1. Dark matter detection with bubble chamber detectors
Miguel Ardid
Dark Matter from aeV to ZeV: 3rd IBS-MultiDark-IPPP Workshop
21-25 November 2016
Lumley Castle

3. DM detection with superheated liquids (bubble nucleation)
Direct DM Basics already reviewed
A bubble chamber is filled with a superheated fluid in meta-stable state.
Energy deposition greater than Eth in a radius less than rc from particle interaction will result in expanding bubble (Seitz “Hot-Spike” Model).
A smaller or more diffuse energy deposit will create a bubble that immediately collapses.
Classical Thermodynamics says: pl pv σ
Surface energy
Latent heat

4. Bubble chambers as nuclear recoil detectors
Thermodynamic parameters are chosen for sensitivity to nuclear recoils but not electron recoils.
Better than rejection of electron recoils (betas, gammas).
Alphas are (were) a concern because bubble chambers are threshold detectors.

5. PICO fast compression bubble chambers
Buffer fluid
(water)
Hydraulic
fluid
Target fluid
(CF3I/C3F8)
to hydraulic controller
Synthetic silica jar
Pressure expansion puts fluid (CF3I or C3F8) in superheated state
I for spin-independent
F for spin-dependent (many fluids possible)
Particle interactions nucleate bubbles
Cameras see bubbles, trigger
Stereo reconstruction of bubble position, multiplicity
Acoustics used to identify alphas
Recompress to reset chamber

6. PICO bubble chambers COUPP-4: superheated fluid 4 kg of CF3I
Observe bubbles with two cameras and piezo-acoustic sensors.
Synchronized measurements of P, T, and control parameters

7. Measured gamma rejection
Bubble nucleation probability from gamma interactions in C3F8 and CF3I
10-10 ~ 3 keV
10-10 ~ 11 keV 7

8. Acoustic discrimination
Discovery of acoustic discrimination against alphas (Aubin et al., New J. Phys.10:103017, 2008)
Alphas deposit their energy over tens of microns.
Nuclear recoils deposit theirs over tens of nanometers.
In PICO bubble chambers alphas are several times louder.
Observable bubble ~mm
~40 μm
~50 nm
Daughter heavy nucleus
(~100 keV)
Helium nucleus
(~5 MeV)

9. Calibration and efficiencies
C3F8
In addition to understand the detector and background for
Nuclear Recoil Calibration and Efficiency :
Am-Be source, but as well
Y-Be source and neutron beams for fluorine near threshold recoils
Pion beams for Iodine recoils
CF3I 9

10. Dark Matter Bubble Chamber Evolution
PICO-60, PICO-40L
PICO-500L 10

11. PICO-60 at SNOLAB

12. PICO-60 PICO-60 Run-1: June 2013 to May 2014.
Filled with 36.8 kg of CF3I.
Run-2 with C3F8 target in 2016.

13. PICO 60 – CF3I SNOLAB Phys. Rev. D, 93 (2016) 052014
Pattern suggests particulate contamination moving on convection currents
Events are shifted systematically higher
in AP than neutrons.
They are systematically louder at larger expansion times.
They are higher in the
vessel at longer
expansion times
A background-free region can be identified
By cutting on AP, Expansion Time, and Z
48.2% of total exposure
Statistical penalties taken into account due to cut optimizations

14. Silica jar is an exact replica of COUPP-4 jar.
PICO-2L
Two liter active mass of C3F8.
Same size as COUPP-4.
Re-uses COUPP-4 location/water shield at SNOLAB.
Better fluorine sensitivity than CF3I:
Twice the F density.
Lower threshold.
Improved efficiency.
More stable chemistry.
Silica jar is an exact replica of COUPP-4 jar.

15. PICO-2L at SNOLAB Filled with 2.9 kg C3F8. Run 1 Oct 2013 - May 2014.
Run 2 in 2015.

16. PICO-2L Run-1 results Phys. Rev. Lett. 114 (2015) 231302
No expected neutron background (<1 event at 3.2keV), and no multiple bubble events observed, so these are not neutron events.
There are timing correlations of events at 3.2keV with previous events, therefore not DM or neutrons.
We set limits by applying a timing cut with MC-derived statistical penalty, similar to PICO-60.

