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Ionization Imaging: a better way to search for 0- v  decay?

Ionization Imaging: a better way to search for 0- v  decay?. David Nygren LBNL - Physics Division. Z. Two Types of Double Beta Decay. A known “background” process and an important calibration tool. 2  . This process not yet observed particle = antiparticle.

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Ionization Imaging: a better way to search for 0- v  decay?

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  1. Ionization Imaging:a better way to search for 0-v  decay? David Nygren LBNL - Physics Division Paris 11-12 December 2006

  2. Z Two Types of Double Beta Decay A known “background” process and an important calibration tool 2  This process not yet observed particle = antiparticle Neutrino effective mass Neutrinoless double beta decay lifetime 0  Paris 11-12 December 2006

  3. 0-vDecay • If 0-v decays occur, then: • Neutrino mass ≠0 (now we know this!) • Decay rate measures effective mass mv  • Neutrinos are Majorana particles • Lepton number is not conserved • Because the physics impact is so great, the experimental result must be robust. Paris 11-12 December 2006

  4. A “robust” experimental result: Only 2-vdecays! Only 0-v decays! Rate No backgrounds above Q-value! 0 Energy Q-value The experimental result is a spectrum of all  events, with very small or negligible backgrounds. Paris 11-12 December 2006

  5. Energy Resolution Germanium Detectors: • Excellent electron and hole mobilities • Complete charge collection • Small level of recombination • Charge collection independent of track topology • Small energy per ion/electron pair • Fluctuations small The Gold Standard:Germanium diodes E/E ~ 1.25 x 10-3 FWHM (at 2.6 MeV) Paris 11-12 December 2006

  6. Energy Resolution “Extra margin in energy resolution is very desirable because non-gaussiancharacteristics are often present in the tails of the experimental distribution.” “To realize an energy resolution near the limit imposed by physical processes, the detector and target must be the same.” Paris 11-12 December 2006

  7. Using Energy to Detect  Get a large quantity of candidate nuclei Put them in an electron detector Shield and purify Acquire data for a few years (“plug and pray”) Cut on energy to select out the neutrinoless events 100’s of kg target - Condensed matter strongly preferred Theory Spectra from Ludwig DeBraekeleer Esum of 2 final state electrons Practice positive signal claim is disputed! Data massaged to give ~4 effect… Spectra from Klapdor Kleingrothaus et. al. Paris 11-12 December 2006  Background rejection is essential - energy resolution may not be enough !

  8. Present Status • Heidelberg-Moscow (H-M) claim: mv= 0.44 +0.14-0.20 eV (best value) disputed! 0v1/2 = (0.8 — 1.83) x 1025 y (95% c.l.) Scale: ~11 kg of 76Ge, for ~7 years No other claim for a positive result exists Paris 11-12 December 2006

  9. CAMEO......... CANDLES..... COBRA......... CUORE......... DCBA............ EXO.............. GERDA......... GSO.............. Majorana....... MOON........... Nano-crystals NEMO + ... Xe................. XMASS......... CdWO4 crystals in liquid scintillator CaF2 crystals in liquid scintillator CdTe semiconductors TeO2 bolometers, Cuoricinonow Nd foils and tracking chambers Xe TPC; liquid now, maybe gas later Germanium crystals in LN Gd2SiO5 crystals in liquid scintillator Segmented Ge crystals Mo foils and plastic scintillators Suspended nanoparticles Foils with tracking Xe dissolved in liquid scintillator Liquid xenon Worldwide Activity Paris 11-12 December 2006

  10. Multi-module approaches: • Cuoricino (130Te): background-limited! • E/E only  Cuore says factor of ~20 enough • Majorana (76Ge): pre-construction stage • E/E + multi-site rejection (x10) • Common to both: • Multi-detector coincidences can reject some backgrounds, but, I argue that: • “Background rejection factors must be improved by at least two orders of magnitude” Paris 11-12 December 2006

  11. 136Xe: Qv = 2.48 MeV • Previous Xe experiments are inconclusive • HPXe TPC in Gotthard tunnel (5 bar, no start time) • Several MWPC experiments in Russia • EXO • EXO-200 underway with LXe @ WIPP • Results eagerly awaited! • Laser tagging of barium daughter: R&D stage • Strong anti-correlation of ionization/scintillation • “Energy resolution seems poor: E/E ~ 3.5% ?” • “Energy resolution appears to be non-Gaussian” Paris 11-12 December 2006

