introduction 800kg detector design summary
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1
1. Introduction2. 800kg detector design3. Summary
XMASS experiment
15th Sep. 2006
A. Takeda for the XMASS collaborationKamioka Observatory, ICRR,
University of Tokyo
IDM2006, Rhodes, Greece
2
1. Introduction
Dark matter
Double beta
Solar neutrino
What’s XMASS
Xenon MASSive detector for solar neutrino (pp/7Be) Xenon neutrino MASS detector ( decay) Xenon detector for Weakly Interacting MASSive Particles (DM search)
Multi purpose low-background experiment with liq. Xe
3
Why liquid xenon
Large Z (=54) Self-shielding effect Large photon yield (~42 photons/keV ~ NaI(Tl)) Low threshold High density (~3 g/cm3) Compact detector (10 ton: sphere with diameter of ~2m) Purification (distillation)
No long life radioactive isotope Scintillation wavelength (175 nm, detected directly by PMT) Relative high temperature (~165 K)
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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
Key idea: self-shielding effect for low energy events
tracking MC from external to Xenon
Blue : γ trackingPink : whole liquid xenonDeep pink : fiducial volume
Background are widely reduced in < 500keV low energy region
U-chain gamma rays
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100kg Prototype 800kg detector
10 ton detector
~ 30cm
~ 80cm
~ 2.5mR&D
Dark matter search
Multipurpose detector
(solar neutrino, …)
Strategy of the scale-up
With light guide
We are here !
Vertex & energy reconstruction Self shielding power BG level
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Trend of Dark matter (WIMPs) direct searches
Scintillation
Phonon Ionization
Ge, TeO2, Al2O3, LiF, etc
NaI, Xe, CaF2, etc.
Ge
Recoiled nuclei are mainly observed by 3 ways
CDMS, EDELWEISS: phonon + ionization
Taking two type of signals simultaneously is recent trend
ray reduction owing to powerful particle ID
However, seems to be difficult to realize a large and uniform detector due to complicated technique
Ge, Si
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Strategy chosen by XMASS
Make large mass and uniform detector (with liq. Xe)
Reduce ray BG by fiducial volume cut (self shielding)
Same style as successful experiments of Super-K, SNO, KamLAND, etc.
Super-K KamLANDSNO
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Main purpose: Dark Matter search
812-2” PMTs immersed into liq. Xe 70% photo-coverage
~80cm diameter
~4 p.e./ keV
External ray BG: 60cm, 346kg 40cm, 100kg
Expected dark matter signal(assuming 10-6 pb, Q.F.=0.2 50GeV / 100GeV,)
pp & 7Be solar
Achieved
2. 800kg detector design
0 100 200 300Energy [keV]
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Expected sensitivities
Large improvements will be expected SI ~ 10-45 cm2 = 10-9 pb SD~ 10-39 cm2 = 10-3 pb
XMASS(Ann. Mod.)
XMASS(Sepc.)
Edelweiss Al2O3
Tokyo LiF
Modane NaI
CRESST
UKDMC NaI
NAIAD
XMASS FV 0.5 ton yearEth = 5 keVee~25 p.e., 3 discoveryw/o any pulse shape info.
10-6
10-4
10-8
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
10-10
10
Basic performances have been already confirmed using 100 kg prototype detector
Status of 800 kg detector
Vertex and energy reconstruction by fitter Self shielding power BG level
Detector design is going using MC Structure and PMT arrangement (812 PMTs) Event reconstruction BG estimation
New excavation will be done soon
Necessary size of shielding around the chamber
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Hex
1PMT 12 pentagons / pentakisdodecahedron
Structure of 800 kg detector
5 triangles make pentagon
34cm
31cm
31cm
10 PMTs per 1 triangle
1
2
3
4
5
6
7
8
9
10
agonal quarts window
HamamatsuR8778MOD
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Total 812 hex PMTs (10PMTs/triangle×60 + 212 @gap) immersed into liq. Xe ~70% photo-coverage Radius to inner face ~44cm
Each rim of a PMT overlaps to maximize coverage
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Event reconstruction
60
50
40
30
20
10
030 3426 3822
GeneratedR = 31cmE = 10keV
= 2.3 cm
Reconstructed position [cm]
Eve
nts
5 keV
10 keV
50 keV100 keV500 keV1 MeV
Fiducial volume
2010 300 40
8
10
6
4
2
0
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Distance from the center [cm]
(r
econ
stru
cted
) [c
m]
Position resolution @Boundary of fiducial volume
10 keV ~ 3.2 cm 5 keV ~ 5.3 cm
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5keV ~ 1MeV
0 5 10 15 20 25 30 35 40 45
50
45
40
35
30
25
20
15
10
5
50Distance from the center [cm]
R_reconstructed(cm) Generated VS reconstructed
• No wall effect
• Out of 45cm, some events occurring behind the PMT are miss reconstructed
• In the 40cm~44cm region, reconstructed events are concentrated around 42cm,
