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 0?2? 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 ?-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
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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
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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
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2. Germanium spectroscopy
Selected results different materials
Introduction
228Th 226Ra 40K 210Pb
Copper 0.012 0.016 0.088
Lead DowRun 0.022 0.029 0.044 ? 0.014 (27? 4)?103
Ancient lead 0.072 0.045 0.27 1300
Teflon 0.023 ? 0.015 0.021 ? 0.009 0.54 ? 0.11
Kapton cable 4 9 ? 6 130 ? 60
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Specific activities in mBq/kg G. Heusser et al.
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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 a-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
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)
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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 10-13 cm2/s
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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.)
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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
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3. Radon detection
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

Introduction
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Copper cleaning tests

• Etching removes most of 210Pb and 210Bi (gt 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
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
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4. Mass spectrometry
Ar and Kr in nitrogen for the BOREXINO experiment
(SOL)
Introduction
Requirements 222Rn lt 7 ?Bq/m3 39Ar lt 0.5
?Bq/m3 85Kr lt 0.2 ?Bq/m3 Ar lt 0.4 ppm Kr lt
0.1 ppt
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
222Rn 8 ?Bq/m3 Results
Ar 0.01 ppm Kr 0.02 ppt
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4. Mass spectrometry
Kr in nitrogen purification tests
Introduction
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
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5. Conclusions

• 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.

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