Transcript スライド 1 - University of Tokyo

Status and background considerations of
XMASS experiment
Yeongduk Kim
Sejong University
for the XMASS collaboration
LRT2006
Oct. 3, 2006
1
Outline
1. Various signal rates in large LXe detector
2. 800kg detector design
3. Gamma backgrounds in 800 kg detector
4. Neutron backgrounds
5. Possible calibration sources
2
Introduction
 What’s XMASS
Multi purpose low-background experiment with liq. Xe
 Xenon MASSive detector for solar neutrino (pp/7Be)
 Xenon neutrino MASS detector (bb decay)
 Xenon detector for Weakly Interacting MASSive Particles (DM search)
Solar neutrino
Dark matter
Double beta
3
Why Liquid Xenon(LXe) ?
Inert gas(liquids)
He
Ne
Ar
Xe
2
10
18
54
Radiat. L. (cm)
756
24
14
2.4
Ionization(eV)
24.6
21.6
15.8
12.1
15
40
42
Lowest threshold
Compact detector
Z
Scint. Eff.
(phtons/keV)
Largest selfshielding effect
Density
0.13
1.2
1.4
3.0
B.P. (K)
4.2
27.1
87.3
165.0
 Cooling easiest
Scint. (nm)
73
80
128
174
No need for WLS
5X10-4
1.8X10-3
0.93
9X10-6
In Air (%)
Relatively expensive
Purification by distillation+ No longlived radioactive isotope
Longest half-life : 127Xe(36.4 days)
Little internal background
ZEPLIN 1(Single), ZEPLIN2,3(Double),
CLEAN : Ne,
WARP:Ar
4
Signals expected with natural LXe
WIMP (SI:10-7
pb, MW=100 GeV)
51Cr
n
Source
(1MCi) @ 1-2m distant
n  11010 B
Current direct limit
n  0
2n2b, T1/2>2.4x1021 yr
Recently, T1/2>1.0x1022 yr
pp
7Be
Solar
n
Energy resolution is not applied.
5
 Key idea:
self-shielding effect for low energy events
U-chain gamma rays
Blue : γ tracking
Pink : whole liquid xenon
Deep pink : fiducial volume
External g ray from U/Th-chain
BG normalized by mass
g tracking MC from external to Xenon
All volume(23ton)
20cm wall cut
30cm wall cut (10ton FV)
Large self-shield effect
0
1MeV
2MeV
3MeV
Background are widely reduced
in < 500keV low energy region
6
 Status of 800 kg detector
 Basic performances have been already confirmed
using 100 kg prototype 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
7
 Structure of 800 kg detector
-- tried to optimize the photocathode coverage.
12 pentagons / 60 triangles
pentakisdodecahedron
1PMT
Hexagonal PMT
6
2
7
1 3 5 8
4 9
10
34cm
10 PMTs / triangle surface
5 triangles make pentagon
8
 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
9
 Event reconstruction(Simulation)
Boundary of fiducial volume
 Position resolution
10 keV ~ 3.2 cm
5 keV ~ 5.3 cm
s (reconstructed) [cm]
60 Generated
R = 31cm
50 E = 10keV
Events
40
30
20
s = 2.3 cm
10
0
22
26
30
34
38
Reconstructed position [cm]
12
10 Fiducial volume
8
5 keV
6 10 keV
4
50 keV
2
0
100 keV
500 keV
1 MeV
0
10
20
30
40
Distance from the center [cm]
10
R_reconstructed(cm)
50
 Vertex reconstructed
45
40
35
30
25
20
15
10
5keV ~ 1MeV
• 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
5
0 5 10 15 20 25 30 35 40 45 50
Distance from the center [cm]
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 800kg BG study
Achieved (prototype detector)
Goal (800kg detector)
 g ray from PMTs ~ 10-2 cpd/kg/keV 1/100
10-4 cpd/kg/keV
→ Increase volume for self shielding
→ Decrease radioactive impurities in PMTs (~1/10)
 Liquid Xenon
238U
= (33±7)×10-14 g/g
→ Remove by filter
232Th
< 23×10-14 g/g (90% C.L.)
→ Remove by filter (Only upper limit)
Kr = 3.3±1.1 ppt
→ Achieve by 2 purification pass
1/33
1×10-14 g/g
1/12
2×10-14 g/g
1/3
1 ppt
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 Estimation of g BG from PMTs
Statistics: 2.1 days
Counts/keV/day/kg
All volume
R<39.5cm
R<34.5cm
R<24.5cm
• U-chain
• 1/10 lower BG PMT
than present R8778
238U
: 1.8×10-2 Bq/PMT
232Th : 6.9×10-3 Bq/PMT
40K
: 1.4×10-1 Bq/PMT
All volume
R<39.5cm
R<34.5cm
R<24.5cm
No event is found below
100keV after fiducial cut
(R<24.5cm)
< 1×10-4 cpd/kg/keV
can be achieved
(Now, more statistics
is accumulating)
Energy [keV]
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 Water shield for ambient g and fast neutron
Necessary shielding was estimated
for the estimation of the size of the new excavation
Generation
point of
g or neutron
wa
Liq. Xe
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
water
MC geometry
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Detected/generated*surface [cm2]
 g attenuation
Initial energy spectrum from the rock
104 g attenuation by water shield
103
102
Deposit energy spectrum (200cm)
10
1
10-1 PMT BG level
10-2
0
100
200
300
Distance from LXe [cm]
More than 200cm water is
Needed to reduce the BG
to the PMT BG level
15
 fast neutron attenuation
water: 200cm, n: 10MeV
• Fast n flux @Kamioka mine:
(1.15±0.12) ×10-5 /cm2/sec
• Assuming all the energies are
10 MeV conservatively
water
< 2×10-2 counts/day/kg
Liq. Xe
No event is found from
the generated neutron
of 105
~200cm water is enough
to reduce the BG to the
PMT BG level
BG caused by thermal neutron
Is now under estimation
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57Co
microsource for internal calibration
Sourc
e wire
welding
cap
space
r
resi
n
~7m
m
Active resin
tub
e
0.9mm
• stainless steel 316L
tube
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




b
220
216
212
Th 
 224
Ra


Rn


Po


Pb


88
86
84
82
228
90
(1.9y)
(3.7d)
(56s)

b
208
Bi 
 208
Tl


81
82 Pb (36%)
(61m)
(10h)
Xe Gas In
Longest T1/2
212
83
(0.15s)
(3m)
(stable)
Since the chamber
volume is small, most
of the Rn gas will enter
to Lxe chamber before
decay.
b

208

 212
Po


84
82 Pb (64%)
Q=2.25MeV
(0.3 s)
V ( b )  V ( )
(stable)
 will give the
position resolution
of beta events.
If we flow 220Rn+Xe
gas for an hour, then
within 10 minutes,
most will be 212Pb.
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• 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.
G4 simulation
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Beta energy spectrum
New excavation will be made in kamioka mine.
※Tentative design
20m
Neutron/γ are reduced
>250cm water shield
-fast neutron : 1/10000 for 2m shield
-<500keV γ : 1/10 for 50cm shield
9m
15m
8m
Water shield tank
Experiment area
excavation
800kg chamber
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
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