1. Expected rates for various targets
For a heavy target nucleus such as Xe, a very low recoil energy threshold is crucial.
The expected rate, integrated above threshold of ~16 keV is 1 events/ kg/day

2. WIMP Direct Searches with Recoil Discrimination

3. Electron vs Nuclear Recoil Discrimination
Electron vs Nuclear Recoil Discrimination (Direct & Proportional Scintillation )
Measure both direct scintillation(S1) and charge
(proportional scintillation) (S2)
Dual Phase Detection Principle Common to All LXe DM Projects
Nuclear recoil from
WIMP
Neutron
Electron recoil from
gamma
Electron
Alpha
Gas
~1μs
anode
grid
Proportional scintillation
depends on type of recoil and applied electric field.
electron recoil → S2 >> S1
nuclear recoil → S2 < S1
but detectable if E large
Drift Time e- E
Liquid
~40ns
cathode

4. The XENON TPC Signals
Three distinct signals associated with typical event. Amplification of primary scintillation light with CsI photocathode important for low threshold and for triggering.
Event depth of interaction (Z) from timing and XY-location from center of gravity of secondary light signals on PMTs array.
Effective background rejection direct consequence of 3D event localization (TPC)

5. The XENON Experiment : Design Overview
The XENON design is modular.
An array of 10 independent 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass, is used to make the 1-tonne scale experiment.
The fiducial LXe volume of each module is self-shielded by additional LXe. The thickness of the active shield will be optimized for effective charged and neutral background rejection.
One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the shield LXe (~50 kg ).
SAGENAP Review of the XENON Project, March 12-13, 2002

6. A Liquid Xenon Time Projection Chamber for Gamma-Ray Astrophysics

7. The Columbia 10 liter LXeTPC
30 kg active Xe mass
20 x 20 cm2 active area
8 cm drift with 4 kV/cm
Charge and Light readout
128 wires/anodes digitizers
4UV PMTs

8. UCLA ZEPLIN II

9. ZEPLIN II

10. ZEPLIN II  ZEPLIN IV
30 kg  1000 kg
The latest design as at DM2002

11. UKDM ZEPLIN III

12. ZEPLIN III

13. The LXe Program at Boulby

14. The LXe Program at Kamioka
XMASS
Cold finger
present
with new PMTs no rejec.
gas filling line
Wire set
(Grid1,Anode
Grid2)
with 99% rejection
PTFE Teflon
(Reflector)
Gas Xe
MgF2 Window
with Ni mesh
(cathode)
Liq. Xe(1kg)
9.5 cm Drift
OFHC vessel
(5cm)
PMT

15. … and for Solar n and 0nbb Decay

16. Ionization and Scintillation in Liquid Xenon
I/S (electron) >> I/S (non relativistic particle)
Alpha scintillation
Electron charge
L/L0 or Q/Q0 (%)
electron scintillation
Alpha charge
Electric Field (kV/cm)

17. The XENON TPC: Principle of Operation
30 cm drift gap to maximize active target  long electron lifetime in LXe demonstrated
5 kV/cm drift field to detect small charge from nuclear recoils  internal HV multiplier (Cockroft Walton type)
Electrons extraction into gas phase to detect charge via proportional scintillation (~1000 UV g/e/cm) demonstrated
Internal CsI photocathode with QE~31% (Aprile et al. NIMA 338,1994) to enhance direct light signal and thus lower threshold  demonstrated
PMTs readout inside the TPC for direct and secondary light  need PMTs with low activity from U/Th/K

18. Detection of LXe Light with a CsI Photocathode
Stable performance of reflective CsI photocathodes with high QE of 31% in LXe has been demonstrated by the Columbia measurements
CsI photocathodes can be made
in any size/shape with uniform response, and are inexpensive.
LXe negative electron affinity Vo(LXe)= eV and the applied electric field explain the favorable electron extraction at the CsI-liquid interface.
Aprile et al. NIMA 338(1994)
Aprile et al. NIMA 343(1994)

19. Assumptions Light Collection Efficiency: MonteCarlo Wph : 13 eV
lph: 1.7 m
Quenching Factor: 25%
Q.E. of PMTs: 26%
Q.E. of CsI : 31%
R.E of Teflon Wall: 90%
Mass of Liquid Xe: 100 kg
37 PMTs (2 inch) array

20. Simulation Results
A 16 keV (true) nuclear recoil gives ~ 24 photoelectrons. The CsI readout contributes the largest fraction of them.
Multiplication in the gas phase gives a strong secondary scintillation pulse for triggering on 2-3 PMTs.
Coincidence of direct PMTs sum signal and amplified light signal from CsI
Main Trigger is the last signal in time sequence post-triggered digitizer read out Trigger threshold can be set very low because of low event rate and small number of signals to digitize. PMTs at low temperature low noise.
Even w/o CsI (replaced by reflector) we still expect ~6 pe . Several possible ways to improve light collection.

