The current status of XMASS • Introduction • Current status of prototype detector • Next step • Summary Cryodet 1 Y.Koshio for XMASS collaboratio n (Kamioka observatory, ICRR)
The current status of XMASS
• Introduction
• Current status of prototype detector
• Next step
• Summary
Cryodet1
Y.Koshiofor XMASS collaboration(Kamioka observatory, ICRR)
What’s XMASS
• Xenon MASSive detector for Solar neutrino (pp/7Be)• Xenon detector for Weakly Interacting MASSive Particles (Dark Matter search)• Xenon neutrino MASS detector (double beta decay)
Why liquid xenon• Large photon yield (~42000photons/MeV)• Self-shield (large Z=54)• Purification (distillation)• Compact detector size (~3g/cm3 10ton=1.5m cubic)• No long life isotope• Scintillation wavelength (175nm, detect directly by PMT)• Relative high temperature (~165K)
Introduction
Co
unt
s/(k
eV
kg
da
y)
23ton Lxe all volume20cm self-shielding30cm self-shielding (10ton FV)
1MeV0 2MeV 3MeV
Large self-shield effect
External ray from U/Th-chain
Cryodet2
100kg Prototype 800kg detector
10 ton detector
~ 30cm~ 80cm
~ 2.5m
R&DDark matter search
Multipurpose detector
(solar neutrino, …)
We are now here
With light guide
Strategy of the scale-up
Cryodet3
Current status of 100kg prototype detector
54 2-inch low BG PMTs
Liq. Xe (30cm)3= 30L= 100Kg
MgF2 window
• Stability check in low temp.• Low background setup• Vertex / energy reconstruction• Demonstration of self-shielding
• Purification system• Particle ID• Attenuation length• Background study
Menu of R&D
Photo coverage: 16% 0.6p.e./keV
Set in Kamioka mine
Cryodet4
1.0m
1.9m
Shield
Rn free air (~3mBq/m3)
material thickness
Polyethylene 15cm
Boron 5cm
Lead 15cm
EVOH sheets 30μm
OF Cupper 5cm
4 shield with doorCryodet
5
=== Background event sample === QADC, FADC, and hit timing information are available for analysis
Reconstructedhere
FADC Hit timing
QADC
Energy and Vertex reconstruction
Using photoelectron map made by MC (only charge,but no timing information)
Vertex: MC hitmapEnergy: Hitmap scale
Maximum likelihood method is used.
PMT
n
nL )
!
)exp(Log()Log(
L: likelihood: F(x , y, z) ×
(total p.e./total acceptance)n: observed number of p.e.
Tuning of parameters correlated to photon tracking is important.
Cryodet6
Results
→ Vertex reconstruction works well
1. Performance of the vertex reconstruction Collimated ray source run from 3 holes (137Cs, 662keV)
DATA
MC
hole Ahole Bhole C
ABC+++
Cryodet7
2. Performance of self shielding z position distribution of the collimated ray source run
→ Data and MC agree well γ
Cryodet8
65keV@peak(677keV)(/E ~ 10%)
Similar peak position in each fiducial.No position bias
3. Performance of the energy reconstruction Collimated ray source run from center hole
137Cs, 662keV
→ Energy reconstruction
works well
All volume20cm FV10cm FV
Cryodet9
4. MC tune using source data
Input parameters:• Photon yield ~42000 photons/MeV• QE of PMT ~25%• Collection efficiency of PMT ~90%• Refractive index of liquid Xe 1.6 (measured)• Absorption• Scattering• Reflection on the surface of cupper
Using hit pattern of source data
tune using total photo electron distribution
Tunable
e.g.) γ beam
front view
30cm 50cm 60cm
side view
γ beam
sensitive to scattering
Change scattering
Fix abs.(100cm) and ref.(20%),
60cm55cm20%
Very preliminaryVery preliminary
Cryodet10
5. Compare background shape b/w data and MC
☆ MC ( ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield
→ Data and MC agree well
→ Self shield effect can be seen
→ Very low background
All volume20cm FV10cm FV
10-2 /kg/day/keV@100-300 keV
Cryodet11
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)
Main sources in liq. Xe are Kr, U-chain and Th-chain
6. Internal backgrounds in LXe were measuredCryodet
12
Now (prototype detector) Goal (800kg detector)
ray BG ~ 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
Very near to the target level!
