“There is always room at the bottom” -R. Feynman, 1962

1. Tracking in the Super-Cryogenic Liquids:LHe, LHD, LNe+ some possible use of similartechniques in LArgon, per request “There is always room at the bottom” -R. Feynman, 1962 Bottom: tiny track width, energies, lowest radiation loss, “long” tracks, low backgrounds Room: big volume, long smooth precise tracks, clean target interaction, kinematics http://www.nevis.columbia.edu/~ebubble/

2. Collaborators & Consultants Lin Jia, Veljko Radeka, Pavel Rehak, Valeri Tcherniatine… thanks to John Warren…BNL Jeremy Dodd, Yonglin Ju, Mikhail Leltchouk, and thanks to Tony Heinz…Columbia Visitors: …. John Martin, University of Toronto, Sampa Bhadra, York University, Bill Metzler, Highland High School, Jennifer Denbow, University of MichiganBen Howell, Columbia UniversityManssa Muhammad, Langston UniversityDana Lindemann, Wake Forest UniversityGeoff Shraga, Bronx High School of ScienceAdam Levine, Scarsdale High School (Harvard '05)Nick Litombe, Fox Lane High SchoolPhil Harris, Byram Hills High School (Cal Tech '05)

3. Application: Solar pp fusion neutrinos, E < 423KeV • There are also some HEP applications: • High energy neutrinos • Full, clean events • s, c, b,  identification • New particle search (FNAL events?) • Super high energy (LHC) neutrinos • Heavy neutrino beams, identification • Neutral/charged Currents, etc. with kinematics • SUSY-type searches • You will hear about some measurements that are considered important in Condensed Matter Physics

4. SNOLAB Call for Letters of Interest,26 January 2004 • “It is anticipated that the extreme depth and cleanliness planned for the new facility will attract a significant number of next generation, international experiments directed at measurement of Dark Matter, Double Beta Decay, Solar and Supernova neutrinos…Next generation experiments to measure solar neutrinos in real time with energy thresholds sensitive to the dominant pp-fusion branch would provide improvements in accuracy of fundamental neutrino properties and detailed studies of stellar astrophysics.” “It is not intended that the new SNOLAB will provide opportunities for next generation nucleon decay or long baseline accelerator based …experiments.”

5. Solar Neutrino Spectrum Solar Neutrino Detector

6. Detector requirement for very low energy neutrino physics • Fine spatial resolution on sub-MeV tracks • For identification of electrons • For Range measurements • For Compton rejection • For track angle measurement • Good total charge measurement for energy • At least 5T, 25 m3 LHe fiducial volume for 1000 solar neutrino events per year • Low background • For even lower energies, for neutrino magnetic moment for example, energy resolution is key

7. The Voxel Challenge (pixel = 2-D detection element, voxel = 3-D element) • We are looking for sub-mm resolution, say 100 microns, so a cubic meter has 1012 voxels. • The challenge is to obtain the fundamental resolution, to supply a thinkable detector structure and to read it out. • For a homogeneous medium, one dimension must use a drift, so the resolution is limited by diffusion. The Einstein-Nernst law for thermal diffusion gives us few handles to get the desired resolution once we have fixed, d, the dimension of the detector: seemingly T and E.

8. For pure noble liquids, the electrons heat up in even modest electric fields, T reaches hundreds or thousands of degrees. • A massive charge carrier, like an ion, remains thermal for much higher E. • Diffusion is thermally driven; lowering the temperature T cuts diffusion. • The equilibrium negative charge carrier in liquid helium (also hydrogen and neon) is not a free electron, but a 2nm bubble containing the electron (or electrons). The positive carrier is a “snowball” with He+ inside, with comparable mobility. • We want to track electron-bubbles or snowballs in LHe; they are massive,very low temperature and subject to manipulation by their atomic properties. • The origin of the bubble is a strong (Pauli) repulsion between the electron and the helium atoms. For heavy atoms like Argon, this is compensated by the polarization of the atom.

