Identification of the Surface Event Background in the CDMS II Experiment. Sunil Golwala for the CDMS II Collaboration Caltech August 21, 2008

Transcription

1 Identification of the Surface Event Background in the CDMS II Experiment Sunil Golwala for the CDMS II Collaboration Caltech August 21, 28

2 Surface Event Background in CDMS II Identification of Surface Event Background in CDMS II/IDM 28 2

3 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Bulk electron recoils Nuclear recoil acceptance region Identification of Surface Event Background in CDMS II/IDM 28 2

4 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Ionization yield suppressed for surface events due to ionization dead layer (tens of µm thick) Bulk electron recoils Low yield surface events Nuclear recoil acceptance region Identification of Surface Event Background in CDMS II/IDM 28 2

5 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Ionization yield suppressed for surface events due to ionization dead layer (tens of µm thick) Surface events arise from low-energy electrons with short penetration depths Bulk electron recoils Low yield surface events Nuclear recoil acceptance region Identification of Surface Event Background in CDMS II/IDM 28 2

6 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Ionization yield suppressed for surface events due to ionization dead layer (tens of µm thick) Surface events arise from low-energy electrons with short penetration depths Bulk electron recoils Low yield surface events Nuclear recoil acceptance region Low-yield event rate ~.37/kg/day 1-1 kev single scatters in inner detectors CDMS Tower of 6 detectors inner detectors endcap detectors Identification of Surface Event Background in CDMS II/IDM 28 2

7 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Ionization yield suppressed for surface events due to ionization dead layer (tens of µm thick) Surface events arise from low-energy electrons with short penetration depths Bulk electron recoils Low yield surface events Nuclear recoil acceptance region Low-yield event rate ~.37/kg/day 1-1 kev single scatters in inner detectors Phonon timing cut removes all events in nuclear recoil acceptance region (.6 leakage expected). CDMS Tower of 6 detectors inner detectors endcap detectors Identification of Surface Event Background in CDMS II/IDM 28 2

8 Surface Event Background in CDMS II Recent results from Cryogenic Dark Matter Search (CDMS) II reported by Jeter Hall on Tuesday CDMS II detectors discriminate nuclear recoils from bulk electron recoils using ionization yield = ionization energy/recoil energy, latter measured by phonons Ionization yield suppressed for surface events due to ionization dead layer (tens of µm thick) Surface events arise from low-energy electrons with short penetration depths Bulk electron recoils Low yield surface events Nuclear recoil acceptance region Low-yield event rate ~.37/kg/day 1-1 kev single scatters in inner detectors Phonon timing cut removes all events in nuclear recoil acceptance region (.6 leakage expected). Would like to identify source of surface events to maintain rate at current levels for SuperCDMS Soudan and to improve for SuperCDMS SNOLAB. inner detectors CDMS Tower of 6 detectors endcap detectors Identification of Surface Event Background in CDMS II/IDM 28 2

9 Surface Event Background: 21 Pb Radon Daughter Environmental 222 Rn in air can deposit long-lived 21 Pb! source on surfaces 21 Pb, 22 yr Expected signatures: low-energy! decay, but final state of 17 kev decay results in peak ~46 kev delayed 1.16 MeV! from 21 Bi delayed 21 Po " 63.6 kev BR 16+3% 21 Bi* <3 ns! kev 4.25±.4% ToI 21 Bi kev, BR 84+3% M e kev 4.3±1.4% e kev 16±5% N e - 46 kev.9±.3% e kev 57±2% L1 e kev 6.±.2% L2 e kev.5±.2% Campbell, J. Phys. A 36, 3219 (23) Literature Review Nucl. Data Tables A4, 1 (1968) Nucl. Data Tables A6, 235 (1969 Nucl. Data Tables A9, 119 (1971 Atom. Data. Nucl. Data Tables A81, 1, (22). L3 24.6±.8% emit Flourescent x-rays NOP Mostly Auger electron emission H. Nelson Identification of Surface Event Background in CDMS II/IDM 28 3

