Transcript XMASS, Status of 800 kg detector design

XMASS, Status of 800 kg
detector design
Ko Abe for the XMASS collaboration
Kamioka Observatory, ICRR,
University of Tokyo
1. 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
 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)
Target for 800kg : Dark Matter search
g tracking MC from external to Xenon
External g ray BG:
U-chain gamma rays
20cm fiducial
volume, 100kg
pp & 7Be solar n
Blue : g tracking
Pink : whole liquid xenon
Deep pink : fiducial volume
Expected dark matter signal
(assuming 10-42 cm2, Q.F.=0.2
50GeV / 100GeV,)
1. Dark matter search
1. With liquid xenon ~1ton, reduce BG below 100 keV to 10-4/day/keV/kg
by self shielding.
2. Search the signal from dark matter in low energy region.
Expected sensitivities
XMASS FV 0.5 ton year
Eth = 5 keVee~25 p.e., 3s discovery
w/o any pulse shape info.
Cross section to nucleon [pb]
10-4
DAMA
10-6
CRESTTII
CREST
Edelweiss
T
ZEPLIN1
CDMSII
ZEPLIN2
Edelweiss2
XENON100
10-8
10-10
XMASS
800kg
SuperCDMS
phase A
 Large improvements will be expected
Two order higher than experimental
results so far.
~10-45 cm2
Plots except for XMASS:
http://dmtools.berkeley.edu
Gaitskell & Mandic
 Status of 800 kg detector
 Basic performances have been already
confirmed using prototype detector
 Method to reconstruct the vertex and
energy
 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
Design of 800kg
1 detector
• 60 triangles
• 10 PMT/triangle x 60 =
600 PMTs
• + 212 PMTs in triangle
boundary region
• Total 812 PMTs
• Photo coverage 67.0%
• Center to photocathode
~45cm
• Fiducial vloume is 25cm
from center.
• PMTs are inside liquid
xenon.
Resolution of event reconstruction and BG
estimation from MC
– Resolution of position.
– BG from PMTs
• Resolution
– Using signals from the
PMTs, vertex position is
calculated so as to
maximize likelyhood.
12
• At boundary of
fiducial volume
10keV ~3cm
5keV ~5cm
Fiducial
volume
10
DR_reconstructed (cm)
• In current design,
performance of
detector was estimated
using Geant4 MC.
8
5keV
6
4
10keV
50keV
100keV
2
0
5
10
15
20
25
30
500keV
1MeV
35
40
R(cm)
Background from PMT 238U
10-5 10-4 10-3 10-2 10-1
1
dru(day-1kg-1keV-1)
• 1.8 x 10-3 Bq/PMT
• <100keV
All volume
5cm self shield 40cm from center
10cm shield 35cm
20cm shield 25cm
Reconstructed Energy(keV)
– 5cm shield ~10-3 dru
– 10cm shield ~10-4 dru
– 20cm shield ~10-5 dru
60Co
Background from PMT
All volume
5cm self shield 39.5cm from center
10cm shield 34.5cm
20cm shield 24.5cm
10-5
10-4
10-3
10-2
10-1
1
dru(day-1kg-1keV-1)
Reconstructed Energy(keV)
• 5.5 x 10-3 Bq/PMT
• <100keV same level as 238U
• We can achieve 10-5 dru level
 Design of 800 kg Detector
2 Water shield for ambient g and fast neutron
 Ambient background g and
neutron is another large
Generation
background source.
point of
 To reduce these background,
g or neutron
use thick water shield.
 Estimated how thick shield is
needed with simple simulation.
wa
Liq. Xe
water
MC geometry
Configuration of the estimation
 Put 80cm diameter liquid Xe ball
 Assume copper vessel (2cm thickness)
 Assume several size of water shield
50, 100, 150, and 200cm thickness
for liquid Xe
Detected/generated*surface [cm2]
 g attenuation
104 g attenuation by water shield
103
102
10
1
10-1 PMT BG level
10-2
0
100
200
300
Thickness of shield [cm]
More than 200cm water is
needed to reduce the BG
to the PMT BG level
 Reach points of fast neutron
Reach points before thermalized
water: 200cm, energy: 10MeV
Generation:107
• Fast n flux @Kamioka mine:
(1.15+0.12)
x10-5 /cm2/sec
-
Z [cm]
• Assuming all neutron’s energies
are 10 MeV very conservatively
water
Liq. Xe
< 2 x 10-4 counts/day/kg
200cm of water is enough
to reduce the fast neutron
X [cm]
Summary
• XMASS 800kg detector
– 1 ton liquid xenon, 90cm diameter, 60 triangles,
812 PMTs
– BG level 10-4 dru(day-1kg-1kev-1)
– Dark matter search 10-45 cm2
• Detector design by simulation
– Resolution of event reconstruction
• 10keV ~3cm
volume
5keV ~5cm at boundary of fiducial
– Background from PMT
•
238U, 60Co
~10-5 dru inside fiducial volume
– Water shield for ambient g and fast neutron
• 200cm shield is enough
• In the event which
interacted close to the wall,
photons emitted toward wall
are difficult to detect.
Current reconstruction
algorithm tend to reconstruct
such events more inner
region.
All volume
5cm shield
10cm shield
20cm shield
Energy (keV)
photon
PMT
PMT
• Use 20cm outer region as
shield fiducial, the effect
from these events are not
large, we can keep BG as
~10-5dru.
• BG level target of 800kg
detector ~10-4dru
 New excavation @Kamioka mine
New excavation for XMASS and other
underground experiment will be made soon
~5 m
~20 m
~15 m
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
Thickness Distance [cm]
More than 200cm water is
needed to reduce the BG
to the PMT BG level
 Key idea of XMASS:
self-shielding effect for low energy events
U-chain gamma rays
Blue : g 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
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
Expected sensitivities
Cross section to nucleon [pb]
10-4
XMASS FV 0.5 ton year
Eth = 5 keVee~25 p.e., 3s discovery
w/o any pulse shape info.
106
DAMA
10-6
CRESTTII
ZEPLIN2
XENON100
CREST
Edelweiss
T
ZEPLIN1
CDMSII
Edelweiss2
10-8
104
102
1
Edelweiss Al2O3
Tokyo LiF
Modane NaI
CRESST
UKDMC NaI
XMASS(Ann. Mod.)
NAIAD
10-10
XMASS
800kg
SuperCDMS
phase A
10-2
XMASS(Sepc.)
10-4
 Large improvements will be expected
Two order higher than experimental
results so far.
Plots except for XMASS:
http://dmtools.berkeley.edu
Gaitskell & Mandic