WIMPS, 0- decay & xenon High-pressure 136 Xe Gas TPC: An emerging opportunity

WIMPS, 0- decay & xenon

High-pressure 136Xe Gas TPC: An emerging opportunity

David Nygren

Physics Division - LBNL

Stockholm October 2007 2

Outline

• 0- decay & WIMPs• Conclusion• Background rejection• Energy resolution • TPC options - “due diligence”• Scaling to the 1000 kg era• Issues• Perspective


WIMPs

• Dark matter: ~23% of universe mass– visible ordinary matter: ≤1%– atoms, ordinary matter ~4%– WIMPs - lightest supersymmetric particle?

• Mass range: ~100 - 1000 GeV/c2

• Interaction strength not pinned down• Recoil energy: ~ 0 - 100 keV

– For a positive claim:

Very robust evidence shall be required!


Two Types of Double Beta Decay

If this process is observed:Neutrino mass ≠ 0

Neutrino = Anti-neutrino!Lepton number is not conserved!

Neutrinoless double beta decay lifetime

Neutrino effective

mass

A known standard model process and an important calibration tool

Z

0

2

22

2

1

1mG

T×Μ×=

.1019

2

1 yrsT ≈


Conclusions:

• An attempt to optimize a detector for a 0- decay search in 136Xe has led me to the following three conclusions:

– High-pressure xenon gas (HPXe ~20 bar) will offer much better resolution than liquid xenon

– Optimal energy resolution is achieved with a proportional scintillation readout plane

– Result: An optimum WIMP detector too!

Two identical TPCs, filled with different isotopes…


Two Identical HPXe TPCs

• “ Detector”– Fill with enriched Xe

mainly 136Xe– Isotopic mix is mainly

even-A – Events include all

events + background– WIMP events include

more scalar interactions

• “WIMP Detector”– Fill with normal Xe or fill

with “depleted” Xe– Isotopic mix is ~50%

odd-A: 129Xe 131Xe– Events include only

backgrounds to – WIMP events include

more axial vector interactions


Double beta decay

Only

2-v decays!

Rate

Energy Q-value

Only

0-v decays!

No backgrounds above Q-value!

The robust result we seek is a spectrum of all events, with negligible or very small backgrounds.

0


Perils of backgrounds

• Sensitivity to active mass deteriorates if backgrounds begin to appear:– m ~ {1/MT}1/2

– m ~ {(bE)/MT}1/4

• b = number of background events/unit energy E = energy resolution of detector system• M = active mass• T = run time of experiment

Ouch!


Current Status• Present status (partial list):

– Heidelberg-Moscow (76Ge) result: mv = 440 +14-20 meV - disputed!

– Cuoricino (130Te): taking data, but - background limited...– EXO (136Xe): installation stage, but - E/E shape/resolution issue...– GERDA (76Ge): under construction at LNGS– Majorana (76Ge): proposal & R&D stage – NEMO Super-NEMO (foils): proposal & R&D stage

• Global synthesis: mi < 170 meV (95% CL)*

100’s to 1000 kg active mass likely to be necessary!• Rejection of internal/external backgrounds in ~1027 atoms! • Excellent energy resolution: E/E <10 x 10-3 FWHM - at least!

• Target (from oscillations): m ~50 meV

*Mohapatra & Smirnov 2006 Ann. Rev. Nucl. Sci. 56 - (other analyses give higher values)


- Background rejection1. Charged particles

• With a TPC, a fiducial volume surface can be defined with all the needed characteristics:– fully closed– Precise spatial definition– 100% active - no partially sensitive surfaces– deadtime-less operation– surface is variable ex post facto

• Charged particles cannot penetrate fiducial surface unseen; – 100.000% rejection


- Background rejection2. Neutral Particles

-rays (most serious source, and needs real work)– Radon daughters (208Tl, 214Bi) plated out on surfaces – Photo-conversion fluorescence: 85% ( ~1 cm @ 20 bar)– Multiple Compton events more likely at large scale– Cannot get adequate self-shielding for MeV -rays

• Neutrons can activate detector components, – Excitation of xenon may be useful calibration signal

