Recent status of the XMASS project

1
Recent status of the XMASS project
Yasuo Takeuchi (Kamioka Observatory, ICRR, Univ.
of Tokyo) for XMASS Collaboration

• Physics goals at XMASS
• Overview of XMASS
• Current status of RD
• Summary

XMASS a multi purpose detector to search rare
phenomena under an ultra low background
environment by using ultra pure liquid xenon
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Physics goals at XMASS
XMASS FV 50 ton year (90CL)
G. Gratta _at_Neutrino2004

• Xenon MASSive Detector for Solar Neutrinos
  (pp/7Be)
• Xenon Detector for Weakly Interacting MASSive
  Particles (Dark Matter Search)
• Xenon Neutrino MASS Detector (Double Beta Decay)

2nbb life time should be measured Isotope
separation would be needed
3
Physics goals at XMASS
XMASS FV 0.5ton year Eth5keV, 3s discovery
Spin Independent
Direct search via nuclear elastic scattering
Eth 20keV 3 events/day/ton
Eth 5keV 200 events/day/ton

• Xenon MASSive Detector for Solar Neutrinos
  (pp/7Be)
• Xenon Detector for Weakly Interacting MASSive
  Particles (Dark Matter Search)
• Xenon Neutrino MASS Detector (Double Beta Decay)

4
Physics goals at XMASS

• Search for 0nbb (2nbb) decay of 136Xe (na 8.87)
• High purity and enriched Xe can be used.
• Energy region is different from solar n / DM.
• PMTs should not be placed near the detector.

136Xe 136Ba e- e-
Q-Value 2.48 MeV
Need another design of the detector! (low
priority, at moment)

• Xenon MASSive Detector for Solar Neutrinos
  (pp/7Be)
• Xenon Detector for Weakly Interacting MASSive
  Particles (Dark Matter Search)
• Xenon Neutrino MASS Detector (Double Beta Decay)

5
Overview of XMASS

• Strategy
• Key ideas (self shielding, distillation)

6
Strategy of the XMASS project
NOW
2.5m
1m
30cm
1 ton detector (FV 100kg) Dark matter search
20 ton detector (FV 10ton) Solar neutrinos Dark
matter search
Prototype detector (FV 3kg) RD
Confirmation of feasibilities of the 1 ton
detector Analysis techniques Self shielding
performance Low background properties Purification
techniques
Dedicated detector for Double beta decay search
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Self shielding
30cm 105 reduction for lt 500keV
Liquid Xe
Volume for shielding
Fiducial volume
PMTs
Reconstruct the vertex and energy based on PMTs
information (light pattern)

• Quite effective for the events below 500 keV (pp
  n DM)
• Not effective for double beta decay experiment

8
Distillation to remove Kr
Boiling point (_at_1 atm)
Xe 165K
Kr 120K

• Very effective to eliminate internal impurities
  (85Kr, etc.)
• We have processed 100kg Xe in March 04

Lower temp.
Off gas Xe 330100 ppb Kr (measured)
Raw Xe 3 ppb Kr
1
3m
Operation 2 atm Processing speed 0.6 kg /
hour Design factor 1/1000 Kr / 1 pass
13 stage of
2cmf
Higher temp.
Purified Xe lt 5 ppt Kr (measured)
99
9
Current status of RD

• Prototype detector
• Results from test runs
• Self shielding
• Internal background
• External background

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XMASS prototype detector

• 30 litter liquid Xenon (100kg)
• Oxygen free copper (31cm)3
• 54 of low-BG 2-inch PMT
• Photo coverage 16
• MgF2 window
• 0.6 p.e. / keV

Polyethylene (15cm)
n
Boric acid (5cm)
g
Lead (15cm)
EVOH sheets (30mm)
OFC (5cm)
Rn
Rn free air (3mBq/m3)
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Test runs with the prototype detector

