Zuzel LAUNCH

Low-level techniques applied in experiments looking for rare events: 

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

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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. Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction

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2. Germanium spectroscopy Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 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) 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 Highly sensitive Ge spectroscopy is a perfect tool for material screening

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2. Germanium spectroscopy Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 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 Sensitivity: ~10 Bq/kg

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

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

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

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3. Radon detection Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 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

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3. Radon detection Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction Proportional counters 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)

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

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

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3. Radon detection Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 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) Bx IV foil: bulk ≤ 15 Bq/kg surface ≤ 0.8 Bq/m2 total = (16  4) Bq/kg (1.2 ppt U eqiv.)

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

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3. Radon detection Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 222Rn daughters on surfaces (M. Wojcik) 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 Copper cleaning tests Etching removes most of 210Pb and 210Bi (> 98 %) but not 210Po 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

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4. Mass spectrometry Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction Noble gas mass spectrometer Detection limits: Ar: 10-9 cm3 Kr: 10-13 cm3 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.

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

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

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5. Conclusions Germanium spectroscopy Radon detection Mass spectrometry Conclusions Introduction 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.

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