10 Surface Event Background: 21 Pb Radon Daughter Environmental 222 Rn in air can deposit long-lived 21 Pb! source on surfaces Expected signatures: low-energy! decay, but final state of 17 kev decay results in peak ~46 kev delayed 1.16 MeV! from 21 Bi delayed 21 Po " Counts Energy [kev] Identification of Surface Event Background in CDMS II/IDM 28 3

11 Surface Event Background: 21 Pb Radon Daughter Environmental 222 Rn in air can deposit long-lived 21 Pb! source on surfaces Expected signatures: low-energy! decay, but final state of 17 kev decay results in peak ~46 kev delayed 1.16 MeV! from 21 Bi delayed 21 Po " Counts Energy [kev] 21 Bismuth! 21 Polonium+! "! 26 Pb(Lead)+# $ 1/2 = 5. days 138 days Stable 1.16 MeV endpoint 5.3 MeV Identification of Surface Event Background in CDMS II/IDM 28 3

12 21 Pb Visible Signatures Identification of Surface Event Background in CDMS II/IDM 28 4

13 21 Pb Visible Signatures 4 Ionization Energy [MeV] Recoil Energy [MeV] Daughter 21 Po "-decay easy to ID sometimes see 26 Pb recoiling nucleus in adjacent detector (~1 kev v. low yield) Identification of Surface Event Background in CDMS II/IDM 28 4

14 21 Pb Visible Signatures 4 Ionization Energy [MeV] :1 scale: 3 in. x 1 cm, 1 mm separation Recoil Energy [MeV] Daughter 21 Po "-decay easy to ID sometimes see 26 Pb recoiling nucleus in adjacent detector (~1 kev v. low yield) Parent 21 Pb!-decay not easy to ID single electrons easy to see, but have no clue to parent: 21 Pb? photon-induced? other contamination? unambiguous 21 Pb ID only when nearest-neighbor double-scatter: see ~46 kev peak in sum energy Identification of Surface Event Background in CDMS II/IDM 28 4

15 21 Pb Visible Signatures 4 Ionization Energy [MeV] Recoil Energy [MeV] Daughter 21 Po "-decay easy to ID sometimes see 26 Pb recoiling nucleus in adjacent detector (~1 kev v. low yield) Parent 21 Pb!-decay not easy to ID single electrons easy to see, but have no clue to parent: 21 Pb? photon-induced? other contamination? unambiguous 21 Pb ID only when nearest-neighbor double-scatter: see ~46 kev peak in sum energy Identification of Surface Event Background in CDMS II/IDM 28 4

16 21 Pb Visible Signatures 4 1 Ionization Energy [MeV] Detector 2 energy [kev] Recoil Energy [MeV] Detector 1 energy [kev] Daughter 21 Po "-decay easy to ID sometimes see 26 Pb recoiling nucleus in adjacent detector (~1 kev v. low yield) Parent 21 Pb!-decay not easy to ID single electrons easy to see, but have no clue to parent: 21 Pb? photon-induced? other contamination? unambiguous 21 Pb ID only when nearest-neighbor double-scatter: see ~46 kev peak in sum energy Identification of Surface Event Background in CDMS II/IDM 28 4

17 21 Pb Visible Signatures 4 Ionization Energy [MeV] Counts/4 kev Recoil Energy [MeV] Nearest-Neighbor Double-Scatter Beta-Beta Event Energy Sum [kev] Daughter 21 Po "-decay easy to ID sometimes see 26 Pb recoiling nucleus in adjacent detector (~1 kev v. low yield) Parent 21 Pb!-decay not easy to ID single electrons easy to see, but have no clue to parent: 21 Pb? photon-induced? other contamination? unambiguous 21 Pb ID only when nearest-neighbor double-scatter: see ~46 kev peak in sum energy Identification of Surface Event Background in CDMS II/IDM 28 4

