Background Reduction in Cryogenic Detectors x Rock Rock U/Th/K/Rn n ,n Detector U/Th/K/Rn Shielding Veto Dan Bauer, Fermilab LRT2004, Sudbury, December 13, 2004
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Background Reduction in Cryogenic Detectors x Rock Rock U/Th/K/Rn n ,n Detector U/Th/K/Rn Shielding Veto Dan Bauer, Fermilab LRT2004, Sudbury, December 13, 2004 Cryogenic Dark Matter Search - CDMS •Dark Matter Search Goal is direct detection of a few WIMPS/year • Detector Tower Signature is nuclear recoil with E<100 KeV •Cryogenic Shield/Muon Veto Cool very pure Ge and Si crystals to < 50 mK •Active Background Rejection Detect both heat (phonons) and charge • Nuclear recoils produce less charge for the same heat as electron recoils •Deep Underground (Soudan) • Dilution Refrigerator Electronics and Data Acquisition Fewer cosmic rays to produce neutrons • Neutrons produce nuclear recoils •Shielding (Pb, polyethylene, Cu) Reduce backgrounds from radioactivity Active scintillator veto against cosmic rays LRT 2004 Dan Bauer CDMS Background Rejection Strategy Detector Rejection of Backgrounds Phonon timing: surface events () gamma cal. Phonon timing Y = Charge/phonons Charge yield: , Erecoil (keV) Position information: locate discrete sources y Multiple-scatters: n (also Si vs Ge rates) Y = Charge/phonons x LRT 2004 Dan Bauer CDMS Background Reduction Strategy Layered shielding (reduce , , neutrons) ~1 cm Cu walls of cold volume (cleanest material) Thin “mu-metal” magnetic shield (for SQUIDs) 10 cm inner polyethylene (further neutron moderation) 22.5 cm Pb, inner 5 cm is “ancient” (low in 210Pb) 40 cm outer polyethylene (main neutron moderator) All materials near detectors screened for U/Th/K Active Veto (reject events associated with cosmics) Hermetic, 2” thick plastic scintillator veto wrapped around shield Reject residual cosmic-ray induced events Information stored as time history before detector triggers Expect > 99.99% efficiency for all , > 99% for interacting MC indicates > 60% efficiency for -induced showers from rock LRT 2004 Dan Bauer The Radon Problem • Radon levels high, vary seasonally at Soudan (200-700 Bq/m^3) Decays include energetic gammas which can penetrate to detectors, and eject betas from Compton scatters (‘ejectrons’) Need to displace Radon from region inside Pb shield Six purge tubes along stem shield penetrations • Purge gas is medical grade breathing air ‘aged’ in metal cylinders for at least 2 weeks to allow decay of 90% of 222Rn Radon variation at Soudan 800 700 600 500 400 300 200 100 0 Jun-01 LRT 2004 Jan-02 Jul-02 Feb-03 Aug-03 Mar-04 Oct-04 Apr-05 Dan Bauer Measured Gamma Backgrounds •Typically “bulk” events High ionization yield in detector bulk Rejection 99.9999% at 70% nuclear recoil efficiency •Sources Residual contamination in the Pb, polyethylene and copper Environmental radon • Comparison of data and MC: Gammas from U/Th/K in Pb, Poly, Cu at assayed level Radon between purged volume & Pb • • Fit concentration to data in summed spectra 35 Bq/m3 compared with ambient ~500 Bq/m3 Fair agreement but actual radon level may be slightly lower based on: • • 609 keV 214-Bi line lower in data 1765 keV 214-Bi line agrees • Three event classes Compton scatters from nearby passive materials have low solid-angle for hitting detectors Compton scatters from nearest neighbor can be vetoed Dominant component is 1 in ~30000 gammas interacting in dead layer: expect <0.1 events in CDMSII (after timing cuts) Radon: fit to data U/Th/K: ~1/4 total rate L. Baudis, UFL LRT 2004 Dan Bauer Measured Beta Backgrounds Typically surface events: rejected at 99.4% in present analysis • Sources • Timing 97% Ionization yield 80% Residual contamination on detector and nearby surfaces: “intrinsic” betas Soft x-rays Pb-210, K-40, C-14 primary focus Auger, SIMS, RBS+PIXE Rates Convolve source spectrum in Monte Carlo to model charge collection Confirm with calibration/TF data Correlate with gammas and alphas surface science techniques • • in situ direct counting • Robust leakage estimates • Identification • • Important to ID and characterize these backgrounds for CDMSII Observe ~0.4/det/day on inner detectors Expect ~7 Events in CDMS-II for present analysis and rate Modest improvements will keep us background free Charge Efficiency • — Charge side — Phonon side J.