1. SAGENAP Review of the XENON Project March 12-13, 2002
A 1 tonne Liquid Xenon experiment for a sensitive Dark Matter Search
Elena Aprile
Physics Department, Columbia University
Elena Aprile
SAGENAP Review of the XENON Project, March 12-13, 2002

2. The XENON Project Overview
SAGENAP Review of the XENON Project March 12-13, 2002
The XENON Project Overview
Outline
Science Motivation and Goals Overview
Dark Matter Direct Searches Worldwide
LXe Properties relevant to WIMP Detection
XENON Instrument Design Overview
Comparison with other LXe Projects
XENON Team Presentations
XENON Organization and Management
Elena Aprile

3. The XENON Collaboration
Columbia University: E. Aprile (Principal Investigator)
T. Baltz, A. Curioni, K-L. Giboni, C. Hailey, L. Hui, M. Kobayashi and K. Ni
Brown University: R. Gaitskell
Princeton University: T.Shutt
Rice University: U. Oberlack
LLNL: W. Craig
Elena Aprile
SAGENAP Review of the XENON Project, March 12-13, 2002

4. Why should NSF support XENON
Because a WIMP experiment with discovery potential will have enormous scientific impact in particle physics and astrophysics. Need to validate discovery with different targets and technology.
Because the timing is right and the proposed XENON concept is based on a relatively simple technology with unique suitability for the 1-tonne scale required by the science.
Because the proposing team combines extensive experience with large scale LXe detectors with complementary experience in other key areas required for a successful realization of the XENON dark matter project!
Elena Aprile

5. The Case for Non-Baryonic Dark Matter
Standard BBN calculations + 4He and D primordial abundance
Ωbh2 = ± 0.001
(APJ, 552, L1, 2001)
Measurements of the matter density
Ωm = 0.2 ~ 0.4
h=H0/100 kms -1Mpc-1 (h = 0.6 ~ 0.8)
Cluster velocity dispersion (Mass to Light ratio)
Galactic rotation curves
Cluster baryon fraction from X-ray gas
CMB anisotropies give Ωmh2 = 0.15 ± 0.05 (APJ, 549, 669, 2001)
and also confirms Ωbh2 ~ 0.02
Ωm >> Ωb
Elena Aprile

6. Non-baryonic Dark Matter Candidates
Neutrinos: hard to make up a significant fraction of mass density with neutrinos, unless much more massive than observed
m < 0.1 eV ( PRL 81(1998)1562)
Axions: strong CP, m ~ 10-5eV, search is in progress using microwave cavities ( PRL 80(1998)2043)
Massive Compact Halo Objects (MACHO): with Mo cannot account for a large fraction of the DM in the Milky Way halo
(ApJ 550(2001)L169)
Weakly Interacting Massive Particles (WIMPS):
Stable (or long lived) particles left over from the BB, decoupling when non-relativistic: their relic density ΩXh2 ~ 1/<X v>
(X ~ weak)  ΩXh2 ~ 1
Elena Aprile

7. Supersymmetry Stabilizes MPL and MZ hierarchy
Unification of coupling constants
Lightest Super Particle is stable
Neutralino
SUSY offers the favorite WIMP candidate
Superposition of photino, zino and higgsinos
SUSY particles were not invented to solve the dark matter problem.
Particles with several 100 GeV/c2 actively being pursued at accelerators.
Direct WIMP searches can probe mass values impossible to reach at colliders.
Typical WIMP nucleon cross sections in the range 10-5 and pb
Elena Aprile

8. Muon g-2 Measurement
BNL results on muon anomalous magnetic moment disagree with Standard Model at 1.6  level (PRL 86(2001)2227)
If discrepancy is due to SUSY, a large neutralino-nucleon cross section (10-9 pb) and a low mass (<500 GeV) are favored
World eagerly awaiting for new results from last run!
Elena Aprile

9. Elastic scattering off nuclei in laboratory target
WIMP Direct Detection
Elastic scattering off nuclei in
laboratory target
 measure nuclear recoil energy
Spin-independent interactions are coherent ( A2) at low energy  dominate for most models. Target with odd isotopes needed for spin-dependent interactions
Energy spectrum and rate depend on local dark matter density 0 :
measured galactic rotation curve : flat out to 50 kpc with vcir220 km/s  spherical halo with 0  GeV/cm3 and M-B velocity distribution with v 220 km/s
Elena Aprile

