Background studies for underground particle astrophysics ... · Background studies for underground particle astrophysics experiments ... EURECA (EDELWEISS + CRESST), XMASS ... (Na);

INT2005 - 23/06/2005 Vitaly Kudryavtsev 1

Background studies for undergroundparticle astrophysics experiments

V. A. Kudryavtsev

Department of Physics and AstronomyUniversity of Sheffield

for ILIAS andthe UK Dark Matter Collaboration


Outline

• ILIAS - European Programme: Integrated LargeInfrastructures for Astroparticle Science.

• Sources of background in underground experiments.• Neutrons from radioactivity and their suppression by

passive shielding.• Neutrons from cosmic-ray muons.• Gamma background and its suppression.• Rejection of backgrounds by active veto.• Summary.


ILIAS• ILIAS is funded by EU within the 6th Framework

Programme (FP6): cooperation of many EU institutionsinvolved in Astroparticle physics - http://ilias.in2p3.fr.Period of funding: April 2004 - March 2009.

• ILIAS was proposed under the coordination and review ofAPPEC - Astroparticle Physics European Coordination).

• 6 Networking activities (N), 3 Joint Research Initiatives(JRA), 1 Transnational Access (TA):– Networking activity - meetings and other contacts between

researchers;– JRA - joint R&D projects between institutions on common subjects;– TA - support the access of research teams to the major EU research

infrastructures - underground laboratories.


ILIAS activities• Networks:

– N2/DUSL - Deep Underground Science Laboratories - LNGS (GranSasso, Italy), LSM (Modane/Frejus, France), Canfranc (Spain),Boulby (UK): operational support to users; health and safety issues; public relations.

– N3/DMD - Direct Dark Matter Detection: assessment of different detector concepts; strategy for future large-scale dark matter experiments in Europe;

– N4/DBD - Search on Double Beta Decay: coordination of double beta decay community; bank of pure isotopes.

– N5/GWA - Gravitational Wave Antenna: exchange of information about existing detectors; develop methodology for joint observation and common strategy for

future detectors in Europe.


ILIAS activities• Networks:

– N6/ENTApP - Theoretical Astroparticle Physics: coordination and integration of theorists with a focus on areas of

experimental programmes, in particular those included in ILIAS; links with experimental groups.

• Joint Research Activities:– JRA1/LBT-DUSL - Low Background Techniques for Deep

Underground Science: Study of different backgrounds in the underground laboratories; ultra-low background facilities in the 4 labs.

– JRA2/IDEA - Integrated Double Beta Decay: isotope enrichment and study of promising nuclides; control of the background radioactivity.

– JRA3/STREGA - Study on Thermal Noise Reduction in GW Detectors: study of materials and new cryogenic technology for future detectors; study of thermo and photo elastic noise.


ILIAS activities• Transnational Access:

– TA1/TA-DUSL - Transnational access - Deep Underground ScienceLaboratories: access for user groups to the infrastructures.


Background studies in ILIAS

• N3 (Direct Dark Matter Detection - DMD) and JRA1 (LowBackground Techniques for Deep Underground Science -LBT-DUSL) have working groups dealing with backgroundstudies - joint working group has been formed. (AnotherWG on low radioactivity materials).

• One of the objectives of ILIAS: to disseminate a newknowledge as widely as possible through the participationin international workshops and conferences, publication ofscientific papers.


Aims of the working group• Simulations of various types of background in underground

laboratories (in particular, for dark matter detectors),comparing the results from different MC codes with eachother and with available experimental data (testing MCcodes) - N3, JRA1.

• Measurements of neutron and gamma fluxes (and spectra)in underground laboratories (Gran Sasso, Modane,Canfranc, Boulby) - one of the main objectives of JRA1 -can be used to check Monte Carlo codes.

• Investigation of methods of background suppression andrejection (passive shielding, active vetoes etc.); formulationof requirements for shieldings and veto systems - mainly N3.

• Calculation of background rates in dark matter detectors -N3.