21. Summary of Previous Nuclear Recoil Measurements (Quenching Factor)
 previous measurements have wide scatter
 no measurements at all at low energies
 results consistent with Lindhard theory

22. We have experience measuring neutron-nuclear recoil efficiency
typical setup for measurement of
nuclear recoil scintillation efficiency
at University of Sheffield
measured low energy nuclear recoil efficiency of liquid scintillator
Hong, Hailey et. al., J. AstroParticle Physics 2001
2.9 MeV neutron beam

23. Why Do Nuclear Recoil Scintillation Efficiency Measurements?
Confirm that measured efficiency at higher energies extends down to lowest energies of interest to a WIMP search
Confirm result in our particular experimental configuration.
Results can vary with Xe purity, light collection efficiency etc.
Measure true nuclear recoil scintillation pulse shapes

24. Charge readout with GEMs: a promising alternative
High gain in pure Xe with 3GEMs demonstrated
Coating of GEMs with CsI
2D readout for mm resolution
See Bondar et al.,Vienna01

25. XENON Technical Heritage: LXeGRIT
A 30 kg Liquid Xenon Time Projection Chamber developed with NASA support. 3D imaging detector with good spectroscopy is the basis of the balloon-borne LXeGRIT, a novel Compton Telescope for MeV Gamma- Ray Astrophysics.
The LXeTPC operation and response to gamma-rays successfully tested in the lab and in the harsh conditions of a near space environment.
Road to LXeGRIT: extensive R&D to study LXe ionization and scintillation properties, purification techniques to achieve long electron drift for large volume application, energy resolution and 3D imaging resolution studies, electron mobility etc.

26. High Purity Xenon for Long Electron Drift and Energy Resolution
And the power of Compton Imaging

27. Compton Imaging of MeV g-ray Sources

28. 3D capability for event discrimination: Flight Data

29. LXeGRIT inflight energy spectra
From the Lab to the Sky: The Balloon-Borne Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)
LXeGRIT inflight energy spectra
Atm/Cosmic Diffuse MC simulation and Data
Compton Imaging Events

30. Background Considerations for XENON
 and  induced background
85Kr (1/2=10.7y): 85Kr/Kr  2 x in air giving ~1Bq/m3
Standard Xe gas contains ~ 10ppm of Kr10 Hz from 85Kr decays in 1 liter of LXe.
Allowing <1 85Kr decay/day i n XENON energy band  <1 ppb level of Kr in Xe
136Xe 2 decay (1/2=8 x 1021y): with Q= 2.48 MeV expected rate in
XENON is 1 x 10-6 cts/kg/d/keV before any rejection
Neutron induced background
Muon induced neutrons: spallation of 136Xe and 134Xe  take 10 mb and Homestake 4.4 kmwe estimate 6 x 10-5 cts/kg/d before any rejection
 reduce by muon veto with 99% efficiency
(,n) neutrons from rock: 1000/n/m2/d from (,n) reactions from U/Th of rock
 appropriate shield reduces this background to 1 x 10-6 cts/kg/d/keV
Neutrons from U/Th of detector materials: within shield, neutrons from U/Th of
detector components and vessel give 5 x 10-5 cts/kg/d/keV
 lower it by x10 with materials selection

31. Background Considerations for XENON
 -rays from U/Th/K contamination in PMTs and detector components dominate the background rate. For the PMTs contribution we have assumed a low activity version of the Hamamatsu R6041 (  100 cts/d ) consistent with recent measurements in Japan with a Hamamatsu R7281Q developed for the XMASS group (Moriyama et al., Xenon01 Workshop).
Numbers are based on Homestake location and reflect 99.5% background rejection but no reduction due to 3D imaging and active LXe shield.

32. How is XENON different from other Liquid Xe Projects?

33. Signals from 1kg XMASS Prototype
42000photon/MeV
Decay time nsec
direct
direct
direct
proportional
drift time
drift time
proportional

34. XMASS Recoil /γ ray Separation
>99% γ ray rejection
22 keV gamma ray
Proportional scintillation(S2)
Recoil Xenon (neutron source)
Direct scintillation(S1)
(Ref. JPS vol.53,No 3,1998, S.Suzuki)

35. XMASS: low activity PMT development
57 Co (122keV)
σ/E = 15 %
2.4 [p.e./keV] at 250[V/cm]
counts
with R7281MgF2 (Q.E.30%) (HAMAMATSU(prototype)
A low activity version of this tube shows ~4.5× Bq!
p.e.
137Cs 662keV
Towards a 20 kg Detector
counts
p.e.

