COUPP : C hicagoland O bservatory for U nderground P article P hysics (FNAL Test Beam Program T-945)

1. COUPP: Chicagoland Observatory for Underground Particle Physics (FNAL Test Beam Program T-945) Development of a bubble chamber technique for dark matter detection. J. Collar, K. Crum, D. Nakazawa, B. Odom, J. Rasmussen, M. Szydagis Kavli Institute for Cosmological Physics The University of Chicago P. Cooper, M. Crisler, H. Nguyen, E. Ramberg,, A. Sonnenschein, R. Tesarek Fermi National Accelerator Laboratory * Engineering and technical support from PPD Technical Centers and Mechanical Department: K. Krempetz, C.M. Lei, R. Schmitt, T. Nebel, J. Zimmerman, C. Lindenmeyer, J. Korienek, D. Butler, J. Krider, D. Erikson, B. Pritchard

2. Basic idea: • Low energy nuclear recoils from WIMP nucleus scattering can produce a bubble in a superheated liquid. • Many liquids could be used, so wide choice of potential target nuclei to check signal dependence on spin and A2. • Under correctly chosen pressure and temperature conditions, background gammas and betas can not produce bubbles.

3. 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)

4. 0.1 mm nucleation sites Liquid Solid Bubble Nucleation in Cracks • Trapped gas volumes in surface imperfections are now known to be the primary source of nucleation. • Most (all?) construction materials have rough surfaces at scales below 1 mm, but some materials much better than others. • Historically, problem was overcome for high energy physics experiments by rapid cycling of chamber in sync with a pulsed beam. Bubbling at walls was tolerated because of finite speed of bubble growth. • A few small “clean chambers” (~10 ml) were built in the 50’s and 60’s, with sensitive times ~1 minute. • Ways to preserve superheated state: • Elimination of porous surfaces in contact with superheated liquid. • Precision cleaning to eliminate particulates. • Vacuum degassing. • … a few other tricks borrowed from chemical engineers

5. Pressure balancing bellows Buffer fluid Steel pressure vessel Thin- walled quartz bell jar 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.

6. Target Liquids Tested in Small Chamber

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

8. Installation of 1 Liter Chamber At Fermilab NuMi Tunnel • Prototypes design features required for chambers up to 1000 liters

9. 160 msec of Video Buffer (20 msec/frame)

10. Muon Track @ 160 psi Vapor Pressure

11. Data From December 1st ‘05 to March 27 ‘06 Video trigger turned on 115 days in run 45k expansions 82 seconds mean expansion time 42.8 live days = 37% of real time 15k bubbles counted 145 GB in Enstore Adjustment of trigger thresholds and compression schedule

12. Spatial Distribution of Single Bubbles, First 42 Days • Duty cycle of chamber allowed 33% live time (14 live days out of 42 real days)

13. Radial Distribution

14. Multiple Bubble Analysis • Statistics of multiple bubble events suggest ~10% of bulk singles (2.8 /day) are due to neutrons. • These should be coincident with cosmic rays-- we’ll see when Muon veto is installed. Counts PRELIMINARY Number of Bubbles

15. Neutron Shielding and Muon Veto • Detector is surrounded by 30 cm of polyethylene neutron moderator. • Very effective for low energy neutrons coming from (a,n) radioactivity in rock. • Simulations show reduction in rate to < 1/month. • Active muon veto: 150 plastic scintillator counters from KTEV. • Goal: < 0.1 m-induced neutrons/ day in 1 liter chamber, requires >98% efficiency. To be installed underground summer ‘06.

16. Alpha decay produces monoenergetic, low energy nuclear recoils. • For example, consider 210Po->206Pb: ER= 101 keV Ea = 5.407 MeV 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. • Solar neutrino experiments (Borexino, Kamland, SNO) have demonstrated feasibility of reduction to ~1 event per day in scintillator and water-- about 2 orders of magnitude lower rates than seen in current-generation dark matter experiments. What Will Our Dominant Background Be?

17. Potential Sensitivity Of 1-Liter Chamber at Fermilab Site Goal for this phase: reduce background to <1 event per liter per day Spin-independent Spin-dependent

18. To Prove Viability of Bubble Chamber Technique, We have demonstrated: • We have a bubble chamber which can remain expanded and sensitive for a significant fraction of the time. • Efficient detection of single bubble events from low energy nuclear recoils. Work at U. Chicago using radioactive sources. • Low backgrounds from radioactivity & cosmic rays. • Installing new radio-pure inner vessel assembly • Muon veto system (KTEV plastic scintillator) • Goal: < 1 background bubble per day

19. Underground Lab Requirements- A Guess Which Henderson DUSEL campus will be most suitable for your experiment? We now have prototyping space at 100 m.w.e., appropriate for 2 kg chamber. We are looking for a deeper site in 2007 (Soudan?). ~100 kg chambers. Need to study potential sensitivity at central campus for 2010 (tons) Deep campus preferred, but not available until 2014. 2) What is the required space? More than most other dark matter experiments, because cost per unit target mass likely 1-2 orders of magnitude lower. E.g. 100 x 1 ton modules = ~ 1000 m2 of floor space (similar to 1 ton TPC in infrastructure matrix) What are the special needs: clean room, cryogenic facility, etc.? Clean rooms? Water shielding? Emergency vent path for gas?