• Neutral particles need serious study - high priority


HPXe, LXe TPC Fiducial volume

-HV planeReadout planeReadout plane

.

ionselectrons

Fiducial volume surface

*

Real event Backgrounds


EXO-200: LXe TPC

Installation now, at WIPP:

• 200kg of 80% enriched 136Xe• Liquid xenon TPC• Localization of the event in x,y,z

(using scintillation for T0 )

• Avalanche Photo-Diodes (APD) used to detect scintillation

• Strong anti-correlation of scintillation & Ionization components

• Expect = 3.3% FWHM for

For a subsequent phase of experiment:

• Barium daughter tagging byoptical spectroscopy

• Electrostatic capture & mechanical transfer of ion to trap - Can this ambitious idea really work?

• Liquid or gas under consideration


EXO Experimental Design


EXO:• Anti-correlation observed between scintillation and ionization signals

•Anti-correlation also observed in other LXe data

= 3.3% FWHM for expected resolution (2480 KeV)

•What about the tails?


Energy resolution in xenon shows complexity: ionizationscintillation

For >0.55 g/cm3, energy resolution deteriorates rapidly!

Ionization signal only


Molecular physics of xenon

• Complex multi-step processes exist: *...– Ionization process creates regions of high

ionization density in very non-uniform way– As density of xenon increases, aggregates form,

with a localized quasi-conduction band– Recombination is ~ complete in these regions– Excimer formation Xe*+ Xe Xe2* h + Xe– Also: Xe*+ Xe* Xe** e- + Xe++ heat– Density dependence of signals is strong!

* not completely understood - most likely, not by anyone


Impact for WIMP search?

But...wait a second!

WIMP searches in LXe use the ratio =

Primary ionization

Primary scintillation

to discriminate nuclear from electron recoils

nuclear < electron


QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.


Impact for WIMP Search...

The anomalous fluctuations in LXe must degrade the nuclear recoil discriminant S2/S1 = ... So:

Maybe HPXe would be better! ...butDo the fluctuations depend on

1. atomic density?

2. ionization density?

3. particle type?

4. all of the above?

Many -induced events reach down to nuclear band in plots of S2/S1 versus energy


Impact for WIMP Search...

The anomalous fluctuations in LXe must degrade the nuclear recoil discriminant S2/S1 = ... So:

Maybe HPXe would be better! ...butDo the fluctuations depend on

1. atomic density?

2. ionization density?

3. particle type?

4. all of the above? - YES, in LXe

Many -induced events reach down to nuclear band in plots of S2/S1 versus energy


QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Gamma events ()

Neutron events (NR)

Latest Xenon-10 results look better, but nuclear recoil acceptance still needs restriction

L og 1

0 S

2/S

1


Fluctuations

• Let R = [log10 {S2/S1()}]

[log10 {S2/S1(N)}]– R for LXe: ~101/2 (from previous slide)

– R for HPXe: unmeasured

• Let D = discrimination power vs density

D = R/T

T = [2 {log10S2/S1()} + 2 {log10S2/S1(N)}]1/2


Fluctuations…

– S2/S1(, HPXe) ( < 0.5 g/cm3) is roughly known• anomalous fluctuations are surely absent; (Q/S ()) is normal in HPXe for sure! But:

– S2/S1([N, , , energy,E-field], HPXe) unmeasured• At least, haven’t found any relevant data

– R could be smaller/larger in HPXe, but:

D could/should be much larger!



What about “nuclear” recoil?

• v/c 0.001 for 50 keV recoiling xenon

• v/c 0.003 for a thermal electron

atomic shells stay with nucleus… recoiling atom, not just nucleus Xe-Xe collisions should be different from , “resonant charge exchange” isolates electrons fast ion moves away quickly isolated electrons are easily swept up by field


“Due Diligence”

Evaluation of TPC operating modes

Emphasis placed on:

energy resolution for decays

Stability of operation

One unlikely mode stands out


Pioneer HPXe TPC detector for 0- decay search

• “Gotthard tunnel TPC”– 5 bars, enriched 136Xe (3.3 kg)– No scintillation detection! no TPC start signal E/E ~ 80 x 10-3 FWHM (1592 keV)

66 x 10-3 FWHM (2480 keV)

Reasons for this poor resolution are not clear...