Backgrounds for dark matter experiments• Several large-scale (~ 1 tonne) experiments are planned for

WIMP dark matter searches down 10-10 pb: SuperCDMS(Ge), EURECA (EDELWEISS + CRESST), XMASS (Xe),XENON, ZEPLIN IV/MAX (Xe), CLEAN (Ne), DRIFT,WARP etc.– Tolerated (non-discriminated) background - no more than a few

events per tonne of target per year (< 5 ev/tonne/year in Xe at 10-50keV recoil energies to reach 10-10 pb);

– Neutrons and gammas from rock - suppression of >106 should beachieved - passive shielding;

– Neutrons, gammas, alphas and betas from detector components -ultra-pure materials, veto (?);

– Neutrons from cosmic-ray muons - large depth, veto;– Radon - gammas, neutrons and alphas (?) - radon removal, gas-tight

sealing and ventilation with radon-free air.


Sources of background

• Neutrons and gammas from radioactivity in rock:U/Th/K.

• Neutrons, gammas, alphas and betas from radioactivityin detector components (including shielding).

• Neutrons produced by cosmic-ray muons.• Background from radon (special case).• Cosmogenic activation.

Review: J. A. Formaggio and C. J. Martoff. Annu. Rev. Nucl. Part. Sci.,54 (2004) 361.


Neutron production in U/Th decay chains• SOURCES-4A (Wilson et al. SOURCES4A, Technical Report LA-13639-MS, Los

Alamos, 1999) - code to calculate neutron flux and energy spectrum arising fromU/Th contamination in various materials.

• Problems:– Alphas below 6.5 MeV only;– Some cross-sections are missing (because the energy threshold for these

reactions is higher than 6.5 MeV);– Cross-sections needed updating;

• Modifications to SOURCES:– 6.5 MeV upper limit removed;– Cross-sections already present in the code library extended to higher

energies using available experimental data;– Some cross-section updated according to recent experimental results (Na);– New cross-sections added (35Cl, Fe, Cu);– Probability of transitions to the excited states at high energies of alphas are

as at 6.5 MeV (overestimate neutron energy);– For new cross-sections - all transitions to the ground state only.


Neutron production spectra• Neutron production spectrum in NaCl

(from modified SOURCES-4A): 60 ppb U,300 ppb Th - mainly (α,n).

• Neutron production rate in NaCl -1.05×10-7 cm-3 s-1 agrees with othercalculations.

• Major problem: neutron energyspectrum in the laboratory (afterpropagation) is softer than measured atModane (Chazal et al. Astropart. Phys. 9(1998) 163; revised recently - Gerbier etal. TAUP2003), Gran Sasso (Arneodo etal. Nuovo Cimento A112 (1999) 819) andCPL (Korea) (Kim et al. Astropart. Phys.20 (2004) 549) and also softer than othersimulations. But - different types of rockmake direct comparison difficult.


Neutron production spectra

SOURCES and GEANT4: Carson et al.Astropart. Phys. 21 (2004) 667 - simulationfor Modane rock; triangles show themeasurements (from concrete?) from Chazalet al. Astropart. Phys. 9 (1998) 163 (fluxrevised recently - Gerbier et al. TAUP2003).

1.00E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06Initial spectrum from RA from rock(4/5 α n + 1/5 fission)

Spectrum after Pb/Cu shield

Spectrum after Pb/Cu+ 30 cm paraffin

MeV

Neutron spectrum (Modane calculation)and its suppression in paraffin(propagation with MCNP) - Gerbier etal. Talk at TAUP2003:http://www.int.washington.edu/talks/WorkShops/TAUP2003/Parallel/


Neutron spectra from rock

Neutronspectrum at CPL- Kim et al.Astropart. Phys.20 (2004) 549.

Neutron spectra calculated for GranSasso (different halls and watercontents in concrete) afterpropagation - MCNP - Wulandari etal. Astropart. Phys. 22 (2004) 313.

Neutron propagation through rock and shielding: MCNP - Briesmeister (Ed.),MCNP - Version 4B (and later), LA-12625-M, LANL, 1997; GEANT4 - GEANT4Collab., NIMA, 506 (2003) 250.

