University of Sheffield Dan Tovey 1 ZEPLIN-MAX Design Options General considerations ZEPLIN-I design...

1 University of SheffieldUniversity of SheffieldDan ToveyDan Tovey

ZEPLIN-MAX Design Options

• General considerations• ZEPLIN-I design• ZEPLIN-II design• ZEPLIN-III design• Charge read-out


IntroductionIntroduction• Small study group formed to study options for a ZEPLIN-

MAX module with view to input to new Proposal.• No design concept considered out-of-bounds.• Studying:

• ZEPLIN-I scale-up with improved light collection• ZEPLIN-II scale-up with improved light collection• ZEPLIN-III scale-up with reduced cost/kg.• Charge read-out concepts.

• Charged with identifying issues requiring further study.• Also common problems/solutions associated with different

designs.• Must identify and solve any potential 'stoppers' !


• We need to be sensitive to majority of SUSY parameter space.

• Will allow us to discover/rule-out SUSY models (e.g. mSUGRA).

General ConsiderationsGeneral Considerations

Complementary to LHC

Most favourable case (tan() = 55)


General ConsiderationsGeneral Considerations• Minimum requirement is sensitivity to p~10-10pb in the crucial

100 - 300 GeV mass region.• Family of sensitivity curves generated using WINDLAS for

different thresholds and 1 upper limit on nuclear recoil rate.• Gaussian statistics assumed:

• Conservative• Less accurate for high discrimination & low exposure

• Given detector discrimination, threshold & background rate can use to find required target exposure.

• Looking at modules in the 100 kg range.• Will need neutron shielding, muon veto, multiple neutron

rejection (a la CDMS?)


General ConsiderationsGeneral Considerations• PMTs expensive and also ….• … high levels of U/Th/K in graded seal.• Gives e-recoil background (but can discriminate).• Also gives nuclear recoil events via neutrons (-n processes)

contained within detector (TJS):• Neutron veto less efficient (although multi-scatter tag?)• Single scatter + absorption is irreducible

• Could limit p > 10-8 pb? potential 'stopper'!

• PFS investigating rate can active neutron veto help?• Alternatives include:

• low background sapphire ET 9226B (~£5000/2" expensive!)• Electrical read-out (see later).• APDs


Gas Phase DetectorGas Phase Detector

• Proposed by James White et al. (DM2002)

• 1 tonne Xe gas in 6m x 2m cylinder (10 bar?)

• Internal PMTs• Central

electroluminescence grid.• Primary/Secondary

discrimination.• Needs:

• No dump• No cooling.

PMTs

PMTs


ZEPLIN-IZEPLIN-I• Consider scaled-up ZEPLIN-I style

detector.• Increased target mass with PMTs in

liquid (factor 2 improvement: NJTS)• Similar to fully filled ZEPLIN-II

• Questions include:• What is maximum depth of xenon?• What effect does faster PMTs/APDs have on

discrim?• How can improve light-yield further:

integrating sphere?• How is Compton veto efficiency effected by

increased depth?• Take low 85Kr Xe into account.

rig diagramrig diagram


ZEPLIN-IIZEPLIN-II• Simple scale-up of ZEPLIN-II.• BUT can threshold be maintained

/reduced?• Questions include:

• What limits the depth: drift field or purity?• Does Xe absorbed in PTFE limit disrim (Also

an issue for ZII)? If yes can veto fringe events a la ZIII? What do Xe-PTFE events look like (primary/secondary)?

• Can improve discrim with PMTs in liquid/two layers of PMTs? PMTs in turrets with S3 type cut to reject background? Efficiency of cut?


ZEPLIN-IIIZEPLIN-III• More expensive to get 1 tonne.

• Not necessary (threshold)?

• Possible scenario (TJS):• Double depth• Increase active diameter to 1250 mm• Gives 20 X Fid. Vol. (100 kg)• Needs 300-400 PMTs.• Must minimise LXe reverse field region.

