1. Dark Matter Detection with Superheated Liquids
Andrew Sonnenschein
Fermilab
Fort Collins, CO, Nov. 18, 2005.

2. Basic idea:
Nuclear recoils from WIMP nucleus scattering can produce a bubble in a superheated liquid.
Under correctly chosen pressure and temperature conditions, background gammas and betas can not produce bubbles.
Almost any liquid can be used, so wide choice of potential target nuclei.

3. Why Do Liquids Not Always Boil When They Pass into the “Vapor” Part of the Phase Diagram?
In the “vapor” region, the equilibrium state is a vapor.
But liquids have surface tension, so there is an energy cost to create a bubble.
This energy barrier may be greater than kt.
a metastable (“superheated”) liquid state may continue to exist for some time.
The liquid will boil violently once the energy barrier to the vapor phase is overcome.

4. Bubble Nucleation in Superheated Liquids by Radiation
(Seitz, “Thermal Spike Model”, 1957)
Pressure inside bubble is equilibrium vapor pressure.
At critical radius Rc surface tension balances pressure.
Bubbles bigger than the critical radius Rc will grow, while smaller bubbles will shrink to zero.
Boiling occurs when energy loss of throughgoing
particle is enough to produce a bubble with radius > Rc Rc
Pvapor
Pexternal
particle
Surface tension s Rc Ec
Radius
Work

5. dE/dX Discrimination in a Small Propane Chamber
Waters, Petroff, and Koski, IEEE Trans. Nuc. Sci. 16(1) (1969)
Plot of event rate vs. “superheat pressure” (= vapor pressure - operating pressure)
electrons
protons
a plateau
(psi)

6. Superheated Droplet Detectors (Apfel, 1979)
Small droplets of superheated liquids suspended in a viscous gel
Mixture has usually ~1% superheated liquid
Usually Freons, such as CCl2F2 , C2ClF5 , C4F10 , C3F8
Freons are hydrophobic (oil-like) and gel is hydrophilic (water-like).
They form an emulsion when mixed.
Emulsion can be stable for months at atmospheric pressure.
After nucleation
Before nucleation
gas
neutron
20 mm
Inert gel

7. Superheated Droplet Detectors
Gel surrounding each droplet inhibits nucleation by container walls and dust.
Detector is gradually “used up” as the ~106 droplets/cc are depleted (~1% loading).
State of the art detectors have ~ 10 g active mass.
(Figures
from
Picasso)

8. 2005 Picasso Results
10 keV threshold
Old data
one WIMP model
(50 GeV/c2, sWp=10 pb)
Current data
Background rate ~ 400/kg day. Attributed to contamination of gel.

9. Background Due to Nuclear Recoils from Alpha Decay
Alpha decay produces monoenergetic, low energy nuclear recoils.
For example, consider 210Po->206Pb:
Ea = MeV
ER= 101 keV
206Pb a
The recoiling nucleus will nucleate a bubble in any chamber that is sensitive to the lower energy (~10 keV) recoils expected from WIMP scattering.
The 238U and 232Th decay series include many alpha emitters, including radon (222Rn) and its daughters.
Radon is highly soluble in bubble chamber liquids.

10. Sensitivity for Spin-dependent Interactions on Proton
NAIAD
Super-K (indirect)
PICASSO ‘05
SIMPLE ‘05
ZEPLIN-I
CDMS

11. Why Not Just Build A Big Bubble Chamber?
Conventional bubble chambers were limited to short sensitive periods (~10 msec per decompression) by surface nucleation.
Liquid
0.1 mm
Solid
nucleation sites
Can this problem be overcome by careful surface preparation?

13. High-Stability Bubble Chamber Test at U. Chicago
3-way valve
Propylene glycol buffer
liquid prevents evaporation
of superheated liquid.
Acoustic sensor
Quartz pressure vessel
Glass dewar with
heat-exchange fluid
Piston
Camera
(1 of 2)

14. Bad surface
Good surface

15. Background Rate
1 event/ (15 minutes) for 18 grams of CF3Br in sub-basement lab (~6 ft. depth).
Observed rates are consistent with measured ambient neutron flux.

16. Neutron Multiple Scattering
Multiple bubbles are present in approximately 4% of events in our “background” data set.
These events can only be caused by multiple neutron scattering, since uniform size of bubbles implies simultaneous nucleation at multiple sites.
Events such as this can be used to measure neutron backgrounds in- situ while searching for recoils due to WIMPs.

17. at Fermilab
COUPP test site
~300 m.w.e.

18. Chicagoland Observatory for Underground Particle Physics
COUPP:
Chicagoland Observatory for Underground Particle Physics
J. Collar, D. Nakazawa, B. Odom, K. O’Sullivan
Kavli Institute for Cosmological Physics
The University of Chicago
M. Crisler, J. Krider, K. Krempetz, C.M. Lei, H. Nguyen, E. Ramberg, R. Schmitt, A. Sonnenschein, R. Tesarek
Fermi National Accelerator Laboratory

19. Design Concept for Large Chambers
Central design issue is how to avoid metal contact with superheated liquid.
Fabrication of large quartz or glass pressure vessels is not practical, but industrial capability exists for thin-walled vessels up to ~ 1 m3 in volume.
Thin- walled quartz bell jar
Steel pressure vessel
Pressure balancing bellows
Buffer fluid

20. 2 kg (1 Liter) Chamber in NuMi Gallery

21. 160 msec of Video Buffer (20 msec/frame)

22. First Peek at Results from 2 Kg Chamber in NuMi Gallery
Optical effects not yet corrected, but wall events are already easy to cut.
Alpha backgrounds? 222Rn?
Rate in fiducial volume is ~100/ day . Similar to Picasso and SIMPLE.

23. Future
Alpha background problems need to be solved for both superheated droplet and bubble chamber technologies.
Solar neutrino experiments have achieved alpha contamination levels 5 orders of magnitude lower in similar liquids.
We are now implementing techniques developed for solar neutrino projects (distillation of liquids, special cleaning procedures and equipment).
Assuming success, both bubble chambers and superheated droplet detectors can be rapidly and relatively inexpensively scaled in target mass.
Picasso and SIMPLE collaboration on 1-ton design (~100 tons of gel).
FNAL/U Chicago studying design issues for larger chambers. Current design seems to scale well to ~1 ton.

24. Lab Infrastructure Requirements
Scalability and simplicity of construction of bubble and droplet chambers may create a demand for significant amounts of deep underground space on a short time scale.
Depth. Must Henderson equal SNOLAB depth to compete effectively for these projects? Would Henderson “central campus” be an acceptable place to be in 2010?
Type of underground space: General purpose, “Gran Sasso style” caverns would allow the lab to quickly adapt to new technologies, while custom excavations force us to plan further ahead and limit flexibility.
Access for large, heavy modules. These technologies probably allow instruments up to ~1 ton active mass to be built outside the lab and brought in on trucks.
Need water for shielding rock neutrons and perhaps for Muon veto Cherenkov counters. Can share infrastructure for water production and discharge with other experiments?
Venting: may require emergency capability to safely vent asphyxiating or mildly toxic gases. Similar requirement will exist for large cryo experiments.

25. Table of Isotopes neutrons Uranium and Thorium Decay Chains
Rare earth group
222Rn
Iodine
Bromine
betas
protons
Fluorine