Bubble Chambers for Dark Matter Detection

Transcription

1 Bubble Chambers for Dark Matter Detection Andrew Sonnenschein Fermilab SLAC Advanced Instrumentation Seminar, March 5, 2008

2 Cartoon of a Galaxy dark matter halo Invisible material of unknown composition >85% of total mass Stars and bright gas ~ 200 kpc

3 Dark Matter From Supersymmetry Standard Model MSSM Higgs Superpartner flavor states Superpartner mass states u,d,s,c,t,b squarks e, µ, τ sleptons ν e,ν µ,ν τ sneutrinos W ± charged winos χ ± 2 Charginos H ± charged higgsinos χ ± 1 mixing γ photino χ 0 4 Z 0 zino χ 0 3 h h higgsino χ 0 2 Neutralinos Mass: GeV G H higgsino χ 0 1 A 0 gravitino Lightest neutralino is dark matter?

4 Spectrum of WIMPs in a Detector on Earth Energy of Nuclear Recoil [kev] Based on simple assumptions: Particles are gravitationally bound to halo, with Maxwellian velocity distribution (V rms =220 Km/s) and local density 0.3 GeV/cm 3 WIMPs are heavy particles, 10 GeV< M WIMP < 1 TeV. Nuclear scattering can efficiently transfer energy to a nucleus, since M nucleus ~M wimp. The signal will be a nuclear recoil, with energy ~10 kev Germanium detector Scattering is non-relativistic. Events/keV-Kg-day Shape of spectrum does not depend on particle physics inputs. Amplitude of spectrum depends on unknown supersymmetry parameters and some astrophysical uncertainties.

5 What Determines Sensitivity? Construction of sensitivity plot is illustrated with data from an early experiment. Environmental radioactivity limits sensitivity. Assume halo density 0.3 GeV/cm 3 Event Rate [events/kev-kg-day] Largest compatible Signal for 1 TeV WIMP-nucleon Cross Section [cm 2 ] Excluded Region Energy [kev] WIMP Mass [GeV/c 2 ]

6 Backgrounds from Radioactivity and Cosmic Rays A long history of successful attempts to reduce by choosing special materials and shielding. Gammas & betas From primordial, cosmogenic, and manmade nuclei: (not an exhaustive list!) 238U, 232 Th + daughters (incl. 222 Rn) 40K, 14 C 85Kr, 137 Cs, 3 H - nuclear tests 68Ge, 60 Co - cosmogenic in detector setups Cosmic Rays (p, π, µ, e ) Can be reduced by going underground. The µ s penetrate to great depth. Neutrons From µ spallation or (α, n) reactions in rocks, with alphas from U/Th chains. Can be shielded with moderator at low energies. ( figure from Brodzinski et al, Journal of Radioanalytical and Nuclear Chemistry, 193 (1) 1995 pp )

7 Discriminating Against Backgrounds WIMPs interact with the nucleus, while most backgrounds are due to electron scattering by gamma and beta rays. The resulting spatial distributions of energy and charge are very different-- this is fundamental physical basis of most discrimination techniques. 40 kev Ar in 40 Torr Ar 13 kev e - in 40 Torr Ar Y (mm) Y (mm) X (mm) X (mm) (Figures from DRIFT collaboration)

8 Observables Which Could be Used to Separate WIMP-nucleus Events from Backgrounds Spatial distribution of charge deposited in a low-pressure drift chamber. Ratio of ionization to deposited energy in a cryogenic detector (CDMS). Pulse shape discrimination in scintillating materials (organic & inorganic). Ratio of ionization to scintillation in liquid noble gases. Ratio of scintillation to deposited energy in a cryogenic detector. Annual modulation due to motion of Earth around the Sun. Daily modulation in direction of ion tracks in a low-pressure drift chamber, due to rotation of the Earth. Efficiency for bubble formation in superheated liquids (bubble chambers).

9 Background Discrimination With CDMS Cryogenic Detectors γ Rays, neutrons, & WIMPs E Temperature sensor phonons electrons & holes Charge Energy [kev] Co-60 γ rays. Cf-252 neutrons. CDMS-I Semiconductor Charge collector Recoil Energy [kev]

10 CDMS-II Detectors Phonon signal From a quadrant Phonon signal from one quadrant Direct detection of WIMP signal Nuclei recoil by elastic scattering Read both phonon signal (4 channels) and ionization signal (inner and outer electrode) 1cm thick, 7.8 cm diameter 250g Germanium, 100g Silicon A B D C Charge signal From inner electrode

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12 Why Bubble Chambers? 1. Large target masses would be possible. Multi ton chambers were built in the 50 s- 80 s. 2. An exciting menu of available target nuclei. No liquid that has been tested seriously has failed to work a bubble chamber liquid (Glaser, 1960). Most common: Hydrogen, Propane But also Heavy Liquids : Xe, Ne, CF 3 Br, CH 3 I, and CCl 2 F Good targets for both spin- dependent and spin-independent scattering. Possible to swap liquids to check suspicious signals. Low energy thresholds are easily obtained for nuclear recoils. < 10 kev easy to achieve according to standard nucleation theory. Backgrounds due to environmental gamma and beta activity can be suppressed by running at low pressure. as

13 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.

