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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


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


2. 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




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




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.




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.




Radon detection

Mass spectrometry

Conclusions

Introduction

Selected results: steel for the GERDA cryostat (MPIK/LNGS)


3. Radon detection


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


3. Radon detection


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)


3. Radon detection


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)


3. Radon detection


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


3. Radon detection


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.)


3. Radon detection


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


3. Radon detection


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


4. Mass spectrometry


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.




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




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


5. Conclusions


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.




Radon detection

Mass spectrometry

Conclusions

Introduction

Comparison of different detectors

Slide from M. Hult

launch - low-energy, astroparticle underground, neutrino physics and cosmology in heidelberg,...

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