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

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)

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

Electron contamination!   

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 CDMS background goals

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

Layout of the CDMS I shield Active Muon Veto Detectors Inner Pb shield Polyethylene Pb Shield plastic scintill. 15 cm Pb 25 cm PE HPCu-cryostat 1cm inner Pb

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 => 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%!) => ev/kg d keV

ZIP Risetime Cut gammas neutrons betas 60 keVbetas t rise >31 s t rise <31 s

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

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!)

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! WIMPS 40 GeV neutrons

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) all single-scatters nuclear recoil candidates Analysis threshold (10 keV) 90% acceptance g Si ZIP (1.6 kg days) 4 single scatter nuclear recoils (2.5/kg/day)

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

Nuclear recoil efficiency Typical background SUF

muon flux reduced x 10 4 ! 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 CDMS II Soudan

CDMS II background goals CDMS I [ev/kg d keV] CDMS II [ev/kg d keV] Gammas Betas Neutrons x factor 3 factor 15 factor 4 x 10 4 ~ 25 events expected for 7 kg yr exposure

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:  flux reduced by factor 10 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

MC simulations with FLUKA standalone 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 (  better estimation of absolute n-flux  correlations between n-hits and veto response

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

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.)

Cosmogenics in Ge 30 d exposure at see level, 1 year storage below ground COSMOSIGMA

Important for CDMS realistic exposure: 4 months above ground estimations from Run 19 3 H: 1.34 x COSMO 68 Ge: 1.26 x COSMO CDMS goal for gammas: 95 /kg yr keV 3 H: 1.34 x 50 -> not a problem ! 68 Ge: 1.26 x 2.5 x 10 3 !

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

However... 3 H production already in the right order of magnitude  avoid any further activation store Ge/Si crystals and Cu in tunnel 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!

The Radon problem 222 Rn -> 210 Pb source: 238 U chain noble gas colorless tasteless odorless plateout: adhesion of Rn daughters on surfaces 1 Bq ~ 5 x 10 5 Rn atoms air: 40/10 Bq/m 3 (in/out)

Radon plateout the long lived 210 Pb accumulates on surfaces and decays:  -: E max = 63 keV: most dangereous -> surface electrons amount of 210 Pb on surfaces depends on: - exposure time: t - Rn concentration in air: A - efficiency 222 Rn (air) -> 210 Pb (surface): p goal for CDMS II: /cm 2 keV d exposurep x tA [Bq/m 3 ] rate /keV cm 2 d stand. CL1 x 7 d203x10 -3 scrubbed CL1 x 7 d0.23x10 -5 scrubbing + low p + low t!

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

Radon Scrubbing Facility continuous particulate + Rn monitoring particulates: better than class 100 Rn: ~ 6 Bq/m 3 but scrubbing not started yet! goal: factor 10 better

CDMS I background goal (1 ev/kg d) SUF 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 Conclusions

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