Background Reduction in Cryogenic Detectors Dan Bauer, Fermilab LRT2004, Sudbury, December 13, 2004 Detector Shielding Veto U/Th/K/Rn ,n U/Th/K/Rn x Rock n  

LRT 2004 Dan Bauer Cryogenic Dark Matter Search - CDMS Dark Matter Search  Goal is direct detection of a few WIMPS/year Signature is nuclear recoil with E<100 KeV Cryogenic  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) Fewer cosmic rays to produce neutrons Neutrons produce nuclear recoils Detector Tower Dilution Refrigerator Shield/Muon Veto Electronics and Data Acquisition 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 y x Phonon timing Y = Charge/phonons E recoil (keV) gamma cal. Charge yield: ,  Phonon timing: surface events (  ) Multiple-scatters: n (also Si vs Ge rates) Position information: locate discrete sources

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 210 Pb) 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 ( 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

LRT 2004 Dan Bauer Measured Gamma Backgrounds Typically “bulk” events  High ionization yield in detector bulk  Rejection % at 70% nuclear recoil efficiency Sources  Residual contamination in the Pb, polyethylene and copper  Environmental radon 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) 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/m 3 compared with ambient ~500 Bq/m 3  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 L. Baudis, UFL U/Th/K: ~1/4 total rate Radon: fit to data

LRT 2004 Dan Bauer Measured Beta Backgrounds Typically surface events: rejected at 99.4% in present analysis  Timing 97%  Ionization yield 80% Sources  Residual contamination on detector and nearby surfaces: “intrinsic” betas  Soft x-rays  Pb-210, K-40, C-14 primary focus Identification  in situ direct counting Correlate with gammas and alphas  surface science techniques Auger, SIMS, RBS+PIXE Rates  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 Important to ID and characterize these backgrounds for CDMSII  Robust leakage estimates Convolve source spectrum in Monte Carlo to model charge collection Confirm with calibration/TF data — Charge side — Phonon side Depth (um) Charge Efficiency J.-P. Thompson, Brown

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 charge Recoil Energy (keV) Events J. Cooley-Sekula, Stanford

LRT 2004 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 nat K and assumption that 40 K 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 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 210 Pb 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  Active (inexpensive) ionization “endcap” detectors to shield against betas, identify multiple-scatters  Add inner ‘clean’ Pb shielding  Improved gamma screening (Rick Gaitskell talk)

LRT 2004 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 S. Kamat, R. Hennings-Yeomans, Case; A. Reisetter, U Minn; J. Sander, H. Nelson, UCSB

LRT 2004 Dan Bauer Neutron Reduction Strategies Depth (meters water equivalent) Muon Flux (m -2 s -1 ) Super SNOLAB Avoid the problem by reducing muon flux by 500x CDMS Soudan Could add inner neutron veto

LRT 2004 Dan Bauer CDMS Goal Maintain Zero Background as MT increases CDMS II Goal 1998 Tower 1: Fall 03 Expected CDMSII end 2005 Expected Tower 1+2 Summer 04 Zero background 58% efficiency Blue points illustrate random fluctuation from experiment to experiment 04/04/14 Currently 45% Z 2,3,5 > 10keV 90% CL upper limit Improvement linear until background events appear Then degrades as √MT until systematics dominate