 222Rn in gases (N2/Ar) - MoREx • 222Rn adsorption on activated carbon • several AC traps available (MoREx/MoRExino) • pre-concentration from 100 – 200 m3 • purification is possible (LTA) 222Rn detection limit: ~0.5 Bq/m3 (STP) [1 atom in 4 m3] Introduction Germanium spectroscopy A combination of 222Rn pre-concentration and low-background counting gives the most sensitive technique for radon detection in gases Radon detection Great importance for BOREXINO, GERDA, EXO, XENON, XMASS, WARP, CLEAN, … Mass spectrometry Conclusions 222Rn/226Ra in water - STRAW • 222Rn extraction from 350 liters • 222Rn and 226Ra measurements possible 222Rn detection limit: ~0.1 mBq/m3 226Ra detection limit: ~0.8 mBq/m3 Production rate: 100 m3/h 222Rn ≤0.5 Bq/m3 (STP)

11. 3. Radon detection 222Rn emanation and diffusion Blanks: 20 l  50 Bq 80 l  80 Bq Introduction Germanium spectroscopy Absolute sensitivity ~100 Bq [50 atoms] Radon detection Mass spectrometry Conclusions Sensitivity ~ 10-13cm2/s

12. 3. Radon detection BOREXINO nylon foil 1 ppt U required (~12 Bq/kg for 226Ra) Ddry = 2x10-12 cm2/s (ddry= 7 m) Dwet = 1x10-9 cm2/s (dwet = 270 m) Adry= Asf + 0.14  Abulk Awet= Asf +Abulk Separation of the bulk and surface 226Ra conc. was possible through 222Rn emanation Very sensitive technique: (CRa ~ 10 Bq/kg) Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Bx IV foil: bulk ≤ 15 Bq/kg surface ≤ 0.8 Bq/m2 total = (16  4)Bq/kg (1.2 ppt U eqiv.)

13. 3. Radon detection Online 222Rn monitoring: electrostatic chamber (J. Kiko) Introduction Germanium spectroscopy • 222Rn monitoring • in gases • Shape adopted to • the electrical field • Volume: 750 l • Sensitivity goal: • ~ 50 Bq/m3 Radon detection Mass spectrometry Conclusions

14. Screening of 210Po with an alpha spectrometer 50 mm Si-detector, bcg ~ 5 /d (1-10 MeV) sensitivity ~ 20 mBq/m2 (100 mBq/kg, 210Po) Screening of 210Bi with a beta spectrometer 250 mm Si(Li)-detectors, bcg ~ 0.18/0.40 cpm sensitivity ~ 10 Bq/kg Screening of 210Pb (46.6 keV line) with a gamma spectrometer 25 % - n-type HPGe detector with an active and a passive shield sensitivity ~ 20 Bq/kg Only small samples can be handled – artificial contamination needed: e.g. discs loaded with 222Rn daughters 3. Radon detection 222Rn daughters on surfaces (M. Wojcik) Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Copper cleaning tests • Etching removes most of 210Pb and 210Bi (> 98 %) but not210Po • Electropolishing is more effective for all elements but proper • conditions have to be found (e.g. 210Po reduction from 30 up to 200) • Etching: 1% H2SO4 + 3% H2O2 Electropolishing: 85 % H3PO4 + 5 % 1-butanol

15. 4. Mass spectrometry Noble gas mass spectrometer VG 3600 magnetic sector field spectrometer. Used to investigate noble gases in the terrestial and extra-terrestial samples. Adopted to test the nitrogen purity and purification methods. Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Detection limits: Ar: 10-9 cm3 Kr: 10-13 cm3

16. 4. Mass spectrometry Ar and Kr in nitrogen for the BOREXINO experiment (SOL) Introduction Requirements: 222Rn: < 7 Bq/m3 39Ar: < 0.5 Bq/m3 85Kr: < 0.2 Bq/m3 Ar: < 0.4 ppm Kr: < 0.1 ppt Germanium spectroscopy Radon detection Mass spectrometry Conclusions 222Rn: 8 Bq/m3 Results: Ar: 0.01ppm Kr: 0.02 ppt

17. 4. Mass spectrometry Kr in nitrogen: purification tests Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions

18. Low-level techniques have “natural” application in experiments looking for rare events. There is a long tradition and a lot of experience at MPI-K in this field (GALLEX/GNO, HDM, BOREXINO, GERDA). Several detectors and experimental methods were developed allowing measurements even at a single atoms level. Some of the developed/applied techniques are world-wide most sensitive (Ge spectroscopy, 222Rn detection). The ”low-level sub-group” is a part of the new division of M. Lindner. 5. Conclusions Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions

19. 2. Germanium spectroscopy Comparison of different detectors Introduction Germanium spectroscopy Radon detection Mass spectrometry Conclusions Slide from M. Hult