9. Compare L Argon to L Helium, H2 • An electron near a large atom: • An electron near a He/H2 atom (Pauli)

10. Work Functions L Argon L He W = + 1.4eV W = - 0.9eV

11. Fate of an electron in L He • If an electron is created suddenly in the body of L He in the presence of an electric field, it will start to move with a large mobility as in Argon, but the repulsive force with the liquid will soon blow a hole in the liquid, creating a cavity empty of helium atoms, containing only the electron: • Scale: nanometers; a “mesoscale” object!

12. Characteristic of eBubbles • Diameter 2nm, i.e. cavity lacking a thousand He. • For E=1KV/m and T=2K, v=100mm/s, s = 100mm averaged over 5m drift. • There are ~ 800 bubbles/mm, i.e. order of magnitude denser than grains on tracks in nuclear emulsion  s = 7mm on 1mm track segment. • eBubble has several resonant absorption lines (E~0.1 and 0.9eV) in the infrared, and strong photoionization, with big cross sections due to the large size of the object, ~10-16 cm2 • eBubble remains thermal up to 40KV/cm, field-ionizes around 400KV/cm. • Liquid-gas interface (with repulsive image force) holds the electron for a time dependant on T and E, adjustable from seconds to microseconds, This delay, or photoionization of the eBubble, can be used for optical gating of signals. (There are as yet no satisfactory experimental measurements of the cross sections.)

13. Repulsive image force at interface liquid He to gas:charge0.06 e•- E X= (e/160liq E) ~50nm _ -

14. Thermal Diffusion in Helium and Neon

15. Characteristics of Tracks in LHe • Density =0.125 gm/cc • Energy loss = 24 KeV/mm minimum ionizing • Range, 100KeV =1.2mm 200KeV=3.6mm 400KeV=11mm • Radiation Length =7550mm • Fit angle to first mm of track, error for 400KeV = 20degrees, for 120KeV = 60 degrees • The last mm of track looks like about 4x minimum • Below 50KeV, we see a dot rather than a track; we intend to bring our threshold down to ~KeV with gas gain: below ~ 10KeV we enter the self shielding range. • Track width from diffusion is used for rough depth measurement. • We intend to allow a Neon fill capability; lose track direction, gain >10 in rate, and self shielding for “dot-like” tracks

16. Low energy backgrounds • There seems to be no solubility of heavier molecules in LHe. People have tried, but only introduced other elements in the liquid, for mobility measurements for example, by implantation from a beam. The purity of the liquid is expected to be perfect from this point of view, and we do not expect attachment in long drifts. • Low temperature physicists found that micropore filters gave “atomically clean” liquid. This seems to eliminate embedded sources of radioactive decays. • Adjacent electronics is probably the main internal source. Item by item study seems to be the only approach. • (Solid) Nitrogen shielding is effective against the radiation from the main cryostat wall, added to water shielding radiation from the external world. • Neutron initiated events will be recognized by the range and total ionization of tracks.

17. The good energy and spatial resolution and large size give a powerful capability for recognizing “Compton clusters,” chains of scatterings initiated by photons entering from outside. Each secondary photon from successive scatters has a lower energy, and a decreased absorption length, leading to events with a number of scattering vertices easily recognized as a Compton cluster. • There can still be events where a high energy photon (~1MeV) enters and scatters only once, giving a low energy electron recoil, accepted as a neutrino-electron scattering. Calculations by Valeri Tcherniatine calculated the rejection by cluster recognition, of the order of hundreds, depending on the source. • Our student Adam Levine followed up, calculating the equilibrium distribution of photons emitted by U and Th and all decay products. After filtering with water and nitrogen shielding, 95% of the photons reaching the detector are from the 2.614 MeV line from Thorium daughter Tl208, at seven per day (reduced from 40 million). The cluster cut and angle cuts can bring backgrounds to low levels. It would seem that the background rates go down with energy. This is important in measuring  magnetic moment. • Also important is that the recognized clusters allow one to measure accurately the photon background as a function of position, energy, angle and time.