-P. Thompson, Brown Depth (um) LRT 2004 Dan Bauer Sources of residual beta background • Pb-210 — from airborne radon daughters Could be dominant source — further analysis needed Complex decay chain with numerous alphas and betas expect and observe roughly equal numbers Detailed simulations to check relative detection efficiency in progress Events charge • Recoil Energy (keV) LRT 2004 Recoil Energy (keV) J. Cooley-Sekula, Stanford Dan Bauer Sources of residual beta background • K-40 — from natural potassium Direct upper limit less than half observed rate 1460 keV gamma: lack of observed photopeak or compton edge sets upper limit of 0.15 betas/det/day RBS+PIXE surface probe for natK and assumption that 40K is in standard cosmogenic abundance limits rate to 0.04 betas/det/day • C-14 — from natural carbon Auger spectroscopy and RBS indicate 2-3 monolayers of “adventitious” carbon 0.3 betas/det/day to 156-keV endpoint 0.05 betas/det/day in 15-45 keV • Work is ongoing Complete Pb-210 analysis Broaden scope to more possible isotopes Just beginning use of new technique: ICP-MS • Inductively coupled plasma mass spectroscopy • Antimony found on test wafer - normalization not known yet R. Schnee, D. Grant, Case; P. Cushman, A. Reisetter, U Minn LRT 2004 Dan Bauer Reduction of EM Backgrounds • Reduce beta contamination via active screening/cleaning Observed alpha rate indicates dominated by 210Pb on detectors • Improved radon purge should help, if this is correct Materials surface analysis (PIXE/RBS/SIMS/Auger) (in progress) • Try to pinpoint source(s) of beta contamination Developing multiwire proportional chamber or cloud chamber as dedicated alpha/beta screener (Tom Shutt talk) • Necessary for 17 beta emitters that have no screenable gammas/alphas • Reduce photon background via improved shielding LRT 2004 Active (inexpensive) ionization “endcap” detectors to shield against betas, identify multiple-scatters Add inner ‘clean’ Pb shielding Improved gamma screening (Rick Gaitskell talk) Dan Bauer Neutron Backgrounds • Predictions based on neutron propagation from rock and shield, normalized to Soudan muon flux Expected <0.05 unvetoed neutrons in first data set - none observed Expected 1.9 vetoed neutrons - none observed (agrees at 85% CL) • Should see ~ 5 vetoed neutrons in second data set Will allow normalization of Monte Carlos Observe one muon-coincident multiple-scatter nuclear recoil so far • Ongoing work to refine estimates Direct measure of muon flux from veto Throw primary muon spectrum in Fluka + Geant4 • • • • Hadron production Correlations of particles from same parent muon Simulate vetoed fraction of externally produced events Predict 60% of “punch through” (>50 MeV) are vetoed by outer scintillator Expect <0.2 unvetoed neutrons in full CDMS-II exposure • Will reach ‘natural’ neutron background limit at Soudan in a few years LRT 2004 S. Kamat, R. Hennings-Yeomans, Case; A. Reisetter, U Minn; J. Sander, H. Nelson, UCSB Dan Bauer Neutron Reduction Strategies Super CDMS @ SNOLAB Could add inner neutron veto Muon Flux (m-2s-1) Avoid the problem by reducing muon flux by 500x CDMS II @ Soudan Depth (meters water equivalent) LRT 2004 Dan Bauer CDMS Goal Maintain Zero Background as MT increases 04/04/14 Currently 45% Z 2,3,5 > 10keV 90% CL upper limit 0.005 Tower 1: Fall 03 Expected CDMSII end 2005 Expected Tower 1+2 Summer 04 CDMS II Goal 1998 Zero background 58% efficiency Improvement linear until background events appear Then degrades as √MT until systematics dominate Blue points illustrate random fluctuation from experiment to experiment LRT 2004 Dan Bauer