A Report on the R&D of the e-Bubble Collaboration

1. A Report on the R&D of the e-Bubble Collaboration Colin Beal Virginia Polytechnic Institute and State University R.M. Wilson Saint Louis University Advisors Dr. Jeremy Dodd, Dr. Raphael Galea & Dr. Bill Willis Nevis Labs, Columbia University REU 2005

2. Some Neutrino Physics Some Holes in Neutrino Physics Goals of the e-Bubble Detector Physics of the e-Bubble Detector Test Chamber Experimental Results Simulation Results Outline

3. Neutral charge • Spin ½ • Massless Wolfgang Pauli, 1930 Cowan & Reines, 1956 Enrico Fermi Using reactor source, “neutrino” -First experimental evidence of neutrino (Italian for “little neutral one”)

4. Neutrinos Weak interactors by the exchange of the W and Z bosons http://www-numi.fnal.gov/public/images/standardmodel.gif

5. p e- Neutrinos W & Interactions with Matter n t e- W e- e- Z e- n,p Z n,p x

6. Neutrinos t … more interactions e- W Z e- e- e+ e- W W x e- e+ e-

7. Neutrinos & the Sun

8. Neutrinos The Solar flux

9. Neutrinos The First Solar Neutrino Detector Homestake • Built at BNL in 1965 • 615 tons tetrachloroethylene • Observed the following solar neutrino reaction… • Saw deficit in solar neutrino flux… http://www.its.caltech.edu/~sciwrite/journal03/A-L2/greissl.html

10. Neutrinos The Solar Neutrino Problem The Solar Standard Model (SSM) is tested… Super-Kamiokande • H2O Cherenkov Detector, 500 metric tons • Minimum ~3 MeV neutrinos • Detects Cherenkov light from scattered electrons • Reported 1/3 expected solar neutrino flux http://ale.physics.sunysb.edu/nngroup/superk/pic/sk-half-filled.jpg The missing neutrinos can be compensated for if a model incorporating new physics is taken into account…

11. Neutrinos They Oscillate Assuming that neutrinos do have some mass, and that their masses are a mixture of the neutrino (say ne and nm) flavor eigenstates… Then the probability that an ne will be detected as an ne a distance L (km) away from its origin is given by… constant Energy of Neutrino (eV) Mass difference

12. Neutrinos The Solar Neutrino Solution Sensitive to electron, muon and tau neutrinos… SNO • D2O Cherenkov Detector, 1000 metric tons • Minimum ~3 MeV neutrinos • Detects Cherenkov light from scattered electrons • Reported expected solar neutrino flux So what else is there to know? http://www.pparc.ac.uk/Nw/Press/sudburysalt.asp

13. Neutrinos There is so much more… • What can we learn from low-energy neutrino experiments? … • Most of the Suns power lies at energies well below the threshold of current real-time neutrino detection experiments. • Our models tell us that high energy neutrino oscillations (governed by the MSW effect) behaves much differently than low energy neutrino oscillations. • Is nuclear fusion the primary source of the Suns energy, or is there something else at work? • The neutrino magnetic moment m is much more accessible for measurement at low energies.

14. e-Bubble The Objective To design, build and implement a real-time low-energy neutrino detector* using a cryogenic liquid detection medium. *The detector will be a tracking detector, i.e. one which utilizes the ionization track of electrons produced in a ne-e scattering event to extract information about the incident particle, in this case, a neutrino.

15. e-Bubble Performance Goals Due to the nature of low-energy neutrinos, we’ll need a detector with the following features… • Excellent spatial resolution (sub-mm) • Excellent energy resolution • Large volume or high event-rate • Low background

16. e-Bubble Tracking Detector 2-D Detection Plane Drifting Ionized Electrons Incident Neutrino n-e interaction e-e ionizations

17. Neutrino-Electron Interaction Origin of the Electron Track Bahcall, John H., Rev. Mod. Phys., 59, 2, 1987.

18. Neutrino-Electron Interaction • Cross-Sections Magnetic Moment m

19. Neutrino-Electron Interaction • Cross-Sections Weak Interactions

20. e-Bubble Tracking Detector

21. e-Bubble Information from Tracks Length of Track Energy of Neutrino Total Ionized Charge Origin of Neutrino Shape of Track

22. e-Bubble The Detector Medium LNe LHe • T = 27K • r = 1.24 g/cm3 • ~1 metric ton • Short tracks ( 700 mm,  300 keV) • Pointing only for highest energy npp • Self-shielding • T = 2K • r = 0.125 g/cm3 • ~5 metric tons • Long tracks (1-7 mm, 100-300 keV) • Good pointing capability • Minimum ionizing (low dE/dx) • Pure (long drifts, low internal background) e-Bubbles