• Block light from fringe events (DRT)?• Reduces PMT granularity (fiducialisation)• Try cylinder of ET low background glass? • Opaque to 175 nm

• Primary PS measurement required.


Charge Read-OutCharge Read-Out• Avoids PMT problems• Various options (using CsI):

• GEMs• PPAC (a la ZIII)• Micromegas

• All will need Xe + CH4 • 2% CH4 saturates in liquid• Light propagation in liquid OK?

• Tests needed:• GEM system (condensation?) - see later• PPAC - is gain high enough?• Micromegas - condensation (uniform

field)?

• Potentially more flexible read-out.

Liquid Xe

Xe Gas

HeCooling

Xe Fill

GEMs

CsI

PTFEReflector

CsI

FieldShapingRings

e-e-

e-

Anode


Other IdeasOther Ideas

• Also avoid PMT problems with other schemes:• APDs• Charge collection in liquid with Si device (needs internal

gain)• Large area gas-filled photon detectors (e.g.

GEMs/Micromegas + CsI, requires window and different gas mixture) work by Peskov et al. (physics/0106070).

• Demonstrates an interest from Torino and Aprile groups.• Much further study needed.


ConclusionsConclusions

• Long term goal of UKDMC must be to gain sensitivity to complete region of parameter space predicted by at least minimal Supergravity SUSY models.

• This requires sensitivity to p ~ 10-10 pb.

• Implies tonne-scale Xe detectors.• Many ideas for how to achieve this.• More ideas welcome.• Must firm up designs / identify / answer key

remaining questions by end of August Baseline Design(s) for Proposal.


Electrical Read-Out of Double-Phase Xenon Detectors

• Concept• Read-out strategies• What have we learned so far?• Other options


ConceptConcept• Need to avoid use of PMTs in ZEPLIN-MAX

• Expensive• Radioactive (gammas) needs veto!• Radioactive (neutrons)• Relatively difficult to instrument large area• Reduced effective QE through internal reflection at window

• Direct read-out of charge a favourable option (if it works!)

• Internal reflective CsI photocathode to detect photons (QE ~ 31% at 175 nm with high E-field)• No windows• Little radioactivity• Simple read-out


Read-Out StrategiesRead-Out Strategies• GEMs

• Believed to work in pure noble gases• Cheap (~1$/cm2)• Low activity (kapton used extensively in

CAST etc.)

• PPAC• Simple (similar to ZIII mirror)• Uniform field (see later)• Robust

• Micromegas• Hybrid of GEM and PPAC• Cheap• Robust• Uniform field

KaptonCu

Cu


What Have We Learned ?What Have We Learned ?• GEMs may work in pure Xe but fragile

• HV trip does not prevent spark damage• Saturated vapour too dense for appreciable gain

• GEMs do work in Xe + CH4 at stp (nothing new)• Quenches UV photons in gas increases gain

• Tischauser et al (1992) 2% CH4 absorbed in liquid but scintillation in liquid unaffected.

• GEMs work (for a while) in Xe + CH4 at 160K • Proportional pulses saturated pulses no pulses (20 mins)!• Gain returns when raise voltage• Xenon condensation in non-uniform field (Vadim)?• Primary pulses from CsI observed? Need photopeak to confirm.

• Next try PPAC (HV high enough?), Micromegas• Field more uniform in avalanche region.


Other OptionsOther Options• If are unable to obtain stable charge gain in Xe, what other

options are there for low background read-out?• Opto-electrical read-out using electroluminescence + large

area gas avalanche photon detection.• Studied by Peskov et al. (physics/0106070)• Use gas avalanche PMT

• Divide chamber in two• Quartz window divider above (or below) Xe volume• Fill second volume with e.g. Ne. Seal.• Electroluminescence in Xe chamber (a la ZII/ZIII)• Transmissive CsI coating on window in second volume• GEM stack / Micromegas +anode for avalanche multiplication.

• Guaranteed to work, but complicated.

Ne

Xe

University of Sheffield Dan Tovey 1 ZEPLIN-MAX Design Options General considerations ZEPLIN-I design...

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