Low-level techniques applied in experiments looking for rare events
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Transcript of Low-level techniques applied in experiments looking for rare events
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
Low-level techniques applied in experiments looking for
rare eventsGermanium spectroscopy
Grzegorz ZuzelMax Planck Institute for Nuclear Physics, Heidelberg, Germany
Radon detection
Mass spectrometry
Conclusions
Introduction
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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.
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
2. Germanium spectroscopy
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Introduction
Selected results: different materials
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
Teflon0.023
0.0150.021
0.0090.54 0.11
Kapton cable ≤ 4 9 6 130 60Specific activities in [mBq/kg]
G. Heusser et al.
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
2. Germanium spectroscopy
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Introduction
Selected results: steel for the GERDA cryostat (MPIK/LNGS)
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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)
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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/h222Rn ≤0.5 Bq/m3 (STP)
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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.)
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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 250 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
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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.
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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 ppmKr: < 0.1 ppt
222Rn: 8 Bq/m3
Results: Ar: 0.01 ppm Kr: 0.02 ppt
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
4. Mass spectrometry
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Introduction
Kr in nitrogen: purification tests
gas
liquid
600-L dewar with Kr-enriched liquid
nitrogen (Westfalen A.G.)
Mass spectr.
N26.0
Samplepurification
LAr300-cm3 column
filled with adsorber bubbler
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
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.
LAUNCH - Low-energy, Astroparticle Underground, Neutrino physics and Cosmology in Heidelberg, 21-23.03.2007
2. Germanium spectroscopy
Germanium spectroscopy
Radon detection
Mass spectrometry
Conclusions
Introduction
Comparison of different detectors
Slide from M. Hult