17. Low AP backgrounds in PICO 2L
“collar” events
Higher rate of “wall”, “collar”, and “surface” events
ALL low AP candidates within 1000 sec of prior nucleation
PICO-2L
Surface bubbles
low AP events
1 / 3 days
Alpha events Z
“wall” events R2

18. Background Source? Particulate Contamination? Activated by Steel
Quartz
Activated by
U, Th contamination
This seemed like the best candidate a
Suppressed AP
218Po

19. Low-AP Backgrounds Four Step Program
Assay
analyze the contamination in the chambers
Mitigate
Control the particulates in the existing chambers
Reproduce
add contamination to test chambers
reproduce the low-AP events
Eliminate
Revised Bubble Chamber Design

20. PICO-2L Run-2 flange First bubble February 27, 2015.
Phys. Rev. D, 93 (2016)
flange
First bubble February 27, 2015.
Physics run started June 12, 2015.
Natural quartz inner vessel flange replaced with low-radioactivity fused silica.
Assembled and filled with particular focus on minimizing particulate contamination.
Technical improvements to cooling and piezo-acoustic sensors (0% sensor failure in Run-2).
Bulk event rate consistent with expected neutron background.
active camera cooling

21. PICO-2L Run-1 vs Run-2 (mitigation works)
Run-1 3.2keV data
32 live days
9 events: much above the estimated
neutron background
Run-2 3.2keV data
66 live days
1 event: consistent with the expected neutron background

22. Present situation: SD WIMP-proton cross-section
From LUX latest SD results: Phys. Rev. Lett. 116, (2016)

23. The Physics Case for 19F

24. Plan Going Forward PICO-60 Run 2 (with C3F8)
Commissioning almost done
Physics Data soon (blind strategy)
PICO-40L (RSU: Right Side Up)
2017 Commissioning
PICO-500L
Proposals for funds submitted this year, decision in 2017
2018 Construction Start

25. PICO-60 Run 2 C3F8 new, all synthetic quartz vessel
doubled the active volume, two extra cameras used
Upgraded instrumentation
Low Stress Seal
Fluid Recirculation
Commissioning Now
Physics Data in Dec.
Large Background Reduction

26. PICO-60 Run 2

27. PICO-60 Run 2: First Bubble

28. PICO-60 Run 2: Multiple Bubbles
25 bubble event during neutron calibration

29. PICO-40L PICO-60 (RSU) active volume hydraulic Fluid 15oC hydraulic
H2O
15 oC
“water piston”
insulation
active volume
fused silica
piston
hydraulic
fluid
-5oC
hydraulic
Fluid
15oC
C3F8
15oC
hydraulic
fluid
-5oC

30. Right Side Up design virtues:
Superheated/normal transition is maintained by a thermal gradient with no buffer fluid
No buffer fluid means no surface tension effects
No buffer fluid means no constraint on target fluid
Works with any refrigerant, hydrocarbon, even xenon.
Thermal gradient is naturally stable
All metals at the bottom
Cold zone, no boiling to liberate particulate
No convection to move particulate up
Geometry naturally lends itself to a recirculation loop

31. Summary
PICO-2L and PICO-60 have already produced world-leading limits on Spin-Dependent WIMP-proton interactions with different target fluids.
PICO with C3F8 also has excellent Spin-Independent limits on low-mass (few GeV) WIMPs. Also interesting CF3I for large masses .
Both experiments observed a population of background events in Run-1 that were mitigated in Run-2.
A very exciting plan forward with PICO-60, PICO-40L (RSU) and PICO-500L.

32. Extra slides

33. Nuclear Recoil Calibration
Iodine recoil calibration is challenging
fluorine cross-section dominates neutron scattering
iodine largely invisible in neutron source spectrum
Solution: 11 GeV pion elastic scattering
Fluorine recoil calibration is challenging
fluorine is light, typical source well above threshold
Y-Be source – very low energy neutrons
fluorine resonant scattering – Montreal accelerator

34. iodine recoil calibration in p- scattering
Silicon pixel array
1 cm diameter
bubble chamber
12 GeV/c p- test beam
1 mm wall thickness

35. calibration of iodine recoil energy

36. Mono-energetic neutron beam

37. Alpha acoustic calorimetry
222Rn
4 d
218Po
3 m
210Pb
22 y
214Bi
20 m
214Pb
26 m
214Po
164µs
α (5.6MeV)
α (6.1MeV)
α (7.9MeV) β

38. PICO-2L Acoustic discrimination
Acoustic cut
94%
acceptance
3 keV threshold
Preliminary
No multiple bubble events in the low background data
Two distinct alpha peaks, clearly separated from nuclear recoils
Timing of events in high AP peaks consistent with radon chain alphas, and indicate that the higher energy 214Po alphas are significantly louder (a new effect not seen in CF3I)
“Clean” alpha triplets
222Rn
α(5.6 MeV)
218Po
α(6.1 MeV)
214Po
α(7.9 MeV)
Preliminary
3 minutes
55 minutes

39. PICO-60 results (ArXiv )
Best SD WIMP-proton cross section limit for large DM mass
Interpretation of DAMA signal from nuclear recoils of I ruled out completely

40. SI Projection

41. The original right side up chamber
Warm active region
Waters, Petroff, Koski 1969
10cc Bubble Chamber
COUPP turned this design upside-down and added the water buffer to build a larger scale device
We need a better way to scale up from this design concept
Thermal gradient
Maintained in thin
neck
Cold plumbing region

42. PICO-500L – PICO-40L
SNOLAB
cage