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Underground Physics 2 Background reduction techniques Susana Cebrin, - - PowerPoint PPT Presentation

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Underground Physics 2 Background reduction techniques Susana Cebrin, Universidad de Zaragoza What is common in the detection of nuclear Double Beta Decay or Dark Matter ? Very low probability: Rare Events Cross sections 10 -10 pb Neutrino

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SLIDE 1

Underground Physics 2

Background reduction techniques

Susana Cebrián, Universidad de Zaragoza

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SLIDE 2

What is common in the detection of nuclear Double Beta Decay or Dark Matter?

Cross sections 10-10 pb

~1 event / t / y ~10-3 events / keV / kg / y

Neutrino effective mass ~50 meV

Very low probability: Rare Events

The study of background and its reduction is very important in Underground Physics

ISAPP2010, S. Cebrián

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SLIDE 3

UP2: Background reduction techniques

Background components Strategies for background reduction

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SLIDE 4

Background components

Cosmic rays

Deep underground operation At sea level Muon flux: ~102 m-2 s-1 (mean energy ~4 GeV) Only muons survive: flux reduced by several orders of magnitude

ISAPP2010, S. Cebrián

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SLIDE 5

Background components

Radioactivity in materials

  • Primordial:

232Th, 238U, 40K, ...

α, β, γ

internal external

  • Production rates R of induced isotopes depend on:
  • production cross sections σ
  • flux of cosmic rays φ
  • Activities A induced on a material depend also on:
  • history of exposure and decay

∫

∝ ) ( ) ( E E dE R

n

φ σ

dec

t t

e e R A

λ λ − −

− = ) 1 (

exp

  • Cosmogenic: cosmic rays at sea level can induce long-lived radioactive

isotopes in set-up materials (mainly by neutron spallation)

60Co in copper or germanium

Rock, components in the set-up, ..

ISAPP2010, S. Cebrián

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SLIDE 6

Background components

Radon in air

222Rn from 238U chain

  • Gaseous
  • T1/2=3.8 days
  • α emitter
  • Daughters: 214Pb, 214Bi → gamma

emissions (351.9 keV, 609.3 keV, …) Typical levels:

  • Outside: a few Bq/m3
  • Underground facility: higher

concentration

ISAPP2010, S. Cebrián

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SLIDE 7

Background components

Neutrons

(n,γ) capture; inelastic scattering → secondary particles elastic scattering → mimic dark matter signal

Production in rock (or other materials)

  • Spontaneous fission: 238U
  • (α,n) reactions on light nuclei

0,1 0,2 0,3 0,4 0,5 0,6 0,001 0,01 0,1 1 10 100 E (MeV) dN/dE

spectrum: up to several MeV, flux: ~10-6 n/cm2/s

ISAPP2010, S. Cebrián

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SLIDE 8

Background components

Neutrons Muon-induced in rock and shields

  • Processes: spallation, photonuclear

spectrum: up to GeV, flux: ~10-9 n/cm2/s (~3 km w.e.)

Muon spectrum at 2500 mwe <Eμ>=216 GeV ISAPP2010, S. Cebrián

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  • Y. F. Wang et al, Predicting neutron production from cosmic-ray muons, PRD 64 (2001) 013012.
  • V. A. Kudryavtsev et al, Simulation of muon-induced neutron flux at large depths underground, NIMA

505 (2003) 688-698.

  • H. Wulandari et al, Neutron Background studies for the CRESST Dark Matter experiment, [arXiv:hep-

ex/0401032].

  • J. M. Carmona et al, Neutron background at the Canfranc Underground Laboratory and its contribution

to the IGEX-DM dark matter experiment, AP 21 (2004) 523-533.

  • M. L. Carson et al, Neutron background in large-scale xenon detectors for dark matter searches, AP

21 (2004) 667-687.

  • M. L. Carson et al, Simulations of neutron background in a time projection chamber relevant to dark

matter researches, NIMA 546 (2005) 509-522.

  • P. F. Smith et al, Simulation studies of neutron shielding, calibration and veto systems for gaseous

dark matter detectors, AP 22 (2005) 409-420.

  • H. M. Araujo et al, Muon-induced neutron production and detection with GEANT4 and FLUKA, NIMA

545 (2005) 398-411.

  • C. Galbiati, J. F. Beacom, Measuring the Cosmic Ray Muon-Induced Fast Neutron Spectrum by (n,p)

Isotope Production Reactions in Underground Detectors, PRC 72 (2005) 025807.

  • D. M. Mei, A. Hime, Muon-Induced Background Study for Underground Laboratories, PRD 73 (2006)

053004.

  • V. A. Kudryavtsev, Neutron background in underground particle astrophysics experiments, AIP CP

897 (2007) 99-104 (LRT2006).

  • L. Pandola et al, Monte Carlo evaluation of the muon-induced background in the GERDA double beta

decay experiment, NIMA 570 (2007) 149-158.

