SCIPP - November 14, Ionization Imaging: a better way to search for 0- v  decay? David Nygren LBNL Physics Division

SCIPP - November 14, Two Types of Double Beta Decay This process not yet observed particle = antiparticle Neutrinoless double beta decay lifetime Neutrino effective mass A known background process and an important calibration tool Z 0  2 

SCIPP - November 14, v  Decay If 0- v decays occur, then: –Neutrino mass ≠0 (now we know this!) –Decay rate measures effective mass  m v  –Neutrinos are Majorana particles –Lepton number is not conserved Because the physics impact is so great, the experimental result must be robust.

SCIPP - November 14, Uncertainties… Hierarchy uncertain oDetermines needed sensitivity Matrix element calculations uncertain oOrder of magnitude in rate Effective mass uncertain oPhases enter:  m v  = |∑ |U ei | 2  i m i | Direct tests by 3 H kinematics uncertain oFor  m v  << 1 eV, technically very difficult! Best experimental approach: uncertain!

SCIPP - November 14, A “Robust” Experiment: Only 2- v decays! Rate EnergyQ-value Only 0- v decays! No backgrounds above Q-value! The experimental result is a spectrum of all  events, with very small or negligible backgrounds. 0

SCIPP - November 14, A robust experiment: Has negligible overlap of 0- and 2- events –excellent energy resolution is essential! Selects 0- v  and 2- v  events identically –does not depend solely on end-point energy! Scales to large active mass –M ~ 1/  m v  2  >1000 kg needed? Rejects all backgrounds effectively –no insensitive surfaces!

SCIPP - November 14, 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: Energy resolution with Germanium detector:  E/E ~ 1.25 x FWHM at 2.6 MeV

SCIPP - November 14, Energy Resolution “Extra margin in energy resolution is very desirable because non-gaussian characteristics 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.”

SCIPP - November 14, Using Energy to Detect  Spectra from Ludwig DeBraekeleer 1.Get a large quantity of candidate nuclei 2.Put them in an electron detector 3.Shield and purify 4.Acquire data for a few years (“plug and pray”) 5.Cut on energy to select out the neutrinoless events 100’s of kg target - Condensed matter strongly preferred Theory Practice Spectra from Klapdor Kleingrothaus et. al. positive signal claim is disputed!  Background rejection is essential - energy resolution may not be enough ! E sum of 2 final state electrons

SCIPP - November 14, Present Status Heidelberg-Moscow (H-M) claim:  m v  = eV (best value) disputed!  0v 1/2 = (8 — 18.3) x y (95% c.l.) Scale: ~11 kg of 76 Ge, for ~7 years No other claim for a positive result exists

SCIPP - November 14, Worldwide Activity CAMEO CANDLES..... COBRA CUORE DCBA EXO GERDA GSO Majorana MOON Nano-crystals Super-NEMO Xe XMASS CdWO 4 crystals in liquid scintillator CaF 2 crystals in liquid scintillator CdTe semiconductors TeO 2 bolometers, Cuoricino now Nd foils and tracking chambers Xe TPC; liquid now, maybe gas later Germanium crystals in LN Gd 2 SiO 5 crystals in liquid scintillator Segmented Ge crystals Mo foils and plastic scintillators Suspended nanoparticles Foils with tracking Xe dissolved in liquid scintillator Liquid xenon

SCIPP - November 14, Present Perspective… Cuoricino ( 130 Te): background-limited –  E/E only  Cuore needs factor of ~20 Majorana ( 76 Ge): pre-construction stage –  E/E + multi-site rejection (x10), but –a factor of several 100 needed beyond HM Common to both: –Multi-detector coincidences can reject many backgrounds, but: –Large rejection factor needed for success

SCIPP - November 14, Xenon Q v = 2.49 MeV ( 136 Xe) Previous experiments inconclusive –HPXe TPC in Gotthard tunnel (5 bar, no start time) –Russian experiments with various MWPC EXO –EXO-200 underway with WIPP –No laser tagging of barium daughter: R&D stage –Strong anti-correlation of ionization/scintillation –Results eagerly awaited

SCIPP - November 14, NUSAG Recommendations: “…support research in two or more 0- v  experiments to explore the region of degenerate neutrino masses (  m v  > 100 meV )…” “The knowledge gained and the technology developed in the first phase should then be used in a second phase to extend exploration into the inverted hierarchy region of (  m v  > meV ) with a single experiment.” But: no explicit encouragement for new ideas!

SCIPP - November 14, 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

SCIPP - November 14, Imaging Ionization Chamber -HV plane Pixel Readout plane. ionselectrons ~99%Xe + ~1% CH 20 bars

SCIPP - November 14, 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?

