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XMASS experiment and its double beta decay option
XMASS experiment for dark matter search and low energy
solar neutrino detection Double beta decay option
18th Sep. 2005,HAW05 workshop,Double beta-decay and neutrino massesS. Moriyama, ICRR
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1. Introduction
Dark matter
Double beta
Solar neutrino
What’s XMASS
Xenon MASSive detector for solar neutrino (pp/7Be) Xenon detector for Weakly Interacting MASSive Particles (DM search) Xenon neutrino MASS detector ( decay)
Multi purpose low-background experiment with liq. Xe
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Why Liquid Xenon?
General properties:Large scintillation yield (~42000photons/MeV ~NaI(Tl))Scintillation wavelength (175nm, direct read out by PMTs)Higher operation temperature (~165K, LNe~27K, LHe~4K)Compact (2.9g, 10t detector ~ 1.5m cubic) Not so expensiveWell-known EW cross sections for neutrinos
External gamma ray background:Self shielding (large Z=54)
Internal background:Purification (distillation, etc)No long-life radio isotopes Isotope separation is relatively easyNo 14C contamination (can measure low energy)
Circulation
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Key idea: self shielding effect for low energy signals
Large Z makes detectors very compact Large photon yield (42 photon/keV ~ NaI(Tl))
Liquid Xe is the most promising material.
PMTs
Single phaseliquid Xe Volume for shielding
Fiducial volume
23ton all volume20cm wall cut30cm wall cut (10ton FV)
BG
nor
ma
lize
d b
y m
ass
1MeV0 2MeV 3MeV
Large self-shield effect
External ray from U/Th-chain
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Strategy of the XMASS project
~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
Good results
Confirmation of feasibilities of the ~1ton detector
Double beta decay option?
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3kg FV prototype detector
In theKamioka
Mine(near the
Super-K)
Liq. Xe (31cm)3
MgF2 window
54 2-inch low BG PMTs
Gamma ray shield
OFHC cubic chamber
• Demonstration of reconstruction, self shielding effect, and low background properties.
16% photo-coverage
HamamatsuR8778
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PMT
n
nL )
!
)exp(Log()Log(
L: likelihood
: F(x,y,z,i)
x total p.e. F(x,y,z,i)
n: observed number of p.e.
Vertex and energy reconstruction
=== Background event sample === QADC, FADC, and hit timing information are available for analysis
F(x,y,z,i): acceptance for i-th PMT (MC)VUV photon characteristics:
Lemit=42ph/keVabs=100cm
scat=30cm
Calculate PMT acceptances from various vertices by Monte Carlo. Vtx.: compare acceptance map F(x,y,z,i) Ene.: calc. from obs. p.e. & total accept.
Reconstructedhere
FADC Hit timing
QADC
Reconstruction is performed by PMT charge pattern (not timing)
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Source run( ray injection from collimators) I
DATA
MC
Collimator A Collimator B Collimator C
A B C
+++Well reproduced.
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Source run( ray injection from collimators) II
Good agreements. Self shield works as expected. Photo electron yield ~
0.8p.e./keV for all volume
Gamma rays
Z= +15Z= -15
137Cs: 662keVDATAMC
PMTSaturationregion
-15 +15cm -15 +15cm
~1/200
~1/10
Reconstructed Z Reconstructed Z
Arb
itrar
y U
nit
10-1
10-2
10-3
10-4
10-5
10-1
10-2
10-3
10-4
10-5
60Co: 1.17&1.33MeV DATA MC
No energycut, onlysaturation cut.BG subtracted=2.884g/cc
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Background data
MC uses U/Th/K activity from PMTs, etc (meas. by HPGe). Good agreement (< factor 2) Self shield effect can be clearly seen. Very low background (10-2 /kg/day/keV@100-300 keV)
REAL DATA MC simulation
All volume20cm FV
10cm FV(3kg)
All volume20cm FV
10cm FV(3kg)
Miss-reconstruction due to dead-angle region from PMTs.
10-2/kg/day/keV
Aug. 04 run
3.9dayslivetime
~1.6Hz, 4 fold, triggered by ~0.4p.e.
Eve
nt r
ate
(/kg
/day
/keV
)
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Current results• 238U(Bi/Po): = (33+-7)x10-14 g/g
• 232Th(Bi/Po): < 63x10-14 g/g
• Kr: < 5ppt
Internal background activities
Factor <~30 (under further study)
Achieved by distillation
Factor ~30, but may decay out further
Goal to look for DM by 1ton detector
1x10-14 g/g
2x10-14 g/g
1 pptx5
x32
x33
Very near to the target level of U, Th Radon and Kr contamination.
