SuperCDMS)Soudan) · 2016-02-23 · Ray Bunker - SuperCDMS 7 Low=threshold)Analysis) Tower3 Sources LowQenergy$WIMP$search:$ $ • $Use7 iZIPs$w/$bestlowQenergy$performance$$ •

1

SuperCDMS Soudan Ray Bunker

for the SuperCDMS Collabora4on

Ray Bunker - SuperCDMS

Detec4on Technique

2

Nuclear Recoil (NR) yields ≈20–30% ioniza<on vs. Electron Recoil (ER)

holes+

e-‐

Nuclear recoil (NR)

Electron recoil (ER)

Quasipar4cle trap Aluminum

collector

Ge or Si crystal

Diffusion

Phonons

Cooper pair

tungsten Transi4on

Edge Sensor (TES)

Phonons provide equal measure of NR & ER

recoil energies

Ioniza4on & Phonons

Ray Bunker - SuperCDMS 3

Detec4on Technique

Ioniza4on & Phonons

Ioniza4o

n yield

Recoil energy (keV)

γ-‐calibra4on ERs

Neutron-‐calibra4on NR

s

detector

copp

er hou

sing

133Ba γ

adjacent detector

252Cf neutron

Residual backgrounds are near surfaces: •  U/Th/K decays à γ-‐induced electrons, sidewall γ’s •  210Pb decay chain à betas, x-‐rays, 206Pb recoils

Low-‐energy region, all of the above plus: •  U/Th/K decays à γ-‐induced bulk ERs •  Internal ac<va<on (e.g., 68Ge) à bulk ERs

Surface events

γ

γ

e-‐

γ

e-‐

210Pb

x-‐ray

210Bi β

210Po 206Pb

Low-‐ene

rgy

region

γ

γ γ

e–

γ


The SuperCDMS iZIP Bulk event à Side-‐symmetric ioniza<on signal

Electrons

Holes

Surface event à Asymmetric ioniza<on signal

Electrons Holes

Significantly improved surface-‐event rejec4on

Phonon sensors @ 0V

Ioniza<on electrodes @ ±2V

Interleaved ioniza<on & phonon sensors

5

SuperCDMS Soudan

Tower 3 Sources

15 detector array of germanium iZIPs:

•  0.6 kg per detector •  in situ 210Pb sources •  Installed in CDMS II shielding •  In opera<on since early 2012 •  Primary iZIP science run ended summer 2014 •  More CDMSlite & calibra<ons through fall 2015

CDMS II Shielding Broad science program:

•  Demonstra<on of surface-‐event discrimina<on •  WIMP-‐search analyses:

•  3 CDMSlite runs à MWIMP < 5 GeV/c2 •  Low-‐threshold iZIP à MWIMP = 5–10 GeV/c2 •  High-‐threshold iZIP à MWIMP > 10 GeV/c2

•  Y/Be & Sb/Be photoneutron calibra<ons •  In general, high degree of experimenta<on to inform SuperCDMS SNOLAB

see talk by Sunil Golwala (next session)


Surface-‐event Discrimina4on 210Pb sources placed above and below 2 detector faces:

•  50 live days à 0 of 132,968 leaked surface events in (symmetric) NR signal region •  More than sufficient face-‐event discrimina<on for SNOLAB iZIP payload

Ioniza<on-‐derived side-‐symmetry parameter

Ioniza<o

n yield

Recoil energy (keV)

Ioniza<on yield

1.2

1

0.8

0.6

0.4

0.2

0

Bulk ERs

Bulk ERs Surface Betas & X-‐rays

Bulk NRs

Bulk NRs

206Pb Recoils

APL 103, 164105 (2013)


Low-‐threshold Analysis

Tower 3 Sources

Low-‐energy WIMP search:

•  Use 7 iZIPs w/ best low-‐energy performance •  Data period = Oct 2012 – July 2013 •  577 kg-‐day exposure (2.4x CDMS II) •  Range of thresholds: 1.6–5 keV •  Improved discrimina<on of sidewall events via phonon-‐derived fiducial volume

