Status XMASS experiment M. Nakahata for XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo Dark2007, September 28, 200
Status XMASS experiment
M. Nakahata for XMASS collaboration Kamioka
Observatory, ICRR, University of Tokyo
Dark2007, September 28, 2007
Dark matter
Double beta
Solar neutrino
What’s XMASS
Xenon detector for Weakly Interacting MASSive Particles (DM search) Xenon MASSive detector for solar neutrino (pp/7Be) Xenon neutrino MASS detector ( decay)
Multi purpose low-background and low-energy threshold experiment with liq. Xe
Strategy of the XMASS project
800kg 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
Confirmation of feasibility of the ~1ton detector
Funded from 2007
Double beta decay optionSo far
Concept of background reductionSelf-shielding
PMTs
Single phaseliquid Xe Volume for shielding
Fiducial volume
23ton all volume (d=240cm)
20cm 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
Low Background region near the center of the fiducial volume
pp, 7Be solar
3kg FV prototype detector
In theKamioka
Mine(near the
Super-K)
Liq. Xe (31cm)3
MgF2 window
54 2-inch low BG PMTs
ray/neutron shield
OFHC cubic chamber
• Demonstration of reconstruction, self shielding effect, and low background properties.
16% photo-coverage
HamamatsuR8778
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
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)
Source run( ray injection from collimators)
DATA
MC
Collimator A Collimator B Collimator C
A B C
+++Well reproduced.
Source run( ray injection from collimators)
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
Background data
MC uses U/Th/K activity from PMTs, etc (meas. by HPGe). Self shield effect can be clearly seen. Very low background (10-2 /kg/day/keV@50-300 keV)
REAL DATA MC simulation
All volume20cm FV
10cm FV(3kg)
All volume20cm FV
10cm FV(3kg)
10-2/kg/day/keV
3.9days livetime
Eve
nt r
ate
(/kg
/day
/keV
)
0 20001000
10-2
1
3000keV
10-1
0 20001000 3000keV
10-2
1
10-1
Distillation to reduce kryptonA distillation system was made and tested.System specification:
Boiling point (@1 atm)
Xe 165K
Kr 120K
~3m
13 stage of Operation: 2 atm(abs.)Processed speed: 0.6 kg / hourDesign factor: 1/1000 Kr / 1 pass
Lower temp.
Higher temp.
~1%
2cm
~99%
Purified Xe: (3.3±1.1) x 10-12 Kr (by a high sensitivity measurement method)
Off gas Xe:330±100 x 10-9 Kr(measured)
Original Xe: ~3 x 10-9 Kr
178±2K in tower
Process speed: 0.6kg Xe/hour Collection efficiency: > 99% Kr concentration after process: < 1/1000
Radioactive contamination in LXe(internal background)
Achieved with the prototype detector
U, Th, Kr near to the goal. Within reach.
Requirement for 800kg detector
238U: < 1x10-14 g/g (~1decay/100kg/d)232Th: < 2x10-14 g/g (~1decay/100kg/d)85Kr: < 1ppt
238U:(9±6) x10-14 g/g Further reduction by filter232Th: < 23 x10-14 g/g Upper limit, use filter85Kr: 3.3±1.1 ppt by a prototype distillation tower
800kg(100kg FV) detector for DM search Low energy threshold immerse PMTs into LXe Ext. BG: from PMT’s Self-shield effect (demonstrated) Internal BG: Kr, radon Removed by distillation, filters…
“Full” photo-sensitive,“Spherical” geometry detector
80cm diameter812 hexagonal PMTs
70% photo-coverage ~5p.e./keVee
ray BG from PMTs: 60cm, 346kg 40cm, 100kg
Achieved
pp & 7Be solar
Expected dark matter signal(assuming 10-42 cm2, Q.F.=0.2, M=50,100GeV)
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±0.2)x10-2 Bq Th (6.9±1.3)x10-3 Bq 40K (1.4±0.2)x10-1 Bq
10keVR=5cm
800kg reconstruction and BG study
Extensive study to optimize the detector ongoing
5 keV
10 keV
50 keV100 keV500 keV1 MeV
Fiducial volume
2010 300 40
8
10
6
4
2
0
12
Distance from the center [cm]
(r
econ
stru
cted
) [c
m]
10keVR=35cm
Photon tracking: absorption, scattering, and reflection are taken into account.