18 21 Pb Visible Signatures Ionization Energy [MeV] Recoil Energy [MeV] Good correlation of 21 Pb surface-event double-scatter and 21 Po "/recoiling nucleus rates strongly supports 21 Pb theory: variations among detectors in contamination levels (esp. between older and newer detectors) give correlated variations in these two rates Nearest-Neighbor Double-Scatter Beta-Beta Event Energy Sum [kev] Identification of Surface Event Background in CDMS II/IDM 28 4 Counts/4 kev surface nearest-neighbor double scatters in 46 kev sum peak [/detector pair/day] "/recoiling nucleus pairs [/detector pair/day]

19 Full Surface Event Model Build a model (64 free parameters, nominally) 21 Pb activity on each detector face (5 inner detector faces of 3 detectors in 5 towers) # of events of various types per 21 Pb decay (independent of detector aside from proportionality to 21 Pb activities) alpha/recoiling nucleus pairs from 21 Po decay nearest-neighbor double-scatters in 46 kev peak from 21 Pb decay single-scatter events from 21 Pb and 21 Bi decay (three energy bins: 1-4, 4-65, 65-1 kev) Geometric factors (solid angle of adjacent detector, etc.) Physics effects (assumed independent of detector) Probability of recoiling nucleus to escape parent detector (<.5 if parent is implanted) Efficiencies for identifying 21 Pb and 21 Bi decay products as surface events (three energy bins) Non- 21 Pb-correlated backgrounds (three energy bins, assumed independent of detector) Some parameters set by Monte Carlo of 21 Pb on detector surfaces Fit to 125 data points from 2-tower and 5-tower runs alpha/recoiling nucleus pairs per detector face (2 unique numbers per detector) nearest-neighbor double scatters in 46 kev peak (1 unique number per detector) single-scatters in three energy bins (3 unique numbers per detector) Of 189 possible data points, 49 discarded a priori, 15 discarded as outliers from fit Fit determines 51 free parameters (44 activities + 7 global parameters) Identification of Surface Event Background in CDMS II/IDM 28 5

20 median = mean =.294 Top.481 Face.17 rms rel. median = rms rel. mean = black = all red = T12 green = T345 black = 1-3 counts/ detector/day blue = 1-3 counts/kg/ day (eff-corr.) Fit Results 21 Pb decays 1-1 kev surface event singles Occurences median = mean =.23 Bottom Face rms rel. median = rms rel. mean = Model-fit 21 Pb decays, by face [1-3 decays/detector face/day] Total, all towers Total, T12 Total, T Pb, all towers 21 Pb, T12 21 Pb, T345 non- 21 Pb, all towers photon expected, all towers 65 ± ± ± ± ± ± ± ± 32 3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± 13 Identification of Surface Event Background in CDMS II/IDM 28 6

21 Fit Results Occurences median = mean = black = all rms rel. median = red =.22 T12 rms rel. mean = green = T Model-fit 21 Pb 1-1 kev single-scatter surface events [1-3 counts/detector/day] Goodness of fit evaluated by simulation: P(poorer fit) = 23% black = 1-3 counts/ detector/day blue = 1-3 counts/kg/ day (eff-corr.) Total, all towers Total, T12 Total, T Pb, all towers 21 Pb, T12 21 Pb, T345 non- 21 Pb, all towers photon expected, all towers 21 Pb decays 1-1 kev surface event singles 65 ± ± ± ± ± ± ± ± 32 3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± 13 Identification of Surface Event Background in CDMS II/IDM 28 6

22 Fit Results x 2 reduction in 21 Pb contamination between T12 and T345 black = 1-3 counts/ detector/day blue = 1-3 counts/kg/ day (eff-corr.) Total, all towers Total, T12 Total, T Pb, all towers 21 Pb, T12 21 Pb, T345 non- 21 Pb, all towers photon expected, all towers 21 Pb decays 1-1 kev surface event singles 65 ± ± ± ± ± ± ± ± 32 3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± 13 Identification of Surface Event Background in CDMS II/IDM 28 6