but they are not mistaken for those occurred in the center
• Up to <~40cm, events are well reconstructed with position resolution of ~2~5cm
• Out of 42cm, grid whose most similar distribution is selected because of no grid data
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PMT PMT
Light leak events
Some scintillation lights generated behind the PMTenter the inner region
It is not problemif light shield is installed
PMT hit map
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Achieved (measured by prototype detector) Goal (800kg detector)
ray from PMTs ~ 10-2 cpd/kg/keV 10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10)
238U = (33±7)×10-14 g/g 1×10-14 g/g → Remove by filter
232Th < 23×10-14 g/g (90% C.L.) 2×10-14 g/g → Remove by filter (Only upper limit)
Kr = 3.3±1.1 ppt 1 ppt → Achieve by 2 purification pass
1/100
1/33
1/12
1/3
800kg BG study
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Estimation of ray BG from PMTs
Energy [keV]
Cou
nts/
keV
/day
/kg
All volume
R<39.5cm
R<34.5cm
R<24.5cm
All volume
R<39.5cm
R<34.5cm
R<24.5cm
Statistics: 2.1 days
• U-chain• 1/10 lower BG PMT than R8778
No event is found below100keV after fiducial cut (R<24.5cm)
(Now, more statisticsis accumulating)
< 1×10-4 cpd/kg/keVcan be achieved
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Water shield for ambient and fast neutron
wa
water
Liq. Xe
Generation point of or neutron
Necessary shielding was estimated for the estimation of the size of the new excavation
MC geometry
Configuration of the estimation
Put 80cm diameter liquid Xe ball Assume several size of water shield 50, 100, 150, and 200cm thickness Assume copper vessel (2cm thickness) for liquid Xe
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attenuation
PMT BG level
attenuation by water shield
0 100 200 300Distance from LXe [cm]
10
104
De
tect
ed/g
ene
rate
d*s
urf
ace
[cm
2]
103
102
1
10-1
10-2 More than 200cm water is needed to reduce the BG to the PMT BG level
Initial energy spectrum from the rock
Deposit energy spectrum (200cm)
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water: 200cm, n: 10MeV
Liq. Xe
water
No event is found fromthe generated neutron
of 105
< 2×10-2 counts/day/kg
fast neutron attenuation
• Fast n flux @Kamioka mine: (1.15±0.12) ×10-5 /cm2/sec
• Assuming all the energies are 10 MeV very conservatively
200cm water is enoughto reduce the BG to the PMT BG level
BG caused by thermal neutronis now under estimation
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New excavation @Kamioka mine
New excavation for XMASS and other underground experiment will be made soon
~5 m
~15 m
~20 m
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3. Summary XMASS experiment: Multi purpose low-background experiment with large mass liq. Xe
800 kg detector: Designed for dark matter search mainly, and 102 improvement of sensitivity above existing experiments is expected
Detector design of 800 kg detector is going
BG estimation Shielding New excavation
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Backup
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800kg detector: Main purpose: Dark Matter search
~800-2” PMTs immersed into liq. Xe 70% photo-coverage
~80cm diam
eter
~4 p.e./keV
External ray BG: 60cm, 346kg 40cm, 100kg
Expected dark matter signal(assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,)
pp & 7Be solar
Achieved
Photoelectrons (p.e.)
5 keV
10 keV
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XMASS collaboration• ICRR, Kamioka Y. Suzuki, M. Nakahata, S. Moriyama, M. Shiozawa,
Y. Takeuchi , M. Miura, Y. Koshio, K. Abe, H. Sekiya, A. Takeda, H. Ogawa, A. Minamino, T. Iida, K. Ueshima• ICRR, RCNN T. Kajita, K. Kaneyuki• Saga Univ. H. Ohsumi• Tokai Univ. K. Nishijima, T. Maruyama, Y. Sakurai• Gifu Univ. S. Tasaka• Waseda Univ. S. Suzuki, J. Kikuchi, T. Doke, A. Ota, Y. Ebizuka• Yokohama National Univ. S. Nakamura, Y. Uchida, M, Kikuchi, K. Tomita, Y. Ozaki, T. Nagase, T. Kamei, M. Shibasaki, T. Ogiwara• Miyagi Univ. of Education Y. Fukuda, T. Sato• Nagoya ST Y. Itow• Seoul National Univ. Soo-Bong Kim• INR-Kiev O. Ponkratenko• Sejong univ. Y.D. Kim, J.I. Lee, S.H. Moon
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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 <~25 % (target) Aiming for 1/10 lower background than R8778
Prototype has been manufactured already Now, being tested
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12
Ceramic dielectric parts to support dynodes
Glass parts for feed through & containment
12
For R8778mod using quartz
For R8778mod Reduce glass material
c.f. R8778 (used for 100kg chamber) U 1.8±0.2x10-2 Bq Th 6.9±1.3x10-3 Bq40K 1.4±0.2x10-1 BqU Th 40K
※measured byHPGe detectorin Kamioka
Improvement result will be coming soon!
