Underground Physics 2 Background reduction techniques Susana Cebrián, 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 effective mass ~50 meV ~10 -3 events / keV / kg / y ~1 event / t / y The study of background and its reduction is very important in Underground Physics ISAPP2010, S. Cebrián
UP2: Background reduction techniques � Background components � Strategies for background reduction
Background components Deep underground operation Cosmic rays Only muons survive: flux At sea level reduced by several orders of magnitude Muon flux: ~10 2 m -2 s -1 (mean energy ~4 GeV) ISAPP2010, S. Cebrián
Background components Rock, components in the set-up, .. Radioactivity in materials • Primordial: α, β, γ 232 Th, 238 U, 40 K, ... external internal • Cosmogenic: cosmic rays at sea level can induce long-lived radioactive isotopes in set-up materials (mainly by neutron spallation) 60 Co in copper or germanium • Production rates R of induced isotopes depend on: • production cross sections σ ∫ ∝ σ φ R dE ( E ) ( E ) n • flux of cosmic rays φ • Activities A induced on a material depend also on: • history of exposure and decay − λ − λ = − t t A R ( 1 e exp ) e dec ISAPP2010, S. Cebrián
Background components Radon in air 222 Rn from 238 U chain • Gaseous • T 1/2 =3.8 days • α emitter • Daughters: 214 Pb, 214 Bi → gamma emissions (351.9 keV, 609.3 keV, …) Typical levels: • Outside: a few Bq/m 3 • Underground facility: higher concentration ISAPP2010, S. Cebrián
Background components Neutrons (n, γ ) capture; inelastic scattering → secondary particles elastic scattering → mimic dark matter signal 0,6 � Production in rock 0,5 (or other materials) 0,4 dN/dE 0,3 • Spontaneous fission: 238 U 0,2 • ( α ,n) reactions on light nuclei 0,1 0 0,001 0,01 0,1 1 10 100 E (MeV) spectrum: up to several MeV, flux: ~10 -6 n/cm 2 /s ISAPP2010, S. Cebrián
Background components Neutrons � Muon-induced in rock and shields • Processes: spallation, photonuclear Muon spectrum at 2500 mwe <E μ >=216 GeV spectrum: up to GeV, flux : ~10 -9 n/cm 2 /s ( ~3 km w.e.) ISAPP2010, S. Cebrián
Background components 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 … ISAPP2010, S. Cebrián
Background components 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) ISAPP2010, S. Cebrián
UP2: Background reduction techniques � Background components � Strategies for background reduction ISAPP2010, S. Cebrián
Strategies for background reduction Passive shields Lead and copper (high Z , high density) to attenuate external γ radiation Cadmium, Boron to Water, polyethylene (low A) to moderate capture neutrons Archaeological lead: (high cross section) neutrons extremely low content in 210 Pb 50 g/cm 2 of CH 2 → reduction of ~6 orders of magnitude IGEX ( International Germanium Experiment ) at Canfranc ISAPP2010, S. Cebrián
Strategies for background reduction Passive shields N 2 gas is flushed into a plastic bag creating an overpressure to avoid radon intrusion IGEX ( International Germanium Experiment ) at Canfranc ISAPP2010, S. Cebrián
Strategies for background reduction Segmented detector Active shields allows to work in anticoincidence to Plastic scintillators reject events hitting Germanium detectors covering the top and simultaneously more inside liquid Argon sides of the set-up to than one crystal scintillator working in veto muons anticoincidence CUORE ( Cryogenic GERDA ( GERmanium Detector Underground Observatory for IGEX ( International Array) at GranSasso Rare Events ) at Gran Sasso Germanium Experiment ) at Canfranc ISAPP2010, S. Cebrián
Strategies for background reduction Radiopurity control � Material treatments: Purification: distillation, zone melting Acid cleaning, electropolishing Electroforming � Screening and selection of materials looking for ultra-low levels of radioactivity: below ppb in Th and U and ppm in K Average concentrations in 1 ppb Th = 4 mBq/kg 232 Th continental crust in Bq/kg 1 ppb U = 12.4 mBq/kg 238 U 40 K 850 1 ppb K = 0.030 mBq/kg 40 K 232 Th 44 238 U 36 ISAPP2010, S. Cebrián
Strategies for background reduction Radiopurity control • 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 Set-up of Ge detector for ultra-low + small samples required activity measurements in Canfranc • Neutron Activation Analysis (NAA) - samples become activated ISAPP2010, S. Cebrián
Strategies for background reduction Radiopurity control 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 ISAPP2010, S. Cebrián
Strategies for background reduction Radiopurity control Database of radiopurity measurements in many materials http://radiopurity.in2p3.fr ISAPP2010, S. Cebrián
Strategies for background reduction Techniques for background discrimination Signal: nuclear recoil � Direct detection of WIMPs in galactic halo spectrum, continuum with exponential decay Discrimination of nuclear from electronic recoils: below ~50 keV relative yield of heat and charge/light signals is different Incident WIMP Scattered WIMP LIGHT Detector-Target HEAT Nuclear recoil LIGHT CHARGE ISAPP2010, S. Cebrián
Strategies for background reduction Techniques for background discrimination Signal: two electrons, � Identification of double beta decay peak at Q (2-4 MeV) Analysis of topology of events to reject background Double beta event in NEMO3 experiment: tracks of electrons registered in Geiger cells Expected double beta event in NEXT experiment: tracks of electrons registered in xenon gas TPC ISAPP2010, S. Cebrián
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
Summary: background components � Cosmic rays Muons � Radiation and particles from environment • Radioactivity in materials (primordial or cosmogenic): 232 Th, 238 U, 40 K, 60 Co, 210 Pb, ... α, β, γ • Radon 222 Rn γ fission, ( α ,n) reactions in rock • Neutrons: induced by muon (n, γ ) capture; inelastic scattering elastic scattering ISAPP2010, S. Cebrián
Recommend
More recommend