Lowest-‐energy background >10x lower than CDMS II,

dominated by sidewall events

PRL 112, 241302 (2014)

CDMS II Si

DAMA Further LT studies:

•  Maximum likelihood analysis using background model from 2014 PRL

•  Understand anomalous high-‐energy candidates using Detector Monte Carlo à Stay tuned… PRD long paper in progress

Anomalous high-‐energy candidates from detector w/ shorted charge channel

8

High-‐threshold Analysis

Tower 3 Sources

High-‐energy WIMP search:

•  Different strategy: target high-‐mass WIMPs •  Higher thresholds to prevent background at low energy due to resolu<on smearing •  Use 10 iZIPs w/ fully opera<onal charge channels •  Mix of WIMP search and γ & neutron calibra<ons

Mar 2012 Sep 2012 Apr 2013 Nov 2013 May 2014

Accumulated

Live Time (days)

0

100

200

300

400

500

Nov 2014 Jun 2015

Blinded HT WIMP-‐search exposure

252 Cf, ne

utron calib

ra4o

ns

133Ba gamma calibra4on (≈daily)

CDMSlite

Run

1

Low-‐threshold WIMP-‐search exposure

CDMSlite

Run

2a

CDMSlite

Run

2b

Extend

ed γ calibra4o

n

CDMSlite

Run

3

Photon

eutron

calibra4o

n

9

High-‐threshold Analysis Analysis challenges: •  Exposure-‐limited à maximize fiducial volume with near-‐zero background •  Surface events (γ-‐induced electrons & 210Pb decay chain):

•  Faces: expect a few hundred such events à demonstrated ≈10-‐5 rejec<on more than sufficient! •  Sidewalls: more challenging to understand low-‐yield event rate vs. radial-‐posi<on es<mator (ongoing)


Detector-‐response Simula<on

True radius squared (cm2) 0 4 8 12 2 6 10

0.0

0.5

1.0

1.5

2.0

2.5

True

z po

si<on

(cm)

Reconstructed radial param

eter

1.0

0.75

0.45

0.15

0.0

0.9

0.6

0.3

14

Detector-‐response Simula<on

True radius squared (cm2) 0 4 8 12 2 6 10

0.0

0.5

1.0

1.5

2.0

2.5

True

z po

si<on

(cm)

Reconstructed z p

aram

eter

1.0

0.5

0.0

-‐0.5

-‐1.0

14


SuperCDMS HT Projected Sensitivity

High-‐threshold Analysis

Projected sensi<vity for:

•  1700 kg-‐day exposure (≈2.75x CDMS II)

•  7-‐15 keV thresholds

•  50-‐65% fiducial volume

Nice improvement vs. CDMS II considering modest exposure

Important demonstra<on of iZIP technology in advance of SuperCDMS SNOLAB


Background Measurements Global γ background:

•  Geant4 simula<on of U/Th/K in materials of construc<on •  Simultaneous fit for ac<vi<es using measured ioniza<on spectrum •  Mystery: dominated by U/Th gammas coming from below! •  Auemp<ng to understand with forensic assay à e.g. dust?

Surface α background:

•  Alphas are easily iden<fied! •  Primary contribu<on from 210Po at 5.3 MeV à Normalizes 210Pb surface contamina<on

•  Decay rate indicates more Po vs. Pb at start •  Ge crystal faces show ≈50 nBq/cm2

•  >10x higher rate at sidewalls à Copper housings or crystal sidewalls? à Planning to directly α assay both!

Bouom iZIP from 4th Tower

Measured spectrum Best-‐fit simula<on

500 1000 1500 2000 2500 3000 0 Ioniza<on energy (keV)

10-‐5

10-‐4

10-‐3

10-‐2

Normalize

d even

t rate All selected events

Alpha selec<on

2000 4000 6000 0 8000

1000

500

2000

1500

3000

2500

Middle iZIP from 2nd Tower

Recoil energy (keV)

Ioniza<o

n en

ergy (keV

)

Fully contained at sidewall

fully contained at faces

12

Photoneutron Calibra4on Purpose:

•  Improve understanding of nuclear-‐recoil energy scale for CDMSlite & iZIP modes

Technique: •  Pair 88Y and 124Sb γ sources with Be wafer à ≈mono-‐energe<c neutrons (152 & 23 keV) •  Look for maximum elas<c scauering shoulder off Ge at 8.1 & 1.3 keV

Approxim

ate trigger

threshold

elas4c scag

er

endp

oint

103

102

101

100

Even

t rate (arb.)

