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Background Issues in the Cryogenic Dark Matter Search: Challenges and Solutions

This document outlines the critical background issues encountered in the Cryogenic Dark Matter Search (CDMS) led by Laura Baudis at Stanford University. It discusses the use of phonon and ionization detectors to measure WIMP-nucleus elastic scattering, the current and future locations of experiments, and methods to discriminate between nuclear and electron recoils. Key challenges identified include muon-induced backgrounds, intrinsic radioactivity, and neutron interactions, along with strategies to mitigate these effects and improve detector sensitivity.

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Background Issues in the Cryogenic Dark Matter Search: Challenges and Solutions

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  1. Background issues for the Cryogenic Dark Matter Search Laura Baudis Stanford University

  2. The Cryogenic Dark Matter Search • phonon and ionization detectors to measure WMP-nucleus elastic scattering • current location: SUF ~ 16 mwe • future location: Soudan ~ 2000 mwe • Run21: 1 tower (4 Ge, 2 Si) • Soudan: total of 6 towers (7 kg Ge, 2 kg Si)

  3. CDMS detectors • measure phonons and ionization • discrimination between nuclear • and electron recoils • nuclear recoils: WIMPs, n • electron recoils: g,e,a • ionization yield Y=ionization/recoil energy • dependent on type of recoil • electron recoil Y=1 • nuclear recoil Y=1/3 > 99.8% gamma rejection external gamma source external neutron source phonon trigger threshold

  4. Electron contamination! 

  5. CDMS background goals CDMS background goals SUF: 1 event/ kg d or 0.01 events/kg d keV Soudan: factor 100 improvement 0.01 events/kg d = 1 event/100 kg d

  6. Background sources Muon induced background internal neutrons: n -> muon capture and low energy photo-nuclear reactions in Cu cryostat and inner Pb shield: 100/kg d (Cu), 243/kg d (Pb) (veto coincident) external neutrons: n produced by muon interactions outside the veto (veto-anticoincident) Intrinsic radioactivity of materials Ambient gamma and neutron background

  7. A c t i v e M u o n V e t o Detectors Pb Shield Polyethylene Inner Pb shield Layout of the CDMS I shield • plastic scintill. • 15 cm Pb • 25 cm PE • HPCu-cryostat • 1cm inner Pb

  8. Electromagnetic background • muon coincident: 60 events/kg d keV • muon anticoincident: 2 events/kg d keV • (veto efficiency > 99.9 %: => < 0.1 events/kg d keV) • residuals: non-muon induced! • radioactivity of materials surrounding the crystals single scatter photon background: 0.5 ev/kg d keV with 99.9% rejection efficiency=> 0.0005 ev/kg d keV (SUF goal: 0.01 ev/kg d keV) surface electron background: 0.3 ev/kg d keV rejection efficiency > 95% (ZIPs 99.7%!) => 0.015 ev/kg d keV

  9. ZIP Risetime Cut trise>31 s neutrons gammas neutrons trise<31 s betas 60 keV betas

  10. Neutrons from Rock Ice Box, concentric Cu cans, outer radius 30 cm • HE m-nuclear interactions => HE n • n with E > 50 MeV penetrate PE shield and produce LE sec. n • ( < 20 MeV) => NR < 100 keV • rate from literature has x4 uncertainty for 17 m.w.e. • MC simulations of m-induced hadron cascades yields n-rate x3 higher than observed veto-anticoincident NR: • due to vetoing of associated m and hadrons (~ 40% rejection from n)? n Cold Stem ~1 kg Ge Detectors 30 cm Poly Shield 15 cm Pb Shield 5 cm Plastic Scintillator Dimensions give approximate radial thickness of layers

  11. External neutron background • absolute flux: difficult to predict! • can be measured: • compare NR rates in Si and Ge • rate of multiple scatters gives a direct • measurement of n background • (WIMPs scatter only once!)

  12. CDMS uses Si and Ge detectors • WIMPs: Ge has ~6x higher interaction rate per kg than Si • Neutrons: Si has ~2x higher interaction rate per kg thanGe • Breaks the final degeneracy in particle discrimination! neutrons WIMPS 40 GeV

  13. Data from 1998 and 1999 Data Runs • 1999: 4x165g Ge BLIP (10.6 kg d) • 13 single scatter nuclear recoils (1.2/kg/day) • 4 multiple scatter nuclear recoils (0.4/kg/day) 1998 100 g Si ZIP (1.6 kg days) 4 single scatter nuclear recoils (2.5/kg/day) all single-scatters nuclear recoil candidates Analysis threshold (10 keV) 90% acceptance

  14. + Data w/ 68% confidence interval Prediction based on Ge mult, Si Predictions based on most likely Nuclear Recoil Events  1 4 3.4  13 16 Comparison with with MC • Ge multiples and Si singles imply large expected neutron • background with large statistical uncertainty

