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

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.

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.

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

dE/dX Discrimination in a Small Propane Chamber Waters, Petroff, and Koski, IEEE Trans. Nuc. Sci. 16(1) 398-401 (1969) Plot of event rate vs. “superheat pressure” (= vapor pressure - operating pressure) electrons protons a plateau (psi)

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

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)

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.

Background Due to Nuclear Recoils from Alpha Decay Alpha decay produces monoenergetic, low energy nuclear recoils. For example, consider 210Po->206Pb: Ea = 5.407 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.

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

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?

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)

Bad surface Good surface

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.

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.

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

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

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

2 kg (1 Liter) Chamber in NuMi Gallery

160 msec of Video Buffer (20 msec/frame)

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.

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.

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.

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