Neutronspectrum atGran Sasso -Arneodo et al.Nuovo Cimento,A112 (1999)819.


Neutron spectra from rock

• Measured and simulated (GEANT4propagation) neutron spectra for IGEX(Ge dark matter detector - Canfranclaboratory) - no significant difference wasfound between simulations for soft (fission)and hard (α,n) spectra - Carmona et al.Astropart. Phys. 21 (2004) 523.

Neutron spectra fromfission and (α,n) reactionsassumed in the simulations- Carmona et al. Astropart.Phys. 21 (2004) 523.


Neutron spectra and yields• Main feature in SOURCES absent (probably) in other calculations:

– Accounting for transitions of the final nucleus to the excited states(GNASH calculations of transition probabilities - excitation functions)- reduces neutron energies.

• But how accurate are these cross-sections and transition probabilities?The code was tested against measured spectra from various neutronsources, but need more accurate measurements in underground labs(including neutron spectra).

• Main problems: cross-sections on many isotopes have not been measured.Calculated cross-sections should be used:– GNASH-FKK code (www.nea.fr/abs/html/psr-0125.html)?– Recipe can also be found in: Estimation of Unknown Excitation Functions and

Thick Target Yields for p, d, 3He and alpha Reactions. Series: Landolt-Börnstein: Numerical Data and Functional Relationships in Science andTechnology - New Series; Group 1: Elementary Particles, Nuclei and AtomsVol. 5: Q-Values and Excitation Functions of Nuclear Reactions. Part c; Keller,K.A., Lange, J., Münzel, H. 1974, VI (Springer).

– a code EMPIRE-2.19 (http://www.nndc.bnl.gov/empire219) calculates thecross-sections to different excited states (excitation functions can be extracted).


Neutron spectra and yields

• Note that Coulomb barrier suppress significantly the cross-sections forhigh-Z elements even if the energy threshold (calculated from the Q-value of the reaction) is small; for Fe-Cu the Coulomb barrier is about7.0-7.2 MeV - for high-Z targets spontaneous fission of U-238dominates.

• Is there any alternative to SOURCES?• Similar approach when calculating neutron production rate in detector

components (including shielding).


Neutron propagation through the shielding

SOURCES and GEANT4: Carson et al.Astropart. Phys. 21 (2004) 667 - simulationfor NaCl; MCNP - Rachid Lemrani (Saclay);GEANT3 - Maryvonne De Jesus (Lyon).

SOURCES and GEANT4(uncorrected and corrected):Matt Robinson (Sheffield).

rock/cavern

20 g/cm2 CH2

rock/cavern

20 g/cm2 CH2


Neutron propagation through the shielding

50 g/cm2

Neutron spectra after various shieldingthicknesses (CH2) - R. Lemrani, talk atN3-BSNS meeting, 12/04/05; simulationsby Lemrani, Robinson, De Jesus.

Neutron spectra after various shieldingthicknesses (lead + CH2) - M. Robinson,personal communication, see also Carsonet al. Astropart. Phys. 21 (2004) 667.


Muon-induced neutrons• Inputs:

– Muon rate - measurements at a particular underground site.– Muon spectrum and angular distribution (normalised to the total rate) -

simulations or measurements (if available) - not a problem (we are usingMUSUN code - Kudryavtsev et al. NIMA, 505 (2003) 688).

– Neutrons from muons - production, propagation, detection together with allother particles (muon-induced cascades): GEANT4 (GEANT4 Coll. NIMA,506 (2003) 250) or FLUKA (Fasso et al. Proc. MC2000 Conf., Lisbon, 2000,p. 159; ibid. p. 995).

• Important: all particles should be produced, propagated and detectedwith one code to look for simultaneous detection of neutrons and otherparticles, such as photons, electrons, muons, hadrons.

• For dark matter experiments: FLUKA does not generate nuclear recoilsin a realistic way.