14 Bubble Nucleation by Radiation (Seitz, Thermal Spike Model, 1957) particle P vapor R c P external Pressure inside bubble is equilibrium vapor pressure. At critical radius R c surface tension balances pressure. R C = 2σ P vapor P external Bubbles bigger than the critical radius R c 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 > R c Surface tension σ Work E c R c Radius

15 de/dx Discrimination in Bubble Chambers Energy to make bubble must be deposited inside critical radius, which depends on vapor pressure. Since heavy particles have higher de/dx, operating conditions can be chosen so that chamber is triggered by heavy nucleus recoils, but not electrons. de/dx Nucleation Threshold de/dx [kev/micron] nuclear recoils Electron recoils Vapor Pressure - Operating Pressure [bar]

16 The Problem with Classical Bubble Chambers Classical bubble chambers were only sensitive for a few milliseconds per cycle. Example: SLAC 1-meter hydrogen chamber. decompression recompression ~ 10 msec Ballam and Watt, 1977.

17 Bubble Nucleation in Cracks Trapped gas volumes in surface imperfections are now known to be the primary source of nucleation. nucleation sites Liquid 0.1 µm Most construction materials have rough surfaces at scales below 1 mm, but some materials, like glass, much better than others. Solid 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 glass 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

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19 High-Stability Bubble Chamber, Waters et al., Lucite container Anti-coincidence counters Hot volume Quartz inner vessel (5-15 ml) Heat exchanger Cold volume Pressure sensor filling volume control Low temp bath inlet Expansion bellows Compressed gas line IEEE Trans. Nuc. Sci. 16(1) (1969)

20 Plot of event rate vs. superheat pressure (= vapor pressure - operating pressure) electrons protons α plateau (psi)

21 Prototype Dark Matter Detector (2003) filter pressure sensor gas Propylene glycol buffer liquid prevents evaporation of superheated liquid. gas air glycol glycol Exhaust to Room Glass dewar with heat-exchange fluid Quartz pressure vessel liquid Camera (1 of 2) Acoustic sensor 3-way valve Compressed 140 psi Piston

22 Possible Target Liquids CF 3 Br CF 3 I C 3 F 8 Mass Fractions 8% C (Z=6) 38% F (Z=9) 54% Br (Z=35) 6% C 29% F 65% I (Z=53) 19% C Density 1.5 g/ cc 2.1 g/ cc 81% F 1.4 g/ cc Xe 100% Xe (Z=54) 3.0 g/ cc Boiling 1 atm -58 C -23 C -37 C -108 C Comments Good for spin-dependent and spin- independent couplings. Spin-dependent and spin-independent Non- ozone depleting Spin- dependent only. Possible use in hybrid scintillating bubble chamber.

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24 High Speed Bubble Chamber Movie 1000 frames/ second 241Am-Be neutron source

25 High Speed Bubble Chamber Movie 1000 frames/ second 241Am-Be neutron source

26 Bad surface Good surface

27 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.

28 The COUPP Collaboration Principal Investigator: Juan Collar (spokesperson) Graduate Students: Nathan Riley Matthew Szydagis Undergraduates: Luke Goetzke Hannes Schimmelpfennig KICP Fellows: Brian Odom University of Chicago Wilson Fellows: Andrew Sonnenschein Staff Scientists: Peter Cooper Mike Crisler Martin Hu Erik Ramberg Bob Tschirhart Principal Investigator: Ilan Levine Undergraduates: Earl Neeley Tina Marie Shepherd Engineers: Ed Behnke Fermi National Accelerator Indiana University Laboratory South Bend Funding National Science Foundation Kavli Institute for Cosmological Physics Department of Energy

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

30 Neutron Flux Underground Dominant neutron sources: Direct cosmic ray neutrons. Neutrons from radioactivity in rock. Neutrons made by muons passing through high-z materials around the detector. U. Chicago Sub-basement lab NuMi Tunnel Heusser, 1995.

31 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 m 3 in volume. PISTON Buffer fluid Pressure balancing bellows S U P E R H E A T E D L I Q U I D Steel pressure vessel Thin- walled quartz bell jar HEAT EXCHANGE FLUID VIEWPORT VIEWPORT VIEWPORT

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33 Installation of 1 Liter Chamber At Fermilab Prototypes design features required for chambers up to 1000 liters

34 160 msec of Video Buffer (20 msec/frame)

35 Muon 160 psi Superheat Pressure

36 Energy Calibration with Neutrons Threshold Seitz bubble nucleation model

37 Gamma Calibration

38 Image Analysis Procedure Starting image Subtracted image Zoom In projection of subtracted image onto horizontal axis projection with noise subtraction

39 Spatial Distribution of Single Bubbles Bulk events: indistinguishable from WIMP interactions on an event-by-event basis. Wall Events: not a background, but they reduce our live time due to the need to decompress afterwards, prohibitive for larger chambers. ~ 300/day ~ events/day

40 Alpha Particle Backgrounds Alpha decay produces monoenergetic, low energy nuclear recoils. For example, consider 210 Po-> 206 Pb: E α = MeV α 206Pb E R = 101 kev 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 238 U and 232 Th decay series include many alpha emitters, including radon ( 222 Rn) 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.