18. Prototype eBubble Detector

19. Readout! • Consider 100m3 with 6m drift in a Solar Neutrino Detector: there are 1014 voxels to read for 0.1mm resolution element. • Drift at 10 cm/s through 6m in 60 seconds. • We see the slow drift time is very useful! Our signals are stored in the detector volume and we deal with them one plane at a time, every millisecond. • Zero suppression reduces this to < a few kHz. • The readout plane must then sample 109 pixels/ms (for 10 m2) • Options for readout plane: • Baseline: direct 2D pixels with active matrix. • Serialized by optical gating • Photo-ionization of eBubble • Photo-excitation of electron across liquid/vapor interface

20. Baseline Readout: 2D Active Matrix Pixels • Pavel Rehak has been working on a number of aspects of the readout, one of the most interesting being performance of CMOS at 4K, both through our own measurements, and investigation of new low noise, low temperature technologies in specialized laboratories. • Charge enters the sensing plane through a new invention, the Rehak Grid, a 2D pattern that approximates the 3D field form of a Frisch Grid: easy to fabricate, rugged, no dead supports, anti-microphonic. • Signal integration time 1 millisecond. • XY matrix readout • Noise: low? • Power: low?

21. Commercial direct charge detection active matrix: Digital x-ray by Hologic,sells to Kodak

22. 14” by 17” with 140 micron Nyquist resolution,OEM price about $80K, many delivered

23. Alternatives to the Baseline: • We want avalanche gain in gas, to lower the energy threshold and allow larger electrode capacitance. • Also • Maybe we cannot get 4K CMOS or practical alternative. Then we need many fewer connections (by factor of 104 say) so that we can back off into gas by some cm. • Maybe the power is too high to stick in the liquid. Then back off as above. • Maybe cost will be the issue. • Maybe we need some shielding against electronics radioactivity. • Maybe we don’t make enough progress on noise with long (1 millisecond) shaping times.

24. Optical gating • Idea: use the ability to manipulate, by light, the electron in its bubble, or trapped at the liquid/vapor interface, to deliver a voxel worth of charge into the Rehak grid (in liquid) or Frisch grid (in vapor) in 100ns, at a known location at a given time, avoiding long integration time problems. (note Sandweiss and Majka did something like this 20 years ago, trapping electrons to negative ions, gating them free with a laser, after the needed delay.) • Each channel can handle 104 pixels=1ms/100ns. • In 1ms we can sweep an optical line of width 100mm across 1m,  to 10,000 anodes/m each 1m long, with a 100ns shaping on each, covering one square meter. There are ten of these modules in the readout plane, for a total of 100,000 readout lines, each at 10MHz, or 1 THz total.

25. There have been some measurements:Photoejection of Electrons from Bubble States in Liquid Helium, by J. Northby and T. Sanders

26. Gating the liquid/vapor interface • There are measurements by Rayfield and Sanders that study the range of temperatures and fields that give delay times of the order of a second. • The dependence on those parameters shows that the process is a combination of tunneling and thermal activation; neither dominates. • It appears that one could choose temperatures and fields similar to our drift fields to set the delay time to 1ms. • Then light could be used to clear a voxel in 100ns. • Only one short pulse is needed; once the electron is in the vapor, it is a home run. • If gating is not desired, our operating conditions include regions where the delay time is small, <1ms; then a two dimensional readout must be provided • We are interested in obtaining gain in the vapor, whether we are using the optical gating or not

27. Delay in Tunneling at the 2-phase interface

28. With extraction into vapor, we are set for gain:Our collaborator A. Buzulutzkov has investigated GEM in high pressure He, to simulate the high density at low temperature. Alexei finds good operation in all noble gases with no quencher. In He, unlike common gases, the performance holds up at high pressure:

30. We need key physical measurements to move forward, as well as real experience: a Test Chamber has been constructed on the 10 cm scale: • Five windows transmitting from the infra-red to the UV • HV feedthroughs • Signal feedthroughs • Temperature and pressure over a wide range, ~1K-300K and up to ten atmospheres (the eBubble is quite compressible and allows tuning of parameters by varying the bubble radius when we want to examine structure in the absorption) • A field cage for uniform drift field • A photoelectric source for accurately timed current pulses from Xe flash lamp • Alpha and spontaneous fission source from 252Cf

31. Program of measurements planned for tracking studies • First cool down was used to examine performance of CMOS electronics • Commission sources and measure drift speed as function of parameters • Measure ion yields on different sources • Photo-ionize drifting cloud, measure power required, free electron velocity and lifetime • Lower liquid level, map delay time down to millisecond times • Study photo ejection at liquid/vapor interface • In September, install Buzulutskov GEM and two phase readout • If all goes well, build 1m Engineering Prototype in 2005…

32. Summary • We have some nice equipment. • We have several options for a solution to the tracking goal. • We have definite ideas for a SNOLAB experiment on solar neutrinos down to KeV energies • A range of interesting neutrino physics problems would benefit from these tracking detectors, at high energies as well as low. • Our investment has produced instrumentation that has not be available to the condensed matter folk up to now. Maybe we can solve some outstanding problems.

33. Applying similar ideas to very large Liquid Argon detectors • We want to: Reduce diffusion to allow us to drift a long way and keep good resolution, so use ion, say O2- with a modest attachment energy =0.4eV Diffusion in Liquid Argon for negative ions, Mobility ~ 5 10**-4 cm2/sV, T(K) = 87.2, N/mm = 8200 -------------------------------------------------------- E field velocity distance sigma y-error(z) kV/m cm/s m micron micron -------------------------------------------------------- | 0.1 867. 9. 2 1 | 1 2743. 30. | 10 8674. 95. | 100 27430. 302. -------------------------------------------------------- | 0.1 274. 3. 20 10 | 1 867. 9. | 10 2743. 30. | 100 8674. 95. -------------------------------------------------------- 200 100 | 1 274.3 3. | 10 867.4 9. | 100 2743.1 30. --------------------------------------------------------

34. Typical Parameters for a large volume • Suppose the voxel in LA is ten times the 0.1mm used in the LHe example, for 1mm each dimension Then the volume of the detector is 103 times larger for the same number of voxels, ~50,000m3 for the readout time 10x longer, for E = 20KV/s. • The optical gating as in slide 24 is used, a line of light scanned across the readout plane, traveling 10m/ms, releasing electrons from the ions. • Two options: • work in liquid phase, electron travels 1mm before recapture by O2 (O2 concentration ~10ppm)or • electron released through phase boundary into gas, where the electron mean path is much longer, allowing gas gain if needed for long readout electrodes.

35. Drift Negative Solvated Ion in LA • Ion is trapped at liquid surface, photoionize with swept-line beam, time gives x, wire gives y coord. => our 3D readout problem is down to 1D • The purity question is redefined: charge is not destroyed, what is the object we are drifting? • A neutral, shielded object:

36. The purity problem redefined • The charge is never lost, but in electron readout it is lost to view when captured into an ionic state • With 10ppm of O2 the electron is quickly captured and over microseconds the ion is solvated, by oxygen molecules in this case, a cluster with little external electric field. • What if there is 1:10-13 of, let’s say, SF6 , does it exchange with the O2 to make a somewhat lower energy ion? Probably, but how fast? • Consequences: the mobility is changed only slightly, since the solvation shells is about the same size. • The threshold for photoionization is increased by about 30%. We like to work with light of about twice the energy of the threshold, so the effect on the yield would be appreciable, but not large. There should be an adequate power margin to give good efficiency in any case. • Conclusion: The system may not require unusually high purity, compared to a large electron drift detector.

37. LA • It looks as if it might be worthwhile to take a look at these ideas • Our small test chamber has the necessary capabilities • Our priorities are elsewhere, but we may have a chance to make some simple measurements that would bear on the feasibility of negative ion drift and optical gating

38. Extraction from LKr

39. eBubble Solar Neutrino Detector