SCIPP - November 14, Proportional Gain: poor resolution for MeV energies Typical  E/E: % 2.5 MeV –Gain variations? gas density, composition mechanics, calibration maps –Extended tracks? ballistic deficit in signal processing impact of space charge on gain –Intrinsic physical phenomena? sensitivity to dE/dx density variations large scintillation/ionization fluctuations

SCIPP - November 14, Ionization Chamber Mode Reason # 1: –“Best energy resolution can only be obtained through direct charge integration” There is a lot to learn here... Reason # 2: –“Gain may be needed at HV plane…” This is a new, but very speculative aspect

SCIPP - November 14, Imaging Ionization Chamber is filled with 136 Xe gas Xe is relatively safe and easy to enrich EXO has 200 kg highly enriched in 136 Xe high density is desirable to contain event But there is an upper limit:  < 0.55 g/cm 3 working density:  ~ 0.1 g/cm 3 (  LXe = 2.95 g/cm 3 ) ~20 bars; critical point P = 58 bars, T = 290° K 1000 kg in ~10 m 3 :  = 200 cm, L =300 cm dn/dx ~ 1 fC/cm = 6000 electron/ion pairs/cm

SCIPP - November 14, UV Scintillation in HPXe N  ~ N electron/ion pairs ~ 118,000 at Q = 2.49 MeV –can use for event start time –ratio depends on E field –spectrum peaks at ~170 nm ~ 7 eV However, small amount of “CH 4 ” 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...

SCIPP - November 14, Imaging Ionization Chamber… “provides adequate S/N for good tracking” –detailed imaging of event topologies “provides fully closed, active fiducial surface” –ex post facto variable definition with mm resolution “provides energy resolution of 1% FWHM” –avoids scintillation/ionization anti-correlation “may permit detection of birth of Ba daughter” –automatic process tags both space and time

SCIPP - November 14, Event Characteristics in IIC –High density of xenon constrains  event: Total track length ~ 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 and fluorescence  ’s Distinct satellite “blobs” may be visible

SCIPP - November 14, 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 10 4 /m 2 ) ~ contiguous “hit” pixels for E = Q dn/dx = ~3000 electrons/(5mm) –ultra-low noise readout electronics - BNL ASIC = ~30 e – rms for 4  s shaping time, with pixel! Other noise terms must be included  = 60 e – rms? –“waveform capture” essential for extended tracks

SCIPP - November 14, Pixel geometry A low capacitance solution: a 7-pixel hexagonal sub-module: Or, a 16 channel 4x4 rectangular array…

SCIPP - November 14, Imaging Ionization Chamber collects electrons on pixel readout plane –all energy information is derived from q =  Idt –current is very small until electrons approach pixel –pixels with no net charge have bipolar currents –drift velocity is small, 0.1< V d <0.5 cm/  s –diffusion after 1.5 m drift is ~ 2 mm rms –event is reconstructed from contiguous hit pixels –noise adds only from hit pixels + some neighbors

SCIPP - November 14, Geminate and Volume Recombination Reduces the yield of free ionization Degrades the energy resolution. Recombination rate depends on ionization density, carrier mobility, relative orientation of track & E field. Occurs in gases, liquids, solids, semiconductors. Electrons that scatter and thermalize, or meander, within the Onsager radius r o = e o 2 /(4  o  r k B T) of an ion will recombine r o ~60 nm in gases A significant effect for 20 bar Xe? Apparently not...

SCIPP - November 14, Energy Resolution… Q-value of 136 Xe = 2.48 MeV W =  E per ion/electron pair = 21 eV N = number of ion pairs = Q/W N  2.49 x 10 6 eV/21 eV = 118,350  N 2 = FN (0.05 < F < 0.17) F = 0.17 for pure noble gases (theory)   N = (FN) 1/2 ~ 140 electrons rms

SCIPP - November 14, Energy Resolution… If ionization were the only issue:  E/E = 2.9 x FWHM Other contributions: –electronic noise from N = 49 pixels in event N 1/2 x if noise is gaussian ~ 7 x 60 = 430 e – –ballistic deficit in signal processing –“locked” charge caused by slow-moving ions  E/E < 10.0 x FWHM

SCIPP - November 14, Photo-ionization in LXe Liquid state: – the IP is typically substantially lowered relative to that in gas IP of TMA (TEA) = 7.82 (7.50) eV (dilute gas) IP of TMA (TEA) = 6.1 (5.9) eV in LXe LXe scintillation: ~7 eV –TMA and TEA photo-ionize 80% of LXe scintillation at a few 10’s of ppm. –What might happen in HPXe gas?