  12. A robust experiment: • Has negligible overlap of 0- and 2-events • excellent energy resolution is essential • Detects both 0-v  and 2-v  topologically • does not depend solely on end-point energy • Scales gracefully to large active mass • M ~ 1/ mv2  >1000 kg needed, perhaps • Rejects backgrounds effectively • no partially sensitive or dead surfaces Paris 11-12 December 2006

  13. Experimental Approach “We believe that an Imaging Ionization Chamber is most likely to meet all criteria imposed for a robust experiment.” An Imaging Ionization Chamber (IIC) is a TPC without gain at the readout plane Paris 11-12 December 2006

  14. Imaging Ionization Chamber -HV plane Pixel Readout plane Pixel Readout plane ~99%Xe + ~1% CH4 @ 20 bars . electrons ions Paris 11-12 December 2006

  15. Proportional Gain: good results for low-energy x-rays MWPC, GEM, micromegas all work well... but: why are there so many events below the peak? Paris 11-12 December 2006

  16. Proportional Gain: poor resolution for MeV energies Typical E/E (observed): 4 - 6.6% FWHM @ 2.5 MeV • Extended tracks? • ballistic deficit in electronics & signal processing • impact of space charge on gain • poor localization of edge effects • Gain stability variations? • gas density, temperature, composition • mechanics, calibration maps Paris 11-12 December 2006

  17. Why Ionization Mode? • Reason # 1: • “Best E/E is by direct charge integration” • Many sensitivities vanish or are minimized • Reason # 2: • “Gain may be needed at HV plane” • Novel idea for detecting barium daughter Paris 11-12 December 2006

  18. Imaging Ionization Chamberis filled with 136Xe gas • xenon is relatively safe and easy to enrich • Natural abundance of 136Xe is ~ 8% • EXO has 200 kg highly enriched in 136Xe • high density is desirable to contain event • But there is an upper limit!  < 0.55 g/cm3 • Beyond this density, E/E deteriorates rapidly! Paris 11-12 December 2006

  19. Energy Resolution by Ionization in xenon shows complex behavior! Paris 11-12 December 2006

  20. IIC Operating Conditions • Working density:  ~ 0.1 g/cm3 (LXe = 2.95 g/cm3) • ~20 bars; (critical point P = 58 bars, T = 290° K) • 1000 kg in ~10 m3 : = 200 cm, L =300 cm • dn/dx ~ 1.5 fC/cm: ~9,000 (electron/ion)/cm • pad size: ~0.5 cm  4500 electrons/pixel • ultra-low noise electronics is critical! • HV needed: • probably >1 kV/cm  150 kV • slightly unpleasant, but hopefully tractable… Paris 11-12 December 2006

  21. UV Scintillation in HPXe • Can use signal for event start time: “t0” • N ~ Nelectron/ion pairs ~ 118,000 at Q = 2.48 MeV • ratiodepends on E field • spectrum peaks at ~170 nm ~ 7 eV • However, small amount of “CH4” is necessary • to cool electron drift and keep diffusion low for tracking • Does methane absorb 170 nm light? - No! • Does methane quench scintillation? - don’t know, but • 2% added to LXe without loss! • Does HPXe behave similarly? maybe... Paris 11-12 December 2006

  22. Event Characteristics in IIC • High density of xenon constrains  event: • total track length ~10 - 20 cm max • Multiple scattering will be prominent in xenon • unclear if B-field would help identification • True  events will have two “blobby” ends • shown to reject background by ~30 in Gotthard TPC • Bremsstrahlung present in many events • bigger detector better to catch ’s • Fluorescence ’s from photoelectric  conversion • Distinct satellite “blobs” may be visible Paris 11-12 December 2006