What is the intrinsic resolution?Can resolution near the intrinsic value be reached?


“Intrinsic” energy resolutionfor HPXe ( < 0.55 g/cm3)

Q-value of 136Xe = 2480 KeVW = E per ion/electron pair = 22 eV (depends on E-field)N = number of ion pairs = Q/WN 2.48 x 106 eV/22 eV = 113,000

N2 = FN (F = Fano factor)

F = 0.13 - 0.17 for xenon gas N = (FN)1/2 ~ 130 electrons rms

E/E = 2.7 x 10-3 FWHM (intrinsic fluctuations only)

Compare: Ge diodes (energy for pair) √3/22 = 0.37 Compare: LXe/HPXe Fano factors: √20/.15 = 11.5 !!


Energy resolution issues in traditional gas detectors

• Main factors affecting ionization:– Intrinsic fluctuations in ionization yield

• Fano factor (partition of energy)

– Loss of signal• Recombination, impurities, grids, quenching,

– Avalanche gain fluctuations• Bad, but wires not as bad as one might imagine...

– Electronic noise, signal processing, calibration• Extended tracks extended signals


Loss of signal

Fluctuations in collection efficiency introduce another factor: L L = 1 - similar to Fano factor (assuming no correlations)

N 2 = (F + L)N

– Loss on grids is small: Lgrid < F seems reasonable• If Lgrid = 5%, then E/E = ~3 x 10-3 FWHM

– Other sources of L include:• Recombination - ionization density, track topology• Electronegative impurities that capture electrons (bad correlations)• Escape to edges• Quenching - of both ionization and scintillation!

Xe* + M Xe + M* Xe + M + heat (similarly for Xe2*, Xe**, Xe2*+… )

Xe+ + e–(hot) + M Xe+ + e–(cold) + M* Xe+ + e–(cold) + M + heat

e–(cold) + Xe+ Xe*


A surprising result: adding a tiny amount of simple molecules - (CH4, N2, H2 ) quenches both ionization and scintillation ( particle)

dE/dx is very high; does this effect depend on too? (yes...) Impact for atomic recoils?…

Gotthard TPC: 4% CH4

how much ionization for particles was lost?

K. N. Pushkin et al, IEEE Nuclear Science Symposium proceedings 2004

(~25 bars)

particles


Avalanche gain

– Gain fluctuations: another factor, “G” N = ((F + G)N)1/2 0.6 < G < 0.8 *

N = ((0.15 + 0.8)N) 1/2 = 328

E/E = 7.0 x 10-3 FWHM - not bad, but:• No more benefit from a small Fano factor• Very sensitive to density (temperature)• Monitoring and calibration a big effort• Space charge effects change gain dynamically• Micromegas, GEM, may offer smaller G

*Alkhazov G D 1970 Nucl. Inst. & Meth. 89 (for cylindrical proportional counters)


“Conventional” TPC

• Pure xenon (maybe a small molecular admixture is OK...)

– Modest avalanche gain is possible – Diffusion is large, up to ~1 cm @ 1 meter– drift velocity very slow: ~1 mm/s– Add losses, other effects,…

N = ((F + G + L)N)1/2

E/E ~ 7 x 10-3 FWHM

Avalanche gain may be OK, but can it be avoided?


“Ionization Imaging” TPC

1. No avalanche gain analog readout (F + L)• dn/dx ~ 1.5 fC/cm: ~9,000 (electron/ion)/cm• gridless “naked” pixel plane (~5 mm pads)• very high operational stability E/E = 3 x 10-3 FWHM (F + L) only

– but, electronic noise must be added! • 50 pixels/event @ 40 e– rms N ~280 e– rms E/E ~ 7 x 10-3 FWHM • Complex signal formation• Need waveform capture too

Ionization imaging: Very good, but need experience


“Negative Ion” TPC

“Counting mode” = digital readout, (F + L) – Electron capture on electronegative molecule– Very slow drift to readout plane;– Strip electron in high field, generate avalanche – Count each “ion” as a separate pulse:

• Ion diffusion much smaller than electron diffusion• Avalanche fluctuations don’t matter, and• Electronic noise does not enter directly, either• Pileup and other losses: L~ 0.04 ?E/E = ~3 x 10-3 FWHM

– Appealing, but no experience in HPXe...