Vertex resolution
5keV(~25p.e.) threshold
Estimated PMT BG
• Activity of PMT – 238U chain 1.8x10-3 Bq/PMT
– 232Th chain 6.9x10-4 Bq/PMT
– 60Co 5.5x10-3 Bq/PMT
– 40K 1.4x10-2 Bq/PMT
• Below 300 keV number of events in the 25cm fiducial volume decreases rapidly.
• Below 300keV, <10-4 /kg/day/keV BG level.
• Below 100 keV,
<10-5 /kg/day/keV BG level.
All volume 40cm Fiducial Volume35cm Fiducial Volume25cm Fiducial Volume
(/kg
/day
/keV
)(/
kg/d
ay/k
eV)
Sensitivity of the 800kg detector
10-45cm2 (10-9pb) for SI and 10-39cm2 (10-3pb) for SD
0.5ton ・ year exposure (100kg, 5yr)3 discovery
Plots exept for XMASShttp://dmtools.berkeley.eduGaitskell & Mandic
103
XENON10
DAMA
SuperCDMS Phase A
XMASS800kg
WARP140kg
J Ellis et al. benchmark
WIMP mass(GeV)10 102 103
10-42
10-44
10-46
10-40
Cro
ss s
ectio
n to
nu
cle
on
(cm
2)
Spin Independent(SI)
10-38
10-40
10-34
10-36
10-30
10-32
Spin Dependent(SD)
SI
CDMS-II
Sep/12/2007
• Shield environmental gammas and neutrons by pure water (available from SK system).
• Active shield by water Cherenkov detector for muon induced background.
Water shield for environmental background
~10m diameter, ~10m highpure water tank.
20-inch PMTs for veto counter.
800kg detector
Simulation of environmental and fast neutron
wa
water
Liq. Xe
Generation point of and neutron
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
External gamma and neutron are another large background sources.
To reduce these background, use thick water shield.
Thickness of shield is estimated by a simulation.
background
PMT BG level
attenuation by water shield
0 100 200 300Thickness of shield [cm]
10
104
De
tect
ed/g
ene
rate
d*s
urf
ace
[cm
2]
103
102
1
10-1
10-2More than 200cm water is needed to reduce the BG to the PMT BG level
Initial energy spectrum from the rock
Deposit energy spectrum (200cm)
water: 200cm, energy: 10MeV
Liq. Xewater
< 2 x 10-4 counts/day/kg
Fast neutron background
• Fast n flux @Kamioka mine: (1.15+0.12) x10-5 /cm2/sec
• Assuming all neutron’s energies are 10 MeV very conservatively
200cm of water is enoughto reduce the fast neutron
Generat:107 MC events, no event in Liq.Xe volume
X [cm]
Z [
cm]
Reach points before thermalized-
New experimental halls at Kamioka
KamLAND
SK
New Halls
21m
15m
15m
The halls will be ready by summer 2008.
170m
Sep/12/2007 taup2007
Excavation status
As of end of September 2007.The halls will be ready by summer 2008.Hall for XMASS
PMT1
PTFEfiller
PMT2
200 mm
40 mm
Liquid Xe
15 mm
Blue LED
Co-57
Test PMT immersed inLiquid Xe
PTFE light guide
• Two PMTs immersed in liquid Xe• Co-57 (122 keV, 136 keV) source between the PMTs
SUS304 chamber
Cold cap
100 mm
PMT 1
PMT 2
PMT immerse test: pulse height distribution
(122keV) const. (p0) : 74.8 mean (p1) : 2088.7 sigma (p2) : 49.9 (136keV) const. (p3) : 6.1 mean (p4) : 2267.2 sigma (p5) : 73.9
Resolution: 2.4 % @122 keV
pe1: p.e. of PMT1pe2: p.e. of PMT2
selected
Light yield: 17.1 p.e. / keV
122keV peak
136 keV peak
Summary
• R&D by the prototype detector– Demonstrated vertex reconstruction method and
self-shield– Developed background reduction methods
• 800 kg detector– Sensitivity of 10-45 cm2 for spin independent– Budget funded in this year and construction
started. – Plan to construct detector within 2-3 years.