23. Solar npp flux 6.2E10 cm-2s-1 • Expect ~674 ton-1year-1 LNe • T = 27K • r = 1.24 g/cm3 • ~1 metric ton • Short tracks ( 700 mm,  300 keV) • Pointing only for highest energy npp • Self-shielding • Minimum ionizing (low dE/dx) • Pure (long drifts, low internal background) e-Bubbles

24. e-Bubbles … A Social Metaphor A Red Sox fan enters Yankee Stadium… Go home r And the “Red Sox Fan”-Bubble phenomenon may be observed…

25. e-Bubbles In LNe (or LHe) • Equilibrium state of free electrons in Low-Z noble liquids (LHe, LNe) • Due to Pauli repulsion between free electron and noble atoms • ~1-2 nm diameter • Displaces ~50-100 atoms of liquid

26. e-Bubbles In LNe (or LHe) Useful Properties… Creates large “drag” in liquid Low mobility Slow drift velocity in electric field Small diffusion due to thermal equilibrium

27. LNe Physics of Ionization Tracks • Two primary forms of charged particle energy loss… • Radiative (Bremsstrahlung) • Ionization

28. LNe Physics of Ionization Tracks

29. LNe Physics of Ionization Tracks

30. LNe Physics of Ionization Tracks

31. LNe Physics of Ionization Tracks

32. LNe Physics of Ionization Tracks 250 keV Recoil Electron Tracks 150 keV Recoil Electron Tracks (Single ionizations, parameterized angular distribution)

33. LNe Pointing Capability How well can we determine the origin of the incident neutrino? • Angular diffusion of the ionization track • Length of ionization track • Diffusion over drift in detector

34. LNe Pointing Capability

35. LNe e-Bubble Drifts Liquid Surface Einstein-Nernst Equation for Thermal Diffusion s Path of e-Bubble Drift Ionization Location

36. LNe e-Bubble Drifts Predicted Mobility… Drift Velocity… E = 1000 V/cm E = 5000 V/cm

37. LNe e-Bubble Drifts Liquid Surface What happens at the liquid surface? Why does it matter?

38. LNe Trapping e-Bubbles at the Liquid-Vapor Interface • Dielectric discontinuity at the interface (el> ev) • Potential well just beneath surface • e-Bubble has some probability of tunneling through potential barrier in time Schoepe, W. and G.W. Rayfield, Phys. Rev. A, 7, 6, 1973.

39. LNe Trapping e-Bubbles at the Liquid-Vapor Interface Barrier Height

40. 2-D Detection Ejecting Charge from Liquid Surface • Method needs to be conducive to maintaining resolution (energy and spatial) • Local high-field pulsing at surface • Photo-emission Due to their large size, e-Bubbles are highly sensitive to photo-excitation. Effective, but noisy

41. 2-D Detection Charge Amplification Due to low ionized charge, a method of amplification is required… GEMs • High localized fields • Charge amplification and light emission (~1000x amplification)

42. 2-D Detection Charge Amplification Due to low ionized charge, a method of amplification is required… GEMs • Commercial CCD Cameras to read out light emission • Pixelated anode • No method for in-liquid detection found effective Garfield simulation of charge amplification and drifts

43. In the mean time… some proof of principle. • Experimental verification of LNe physics • Simulated LNe drifts All essential in constructing a large scale detector

44. Research and Results • Outline: • e-Bubble Test Chamber Setup • Experimental Data • Computer Simulation Results

45. Experimental Run:Design e-Bubble experiment is set up at Brookhaven National Lab A cryostat uses liquid Helium (~4K) and liquid Nitrogen (~77K) to cool the test chamber. Optical windows enable “first-hand” observation of the experimental runs

46. Experimental Run:Test Chamber Setup Electrons must be “artificially” inserted into the test chamber • Goals: • - Test electron sources • - Make electron bubble drift measurements

47. Experimental Run:Electron Sources • Photo-Cathode • High Voltage Tip • Radioactive Alpha Source

48. Experimental Run:Drift Time Experimental Theoretical Although the experimental drift time differs from the predicted time by only a few ms, many approximations were used. …stay tuned Drift time is 78 ms @ 4 kV/cm Using µ = 1.6E-3 (cm2/Vs) Drift time is ~80 ms @ 4 kV/cm

49. Experimental Run:Mobility • Using the predicted drift time equation, mobility was fitted as a free parameter 1.66E-3 < µ < 1.9E-3 (cm2/Vs) • The derived mobility was consistent with previously determined electron bubble mobility in LNe (Storchak, Brewer and Morris). Drift time (ms) E-Field (kV/cm) Drift time (ms) E-Field (kV/cm) C (cm2/V) is a constant to compensate for omitting the emission and anode regions

50. Experiment Run:Drift Velocity • The electron bubble drift velocity can be determined using: V=µE For µ=1.6E-3 (cm2/Vs) and E=4 kV/cm; V = 6.64 cm/s.