1/100
1/33
1/12
1/3
Summary of BG measurement Cryodet13
5. Compare background shape b/w data and MC
☆ MC ( ray background) ・ Outside of the shield ・ RI sources in PMTs ・ 210Pb in the lead shield
→ Data and MC agree well
→ Self shield effect can be seen
→ Very low background
Miss-reconstruction
All volume20cm FV10cm FV
10-2 /kg/day/keV@100-300 keV
True vertex
Cryodet14
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
(expected 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
Wall effect Cryodet15
With light guide
Remove the wall effect
PTFE light guide(UV reflection)
Active veto
Fiduciale
ven
t ra
te (
/keV
/day
/kg
)
PMT KPMT ThPMT U
BG simulation(light guide MC)
energy (keV)
Fast neutron upper limit (90%C.L.)
Cryodet16
Energy [keV]
Co
unt
s
10cm fiducial
fiducialvolumeC
ou
nts
Energy [keV]
w/o light guide
Reduce the events due to the wall effect
Hole-B
Comparison of data w/ and w/o light guide
with light guide
Collimated ray source from hole-B (137Cs, 662keV)
Further analysis of Low energy background is under study
Cryodet17
800kg Detector : WIMP search
840-2” PMTs immersed into liq. Xe 70% photo-coverage
~80cm diameter
~5 keVee threshold
External ray BG: 60cm, 346kg 40cm, 100kg
pp & 7Be solar
Achieved
Expected dark matter signal(assuming 10-42 cm2, Q.F.=0.2 50GeV / 100GeV,)
Next step Cryodet18
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
10-10
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
Cryodet19
Detector design Geometry : 12 pentagons /
pentakisdodecahedronA tentative design (not final one)
Hamamatsu R8778MOD(hex)
12cm5.
4cm
5.8c
m(e
dg
e to
ed
ge)
0.3cm(rim)
Hexagonal quartz window Effective area: 50mm (min) QE <~25 % (target) Aiming for 1/10 lower background than R8778
Total 840 hex PMTs immersed into liq. Xe 70% photo-coverage Radius to inner face ~43cm
Cryodet20
Performance check by MC Using Geant4 intput parameter : Xe light yield =42000p.e.
abs. length =100cm, scat.length = 30cm position recon. Method : same with 100kg prototype
(hitmap making & likelihood calculation)
10keV 5cm from center
10keV 35cm from center
Cryodet21
5keV
10keV
100keV 500keV
1MeV
50keV
Fiducial volume
Distance from center (mm)
σ(
real
po
s –
rec
on
po
s)
(m
m)
0 100 200 300 400
0 100 200 300 400
5keV
10keV
100keV 500keV
1MeV
50keV
5keV10keV
100keV
500keV
1MeV
50keV
Npe Light yield Energy resolution
Distance from center (mm)
res %
Distance from center (mm)
Position dependence of reconstructed position resolution
Position dependence of Light yield & resolution
At division of F.V. ( 25cm from center)1MeV ~6mm 5keV ~80mm
Photo yield 4.2 p.e./keV
( at center )Resolution:
1MeV 1.5% 5keV 20%
Cryodet22
Summary
• XMASS experiment: Large volume liquid Xe detector for multi-purpose e.g. DM, pp/Be solar neutrino and measurement. Key idea : Self shielding of liquid Xe
• 100kg prototype detector: The detector performance is as expected. Most of the performances required for 800kg detector are confirmed.
• 800kg next detector: Mainly for dark matter search. 2 orders improvement of sensitivity above existing experiments is expected.
Cryodet23