SCIPP - November 14, IIC and Imaging Power The 3-D imaging of the IIC provides: –Topology reconstruction –Energy resolution independent of scale –Active and continuous fiducial surfaces –Variable fiducial surfaces ex post facto Rejection of ionizing backgrounds from surfaces can be essentially 100%

SCIPP - November 14, Perspective The basic IIC concept offers: –Stable operation: ionization mode –Excellent energy resolution: ~1% FWHM –Good scaling: active mass ~1000 kg –No dead surfaces: 3-D event placement –Active, adjustable fiducial boundaries –Topological rejection of backgrounds –Possibility to evolve further…

SCIPP - November 14, Barium Daughter Atom –In a volume of ~10 27 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?

SCIPP - November 14, Barium Daughter Atom –In xenon/CH 4, the Ba ++ ion will survive: IP(Xe) = eV, IP(CH 4 ) = 13.0 eV First IP(Ba + ) = eV Second IP(Ba ++ ) = eV –if impurities exist with IP less than 10 eV: Ba ++ becomes Ba + through charge exchange

SCIPP - November 14, 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, Right??

SCIPP - November 14, 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 resonant charge exchange is a long-range process; glancing collisions = back-scatter –RCE increases viscosity of ions

SCIPP - November 14, Ion Mobilities in Xenon –Mobility differences have been measured at low pressures, where clustering effects are small:  (Xe + ) = 0.6 cm 2 /V-sec  (Cs + ) = 0.88 cm 2 /V-sec (Cs is between Ba and Xe) –So, the barium ion should move faster by ~50%! (maybe even faster if Ba ++ is stable) RCE can provide a way to detect Ba daughter!

SCIPP - November 14, Ion mobility in dense gases? Ion mobility data at high pressure does not apparently exist in the literature. –Clustering may be prominent at 20 bars. –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

SCIPP - November 14, Ba Daughter Detection If we 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.

SCIPP - November 14, Arrival Time Separation Assume low-pressure data… –Assume drift distance: L=  T = 250 mm –Thermal transport diffusion:  ~.25 mm  /L = 0.25/250 = 1/1000  /(  L) = 2 1/2 /(  Ba -  Xe )T ~ 1/235 –Arrival times are very precisely determined

SCIPP - November 14, Detection of Ion Arrival Detection of ion arrival may be 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 Hoped-for outcome: ≥1 electron appears

SCIPP - November 14, Blind GEM or “Microwell” Drift region: Low E-field ions Very High E-field inside pore; low work-function surface? Resistive back side blocked to electrons HV plane

SCIPP - November 14, 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 –Electron pulse returns to pixel plane, at a spot on the projected event track. –This spot on the projected track is very close to origin of the barium daughter.

SCIPP - November 14, “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”

SCIPP - November 14, The Return Image Echo The Xe + ions will also enter HFP, producing an “echo” of the track. The track echo time will be distinct from the pulse due to the barium daughter. Maybe: transfer charge to C 2 H 4 : IP =11.6 eV –Complex organic molecule may be much less likely to liberate electrons than Ba ++ or Ba + –Will a mobility difference still exist?

SCIPP - November 14, Event Quality strong primary UV scintillation gives t o –Enough intensity, even at low visible energy electron track image provides topology –Energy resolution limited by electronic noise ion track echo also places event in space –Don’t need all 115,000 echoes from HV plane barium daughter echo is elegant tag method –Can some detection scheme be found? an over-constrained reconstruction possible.

SCIPP - November 14, Imaging Ionization Chamber -HV plane Pixel Readout plane. ionselectrons

SCIPP - November 14, What to do? An R&D ( and library) effort is needed to: –Develop good simulation tools –Optimize S/N with real electronics –Measure  E/E in HPXe IIC with  rays –Investigate benefits of organic additives –Determine ion mobilities in HPXe. –Explore ion-induced avalanche processes

SCIPP - November 14, What to do? An R&D ( and library) effort is needed to: –Develop good simulation tools –Optimize S/N with real electronics –Measure  E/E in HPXe IIC with  rays –Investigate benefits of organic additives –Determine ion mobilities in HPXe. –Explore ion-induced avalanche processes A proposal has been rejected by DOE!

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SCIPP - November 14, Summary A novel concept for a robust 0-  decay search has been developed: –  E/E ~ 1% FWHM –Detailed & constrained 3-D event topology –Active, variable fiducial boundaries –Identification of Ba daughter possible, in principle, by exploitation of macroscopic quantum mechanical phenomenon, RCE

SCIPP - November 14, Acknowledgments Azriel Goldschmidt - LBNL Adam Bernstein - LLNL Mike Heffner - LLNL Jacques Millaud - LLNL/LBNL Leslie Rosenberg - UW