Background Reduction in Cryogenic Detectors

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

2. 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 Shield/Muon Veto Dilution Refrigerator Electronics and Data Acquisition • Shielding (Pb, polyethylene, Cu) • Reduce backgrounds from radioactivity • Active scintillator veto against cosmic rays

3. gamma cal. A D Y = Charge/phonons y Phonon timing C B x Erecoil(keV) Y = Charge/phonons CDMS Background Rejection Strategy Detector Rejection of Backgrounds • Phonon timing: surface events () Charge yield: ,  • Multiple-scatters: n • (also Si vs Ge rates) Position information: locate discrete sources

4. CDMS Background Reduction Strategy Layered shielding (reduce g, b, 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 m, > 99% for interacting m MC indicates > 60% efficiency for m-induced showers from rock

5. 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

6. 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 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/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 Measured Gamma Backgrounds Radon: fit to data U/Th/K: ~1/4 total rate L. Baudis, UFL

7. — Charge side — Phonon side Charge Efficiency Depth (um) 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 J.-P. Thompson, Brown

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

9. 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

10. 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 • Active (inexpensive) ionization “endcap” detectors to shield against betas, identify multiple-scatters • Add inner ‘clean’ Pb shielding • Improved gamma screening (Rick Gaitskell talk)

11. 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

12. Neutron Reduction Strategies • CDMS II @ Soudan • Could add inner neutron veto Super CDMS @ SNOLAB Avoid the problem by reducing muon flux by 500x Muon Flux (m-2s-1) Depth (meters water equivalent)

13. 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 Blue points illustrate random fluctuation from experiment to experiment CDMS GoalMaintain Zero Background as MT increases Improvement linear until background events appear Then degrades as √MT until systematics dominate