1. SuperCDMS Energy-scale, Resolution, and Sensitivity
May 18th 2015
SURF Science SDSMT
SuperCDMS Energy-scale, Resolution, and Sensitivity
A.N. Villano
University of Minnesota
(for the SuperCDMS collaboration)

2. CDMS Challenge: low threshold
May 18th 2015
SURF Science SDSMT
CDMS Challenge: low threshold
Control our noise: our athermal phonon readout gives world class energy resolution, and it’s going to get better
Extend our data reduction: we use algorithms to extract our energy/timing parameters which account for our noise environment, and new algorithms have been developed to optimize further
Exploit all calibration handles: toward lower energies it becomes more difficult to find calibration standards, we broadened our low-energy standards for SuperCDMS Soudan and will go further for SuperCDMS SNOLAB

3. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
SuperCDMS Detectors

4. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
The iZIP Detector
Thickness: 1” / Diameter: 3”

5. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
Luke Effect & Yield
Extra phonons are created in response to the moving charges in the semiconductor detectors, this is called the Luke effect, and needs to be taken into account
phonon energy
ionization energy
true recoil energy
yield

6. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
The iZIP Detector
Electrons (e-)
Holes (h+)
If event near surface, all ionization collected locally
Holes (h+)
Electrons (e-)
If event in bulk, ionization shared equally between sides

7. Yield & Charge FV Discrimination (iZIP)
May 18th 2015
SURF Science SDSMT
Yield & Charge FV Discrimination (iZIP)
Clearly identify bulk ERs & surface NRs (210Pb source)
Very few surface events are “charge symmetric”
Clearly identify bulk NRs (252Cf calibration)

8. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
Luke Gain for CDMSlite
Forget about quantifying ionization yield, just turn up the voltage to use the high-resolution phonon sensors to get to the lowest threshold!
boost voltage to ~100 V or more
total phonon energy
true recoil energy
(unmeasurable)
ERs and NRs inseparable and have different energy scales; measure ionization precisely, maybe down to e-/h+ pair events

9. Threshold Drives Mass Reach
May 18th 2015
SURF Science SDSMT
Threshold Drives Mass Reach

10. Ge Results with Different Thresholds
May 18th 2015
SURF Science SDSMT
Ge Results with Different Thresholds
R. Agnese et. al., Phys. Rev. Lett 112, (2013)
R. Agnese et. al., arXiv:
CDMSlite mode limit (red) and iZIP limit (black) (see Luke Gain for CDMSlite in backup)
Get to ~4 GeV/c2 and ~7 GeV/c2 WIMP mass before steep rise
Thesholds ~170 eV and ~1 keV
CDMS II strongest high-mass limit (black solid), and some low-mass reach (black dashed)
Get to ~20 GeV/c2 and ~15 GeV/c2 before steep rise
Thresholds 10 keV and 5 keV

11. Noise Drives Threshold
May 18th 2015
SURF Science SDSMT
Noise Drives Threshold

12. Phonon Channel (QET) Readout
May 18th 2015
SURF Science SDSMT
Phonon Channel (QET) Readout
L/R of SQUID input branch of QET current impacts signal current bandwidth ~ 65 kHz
SQUID
Input coil
L/R bandwidth

13. Phonon Channel (QET) Readout
May 18th 2015
SURF Science SDSMT
Phonon Channel (QET) Readout
Rp (parasitic resistance)
SQUID
Input coil
Readout mechanism: sense current through SQUID input coil which is related to resistance of QET
Major sources of Johnson-Nyquist noise indicated at left
SQUID input noise at the level of ~ 1 nA/√Hz

14. Phonon Noise Mitigation
May 18th 2015
SURF Science SDSMT
Phonon Noise Mitigation
The Johnson-Nyquist noise depends on resistances and temperature ( )
Some typical values for CDMS II and SuperCDMS Soudan:

15. Improvements for SNOLAB
May 18th 2015
SURF Science SDSMT
Improvements for SNOLAB
What can we tinker with for SNOLAB:
Rsh, Tsh: decrease the shunt resistance and move it to base temp
Rs, Ts: (harder) decrease resistance and operating temp of sensor
Rp: (hardest) decrease parasitic resistance back to ground
Tp: (near impossible) resistance spread over many cryostat components at many temperature stages.