Modified GEANT3 and FLUKA

• Modified GEANT3 - correct calculation of muon inelastic scattering (Karlsruhegroup).

• Good agreement between GEANT3 (G. Gerbier, talk at IDM2004; simulations byM. De Jesus) and FLUKA, although GEANT3 does not simulate photoproduction.


Muon-induced neutrons: GEANT4 vsFLUKA

• Comparison between different models in GEANT4 andFLUKA - Bauer et al. Proc. IDM2004,http://www.shef.ac.uk/physics/idm2004.html


Muon-induced neutrons: GEANT4 vsFLUKA

• Neutron production rate in(CH2)n (liquid scintillator):

Araujo et al., NIMA 545 (2005),398; hep-ex/0411026;

FLUKA (Paper 1) -Kudryavtsev et al. NIMA,505 (2003) 688;

FLUKA (Paper 2) - Wang et al.Phys. Rev. D, 64 (2001)013012.

See also A. Hime, Seminar atINT2005.


Muon-induced neutrons: processes

• Contribution of different processes: real photonuclear disintegrationdominates in GEANT4 at all energies and for (almost) all materials.

GEANT4: Araujo et al.FLUKA: Wang et al.


Muon-induced neutrons: A-dependence

Contribution of different processesin various materials - GEANT4:Araujo et al.

A-dependence of neutron production rate -GEANT4: Araujo et al., FLUKA: Kudryavtsevet al. FLUKA gives twice as many neutronscompared to GEANT4 in most materials tested.


Muon-induced neutrons: spectrumand lateral distribution

Neutron production spectrum -GEANT4: Araujo et al., FLUKA: Wanget al. ; data - LVD: LVD Collab., Proc.26 ICRC (Salt Lake City, 1999), vol. 2, p.44; hep-ex/9905047.

Neutron lateral distribution (from muontrack) - GEANT4: Araujo et al., FLUKA:Kudryavtsev et al. ; data - LVD: Proc. 26ICRC; hep-ex/9905047. Simulations didnot include detector specific features.


Neutron multiplicity• Neutron multiplicity in CH2.• µ-induced spallation (in most

cases only one neutron per event)produces few hadrons.

• Secondary neutrons in hadroniccascades - on average 4-5neutrons per event

• Maximum multiplicity ~100 -1000(depending on material and energy, e.g. Pb, Eµ=1 TeV).

• M. Horn, talk at N3-BSNSmeeting in Valencia, April 2005.

Recent comparison between simulations and experiments (Galbiati and Beacom,hep-ph/0504227) confirmed the validity of FLUKA models (light elements).


Muon-induced neutrons: problems• Differential cross-section of

neutron production in thintargets for 190 GeV muons(En>10 MeV). Upper (thick)histograms - GEANT4; dashedline - FLUKA (Araujo et al.);data - NA55 (Chazal et al. NIMA,490 (2002) 334).

• Other data for lead (Bergamascoet al. Nuovo Cim. A, 13 (1973)403; Gorshkov et al. Sov. J. Nucl.Phys., 18 (1974) 57) are old andcontroversial but also showsignificantly higher neutronproduction compared withsimulations.

More data (with simulations) are welcome to check neutron production by muons indifferent materials (especially with high A): LVD, SNO, KamLAND, Borexino CTF?


Geometry

lead

C10H20

n

µ

salt

xenon

air


Spectra in the laboratory

Neutron spectra in the lab before and after shielding - GEANT4: Araujo et al.; alsoAraujo&Kudryavtsev, talk at IDM2004; FLUKA: Kudryavtsev et al. - goodagreement for all energies of interest (within 50%).


Events in xenon detector

Nuclear recoil event rate as a function of measuredenergy (quenching = 0.2 for xenon) - GEANT4 andFLUKA: Araujo et al. - good agreement (within 30%).

Energy spectrum of all events- GEANT4 and FLUKA:Araujo et al.

Only <10% of nuclear recoil events contain nuclear recoils only; others have largeenergy deposition from other particles. To estimate the background from nuclearrecoils only, all particles should be produced, propagated and detected.