  15. Typical background spectra @ SUF Nuclear recoil efficiency

  16. CDMS II Soudan • muon flux reduced x 104! • 7 towers each with 3 Ge & 3 Si ZIP detectors • Total mass of Ge = 7 X 3 X 0.25 kg > 5 kg • Total mass of Si = 7 X 3 X 0.10 kg > 2 kg

  17. CDMS II background goals factor 3 factor 15 factor 4 x 104 ~ 25 events expected for 7 kg yr exposure

  18. Is this achievable? Gammas: 99.5 % discr. eff. assumed (99.9 % reached) understand residual background Betas: 95% discr. eff. assumed (99.7 % for ZIPs) avoid surface contaminations Neutrons:mflux reduced by factor 104 @ Soudan internal: 99 % eff. muon veto sufficient external: 1/3 of total expected background (MC) (25 events for ~ 7 kg yr exposure) better MC needed

  19. MC simulations with FLUKA • standalone FLUKA (http://fluka.web.cern.ch/fluka): • most complete treatment of physical processes at • high AND low energies (but not very user friendly...) • simulate muon propagation + hadron shower generation • in tunnel; save HE neutrons entering the tunnel and • transport them in GEANT and/or in FLUKA • later requires complete geometry in FLUKA, doable with • help of ALIFE (http://AliSoft.cern.ch/offline/fluka/ALIFE.html) • better estimation of absolute n-flux • correlations between n-hits and veto response

  20. What other backgrounds do we fear? • cosmogenics • surface contaminations (Rn-plateout)

  21. Cosmogenics Activation of Si/Ge crystals and other materials during production and transportation at the Earth‘s surface A precise calculation requires: cosmic ray spectrum (varies with geomagnetic latitude) cross sections for the production of isotopes Problem: cross sections! only few measured production is dominated by (n,x) reactions: 95% (p,x) reactions: 5% Existing programs use: semiempirical formulas based on data to calculate cross sections: COSMO (Martoff et al.) SIGMA (J. Bockholt et al.)

  22. Cosmogenics in Ge 30 d exposure at see level, 1 year storage below ground COSMO SIGMA

  23. Important for CDMS realistic exposure: 4 months above ground estimations from Run 19 3H: 1.34 x COSMO 68Ge: 1.26 x COSMO CDMS goal for gammas: 95 /kg yr keV 3H: 1.34 x 50 -> not a problem ! 68Ge: 1.26 x 2.5 x 103 !

  24. Cosmogenics in Si Martoff, Science87 Modif. Cosmo 3 months at see level, 1 yr below ground 3H: 47 ev/kg yr keV for ~ 4 month exposure; not a problem!

  25. 3H production already in the right order of magnitude • avoid any further activation • store Ge/Si crystals and Cu in tunnel C @ SUF • transport detectors via ground: 10 h of flight ~ 125 d exposure! • install PE shield box at SNF (fabrication site): • 10 cm of PE reduce n-flux by factor of 30! However...

  26. The Radon problem 222Rn -> 210Pb source: 238U chain noble gas colorless tasteless odorless plateout: adhesion of Rn daughters on surfaces 1 Bq ~ 5 x 105 Rn atoms air: 40/10 Bq/m3 (in/out)

  27. Radon plateout the long lived 210Pb accumulates on surfaces and decays: b-: Emax = 63 keV: most dangereous -> surface electrons amount of 210Pb on surfaces depends on: - exposure time: t - Rn concentration in air: A - efficiency 222Rn (air) -> 210Pb (surface): p goal for CDMS II: 10-4/cm2 keV d scrubbing + low p + low t!

  28. Radon Scrubbing Facility @ Stanford • use for cleaning and assembly of ZIP detectors and towers inner room < class 100 foyer wetbench antiroom

  29. Radon Scrubbing Facility continuous particulate + Rn monitoring particulates: better than class 100 Rn: ~ 6 Bq/m3 but scrubbing not started yet! goal: factor 10 better

  30. Conclusions • CDMS I • background goal (1 ev/kg d) reached @ SUF • sensitivity @ SUF limited by external n-background • CDMS II • ZIP technology: 99.9 % discrimination of bulk e-recoils • 99.7 % discrimination of surface e • still have to keep track of possible background sources! • reach 100 times better sensitivity ~1 event / 100 kg d

  31. Non-Neighbor interaction B3 B4 B5 Neighbor interaction B6 Neutron Multiple Scatters in Ge BLIPs • Observe 4 neutron multiple scatters in • 10-100 keV multiple events • 3 neighbors, 1 non-neighbor • Calibration indicates negligible contamination by electron multiples Neighbors Non-Neighbors surface electrons photons photons Ionization Yield B5,6 Ionization Yield B6 neutron neutrons Ionization Yield B4,5 Ionization Yield B4

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