Proportional Scintillation (PS): “HPXe PS TPC”

• Electrons drift (m) to high field region (5 mm)– Electrons gain energy, excite xenon, make UV– Photon generation up to ~1000/e, but no ionization– Multi-anode PMT readout for tracking, energy– PMTs see primary and secondary light

• “New” HPXe territory, but real incentives exist– Sensitivity to density smaller than avalanche– Losses should be very low: L < F– Maybe: G ~ F ?


H. E. Palmer & L. A. Braby

Nucl. Inst. & Meth. 116 (1974) 587-589


From this spectrum: G ~0.19


Fluctuations in Proportional Scintillation

• G for PS contains three terms:• Fluctuations in nuv (UV photons per e): uv = 1/√nuv

– nuv ~ HV/E = 6600/10 eV ~ 660

• Fluctuations in npe (detected photons/e): pe = 1/√npe

– npe ~ solid angle x QE x nuv x 0.5 = 0.1 x 0.25 x 660 x 0.5 ~ 8

• Fluctuations in PMT single PE response: pmt ~ 0.6

G = 2 = 1/(nuv) + (1 + 2pmt)/npe) ~ 0.17

Assume G + L = F, then

Ideal energy resolution (2 = (F + G + L) x E/W):

E/E ~4 x 10-3 FWHM


HPXe PS TPC is the “Optimum” detector

• Readout plane is high-field layer with PMT array– Transparent grids admit both electrons and UV scintillation

– Multi-anode PMT array “pixels” match tracking needs

– Two-stage gain: optical + PMT: Noise < 1 electron!– No secondary positive ion production– Much better stability than avalanche gain

• Major calibration effort needed, but should be stable– Beppo-SAX: 7-PMT HPXe (5 bar) GPSC in space!




Some Gas Issues:– Operation of PS at 20 bar? should work...

– Maintain E/P, but surface fields are larger…

– Large diffusion in pure xenon tracking good enough? (I think so… needs study)

– Integration of signal over area? (… needs study)– are pixels on the track edge adding signal or noise?

– Role of additives such as: H2 N2 CH4 CF4?– molecular additives reduce diffusion, increase mobility, but

– Do molecular additives quench signals? (atomic recoils?)


More gas issues: Neon

– Multiple roles of neon (must add a lot)– to diminish Bremmstrahlung?– to increase drift velocity?– to diminish multiple scattering?– to add Penning effect, thereby reducing F?– to facilitate topology recognition by B field?– to add higher energy WIMP-nuclear recoil signals?

– Operation of PS with neon? should work...– neon excitation energy transfers to xenon– May be “invisible”


More issues

Detection of xenon primary UV scintillation (175 nm - ~7.3 eV)

• nelectrons nphotons (depends on E-field, particle, etc)• 0- decays (Q = 2480 keV): No problem!• WIMP signals (5 - 100 keV): Need very high efficiency!

Design augmentation: Must maximize primary UV detection!Add reflective surfaces along sides - for sure... Add optimized dielectric mirror to cathode plane(corner reflectors?)


Even more issues

• For given mass M, HPXe detector has ~9 x more surface area than LXe…

backgrounds larger !?

– Background rejection may be so much better that overall performance is better.