10. Experimental Challenges
Recoil energy is small  few keV  detectors with low threshold
Event rates are low << radioactive background 
detectors with low radioactivity, deep underground and with active background rejection
With E0 = 1/2MX(0c)2
r = 4 MX MA /(MX+ MA )2
R0 = T0c0
c10.78 and c20.58
F=form factor (see Phys.Rept.267(1996)195
Elena Aprile

11. Background Rejection Methods
Reject events more likely to be due to g, e, a radioactivities
 multiple-scatters (WIMPs interact too weakly) HDMS
 single-scatters localized near detector walls (WIMPs interact anywhere) CDMS ZIP detectors
 electron recoils (WIMPs more likely interact with nucleus)
CDMS, EDELWEISS (CRESST, ZEPLINs, DRIFT)
A 3D LXeTPC like XENON will combine all these rejection capabilities
Use motion of Earth/Sun through WIMP halo
 direction of recoil DRIFT
 annual modulation  DAMA, NAIAD
Elena Aprile

12. 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
Elena Aprile

13. WIMP Direct Searches with Recoil Discrimination
Elena Aprile

14. Current and Projected Limits of Spin-Independent WIMP Searches
Projection for CDMS Soudan (7kg Ge+Si) and competing experiments in Europe, including LXe projects of the UKDM program is ~1 event / kg / yr
It will take a target mass at 1 tonne scale and similar background discrimination power to reach a sensitivity of ~1 event / 100kg / yr or s ~ cm2
LXe attractive target for scale-up.
Projection for XENON based on Homestake, 99.5% recoil discrimination, 16 keV true recoil energy threshold and an overall 3.9x 10-5 cts /kg /d /keV background rate.
Elena Aprile

15. Why is Liquid Xenon Attractive for Dark Matter
High mass Xe nucleus good for scalar interaction of WIMPs
High atomic number (Z=54) and density (r=3g/cc) good for compact and flexible detector geometry. “Easy” cryogenics at –100C
High ionization (W=15.6eV) yield and small Fano factor for good DE/E
High electron drift velocity (v=2 mm/ms) and low diffusion for excellent spatial resolution. Calorimetry and 3D event localization powerful for background rejection based on fiducial volume cuts and event multiplicity
High scintillation (W~13 eV) yield with fast response and strong dependence on ionizing particle for event trigger and background discrimination with PSD
Distinct charge/light ratio for electron/nuclear energy deposits for high background discrimination
Available in large quantity and “easy” to purify with a variety of methods. Demonstrated electron lifetime before trapping of order 1 millisecond for long drift. No long-lived radioactive isotopes. 85Kr contamination reducible to ppb level
Elena Aprile

16. … and for Solar n and 0nbb Decay
Elena Aprile

17. 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)
Elena Aprile

18. 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
Elena Aprile

19. 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 ).
Elena Aprile
SAGENAP Review of the XENON Project, March 12-13, 2002

20. 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
Elena Aprile

21. 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)
Elena Aprile

22. 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)
Elena Aprile

23. 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
Elena Aprile

24. 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.
Elena Aprile

25. 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
Elena Aprile

26. 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
Elena Aprile

27. 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
Elena Aprile

28. 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
Elena Aprile

29. 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.
Elena Aprile

30. A Liquid Xenon Time Projection Chamber for Gamma-Ray Astrophysics
Elena Aprile

31. 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
Elena Aprile

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

33. Compton Imaging of MeV g-ray Sources
Elena Aprile

34. 3D capability for event discrimination: Flight Data
Elena Aprile

35. 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
Elena Aprile

36. 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
Elena Aprile

37. 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.
Elena Aprile

38. How is XENON different from other Liquid Xe Projects?
Elena Aprile

39. UCLA ZEPLIN II
Elena Aprile

40. ZEPLIN II
Elena Aprile

41. 30 kg  1000 kg ZEPLIN II  ZEPLIN IV The latest design as at DM2002
Elena Aprile

42. UKDM ZEPLIN III
Elena Aprile

43. ZEPLIN III
Elena Aprile

44. The LXe Program at Boulby
Elena Aprile

45. 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
Elena Aprile

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

47. 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)
Elena Aprile

48. 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.
Elena Aprile

49. 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.
Elena Aprile

50. 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)
Elena Aprile

51. 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..
Elena Aprile

52. 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.
Elena Aprile

53. XENON Organization
Subsystem responsibility is allocated amongst the team of experienced co-investigators.
Elena Aprile

54. 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.
Elena Aprile

55. 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
Elena Aprile

56. 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
Elena Aprile

57. 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.
Elena Aprile

58. Team Members Expertise
Elena Aprile

59. 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
Elena Aprile

60. Team Members Presentations
Elena Aprile

61. 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.
Elena Aprile