  • M. G. Marino et al, Validation of spallation neutron production and propagation within GEANT4, NIMA

582 (2007) 611-620 …

Background components

ISAPP2010, S. Cebrián

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Compilation of measurements of radon levels and muon, gamma and neutron fluxes at the four european underground labs (Boulby (UK), Canfranc (Spain), Gran Sasso (Italy), Modane (France))

http://www.lngs.infn.it/lngs_infn/contents/lngs_en/research/europe/web_ILIAS_A1/JRA1/ilias- last/muon.htm

Integrated Large Infrastructures for Astroparticle Science (2004-2009)

Background components

ISAPP2010, S. Cebrián

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SLIDE 11

UP2: Background reduction techniques

Background components Strategies for background reduction

ISAPP2010, S. Cebrián

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SLIDE 12

Strategies for background reduction

Passive shields

Lead and copper (high Z , high density) to attenuate external γ radiation

IGEX (International Germanium Experiment) at Canfranc

Water, polyethylene (low A) to moderate neutrons Cadmium, Boron to capture neutrons (high cross section) Archaeological lead: extremely low content in 210Pb 50 g/cm2 of CH2 → reduction of ~6 orders of magnitude

ISAPP2010, S. Cebrián

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Strategies for background reduction

N2 gas is flushed into a plastic bag creating an

  • verpressure to

avoid radon intrusion

Passive shields

ISAPP2010, S. Cebrián

IGEX (International Germanium Experiment) at Canfranc

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SLIDE 14

Strategies for background reduction

Segmented detector allows to work in anticoincidence to reject events hitting simultaneously more than one crystal Germanium detectors inside liquid Argon scintillator working in anticoincidence

Active shields

IGEX (International Germanium Experiment) at Canfranc

Plastic scintillators covering the top and sides of the set-up to veto muons

GERDA (GERmanium Detector Array) at GranSasso CUORE (Cryogenic Underground Observatory for Rare Events) at Gran Sasso

ISAPP2010, S. Cebrián

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Strategies for background reduction

Radiopurity control Screening and selection of materials looking for ultra-low levels of radioactivity: below ppb in Th and U and ppm in K

1 ppb Th = 4 mBq/kg 232Th 1 ppb U = 12.4 mBq/kg 238U 1 ppb K = 0.030 mBq/kg 40K

Average concentrations in continental crust in Bq/kg

40K

850

232Th

44

238U

36

Material treatments: Purification: distillation, zone melting Acid cleaning, electropolishing Electroforming

ISAPP2010, S. Cebrián

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SLIDE 16

Set-up of Ge detector for ultra-low activity measurements in Canfranc

Strategies for background reduction

  • Ge spectroscopy underground:

+ very good energy resolution + good detection efficiency + radiopure material + non-destructive technique

  • long measurements needed
  • massive samples convenient
  • Inductively Coupled Plasma Mass

Spectrometry (ICPMS) and Glow Discharge Mass Spectrometry (GDMS) + fast + small samples required

  • Neutron Activation Analysis (NAA)
  • samples become activated

Radiopurity control

ISAPP2010, S. Cebrián

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SLIDE 17
  • M. Laubenstein et al, "Underground measurements of radioactivity", Applied Radiaiton

and Isotopes 61 (2004) 167

  • D. S. Leonard et al, "Systematic study of trace radioactive impurities in candidate

construction materials for EXO-200", NIMA 591 (2008) 490 C Arpesella et al, "Measurements of extremely low radioactivity levels in BOREXINO",

  • Astropart. Phys. 18 (2002) 1.
  • W. Maneschg et al, "Measurements of extremely low radioactivity levels in stainless steel

for GERDA", NIMA 593 (2008) 448

  • D. Budjas et al, "Highly sensitive gamma-spectrometers of GERDA for material

screening: Part I", arxiv:0812.0723

Strategies for background reduction

Radiopurity control

ISAPP2010, S. Cebrián

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SLIDE 18

Database of radiopurity measurements in many materials http://radiopurity.in2p3.fr

Strategies for background reduction

Radiopurity control

ISAPP2010, S. Cebrián

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Strategies for background reduction

Techniques for background discrimination Direct detection of WIMPs in galactic halo

Signal: nuclear recoil spectrum, continuum with exponential decay below ~50 keV

Incident WIMP Scattered WIMP Nuclear recoil Detector-Target

HEAT CHARGE LIGHT LIGHT

Discrimination of nuclear from electronic recoils: relative yield of heat and charge/light signals is different

ISAPP2010, S. Cebrián

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SLIDE 20

Strategies for background reduction

Identification of double beta decay

Signal: two electrons, peak at Q (2-4 MeV) Analysis of topology of events to reject background

Techniques for background discrimination

Double beta event in NEMO3 experiment: tracks of electrons registered in Geiger cells

ISAPP2010, S. Cebrián

Expected double beta event in NEXT experiment: tracks of electrons registered in xenon gas TPC

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Strategies for background reduction

Monte Carlo simulations

Monte Carlo simulations of the interaction of background particles in matter allow to evaluate the response of a detector system to the different background components in order to:

  • Understanding of measured data → background models
  • Assessment of effect of background reduction strategies: shieldings,

vetoes, discrimination methods

  • Evaluation of sensitivity of future experiments

Tools: standard packages like GEANT4, FLUKA, MCNP, …

ISAPP2010, S. Cebrián

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SLIDE 22

Summary: background components

Cosmic rays Radiation and particles from environment Muons

  • Radon 222Rn
  • Neutrons:

fission, (α,n) reactions in rock induced by muon

  • Radioactivity in materials (primordial or cosmogenic):

232Th, 238U, 40K, 60Co, 210Pb, ...

(n,γ) capture; inelastic scattering elastic scattering α, β, γ γ

ISAPP2010, S. Cebrián

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– Passive shielding

  • Lead, copper γ
  • Water, polyethylene, cadmium

n

Summary: strategies for background reduction

– Active shielding:

  • Vetos μ
  • Anticoincidence

– Radiopurity of materials – Techniques for background rejection:

  • Topology of events
  • Simultaneous measurements of heat and light/ionization signals
  • …

Monte Carlo simulations of background

ISAPP2010, S. Cebrián

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References:

ISAPP2010, S. Cebrián

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References:

ISAPP2010, S. Cebrián