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Distillation to reduce Kr (1/1000 by 1 pass) Very effective to reduce internal impuritie
s (85Kr, etc.) We have processed our Xe before the m
easurement.
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 after Kr-enrichment)
Off gas Xe:330±100 ppb Kr(measured)
Original Xe: ~3 ppb Kr
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1 ton (100kg FV) detector for DM Search
Solve the miss reconst. prob. immerse PMTs into LXe Ext. BG: from PMT’s Self-shield effect demonstrated Int. BG: Kr (distillation), Radon Almost achieved
external ray (60cm, 346kg)
external ray:(40cm, 100kg )
/kg/
day/
keV
Dark matter(10-6 pb, 50GeV,100 GeV)Q.F. = 0.2 assumed
pp
7Be
8x10-5/keV/kg/d
“Full” photo-sensitive,“Spherical” geometry detector
80cm dia.
Achieved
0 100 200Energy(keVee)~800-2” PMTs (1/10 Low BG)
70% photo-coverage ~5p.e./keVee
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More detailed geometrical design A tentative design (not final one) 12 pentagons / pentakisdodecahedron
This geometry has been coded in a Geant 4 based simulator
Hexagonal PMT~50mm diameter
Aiming for 1/10 lower BG than R8778 R8778: U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq 40K 1.4±0.2x10-1 Bq
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Expected sensitivity
Large improvements expected.
XMASS(Ann. Mod.)
XMASS(Sepc.)
Edelweiss Al2O3
Tokyo LiF
Modane NaI
CRESST
UKDMC NaI
NAIAD
XMASS FV 0.5ton yearEth=5keVee~25p.e., 3 discoveryW/O any pulse shape info.
10-6
10-4
10-8
10-10
Cro
ss s
ectio
n to
nuc
leon
[p
b]
10-4
10-2
1
102
104
106
Plots except for XMASS: http://dmtools.berkeley.eduGaitskell & Mandic
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Double beta decay option
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BG for double beta decay signals with conventional XMASS detector
2not yet observed. NA 8.9% Q=2.467MeV, just belo
w 208Tl 2.615MeV rays
Self shielding of liquid xenon is not very effective for high energy rays.
rays from rock & PMTs need to be shielded.
23ton 2.5m dia. sphere
15ton 2.1m dia.
10ton (1.9m dia. )
100% 136Xe01025yr<m>~0.2-0.3eV
100% 136Xe 28x1021yr
Eve
nt r
ate
(keV
-1kg
-1y-1
)
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One of possible solutions
Put room temperature LXe intoa thick, acrylic pressure vessel (~50atm).Symbolically…
Wavelength shifter inside the vessel.
We already have 10kg enriched 136Xe.
Merit: Xe can be purifiedeven after experiment starts!
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Expected 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 transp., 20% PMT coverage, 25% QE 57keVrms @ Q=2.48MeV
1yr, 10kg measurement 1.5 x 1025 yr <m>=0.2~0.3eVc.f. DAMA > 7 x 1023 yr (90%)If U/Th ~ 10-16 g/g + larger mas
s <m>~0.02-0.03eV will not be BG thanks to high resolution!!
57keVrms
expected
U+Th normalized for 10kg, 1yr
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R&D items Pressure test Wavelength shifter Scintillation yield Possible creep effect on acrylic material Degas from acrylic surface BG consideration (time anal., plastic scinti. vessel) Detector design
c.f. wavelength shifter: M.A.Iqbal et al., NIMA 243(1986)459 L. Periale et al., NIMA 478(2002)377 D. N. McKinsey et al., NIMB 132 (1997) 351
Pressure vessel PMTs
Useful for any scintillators
Double focus detector• Cheap• Easy• Safe
Water sheild
Scintillation light
-
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Pressure test vessel
Test vessel held 80 atm water!!
valve
110mm-dia.
120mm length
50mm-dia., 50mm length~98cc
water
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R&D study for wavelength shifter DC light source: excimer xenon lamp
Vacuum vessel, signal PMT and monitor PMT
• Vacuum vessel ~80cm diameter• Signal and monitor PMTs R8778 for XMASS• Sample fixed in 50mm dia. holder• Beam splitter: MgF2 tilted by 45 deg.