• December 2003 run
• First test run
• 6 days (2day normal runs for BG estimation)
• Test analysis tools
• Confirmation of the self shielding performance
• Measurements of the internal and external BGs
• August 2004 run
• August 3 11, 2004, 9days (6day normal runs)
• Used purified xenon (by distillation)
• Longer baking time of the system
• New electronics (TDC, etc.)
• Re-measurements of the internal and external BGs

NEW
Photon yield x 1.7
12
Self shielding performance
g
Reconstructed vertex position of collimated
source runs
Z 15
Z -15
60Co (1173 1333keV)
137Cs (662keV)
Data MC
Data MC
Dec.03 run
Remove events (PMT saturation)

• MC reproduces data very well
• We have demonstrated the self-shield actually
  works

13
Internal BG source 222Rn
Aug.04 run Preliminary
238U
222Rn
DT lt 1ms
214Bi 214Po 210Pb
(1.8days)
b (Emax3.3MeV) a (7.7MeV)
3.5MeV
t1/2 164msec
67ev

• 2 separate runs to check 222Rn decay
  (t1/23.8day)
• 4th Aug. 0.8day
• 238U(72-11)x10-14 g/g
• 10th Aug. 1.0day
• 238U(33-7)x10-14 g/g
• Consistent with expected 222Rn decay
• ((30-5)x10-14 )

DT t1/2 141-51msec
DL
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Internal background sources
Preliminary
Goal (1ton) 1x10-14 g/g

• Current results
• 238U (33-7)x10-14 g/g
• 232Th lt 63x10-14 g/g
• Kr lt 5ppt

NEW
Factor 30, but may decay out further
2x10-14 g/g
Factor lt30 (under further study)
1 ppt
NEW
Almost achieved by the distillation process
15
External background sources

• Background level was estimated from known sources

MC estimation for full volume

• g rays from outside shield
• PMTs origin
• 238U series
• 40K
• 232Th series
• 210Pb in the lead shield

count/keV/day/kg (dru)
Energy (keV)
16
Measured background level
Aug.04 run Preliminary
Geometrical effect only for prototype detector
Measurements
Simulation
All volume
All volume
20cm FV
20cm FV
10cm FV
10cm FV

• Self shielding works
• Good agreement with expectation (lt factor 2)

17
Alpha vs Gamma separation
Aug.04 run Preliminary
FADC data
Charge
Alpha-like
Gamma-like
Pulse width (ns)
Alpha-gamma separation by using FADC wave form
would be possible (under further investigation)
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Summary

• XMASS is aiming to search rare phenomena under an
  ultra low background environment by using ultra
  pure liquid xenon.
• 2nd test run with the prototype detector was just
  finished.
• The data were taken using distilled xenon with
  low level krypton (Kr/Xe lt 5ppt).
• Some part of remaining 222Rn in liquid xenon
  looks contaminated in outside of the chamber.
• The background level is consistent with
  expectation within factor about 2.
• The next step (1ton scale) would be feasible,
  and a dark matter search around 10-44 cm2 level
  would be possible.

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Supplement
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Why liquid xenon scintillator

• High photon yield
• Low threshold, good energy resolution,
• Can be directory read by PMT
• Large atomic number
• Radiation length 2.4cm
• Self shielding against external backgrounds
• Compact (R1.22m for 23 tons)
• Easy to liquefy
• Liquid N2 can be used
• Various purification method
• Distillation, circulation during experiment,
• Effective reduction against internal backgrounds
• No long life radioactive isotopes
• 136Xe is a bb decay candidate

Scintillation light 42photon/keV
Scintillation light wave length 175nm
Scintillation light width 40nsec
Atomic number 54
Atomic weight 131.29 amu
Density 3.0 g/cm3
Melting (boiling) point 161.4K (165.1K)
Chemical series Noble gases