0 1 2 3 4 5 6 7

Detected recoil energy (keV)

Geant4 simula<on with approximate

shielding geometry

Pb γ shield

Outer poly (40 cm)

Lead shield (22cm)

Inner poly (10cm)

Copper vacuum cans

Detectors Source

Energy (a.u.)

Even

ts per hou

r

PRELIMINARY

124Sb 124Sb9Be

Rates match in tails where no

neutrons expected

Excess due to neutrons

CDMSlite mode


Summary

January 2016 Collabora4on Mee4ng at PNNL

•  SuperCDMS Soudan is a rich source of data for improving WIMP-‐search sensi<vity and understanding SuperCDMS detectors

•  The iZIP technology significantly improves rejec<on of the CDMS II limi<ng backgrounds •  e.g., demonstrated face-‐event rejec<on beuer than needed for SuperCDMS SNOLAB & improved phonon-‐sensor layout enables >10x reduc<on of low-‐energy backgrounds

•  High-‐threshold analysis is ongoing (expect result this year) + backgrounds & calibra<ons


Backup Slides

14

15

CDMS II ! Background-‐Limited Ioniza<o

n yield

0.6

0.4

0.2

0.0

-‐0.2

40

30

20

10

0

Normalize

d ioniza<o

n yield

10 15 5 0 -‐5 Normalized phonon <ming parameter

-‐10

100 10 2 Recoil energy (keV)

PRL 106, 131302 (2011)

Science 327, 1619 (2010) High-‐threshold analysis:

•  Final CDMS II dataset •  612 kg-‐day Ge exposure •  10 keV threshold •  Limited by near-‐face ERs in high-‐energy region

Bulk ERs

Surface ERs

Signal Region

Low-‐threshold analysis:

•  Final CDMS II dataset •  241 kg-‐day Ge exposure •  2 keV threshold •  Limited by sidewall events in low-‐energy region

Bulk ERs

Near-‐face ERs

Sidewall Events

Signal Region


LT Analysis – Detector Monte Carlo

True radius (cm)

True

z po

si<on

(cm)

Fully Func<onal Detector

Reconstructed Ch

arge Radius (top)

True radius (cm)

True

z po

si<on

(cm)

Shorted-‐Channel Detector

Reconstructed Ch

arge Radius (top)

Charged-‐based radial preselec<on not effec<ve on top side

PRELIMINARY

PRELIMINARY


LT Analysis – Detector Monte Carlo

True radius (cm)

True

z po

si<on

(cm)

Fully Func<onal Detector

Reconstructed Ph

onon

Radius

Phonon-‐based radial fiducializa<on less effec<ve near top corner

True radius (cm)

True

z po

si<on

(cm)

Shorted-‐Channel Detector

Reconstructed Ph

onon

Radius

Slide courtesy Lauren Hsu (FNAL) 18

Photoneutron Calibra4on

lead shield (22cm) x-‐axis units are not calibrated…but they are the same scale shown on previous slide

arbitrary energy units arbitrary energy units

Even

ts per hou

r

Even

ts per hou

r

88Y9Be subtracted

124Sb9Be subtracted

Plots shown here are result axer subtrac<ng neutron-‐off from neutron-‐on spectra; these plots are not efficiency corrected

approxim

ate trigger thresho

ld

approxim

ate trigger thresho

ld

error bars shown are sta=s=cal; data shown is 5 weeks of Sb and 4 weeks of Y (addi=onal data not yet analyzed)

Documents

SuperCDMS)Soudan) · 2016-02-23 · Ray Bunker - SuperCDMS 7 Low=threshold)Analysis) Tower3 Sources LowQenergy$WIMP$search:$ $ • $Use7 iZIPs$w/$bestlowQenergy$performance$$ •