Post on 10-Feb-2016
description
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Status and background considerations of XMASS experiment
Yeongduk KimSejong University
for the XMASS collaboration
LRT2006Oct. 3, 2006
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1. Various signal rates in large LXe detector2. 800kg detector design3. Gamma backgrounds in 800 kg detector4. Neutron backgrounds5. Possible calibration sources
Outline
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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
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Why Liquid Xenon(LXe) ?
He Ne Ar XeZ 2 10 18 54 Largest self-shieldi
ng effectRadiat. L. (cm) 756 24 14 2.4Ionization(eV) 24.6 21.6 15.8 12.1
Lowest thresholdScint. Eff. (phtons/keV)
15 40 42
Density 0.13 1.2 1.4 3.0 Compact detectorB.P. (K) 4.2 27.1 87.3 165.0 Cooling easiestScint. (nm) 73 80 128 174 No need for WLSIn Air (%) 5X10-4 1.8X10-3 0.93 9X10-6 Relatively expensive
Purification by distillation+ No longlived radioactive isotope Longest half-life : 127Xe(36.4 days)
Little internal background
Inert gas(liquids)
ZEPLIN 1(Single), ZEPLIN2,3(Double), CLEAN : Ne, WARP:Ar
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Signals expected with natural LXe
, T1/2>2.4x1021 yr
WIMP (SI:10-7 pb, MW=100 GeV)
7Be
pp
B10101
Current direct limit
51Cr Source(1MCi) @ 1-2m distant
0
Solar Energy resolution is not applied.
RecentlyT1/2>1.0x1022 yr
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All volume(23ton)20cm wall cut30cm wall cut (10ton FV)
BG
nor
mal
ized
by
mas
s
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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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 under progress 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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1PMT
12 pentagons / 60 triangles pentakisdodecahedron
Structure of 800 kg detector-- tried to optimize the photocathode coverage.
5 triangles make pentagon
34cm
31cm
31cm
12
34
5
67
8910
Hexagonal PMT
10 PMTs / triangle surface
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Total 812 hexagonal PMTs 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(Simulation)
60
50
40
30
20
10
030 3426 3822
GeneratedR = 31cmE = 10keV
= 2.3 cm
Reconstructed position [cm]
Eve
nts
5 keV10 keV
50 keV100 keV500 keV1 MeV
Fiducial volume
2010 300 40
8
10
64
2
0
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Distance from the center [cm]
(r
econ
stru
cted
) [cm
]
Position resolution Boundary of fiducial volume
10 keV ~ 3.2 cm 5 keV ~ 5.3 cm
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• Up to <~40cm, events are well reconstructed with position resolution of ~2~3cm
• Out of 42cm, grid whose most similar distribution is selected because of no grid data
• In the 40cm~44cm region, reconstructed events are concentrated around 42cm,
but they are not mistaken for those occurred in the center
• No wall effect
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) Vertex reconstructed
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Achieved (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)
Liquid Xenon238U = (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 BG from PMTs
Energy [keV]
Cou
nts/
keV
/day
/kg
All volumeR<39.5cm
R<34.5cmR<24.5cm
All volumeR<39.5cm
R<34.5cmR<24.5cm
Statistics: 2.1 days• U-chain• 1/10 lower BG PMT than present R8778
No event is found below100keV after fiducial cut (R<24.5cm)
(Now, more statisticsis accumulating)
< 1×10-4 cpd/kg/keVcan be achieved
238U : 1.8×10-2 Bq/PMT232Th : 6.9×10-3 Bq/PMT40K : 1.4×10-1 Bq/PMT
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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]
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104
Det
ecte
d/ge
nera
ted*
surfa
ce [c
m2 ]
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 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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0.9mm
~7mm
• stainless steel 316L tube
welding
tube
cap
Source wire
resin
spacer
57Co microsource for internal calibration
Active resin
57Co source
For lower energy and position calibration, need X-ray source.Electro deposition of I-125 on 20 micrometer metal wire is planned.
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220Rn(Thoron) source for position calibration 228 224 220 216 212
90 88 86 84 82
(1.9y) (3.7d) (56s) (0.15s) (10h) Th Ra Rn Po Pb
212 208 20883 81 82
212 20884 82
(36%)(61m) (3m) (stable)
(64%) (0.3 s) (stable)
Bi Tl Pb
Po Pb
Xe Gas InLongest T1/2 Since the chamber volume is small, most of the Rn gas will enter to Lxe chamber before decay.
If we flow 220Rn+Xe gas for an hour, then within 10 minutes, most will be 212Pb.
Q=2.25MeV
( ) ( )V V
will give the position resolution of beta events.
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G4 simulation
• For 105 beta-alpha coincidence events below E(beta)<100 keV during 10 hours data taking, the activity of 228Th should be ~ 20 kBq.• Commercilly available.• Whole volume of LXe can be studied.
Beta energy spectrum
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800kg chamber
Water shield tank
Experiment areaexcavation
20m15m
15m
9m
8m
New excavation will be made in kamioka mine. ※Tentative design
Neutron/γ are reduced>250cm water shield-fast neutron : 1/10000 for 2m shield-<500keV γ : 1/10 for 50cm shield
Other similar scale experiment such as DBD will be housed.
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Summary Multi-purpose ultra low background experiment with large mass liquid Xe.(ton scale)
800 kg detector: Dark Matter Search 102 improvement of sensitivity above existing experiments is expected
Design of 800 kg detector is under progress.
BG estimation Shielding New excavation