Gamma background

Gamma fluxes from radioactivity in rock(NaCl: 60 ppb U, 300 ppb Th, 1300 ppm K)simulated with GEANT4: Carson et al.,NIMA (2005), in press; see also talk atIDM2004: A - rock/cavern interface;B, C, D, E - after 5, 10, 20, 30 cm of lead;F - after 20 cm of lead and 40 g/cm2 of CH2

Energy deposition spectra in 250 kg xenonfrom gammas simulated with GEANT4:Carson et al. - A - 222Rn (10 Bq/m3); B -ultra-low background PMTs - HamamatsuR8778; C - 85Kr (5 ppb); D - copper vessel(0.02 ppb, 350 kg); E (F) - from rock after10 (20) cm of Pb and 40 g/cm2 of CH2.


Veto performance

Veto efficiency for neutron rejection(from detector components): 250 kg ofxenon; recoil energy - 10-50 keV; veto -Gd loaded liquid scintillator (40 g/cm2);GEANT4: Carson et al. NIMA (2005), inpress. Circles - detection of protonrecoils + neutron capture; triangles -gammas from n-capture only.

Veto efficiency for neutron rejection(from detector components) as a functionof veto thickness. Detector - 250 kg ofxenon; measured energy - 2-10 keV; veto- Gd loaded (0.2%) liquid scintillator;gammas from n-capture only.GEANT4: Carson et al. NIMA (2005), inpress.


Summary - I• Low-energy neutron production:

– modified code SOURCES is probably OK for this job - tested, but…– large discrepancy in the neutron energy spectrum between simulations

(SOURCES) and measurements (in underground labs). More measurementsunderground are needed;

– Absence of measured cross-sections and excitation functions for manyelements; calculations with EMPIRE: what is the accuracy - factor of 2?

• Low-energy neutron propagation and detection:– MCNP vs GEANT4 - good agreement (after cross-section correction): 50%

difference after 6 orders of magnitude suppression by shielding.– Designing dark matter detector:

55 g/cm2 of CH2 (or 20 cm of lead + 45 g/cm2 of CH2) should suppress theneutron flux below 1 event/ton/year at 10-50 keV nuclear recoil energies;

the required thickness of CH2 depends on the fraction of H, density, use oflead (10 cm of lead is equivalent to 5 g/cm2 of CH2), holes, neutron energyspectrum;

detector components (including shielding) may become the dominantbackground.


Summary - II

• Muon-induced neutrons:– FLUKA and GEANT4 (also modified GEANT3) agree within a factor of 2

(even better for some materials and energies).– FLUKA: multigroup approach, kerma factors for neutron energy deposition -

not realistic, potential problem for dark matter searches.– Most experimental data (although with large uncertainties) are also in

agreement with simulations within a factor of 2; some data show significantlyhigher neutron yield in heavy materials;

– Designing dark matter detector: 40 g/cm2 of CH2 (after lead) suppress the muon-induced neutron flux

down to a few events/year/tonne; self-vetoing is important: most neutron events have accompanying

particles - gammas, electrons, muons; (3-5)×10-10 pb can be reachable at >2.5 km w.e. even without active veto.


Summary - III

• Gammas:– Physics is very well known and surprises are not expected (?).– Designing shielding for dark matter detectors: 20 cm of ultra-low-

background lead should reduce the external flux below the level expectedfrom detector components.

• Active veto (Gd-loaded liquid scintillator) for dark matter detectors(simulations for Xe) (just around the detector - can substitute passiveCH2 shielding):– in an ideal configuration (4π coverage, infinite time window for neutron

capture detection, low energy threshold) the neutron rejection efficiency ofup to 90% is expected (neutrons from detector components);

– in reality - 70%;– up to 40% rejection efficiency for gammas;– against muon-induced neutrons - more than 90% efficiency.

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Background studies for underground particle astrophysics ... · Background studies for underground particle astrophysics experiments ... EURECA (EDELWEISS + CRESST), XMASS ... (Na);