  23. Imaging Ionization Chamber has a fully “decorated” pixel readout plane • no grids or wires: eliminates microphonics • pixel size is 5 mm x 5 mm (4 x 104 /m2) • ~ 40 - 80 contiguous “hit” pixels for E = Q • dn/dx = ~4500 electrons/(5mm) • ultra-low noise readout electronics - BNL ASIC • <n> = ~30 e– rms for 4 s shaping time, with pixel! • Other noise terms must be included  <n> = 60 e– rms? • “waveform capture” essential for extended tracks Paris 11-12 December 2006

  24. Pixel geometry A low capacitance solution: a 7-pixel hexagonal sub-module: Hexagonal pads may be optimal Paris 11-12 December 2006

  25. Imaging Ionization Chamber collects electrons on pixel readout plane • all energy information is derived from q =  I dt • current is very small until electrons approach pixel • pixels with no net charge have bipolar currents • drift velocity is small, 0.4< Vd <0.8 cm/s • electron diffusion after 1.5 m drift is ~ 2 mm rms • event reconstruction from “contiguous hit pixels” • noise adds only from hit pixels + some neighbors Paris 11-12 December 2006

  26. 2 1 0 Simulated induced signals for a single cluster at pad 0 centre 5 mm diameter padsv(drift) = 1.25 cm/s, drift gap = 2.5 cm Pad 2 Current signal Charge signal Pad 1 Pad 0 Charge preamp risetime 400 ns, decay time 20 s Credit: Madhu Dixit - Carleton U

  27. “Intrinsic” Energy Resolution Q-value of 136Xe = 2.48 MeV W = E per ion/electron pair = 21 eV N = number of ion pairs = Q/W N  2.49 x 106 eV/21 eV = 118,000 N2 = FN (0.05 < F < 0.17) F = 0.17 for pure noble gases (theory)  N = (FN)1/2 ~ 140 electrons rms Paris 11-12 December 2006

  28. Energy Resolution… • If ionization were the only issue: E/E = 2.9 x 10-3FWHM • Other contributions: • electronic noise from N = 49 pixels in event • N1/2 x <n> if noise is gaussian ~ 7 x 60 = 430 e– • any uncorrected deficits in signal processing • “locked” charge caused by slow-moving ions E/E < 10.0 x 10-3 FWHM Paris 11-12 December 2006

  29. Imaging Ionization Chamber -HV plane Pixel Readout plane Pixel Readout plane Fiducial volume . CsI photo-cathode? Wave-shifter bars, for t0 ? electrons ions Paris 11-12 December 2006

  30. Backgrounds • Ionizing backgrounds from surfaces: • Rejection of can be essentially 100.0% • External non-ionizing backgrounds: •  conversion: probably most serious TBD • multiple Compton events detectable: TBD • neutron activation of target gases: TBD • must go deep underground: TBD • vapor pressure of uranium/thorium series? • ionized daughters swept out quickly... Paris 11-12 December 2006

  31. Imaging Ionization Chamber… • “provides excellent tracking & 3-D topology” • spatial resolution much smaller than pad size • “provides fully closed, active fiducial surface” • ex post factovariable definition with mm resolution • “provides energy resolution of 1% FWHM” • direct charge integration offers stable operation • “seamless and graceful scaling behavior” • active mass to ~1000+ kg Paris 11-12 December 2006

  32. The barium daughter atom • In a volume of ~1027 xenon atoms, a  event creates one barium atomic ion. • The Ba ion drifts out to the HV plane, and in ~ 1 second, the ion will be lost! • Is this a hopeless situation? Paris 11-12 December 2006

  33. Barium Daughter Atom • In pure xenon/CH4, the Ba++ ion will survive: • IP(Xe) = 12.13 eV, • IP(CH4) = 13.0 eV • First IP(Ba+) = 5.212 eV • Second IP(Ba++) = 10.004 eV • if impurities exist with IP less than 10 eV: • Ba++ may become Ba+ through charge exchange Paris 11-12 December 2006

  34. Ion Mobilities Is there a straightforward way to detect and identify the barium daughter? • Ba and Xe ion masses are ~identical… • Ba+ and Xe+ ion charges are identical… • Ion mobilities should be the ~same, ??? Paris 11-12 December 2006

  35. Ion Mobilities… • But: Ion mobilities are quite different! • The cause is resonant charge exchange • RCE is macroscopic quantum mechanics • occurs only for ions in their parent gases • no energy barrier exists for Xe+ in xenon • energy barrier exists for Ba ions in xenon • RCE is a long-range process: R >> ratom • glancing collisions = back-scatter RCE increases viscosity of majority ions Paris 11-12 December 2006