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

42 Radium Decay Chain: Dominant Source of Environmental α s Red= alpha decay Radium T 1/2 =1600 y Radon T 1/2 =3.8 d Present at significant levels in most natural and man-made materials Noble gas- highly diffusive, universally present in air, water Blue= beta decay Polonium T 1/2 =3.1 m Bismouth T 1/2 = 5 d Lead T 1/2 = 27 m Lead T 1/2 = 22 y Bismouth T 1/2 = 20 m Poloniu m T 1/2 =0.2 ms Attracted to surfaces by electrostatic charge Polonium T 1/2 = 138 d Lead Stable Long half-life, implanted in surfaces by 100 kev recoil from decay of parent Hard to clean off!

43 Radium Decay Chain: Dominant Source of Environmental α s O-rings BULK Radium T 1/2 =1600 y Radon T 1/2 =3.8 d Present at significant levels in most natural and man-made materials Noble gas- highly diffusive, universally present in air, water Lead Polonium T 1/2 =3.1 m T 1/2 = 27 m Bismouth T 1/2 = 20 m Red: alpha decay Bismouth T 1/2 = 5 d Lead T 1/2 = 22 y Poloniu m T 1/2 =0.2 ms BULK Blue: beta decay Polonium T 1/2 = 138 d Lead Stable WALL? Long half-life, implanted in surfaces by 100 kev recoil from decay of parent Hard to clean off!

44 Radon Emanation from Viton Rubber Seals Viton rubber O-ring metal bellows quartz vessel radon water volume sensitive volume (CF 3 I) Borexino collaboration has measured Viton to release radon at a rate of 15 atoms/cm 2 -day => Our O-rings should each emit 50 decays/day of radon Expect a total of 300 alpha decays per day in inner vessel, accounting for 2 O-rings and 3 alpha emitters in the decay chain. We observe only a fraction of this decay rate in the sensitive volume ( per day). Decays in water are undetectable. Variation in counting rate believed due to changes in efficiency of radon transport with temperature and operating conditions.

45 New Low-Background Vessel Teflon-coated inconel seals replace Viton. Radon emanation <1% of previous design Water Quartz etched with HF acid by vendor to remove implanted radon daughters. All parts cleaned and assembled in Accelerator Division RF cavity cleaning facility. CF 3 I Very high purity, well-studied water from Sudbury Neutrino Observatory. Bellows made using non-thoriated welding electrodes.

46 New Run of 1-liter Chamber New run started July 30, Some radon introduced during filling, now decayed to equilibrium.

47 Muon Veto System An array of counters is used to tag events with an associated muon. This system will extend sensitivity to below 1 event/kg-day at Fermilab site.

48 Muon and Beam Coincident Events There are ~ 10 events a day coincident with NuMi neutrino beam and muon veto hit. Neutrinos -> Muons -> Neutrons-> nuclear recoils NUMI Veto Acoustic Pressure Time (microseconds)

49 Precision Cleaning at Fermilab Accelerator Division has superb cleaning facilities developed for RF cavities: Class 10 assembly area, large ultrasound machines, high pressure rinse machine. Using SNO- recommended special detergent formulation to remove non-implanted fraction of 210 Pb and 210 Po surface contamination. High pressure rinse to remove dust -- Goal is 1 microgram per liter for 0.01 bubbles/day. Inner vessel high pressure rinse test

50 Data from 2006 Run Data from pressure scan at two temperatures. Fit to alphas + WIMPs Energy Threshold In KeV Radon background Solid lines: Expected WIMP response for σ SD(p) =3 pb

51 Results from 2006 Run We have competitive sensitivity for spin-dependent scattering, despite high radon background Now published, Science, 319: (2008). Spin-dependent Spin-independent

52 COUPP - 60 Kg Bubble Chamber Engineering Layout Solid Model Assembly Drawing

53 60 Kg Chamber

54 20 Kg Chamber Camera array

55 2 weeks ago, 20 kg installation 16 of 17 UC HEP seminar J.I. Collar Feb. 25 UC HEP seminar J.I. Collar Feb. 25

56 Summary: First results from 2-kg chamber now published. Very competitive spin-dependent sensitivity. Radon problem seems to be solved. 20 Kg and 60 Kg chambers in advanced stages of construction, will come on line in next year. Approaching limits of our shallow underground site.