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Super-K
KamLAND (>0.8MeV)
Heidelberg Moscow
Kamioka Ge
ZEPLIN before PSD cut
DM signal for LXe100GeV 10-6pb
CDMS IIAfter PID
DAMANaI
Current XMASS (new improvement!)
XMASS800kg
Eve
nts/
kg/k
eV/d
ay
BG levels
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100kg prototype
With light guide version
R&D status using prototype detector
Main purpose
Confirmation of estimated 800 kg detector performance
BG study
Vertex and energy reconstruction by fitter Miss fitting due to dead angle of the cubic detector (“wall effect”, will be explained later) can be removed with light guide Self shielding power
Understanding of the source of BG Measuring photon yield and its attenuation length
~30 cm cube3 kg fiducial
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In theKamioka Mine
(near the Super-K)
Gamma ray shield
OFHC cubic chamber
Liq. Xe (31cm)3
MgF2 window
54 2-inch low BG PMTs
16% photo- coverage
Hamamatsu R8778
2,700 m.w.e.
100 kg prototype detector
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1.0m
1.9mRn free air (~3mBq/m3)
material thickness
Polyethylene 15cm
Boron 5cm
Lead 15cm
EVOH sheets 30μm
OF Cupper 5cm
4 shield with door
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100 kg Run summary
1st run (Dec. 2003)
2nd run (Aug. 2004)
3rd run (Mar. 2005) with light guide
Confirmed performances of vertex & energy reconstruction Confirmed self shielding power for external rays Measured the internal background concentration
Succeeded to reduce Kr from Xe by distillation Photo electron yield is increased Measured Rn concentration inside the shield
Confirmed the miss fitting (only for the prototype detector) was removed Now, BG data is under analysis
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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.
=== 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)
Vertex and energy reconstruction
34→ Vertex reconstruction works well
Collimated ray source run from 3 holes (137Cs, 662keV)
DATA
MC
hole Ahole Bhole C
ABC+++
Performance of the vertex reconstruction
35
65keV@peak(/E ~ 10%)
Similar peak position in each fiducial.No position bias
Collimated ray source run from center hole (137Cs, 662keV)
→ Energy reconstruction
works well
All volume20cm FV10cm FV
Performance of the energy reconstruction
36
z position distribution of the collimated ray source run
→ Data and MC agree wellγ
Demonstration of self shielding effect
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Shelf shielding for real data and MC
Good agreement (< factor 2) Self shielding effect can be seen clearly. 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.
Eve
nt r
ate
(/kg
/day
/keV
)
10-2/kg/day/keV
Aug. 04 runpreliminary
3.9dayslivetime
~1.6Hz, 4 fold, triggered by ~0.4p.e.
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Kr = 3.3±1.1 ppt (by mass spectrometer)
→ Achieved by distillation
U-chain = (33±7)x10-14 g/g (by prototype detector)
Th-chain < 23x10-14 g/g(90%CL) (by prototype detector)
1/2=164s (Q=3.3MeV) (7.7MeV)
214Bi 214Po 210Pb
1/2=299ns (Q=2.3MeV) (8.8MeV)
212Bi 212Po 208Po
Delayed coincidence search
Delayed coincidence search
(radiation equilibrium assumed)
(radiation equilibrium assumed)
Internal backgrounds in liq. Xe were measured
Main sources in liq. Xe are Kr, U-chain and Th-chain
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Kr 0.1ppm
DM signal(10-6 pb, 50GeV,100 GeV)
0 200 400 600 800
1
102
10-2
10-4
10-6
cpd/
kg/k
eV
energy (keV)
Target = Xe
Kr concentration in Xe
85Kr makes BG in low enegy region
Kr can easily mix with Xe because both Kr and Xe are rare gas
Commercial Xe contains a few ppb Kr
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XMASS succeeds to reduce Kr concentration in Xe
from ~3[ppb] to 3.3(±1.1)[ppt] with one cycle (~1/1000)
~3mLower
Higher
~1%
~99%
Purified Xe: 3.3±1.1 ppt Kr (measured)
Off gas Xe:330±100 ppb Kr(measured)
Raw Xe: ~3 ppb Kr
(preliminary)
(178K)
(180K)Operation@2atm
• Processing speed : 0.6 kg / hour• Design factor : 1/1000 Kr / 1 pass• Purified Xe : Off gas = 99:1
Xe purification system
Boiling point
(@2 atm)
Xe 178.1K
Kr 129.4K
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HIT
HIT
HIT
HIT
HIT
?
MCIf true vertex is usedfor fiducial volume cut
10-2
1
10-1
Energy (keV)0 1000 2000 3000
No wall effect
Remaining problem: wall effect (only for the prototype detector)
Scintillation lights at the dead angle from PMTs give quite uniform 1 p.e. signal for PMTs, and this cause miss reconstruction as if the vertex is around the center of detector
Deadangle
This effect does not occur with the sphere shape 800 kg detector
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800 kg detector
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