172nm
monitor PMT
PMT
sample
0
100
160 170 180 190 (nm)
Wavelength ~ LXe scintillation light
Arb
itrar
y un
it
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TPB in PS Famous WLS for VUV lights TPB: Tetraphenyl butadiene This measurementTPH: p-terphenylDPS: Dephenyl stilbeneSodium salicylate Doped in a polystyrene films 0.5, 1.0, 2.0, 4.0, 8.0, 16.0% (in weight)Ref. systematic study ondoped films for 58nm and 74nm,D. N. McKinsey et al.,NIMB, 132 (1997) 351-358
0.5% TPB doped PS, 100m
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TPB 0.5% doped PS Two measurements for systematic study (1) Gap between PS and PMT: PS n=1.59 Due to total reflection<39deg. light go into PMT Solid angle 11% Correction applied(2) Optical grease btw PS and PMT: grease n=1.47 Light to orange region go into PMT (67deg., 39deg.) Solid angle 50% Correction applied Efficiency 37+/-6% is obtained for 0.5% TPB PS. However, 2% TPB PS does not give consistent
results. Further careful study needed.
PMT39deg. Quartz
n=1.5-1.6
PMT
67deg.
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Background due to 8B solar neutrinos
8B solar neutrinos for double beta decay searchA. A. Klimenko, hep-ph/0407156,
8B solar neutrinos willbe background for136Xe double beta decay search.4.0x1027y 23meV
Need Ba daughter tag,two track ID,or ??
Ge is safe because ofits high energy res.
T21/2 = 2.2x1022y
T21/2 = 4.0x1027y
FWHM=60keV
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Summary XMASS experiment: dark matter, low energy solar neutrino, a
nd double beta decay observation. With 3kg FV R&D detector, we have demonstrated event reco
nstruction, self shielding, and low radioactive contamination in xenon.
(1)238U(Bi/Po) = (33+-7)x10-14 g/g
(2) 232Th(Bi/Po) < 63x10-14 g/g
(3) Kr < 5ppt.
(4) Background @ 200keV ~10-2/kg/keV/day 100kg FV XMASS detector is expected to give ~100 improvem
ent for current dark matter search. For double beta decay option, another design is discussed. Wavelength shifter (0.5% TPB in PS) gives 37+/-6% conversi
on efficiency for 172nm light. Further R&D is ongoing for double beta decay option.
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Ge detector case
Ge case, 4.2x1027y is safe (FWHM 5keV)
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Estimation for the size of the water tank
Back of an envelope estimation:
1. 208Tl 2.6MeV gamma rays from the rock
2. Compton scattered gamma by water does not contribute.
3. Gamma flux of 208Tl based on measured value in the mine.
Attenuation coeff. of 2.614MeV in water : 4.27x10-2 cm-1 (1/70 by 1m)Gamma ray flux in the mine, 2.615MeV: 0.07cm2/secSurface area of 10kg liquid xenon (24cm diameter): 2x103cm2
Probability to deposit 2.6MeV when the gamma ray incident : 10%Water thickness: t (m)
BG 1event/1yr ( 2x10-9! )0.07 x 2x103 x 0.1 x exp(-4.27 t) x 86400 x 365 = 1 t =4.7m
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Distance to the PMT’s 20’’ PMT for Super-Kamiokande 208Tl gamma ray ~10/sec/PMT Use 10 PMT’s To make 1event/1yr background, 10 x 10 x (0.122)/4t2 x exp(-4.27 t) x 86400 x 365 = 1
t=3.3m 3.3m 13m
10m
Water tank
Rough calculation gives: 13m in height 10m in diameter
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0.01
0.10
1.00
10.00
100.00
160 260 360 460 560 660 760
Wavelength [nm]
Pho
toca
thod
e R
adia
nt S
ensi
tivity
[mA
/W]
Qua
ntum
Effi
cien
cy [%
]
Quantum Efficiency
Quantum Efficiency
Fig.1. Xe2*エキシマランプの分布スペクトル
0
10
20
30
40
50
60
70
80
90
100
150 155 160 165 170 175 180 185 190 195 200波長(nm)
相対
強度
spectral
0
10
20
30
40
50
60
70
80
90
100
3500 4000 4500 5000 5500 6000
wave length A
spectr
al
spectral
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spectrum (light source)
0
10
20
30
40
50
60
70
80
90
100
150 155 160 165 170 175 180 185 190 195 200
Wave length [nm]
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0.75
0.8
0.85
0.9
0.95
1
150 155 160 165 170 175 180 185 190 195 200
tran
smitta
nce
MgF2 transmittance
Wave length [nm]
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Hamamatsu R8778MOD(hex)
12cm
5.4c
m5.
8cm
(ed
ge
to e
dg
e)
0.3cm(rim)
c.f. R8778 U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq40K 1.4±0.2x10-1 Bq
Hexagonal quartz window Effective area: 50mm (min) QE ~30 % (target) Aiming for 1/10 lower background than R8778
Prototype has been manufactured already Now, being tested
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McKinsey: Excitation by 58, 74nm