recent status of the xmass project

Y.Takeuchi @ICHEP04 in BeijingAugust 18, 2004

Recent status of the XMASS projectRecent status of the XMASS project

Physics goals at XMASS Overview of XMASS Current status of R&D Summary

Yasuo Takeuchi(Kamioka Observatory, ICRR, Univ. of Tokyo) for XMASS Collaboration

XMASS = a multi purpose detector to search rare phenomena under an ultra low background environment by using ultra pure liquid xenon


Physics goals at XMASSPhysics goals at XMASS

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)

XMASS FV 50 ton year(90%CL)

G. Gratta@Neutrino2004

http://www.sns.ias.edu/~jnb/pp+-1%

Measure pp via + e + e

2 life time should be measuredIsotope separation would be needed


Expected signal

Physics goals at XMASSPhysics goals at XMASS



Direct search via nuclear elastic scattering

XMASS FV 0.5ton yearEth=5keV, 3 discovery

Eth = 5keV ~200 events/day/ton

Eth = 20keV ~3 events/day/ton

Spin Independent




Physics goals at XMASSPhysics goals at XMASS Search for 0 (2) decay of

136Xe (na 8.87%) High purity and enriched Xe can

be used. Energy region is different from

solar / DM. PMTs should not be placed near

the detector.

Need another design of the detector!(low priority, at moment…)

136Xe 136Ba + e- + e-

Q-Value: 2.48 MeV

2 1/2 theory=8 x 1021 y

Overview of XMASSOverview of XMASS

Strategy

Key ideas (self shielding, distillation)


Strategy of the XMASS projectStrategy of the XMASS project

Dedicated detector forDouble beta decay search

~1 ton detector(FV 100kg)Dark matter search

~20 ton detector(FV 10ton)Solar neutrinosDark matter search

Prototype detector (FV 3kg) R&D

~2.5m~1m~30cm

NOW

Confirmation of feasibilities of the ~1 ton detectorAnalysis techniquesSelf shielding performanceLow background propertiesPurification techniques


Self shieldingSelf shielding

PMTs

Liquid Xe Volume for shielding

Fiducial volume

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

Reconstruct the vertex and energy based on PMTs information (light pattern)

30cm 105 reduction for < ~500keV


Distillation to remove KrDistillation to remove Kr Very effective to eliminate internal impurities

(85Kr, etc.) We have processed 100kg Xe in March ‘04

Boiling point

(@1 atm)

Xe 165K

Kr 120K

~3m

13 stage of Operation: 2 atmProcessing speed: 0.6 kg / hourDesign factor: 1/1000 Kr / 1 pass

Lower temp.

Higher temp.

~1%

2cm

~99%Purified Xe: < 5 ppt Kr (measured)

Off gas Xe:330±100 ppb Kr(measured)

Raw Xe: ~3 ppb Kr

Current status of Current status of R&DR&D

Prototype detectorResults from test runs

Self shieldingInternal background External background


XMASS prototype detectorXMASS 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)

Boric acid (5cm)Lead (15cm)EVOH sheets (30mm)OFC (5cm)Rn free air (~3mBq/m3)

1.0m

1.9m

n

Rn


Test runs with the prototype detectorTest 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


DataMC

DataMC

Z= +15Z= -15

Remove events (PMT saturation)

60Co (1173 & 1333keV)137Cs (662keV)

Self shielding performanceSelf shielding performanceReconstructed vertex position of collimated source runs

Dec.03 run

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


Internal BG source: Internal BG source: 222222RnRn

2 separate runs to check 222Rn decay (1/2=3.8day)

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 )

Aug.04 runPreliminary

T: 1/2 = 141+-51sec

T < 1ms

(1.8days)

3.5MeV

67ev

214Bi 214Po 210Pb

1/2 =164sec

(Emax=3.3MeV) (7.7MeV)

238U

222Rn

L


Internal background sourcesInternal background sources

Current results238U: = (33+-7)x10-14 g/g

232Th: < 63x10-14 g/g

Kr: < 5ppt

Goal (~1ton)

1x10-14 g/g

Factor <~30 (under further study)

Almost achieved by the distillation process

2x10-14 g/g

1 ppt

Factor ~30, but may decay out further

Preliminary

NEW

NEW


[co

un

t/ke

V/d

ay/k

g]

(=d

ru)