– Scintillator shell along sides may help…


Scaling to 1000 kg

• Minimum S/ V for cylindrical TPC: L = 2r• Fixed mass M & track size l:

– l/L -2/3 – higher density less leakage

• High voltage L M1/3 (fixed ) • Number of pixels involved in event:

– Diffusion L 1/2 so: very little change

Scaling to large M is fairly graceful


1000 kg Xe: = 225 cm, L =225 cm ~ 0.1 g/cm3 (~20 bars)

A. Sensitive volume

B. HV cathode plane

C. GPSC readout planes, optical gain gap is ~1-2 mm

D. Flange for gas & electrical services to readout plane

E. Filler and neutron absorber, polyethylene, or liquid scintillator, or …

F. Field cages and HV insulator, (rings are exaggerated here) possible site for scintillators


Is HPXe PS TPC the “Optimum” WIMP detector?– Do the fluctuations persist in “nuclear” recoils at

HPXe densities of interest?• threshold density ~ 0.2 g/cm3 for protons, tritons*• Recoil atoms appear to behave differently

– Can we bridge the dynamic range?• Yes, instantaneous ionization signal is ~50 keV• But, scintillation signal is instantaneous• Augmentations needed: • #1: scintillation signal needs high dynamic range• #2: photon detection maximum efficiency

* private communication, Aleksey Bolotnikov (n + 3He 3H + p)


An HPXe System at LLNL


An HPXe System at LLNL

• Built for Homeland Security

spectroscopy goals• Dual 50 bar chambers!• Bake-out, with pump• Gas purification• Gas storage• Excellent R&D platform


Two identical HPXe TPCsTwo distinct physics goals

• “ Detector”– Fill with enriched Xe

mainly 136Xe– Events include all

events + backgrounds

– Isotopic mix is mainly even-A

– WIMP events include more scalar interactions

• “WIMP Detector”– Fill with normal Xe or fill

with “depleted” Xe– Events include only

backgrounds to

– Isotopic mix is ~50% odd-A: 129Xe 131Xe

– WIMP events include more axial vector interactions


What to do?

1. Measure response of HPXe to n & ’s

• ratio of ionization (S2)/scintillation (S1)

• resolution of S2 & S1 signals (fluctuations!)

• must use a good proportional scintillation system

• measure as f (E- field, density, Neon, N2 ,… ?)

• Most of the apparatus and expertise needed has been developed already by the WIMP community (TAMU)

• Can two distinct communities collaborate?• LXe effort EXO just getting going at WIPP, NM• LXe and WIMP community: enthusiasm under control for

new approach by outsider, at least so far...


Perspective

It is extremely rare that two distinct physics goals are not only met, but enhanced, by the

realization of two essentially identical detector systems (differing only in isotopic content).

Because the impact of success would be so significant, and future costs so large, this approach needs to be explored seriously.

57Stockholm October 2007

Thanks to:

• Azriel Goldschmidt - LBNL

• Katsushi Arisaka -UCLA• Adam Bernstein - LLNL• Alexey Bolotnikov - BNL • Doug Bryman -

TRIUMF• Gianluigi De Geronimo - BNL• Madhu Dixit -

Carleton• J J Gomez-Cadenas - Valencia• Cliff Hargrove -

Carleton

• Mike Heffner - LLNL• John Kadyk - LBNL• Norm Madden - LLNL• Jeff Martoff - Temple• Jacques Millaud - LLNL• Leslie Rosenberg - U Wash• Hanguo Wang - UCLA• James White - Texas

A&M

Stockholm October 2007Stockholm October 2007 5858

Stockholm October 2007Stockholm October 2007 5959


Barium daughter tagging and ion mobilities…

• Ba+ and Xe+ 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


Barium daughter tagging and ion mobilities…

– Ba++ ion survives drift: IP = 10.05 eV

– Ba++ ion arrives at HV plane, well ahead of all other Xe+ ions in local segment of track

– Ba++ ion liberates at least one electron at cathode surface, which drifts back to anode

– arriving electron signal serves as “echo” of the Ba++ ion, providing strong constraint

– Clustering effects are likely to alter this picture!


A small test chamber can show whether ion mobility differences persist at higher gas density (no data now).

This could offer an auto-matic method to tag the “birth” of barium in the decay, by sensing an echo pulse if the barium ion causes a secondary emission of one or more electrons at the cathode.




Documents

WIMPS, 0- decay & xenon High-pressure 136 Xe Gas TPC: An emerging opportunity