16. SuperCDMS iZIP Data Reduction
May 18th 2015
SURF Science SDSMT
SuperCDMS iZIP Data Reduction

17. Standard Optimal Filter
May 18th 2015
SURF Science SDSMT
Standard Optimal Filter
phonon pulse
best-fit template
time (0.8 ms/bin)
signal
template
noise
fit amplitude
Power Spectral Density
Used for energy estimators since CDMS-I
Great resolution at low energy where noise causes simple integral to fluctuate
In phonon signals these quantites have significant position dependence

18. Non-Stationary Optimal Filter
May 18th 2015
SURF Science SDSMT
Non-Stationary Optimal Filter
iZIP lower aluminum coverage, have long phonon tails – good for relative calibration and making analog sum pulse
Used on only the summed pulse with iZIPs – where position dependence is small already
Creates better resolution energy estimators for events distributed across the crystal
Position information mostly at the leading edge of the pulse, so NSOF emphasizes the iZIPs long tail for improved energy resolution
freq. correlation matrix
Frequency-correlated position-dependent “noise” obtained from OF residuals

19. Two Low-Threshold Publications using NSOF for Energy Scale
May 18th 2015
SURF Science SDSMT
Two Low-Threshold Publications using NSOF for Energy Scale
R. Agnese et. al., Phys. Rev. Lett 112, (2013)
CDMSlite with impressive HV technique for low mass reach
SuperCDMS low threshold limit
R. Agnese et. al., Phys. Rev. Lett 112, (2013)

20. Energy Resolution & Threshold
May 18th 2015
SURF Science SDSMT
Energy Resolution & Threshold
For standard iZIP mode have ~200 eVee and for CDMSlite mode have ~42 eVee
K.D. Irwin, Appl. Phys. Lett. 66, 1998 (1995)
QET heat capacity
n and a are QET properties related to heat conduction to the substrate and steepness of the transition in R/T
For SNOLAB will get to ~10 eVee resolution for CDMSlite – “fundamental” limit probably ~1 eVee (see Irwin)

21. Precision Calibration
May 18th 2015
SURF Science SDSMT
Precision Calibration

22. Low-energy Ge Activation Lines
May 18th 2015
SURF Science SDSMT
Low-energy Ge Activation Lines
For CDMSlite running at SNOLAB low-energy calibration lines are key, here we have neutron/cosmogenically activated 68Ge/71Ge EC lines
EC is really great because it’s (almost) pure ER and very well-defined energy

23. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
The Lindhard Curve
Charge collection is quenched with respect to the phonon or heat production
Measurements done at 77 K
and high fields
Sparse at E < 1 keV

24. Examples of Effects on High-Precision Low-Mass Limits
May 18th 2015
SURF Science SDSMT
Examples of Effects on High-Precision Low-Mass Limits
CDMSlite limits can change depending on the NR yield model
For other analyses the systematics of the signal identification region can be significantly increased with yield model uncertainty

25. Can use 88Y-Be source potentially
May 18th 2015
SURF Science SDSMT
Can use 88Y-Be source potentially
Near-threshold resonance produces essentially isotropic 9Be(g,n)
88Y has two decays that are slightly off-resonance

26. Neutron Reflector Shielding
May 18th 2015
SURF Science SDSMT
Neutron Reflector Shielding
Shielding configurations can bring
mono-energetic neutron max scattering edge
over background
Disclaimer: picture at left taken from a representative setup at test facility, Soudan has better background situation and specialized trigger

27. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
Projections

28. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
Summary
Control our noise: we’re doing this by lowering Tc and putting resistors that contribute high Johnson noise at lower temperature stages
Extend our data reduction: the non-stationary optimal filter provides a position-dependence-mitigated energy estimator, phonon fiducial volumes have produced improvements in analysis
Exploit all calibration handles: we are exploiting our internal Ge activation lines at 1 and 10 keV to the fullest extent. The M-shell line at ~160 eV could also prove very useful. We are also exploring lines in Si (difficult) and direct nuclear recoil calibrations

29. SURF Science Conference @ SDSMT
May 18th 2015
SURF Science SDSMT
Backup Slides

30. Leverage on Backgrounds
May 18th 2015
SURF Science SDSMT
Leverage on Backgrounds

31. Surface Events and Neutron Events
May 18th 2015
SURF Science SDSMT
Surface Events and Neutron Events
Very few surface events are “charge symmetric”
Can clearly identify bulk ERs and bulk NRs

32. iZIP Surface Event Rejection & Passage Efficiency
May 18th 2015
SURF Science SDSMT
iZIP Surface Event Rejection & Passage Efficiency
Neutron passage fraction above ~90% for important low energy region
Rejection factor for FV is ~1/33,333 in “beta band” for 10 keV threshold
R. Agnese et. al., Appl. Phys.. Lett 103, (2013)

33. CDMS-II Detectors (oZIPs)
May 18th 2015
SURF Science SDSMT
CDMS-II Detectors (oZIPs)
Biased with an electric field 3 V/cm
Dark Matter would appear as “bulk” nuclear recoils (NRs)
Need phonon timing to remove surface events
Side 1
Side 2
Cryogenic Semiconductor
Four athermal phonon TES arrays

34. Exposures at Soudan run num. towers detector types Ge dets Si dets
May 18th 2015
SURF Science SDSMT
Exposures at Soudan
run
num. towers
detector types
Ge dets
Si dets
Publications (pre-2010)
118 1
oZIP (6) 4 2
PRD 72, (2005)
119
oZIP (12) 6
PRL 96, (2006)
PRD 73, (2006)
123 5
oZIP (30) 19 11
PRL 102, (2009)
124
125
Science 327,1619(2010)
126
127
128
132 3
oZIP (4), mZIP (10), iZIP (3) 17
(engineering)
133
iZIP (15) 15
(3 publications spring ‘13 - fall `14)

35. SuperCDMS iZIP Detector Tech
May 18th 2015
SURF Science SDSMT
SuperCDMS iZIP Detector Tech

36. New, Larger (100 mm dia.) iZIPs!
May 18th 2015
SURF Science SDSMT
New, Larger (100 mm dia.) iZIPs!
12 Phonon channels
4 charge channels
Interleaved phonon and charge detection
Larger size increases science impact for cost
More phonon channels give better position information
133Ba peak
Noise performance comparable to current iZIP designs
Well resolved charge signals achieved at UMN test facility for 356 keV line

37. New, Larger (100 mm dia.) iZIPs!
May 18th 2015
SURF Science SDSMT
New, Larger (100 mm dia.) iZIPs!
12 Phonon channels
4 charge channels
Interleaved phonon and charge detection
Larger size increases science impact for cost
More phonon channels give better position information
Noise performance comparable to current iZIP designs
Well resolved charge signals achieved at UMN test facility for 60 keV line
241Am peak

38. New, Larger (100 mm dia.) iZIPs!
May 18th 2015
SURF Science SDSMT
New, Larger (100 mm dia.) iZIPs!
12 Phonon channels
4 charge channels
Interleaved phonon and charge detection
Larger size increases science impact for cost
More phonon channels give better position information
Promises excellent sensitivity for SuperCDMS-SNOLAB!

39. UMN 100 mm Phonons & Ionization
May 18th 2015
SURF Science SDSMT
UMN 100 mm Phonons & Ionization