23 Fit Results x 2 reduction in 21 Pb contamination between T12 and T345 good consistency between non- 21 Pb rate and photon expectation; no major unaccountedfor sources black = 1-3 counts/ detector/day blue = 1-3 counts/kg/ day (eff-corr.) Total, all towers Total, T12 Total, T Pb, all towers 21 Pb, T12 21 Pb, T345 non- 21 Pb, all towers photon expected, all towers 21 Pb decays 1-1 kev surface event singles 65 ± ± ± ± ± ± ± ± 32 3 ± ± ± ± ± ± ± ± ± ± ± ± ± ± 13 Identification of Surface Event Background in CDMS II/IDM 28 6

24 Sensitivity Implications CDMS II, # = 2 x 1-44 cm 6 GeV/c 2 Require x3 gain in phonon timing misid for zero-background in final exposure (CY 28). Only a mild loss of efficiency! Soudan (16 kg), # = 6 x 1-45 cm 6 GeV/c 2 Add ionization-only endcap detectors Expect gains of: x2.5 in detector thickness (volume/area ratio, 1 cm! 1 inch) x2 in surface event backgrounds (T345 levels; no further improvement assumed!) Timing Parameter (µs) x2 in ionization-yield rejection (better electrodes, proven in CDMS I) FIG. 1: Ionization yield versus timing parameter (see text) for x4 in phonon timing rejection (x2 from new calibration sensor data design, inx2 onefrom of our more Ge detectors. sophisticated Theanalysis) yield normalized to unity for typical bulk-electron recoils (dots; from x4 easily provides necessary reach (only 133 need Ba gamma x4; assumes rays). Low-yield no neutron 133 Babgnd Soudan) attributed to surface electron recoils, have small timing parameter values, allowing discrimination from neutron-induced nuclear recoils x2 in surface event bgnds (x2 in photon from bgnd 252 (4 Cf dru ( ), $ which 2 dru), showx2 a wide in 21 range Pb contam.) of timing parameter values. The vertical dashed line indicates the minimum timing parameter allowed for candidate dark matter events in this x2 in detector rejection improvements x16 overall relative to CDMS II, only need detector, x7 and the box shows the approximate signal region, which is in fact weakly energy dependent. (Color online.) SNOLAB (1 kg), # = 3 x 1-46 cm 6 GeV/c 2 Ionization Yield Identification of Surface Event Background in CDMS II/IDM Bulk electron recoils Surface electron recoils Nuclear recoils recoil energy 1!1 kev Bulk Electron Recoils Surface Electron Recoils move cut to reduce leakage Accept as WIMP candidates

25 The CDMS II Collaboration Brown University M. Attisha, R. Gaitskell, J.-P. Thompson Caltech Z. Ahmed, S. R. Golwala, D. Moore, R. W. Ogburn Case Western Reserve University D. S. Akerib, C. N. Bailey, K. Clark, M. Danowski, M.R. Dragowsky, D. R. Grant, R. Hennings-Yeomans Fermilab D. A. Bauer, M. B. Crisler, D. DeJongh, J. Hall, D. Holmgren, L. Hsu, E. Ramberg, J. Yoo MIT E. Figueroa-Feliciano, S. Hertel, S. Leman, K. McCarthy, P. Wikus NIST K. Irwin Queens University W. Rau Santa Clara University B. A. Young Syracuse University M. Kiveni, M. Kos, R.W. Schnee Texas A&M R. Mahapatra University of California, Berkeley M. Daal, J. Filippini, N. Mirabolfathi, B. Sadoulet, D. Seitz, B. Serfass, K. Sundqvist University of California, Santa Barbara R. Bunker, D. O. Caldwell, H. Nelson, J. Sander University of Colorado at Denver M. E. Huber University of Florida A. Achelashvili, T. Saab, D. Balakishiyeva, G. Sardane University of Minnesota P. Cushman, L. Duong, M. Fritts, V. Mandic, X. Qiu, A. Reisetter, O. Kamaev University of Zurich S. Arrenberg, T. Bruch, L. Baudis, M. Tarka Stanford University P.L. Brink, B. Cabrera, J. Cooley, M. Pyle, S. Yellin Identification of Surface Event Background in CDMS II/IDM 28 8