  36. Ion Mobilities in Xenon • (Xe+) = 0.6 cm2/V-sec • (Cs+) = 0.88 cm2/V-sec (Cs is between Ba & Xe) So, the barium ion could move faster by ~50%! (maybe even faster if Ba++ is stable) RCE may provide a way to distinguish a barium daughter ion from all the xenon ions! • But: impurities can confound measurements • Mobility differences have been measured at low pressures, where mass spectrometry works Paris 11-12 December 2006

  37. Ion mobility in dense gases? • Ion mobility data at high pressure does not apparently exist in the literature. • Clustering may be prominent in HPXe • Clustering phenomena are complex, and may introduce very different behavior • Not clear whether this will help or hurt! • Low pressure measurements not adaptable to high pressures like 20 bars - need new method Paris 11-12 December 2006

  38. Ba Daughter Detection • Let’s assume that barium ion mobility is not identical to xenon ion mobility; then: • A barium ion will arrive at the HV plane at a different time than the Xe+ ion track image. • If event time origin and mobilities of the barium and xenon ions are known, an arrival time for the barium daughter at HV plane is predicted. • The unique t between Xe+ and Ba+ ions is a robust signature for a true  event. Paris 11-12 December 2006

  39. Detection of Ion Arrival • Detection of ion arrival maybe possible: • Ions drift at thermal energies to HV plane… Then: • Ions are attracted to “high field pore (HFP)” • Very high electric field inside HFP • Ions can enter, but electrons are blocked • High energy tail of M-B distribution relevant • Ba++ may be critical for desired outcome • desired outcome: ≥1 electron appears! Paris 11-12 December 2006

  40. “Blind GEM, CAT, Microwell” -HV plane Drift region: Low E-field Resistive back side: blocked to electrons ions Very High E-field inside pore open to entry by ions Paris 11-12 December 2006

  41. The barium daughter “Echo” • If a single electron appears, high E-field in blind GEM causes electron avalanche. • Electron avalanche will saturate, producing a large pulse of electrons, more than 103. • Electron pulse returns to pixel plane, at a spot on the projected event track. • This spot on the projected track is very close toorigin of the barium daughter. Paris 11-12 December 2006

  42. “Birth Detection” Because the echo tagging is so precise in space and time (if it can be done at all) I refer to this process as “Birth detection” Paris 11-12 December 2006

  43. The Return Image Echo • The (100,000) Xe+ ions also enter HFP, but well-separated from the barium daughter ion. • An “echo” of the entire event tracks is created, (assuming ion detection works) • Image echo times will be distinct from the pulse due to the barium daughter. Paris 11-12 December 2006

  44. What to do? • An R&D (and library) effort is needed to: • Develop/exploit simulation tools • Optimize S/N with real electronics • Measure E/E in HPXe IIC with  rays • Determine ion mobilities in HPXe • Explore ion-induced avalanche possibilities Paris 11-12 December 2006

  45. Paris 11-12 December 2006

  46. HPXe System at LLNL! • Built for  spectroscopy • Unused at present! • Available now! • Dual 50 bar chambers • Gas purification • Gas storage • Perfect R&D platform Paris 11-12 December 2006

  47. Paris 11-12 December 2006

  48. Paris 11-12 December 2006

  49. Summary • A novel concept for a robust0- decay searchhas been developed: • E/E ~ 1% FWHM • Detailed & constrained 3-D event topology • Active, complete, & variable fiducial boundaries • Identification of Ba daughter possible, perhaps, by a macroscopic quantum mechanical phenomenon: Resonant Charge Exchange: RCE • Maybe, this new concept might work! Paris 11-12 December 2006

  50. Thanks to: • Azriel Goldschmidt - LBNL • John Kadyk - LBNL • Jacques Millaud - LBNL / LLNL • Adam Bernstein - LLNL • Mike Heffner - LLNL • Leslie Rosenberg - UW • Madhu Dixit - Carleton U • Cliff Hargrove - Carleton U Paris 11-12 December 2006

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