36. Answer to Question
LXe long recognized as promising WIMP target for a large scale experiment with relatively simple technology. So far however development effort has been subcritical.
Low energy threshold and background rejection capability yet to be fully demonstrated.
Recent move to an underground lab - 1 kg XMASS detector in Kamioka- an important milestone. Scale up to a 20 kg detector of same design (7 PMTs vs 1) started.
UCLA ZEPLIN II is similar in size and design to XMASS: drift in LXe over ~ 10 cm with low electric. Secondary light pulse from low energy nuclear recoils hard to detect. Scale up to 1 tonne with a monolithic detector (ZEPLIN IV) too risky and unpractical.
UKDM ZEPLIN III better discrimination power and lower threshold due to high electric field. Design does not present an easy scale up from 6 kg to sizable modules of order 100 kg.
XENON combines the best of the techniques with a design which can be easily scaled. Strength of experience with a 30 kg LXeTPC for gamma ray astrophysics + critical mass at Columbia with collaborators key experiences in DM searches.

37. XENON Phase 1 Study: 10 kg Chamber
Demonstrate electron drift over 30 cm (Columbia)
Measure nuclear recoil efficiency in LXe (Columbia)
Demonstrate HV multiplier design (Columbia)
Measure gain in Xe with multi GEMs (Rice and Princeton)
Test alternative to PMTs, i.e. LAAPDs (Brown)
Selection and test of detector materials (LLNL)
Monte Carlo simulations for detector design and background studies (Columbia /Princeton/Brown)
Study Kr removal techniques (Princeton)
Characterize 10 kg detector response and with g and neutron sources (Entire Collaboration)

38. What next? XENON and NUSL
The result of the 2yr Phase 1 will be a working 10 kg prototype with demonstrated low ER threshold and recoil discrimination capability. Its move to a deep underground location will initiate science return.
Phase 2 is for construction and operation of a 100 kg module as 1st step towards 1 tonne. We plan to seek DOE and NSF support and more collaborators
By this time the situation of a NUSL will be clear. If NUSL is delayed, several alternative locations possible ( Boulby, GS, WIPP, etc.)…but deeper the better..

39. Summary
Liquid Xenon is an excellent detector material well suited for the large target mass required for a sensitive Dark Matter experiment.
The XENON experiment is proposed as an array of ten independent, self shielded, 3D position sensitive LXeTPCs each with 100 kg active mass.
The detector design, largely based on established technology and >10 yrs experience with LXe detectors development at Columbia, maximizes the fiducial volume and the signal information useful to distinguish the rare WIMP events from the large background.
With a total mass of 1-tonne, a nuclear recoil discrimination > 99.5% and
a threshold of ~ 16 keV, XENON expected sensitivity of  events/kg/day in 3 yrs operation, will cover most SUSY predictions.

40. XENON Organization
Subsystem responsibility is allocated amongst the team of experienced co-investigators.

41. XENON Management Approach
Phase I of the XENON project spans a 2 year period from the funding start date. This instrument development effort has the focused goal of a clear demonstration of the capabilities of a 10 kg LXe detector for a sensitive Dark Matter search.
The 10 kg prototype defines the roadmap to the Phase II development of a 100 kg detector as one unit of a 1 tonne scale XENON experiment.
In complexity, the XENON Phase I development does not exceed the NASA funded LXeGRIT experiment and we adopt the successful practices developed during this project.
We have the required critical mass with extensive expertise in LXe detector technology and other areas relevant to a Dark Matter experiment. This, plus sensible management practices will insure meeting the milestones promised by the end of the 2nd year of Phase I.

42. Management Activities
To coordinate the efforts and insure the appropriate level of communication and exchange of information between the Columbia team and team members at Brown, Princeton, Rice, and LLNL the PI will:
organize bi-weekly videoconference meetings
obtain monthly progress reports on all sub systems
organize semi annual project reviews with participation of collaborators and external advisors
prepare yearly progress reports for NSF
encourage student/minority involvement in the research
take full responsibility for the key deliverables to NSF by end of Phase I

43. Development Schedule Year 1 activities concentrate on:
Monte Carlo simulations to guide the design
Gas system construction and testing
Neutron recoil efficiency measurements
Baseline detector development
Alternative detector development
Materials selection and testing

44. Development Schedule (2)
Year 2 activities concentrate on:
Build of the 10kg prototype
Demonstration of Krypton reduction
Design of the 100kg instrument
End of Phase I results in near final design of 100kg module and demonstration of all key technologies in the 10 kg prototype.

45. Team Members Expertise

46. Budget Details Year 1 request 823k$ Year 2 request 873k$
Budget breakout (of 2 year total) is consistent with our fast track development of a working prototype

47. Team Members Presentations

48. Materials Selection and Testing
Bill Craig (LLNL)
Candidate material selection will begin with study of existing databases assembled for other projects.
LLNL personnel (Craig, Ziock) are associated with ongoing projects requiring low background and will use this existing infrastructure to do testing of candidate materials.
Close coupling between this effort and the XENON 10/100 kg design team to ensure optimal material choices are incorporated as quickly as possible.