External background sourcesExternal background sources Background level was estimated from known

sources

MC estimation for full volume rays from outside shield

PMTs origin 238U series 40K 232Th series

210Pb in the lead shield

Energy (keV)


Measured background levelMeasured background level

Self shielding works Good agreement with expectation (< factor 2)

MeasurementsAll volume20cm FV10cm FV

SimulationAll volume20cm FV10cm FV


Geometrical effect only for prototype detector


Alpha vs Gamma separationAlpha vs Gamma separationAug.04 runPreliminary

Alpha-gamma separation by using FADC wave form would be possible (under further investigation)

Pulse width (ns)

Cha

rge

Alpha-like Gamma-like

FADC data


SummarySummary 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 < 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.


Supplement


Why liquid xenon scintillatorWhy 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 (R=1.22m 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 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


U 1.5±0.3x10-3 Bq Th 3.2±4.6x10-4 Bq40K 1.7±2.9x10-3 Bq

Development of the low BG PMTDevelopment of the low BG PMT

Aiming for another order of magnitudes improvementHexagonal PMTto accomplish 70-80% PMT coverage

Q.E. ~ 30% @ 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


Energy/vertex reconstructionEnergy/vertex reconstruction

137Cs 662keV Gamma ray ( from a collimator)

F(x,y,z,i): hitmap made by MCVUV photon characteristics:

Lemit=42ph/keVabs=34cm scat=30cm

Reconstructedhere

PMT

n

nL )

!

)exp(Log()Log(

L: likelihood: F(x,y,z,i)/ F(x,y,z,j)

x(total p.e.)n: observed number of p.e.

.

Real data

j

Using photoelectron map made by MC (not timing, but charge information)

Vertex: MC hitmapEnergy: Hitmap scale

Dec.03 run


Event reconstructionEvent reconstruction

ABC+++

Collimated gamma rays for three different positions

Hole AHole BHole C

Real data

MC

Reconstruction works well

137Cs

Dec.03 run


Stability of the energy scaleStability of the energy scaleAug.04 runPreliminary

No degrading of the energy scale Stable within +-0.5%

60Co calibration data

Peak position from simple gaussian fit+-0.5%


Observed light yieldObserved light yield

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



8585Kr: 687keV beta analysisKr: 687keV beta analysis

The event rate around 200~400 keV in the Normal runs could be explained by 2~3ppb of Kr.

Dec.03 run


Measured background levelMeasured background level

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

Simulation


Dec. 2003 run Aug. 2004 runAll volume20cm FV10cm FV


232232Th seriesTh series

212Bi 212Po 208Pb

1/2 =299nsec

(Emax=2.3MeV) (8.8MeV)

-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: < 2000p.e. (efficiency ~100%) T=160~6000nsec in Flash ADC (efficiency = 69%)

2nd peak: 500 ~ 4500p.e. (efficiency ~100%)

(BR=64%)

1 event remained

Dec.03 run


Typical FADC data & peak searchTypical FADC data & peak search

54 PMT analog sum 1 FADC

Range: -8 ~ +8sec 80~240nsec window

Threshold: 70 count (4~12p.e.)

Most of peaks

= after pulses from PMTnsec

FADC count

Peak position

Pedestal(80nsec)

Trigger timing

Dec.03 run


232232Th series: Bi-Po analysis (FADC)Th series: Bi-Po analysis (FADC)

1 candidate event 232Th < 63x10-14 g(232Th)/g(Xe) (90%CL)

Trigger timing

1st peak 2nd peak (T~700nsec)

nsec

FA

DC

co

un

t

Keep this eventconservatively,for now

(only stat. error)

(OK) (very high energy?)

Dec.03 run


An idea of dedicated detector forAn idea of dedicated detector for

Put room temperature LXe intoa thick, acrylic pressure vessel (~50atm).

Wavelength shifter inside the vessel.

We already have 10kg enriched 136Xe.

Test vessel held 80 atm water

(symbolically)


Expected sensitivityExpected sensitivity Assume acrylic material U,Th~10-12g/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 @ Q=2.48MeV

1yr, 10kg measurement 1.5 x 1025 yr <m>=0.2~0.3eV

If U/Th ~ 10-16 g/g + larger mass <m>~0.02-0.03eV2 will not be BG thanks to high resolution

U+Th normalized for 10kg, 1yr

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