Transcript Document
Ionization Imaging: a better way to search for 0-
v
decay?
David Nygren LBNL Physics Division SCIPP - November 14, 2006 1
Z
Two Types of Double Beta Decay
A known background process and an important calibration tool
T
1 2 10 19
yrs
.
2 0 This process not yet observed particle = antiparticle 1
T
1 2
G
2
m
2 Neutrinoless double beta decay lifetime SCIPP - November 14, 2006 Neutrino effective mass 2
0-
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, 2006 3
Uncertainties…
• • • • • Hierarchy uncertain o Determines needed sensitivity Matrix element calculations uncertain o Order of magnitude in rate Effective mass uncertain o Phases enter:
m v
= |∑ |U ei | 2 i m i | Direct tests by 3 H kinematics uncertain o For
m v
<< 1 eV, technically very difficult!
Best experimental approach: uncertain !
SCIPP - November 14, 2006 4
A “Robust” Experiment:
Rate Only 2-
v
decays!
Only 0-
v
decays!
No backgrounds above Q-value!
0 Energy Q-value The experimental result is a spectrum of all events , with very small or negligible backgrounds.
SCIPP - November 14, 2006 5
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, 2006 6
Energy Resolution
The Gold Standard:
Energy resolution with Germanium detector:
E/E ~ 1.25 x 10 -3 FWHM
at 2.6 MeV 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 SCIPP - November 14, 2006 7
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, 2006 8
1.
2.
3.
4.
5.
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
Esum of 2 final state electrons
Spectra from Ludwig DeBraekeleer Practice positive signal claim is disputed!
Spectra from Klapdor Kleingrothaus et. al. SCIPP - November 14, 2006 9
Background rejection is essential - energy resolution may not be enough !
Present Status
• Heidelberg-Moscow (H-M) claim:
m v
0
v
1/2 =
0.44 +0.14
-0.20
eV (best value)
disputed!
= (8 — 18.3) x 10 24 y (95% c.l.) Scale: ~11 kg of 76 Ge, for ~7 years No other claim for a positive result exists SCIPP - November 14, 2006 10
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, 2006 11
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, 2006 12
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 LXe @ WIPP – No laser tagging of barium daughter: R&D stage – Strong anti-correlation of ionization/scintillation – Results eagerly awaited SCIPP - November 14, 2006 13
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
> 10 - 20 meV ) with a single experiment.” But: no explicit encouragement for new ideas!
SCIPP - November 14, 2006 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, 2006 15
Imaging Ionization Chamber
-HV plane Pixel Readout plane Pixel Readout plane .
electrons ions ~99%Xe + ~1% CH 4 @ 20 bars SCIPP - November 14, 2006 16
Proportional Gain:
good
results for low-energy x-rays
SCIPP - November 14, 2006 MWPC, GEM, micromegas all work well...
but: why are there so many events below the peak?
17
Proportional Gain:
poor
resolution for MeV energies
Typical E/E: 4 - 6.6% FWHM @ 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, 2006 18
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, 2006 19
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 • ~20 bars; critical point P = 58 bars, T = 290 ° • 1000 kg in ~10 m 3 : = 200 cm, L =300 cm • dn/dx ~ 1 fC/cm = 6000 electron/ion pairs/cm K 3 ) SCIPP - November 14, 2006 20
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, 2006 21
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, 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 and fluorescence ’s • Distinct satellite “blobs” may be visible SCIPP - November 14, 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 10 4 /m 2 ) • ~ 40 - 80 contiguous “hit” pixels for E = Q • dn/dx = ~3000 electrons/(5mm) – ultra-low noise readout electronics - BNL ASIC •
• Other noise terms must be included
– “waveform capture” essential for extended tracks SCIPP - November 14, 2006 24
Pixel geometry
A low capacitance solution: a 7-pixel hexagonal sub-module: Or, a 16 channel 4x4 rectangular array… SCIPP - November 14, 2006 25
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/ – diffusion after 1.5 m drift is ~ 2 mm rms s – event is reconstructed from contiguous hit pixels – noise adds only from hit pixels + some neighbors SCIPP - November 14, 2006 26
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, 2006 28
Energy Resolution…
• If ionization were the only issue: E/E = 2.9 x 10 -3 FWHM • Other contributions: – electronic noise from N = 49 pixels in event • N 1/2 x
E/E < 10.0 x 10 -3 FWHM
SCIPP - November 14, 2006 29
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, 2006 31
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, 2006 32
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, 2006 33
Barium Daughter Atom
– In xenon/CH 4 , the Ba ++ ion will survive: • IP(Xe) = 12.13 eV, IP(CH 4 ) = 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 ++ becomes Ba + through charge exchange SCIPP - November 14, 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, Right??
SCIPP - November 14, 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 • resonant charge exchange is a long-range process; glancing collisions = back-scatter – RCE increases viscosity of ions SCIPP - November 14, 2006 36
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, 2006 37
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, 2006 38
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 + robust signature and Ba + for a true event.
ions is a SCIPP - November 14, 2006 39
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, 2006 40
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, 2006 41
Blind GEM or “Microwell”
HV plane Drift region: Low E-field ions Resistive back side blocked to electrons Very High E-field inside pore; low work-function surface?
SCIPP - November 14, 2006 42
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 10 3 .
– 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, 2006 43
“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, 2006 44
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, 2006 45
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, 2006 46
Imaging Ionization Chamber
-HV plane Pixel Readout plane Pixel Readout plane .
electrons ions SCIPP - November 14, 2006 47
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, 2006 48
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!
SCIPP - November 14, 2006 49
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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, 2006 53
Acknowledgments
• Azriel Goldschmidt - LBNL • Adam Bernstein - LLNL • Mike Heffner - LLNL • Jacques Millaud - LLNL/LBNL • Leslie Rosenberg - UW SCIPP - November 14, 2006 54