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Development of the low BG PMT
Q.E. 30 _at_ 175nm Collection eff. 90 Quartz
window Metal tube (Low BG) Selection of the
parts (measured by HPGe) ? Low BG PMT base 1/10
of the usual ones
U 1.50.3x10-3 Bq Th 3.24.6x10-4 Bq 40K
1.72.9x10-3 Bq
Aiming for another order of magnitudes improvement
Hexagonal PMT to accomplish 70-80 PMT coverage
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Energy/vertex reconstruction
Dec.03 run
Real data
Using photoelectron map made by MC (not timing,
but charge information) Vertex MC
hitmap Energy Hitmap scale
Reconstructed here
L likelihood
m

F(x,y,z,i)/S F(x,y,z,j) x(total p.e.)
j
.
n observed number of p.e.
F(x,y,z,i) hitmap made by MC VUV photon
characteristics Lemit42ph/keV tabs34cm
tscat30cm
137Cs 662keV Gamma ray ( from a collimator)
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Event reconstruction
Collimated gamma rays for three different
positions
Real data
Hole A
Hole B
Hole C
MC
137Cs
Reconstruction works well
Dec.03 run
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Stability of the energy scale
Aug.04 run Preliminary
60Co calibration data Peak position from simple
gaussian fit
-0.5

• No degrading of the energy scale
• Stable within -0.5

25
Observed light yield
Aug.04 run Preliminary

• Observed number of photons for source runs are
  increased by factor 1.7
• xenon purification
• longer baking time
• removal of unnecessary material in the chamber

26
85Kr 687keV beta analysis
Dec.03 run
The event rate around 200400 keV in the Normal
runs could be explained by 23ppb of Kr.
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Measured background level
Aug.04 run Preliminary
Simulation
Dec. 2003 run
Aug. 2004 run
All volume
20cm FV
10cm FV

• Excess in 200-400keV in Dec. 2003 run may be due
  to 85Kr

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232Th series
Dec.03 run
212Bi 212Po 208Pb
b (Emax2.3MeV) a (8.8MeV)
(BR64)
t1/2 299nsec

• a-tagged beta events of 212Bi and 212Po
• High- and Middle-gain normal runs 1.66day
• 20cm fiducial volume cut (to reject
  external events)
• 1st peak lt 2000p.e.
  (efficiency 100)
• DT1606000nsec in Flash ADC (efficiency 69)
• 2nd peak 500 4500p.e.
  (efficiency 100)

1 event remained
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Typical FADC data peak search
Dec.03 run

• 54 PMT analog sum 1
  FADC
• Range -8 8msec
• 80240nsec window
• Threshold 70 count (412p.e.)
• Most of peaks
• after pulses from PMT

FADC count
Pedestal (80nsec)
Peak position
nsec
Trigger timing
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232Th series Bi-Po analysis (FADC)
Dec.03 run
1st peak
2nd peak (DT700nsec)
(OK)
(very high energy?)
FADC count
Keep this event conservatively, for now
nsec
Trigger timing
1 candidate event 232Th lt 63x10-14
g(232Th)/g(Xe) (90CL)
(only stat. error)
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An idea of dedicated detector for 0nbb
Put room temperature LXe into a thick, acrylic
pressure vessel (50atm).
(symbolically)
Test vessel held 80 atm water
Wavelength shifter inside the vessel.
We already have 10kg enriched 136Xe.
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Expected sensitivity

• Assume acrylic material U,Th10-12g/g, no other
  BG.
• Cylindrical geom. (4cm dia. LXe, 10cm dia.
  Vessel)
• 10kg 136Xe
• 42000photon/MeV but 50 scintillation yield, 90
  eff. shifter, 80 water transparency, 20 PMT
  coverage, 25 QE ? 57keVrms _at_ Qbb2.48MeV

UTh normalized for 10kg, 1yr
1yr, 10kg measurement 1.5 x 1025 yr ?
ltmngt0.20.3eV
If U/Th 10-16 g/g larger mass ?
ltmngt0.02-0.03eV 2nbb will not be BG
thanks to high resolution