XMASS experiment for double beta decay K.Ueshima (ICRR) for XMASS collaboration June 11 @DBD07 in Osaka 1,Strategy of XMASS project 2,Design of detector for double beta decay 3,R&D status for double beta decay 4,800kg detector for Dark Matter search
Strategy of the XMASS project ~1m ~2.5m ~30cm ~1 ton detector ~20 ton detector (FV 100kg) Prototype detector (FV 10ton) Dark matter search (FV 3kg) Solar neutrinos Low BG at ~ keV Confirmation of feasibilities Dark matter search of the ~1ton detector Double beta decay option Low BG at ~ MeV
Current target sensitivity of 0 νββ Inverted hierarchy 136 Xe (N.A. 8.9%) is one of double beta nucleus. 〈 〈 〈 m ββ 〈 ββ 〉 〉 eV 〉 〉 ββ ββ 1 The results of neutrino oscillation disfavoured by 0 ν 2 β experiments suggest that the effective mass of neutrino is above 10 -1 10meV if hierarchy is inverted. 2 < 0 ∆ m 23 Inverted |m ee | in eV disfavoured by cosmology 10 -2 Predicted half life with this effective mass ~ 10 27 -10 28 year 2 > 0 ∆ m 23 10 -3 10ton ( 136 Xe natural) More or 99% CL (1 dof) than 10 -4 1 10 -4 10 -3 10 -2 10 -1 1ton (enriched) lightest neutrino mass in eV
Large scintillation yield reduces 2 νββ background Ultimate energy resolution is 0.6% @Q=2.5MeV since we have large scintillation yield (~42000photons/MeV). 10ton natural, 5 year 0.01 – 10 28 0.01- 10 4 from 1 σ region lifetime limit (95% C.L.) [yr] 0.03eV w/o smear from 2 σ region 0.03eV [cts/keV/5years/10ton] 2% at 2.5 MeV 10 3 from 3 σ region 10 27 4% at 2.5 MeV ∆ E=0.6%RMS Our goal 6% at 2.5 MeV 10 2 peak position 0.1 – 10 26 0.1- 10 1 0.3eV 0.3eV Signal region 10 25 10 0 8x10 21 year 10 -1 10 24 1- 3eV 1 – 3eV 0 5 10 15 20 25 30 10 -2 resolution (at 100keV) [%] 0 500 100015002000250030003500 0 1 2 3 4 5 6 [keV] 2200 2400 2600 Resolution at 2.5 MeV (RMS) keV E resolution < 2% is needed for reducing 2 νββ νββ νββ νββ BG .
Self shielding effect of liq. Xe Many PMTs Low energy: attenuation for 23ton liquid Xe detector event rate gamma rays are very large BG spectrum cnt/day/keV/kg � realize low background event rate (dru) -1 10 -1 23t 2.5m dia. Sphere 10 Small High energy: gamma rays -2 10 Large effect all volume effect -3 penetrate liquid xenon easily 10 -3 10 < 300keV 2.1mFV -4 � not enough for 0 νββ exp. 10 -5 10 -5 10 γ γ γ rays from PMTs, γ -6 10 1.9m FV10t -7 pp, 7 Be 10 -7 10 rocks, etc. need to -8 2 νββ νββ νββ νββ 10 -9 be shielded by 10 -9 10 -10 0 νββ νββ νββ νββ 10 other material -11 T 1/2 =4x10 27 yr 10 -11 10 -12 50keV RMS 1 2 3 10 0 0 500 1000 1500 2000 2500 3000 Need a different design energy (keV)
One solution for reducing BG Water shield & 136 Xe in very low BG vessel Wave length shifter 136 Xe Acrylic mirror vessel Scintillation light Acrylic PMT vessel γ γ γ γ 136 Xe Pure water Room temperature, high pressure liquid Xe.
Optics of the elliptic water tank � ELliptic water Tank (ELT) detector has an interesting optical property. � More than 90% of emitted photons can be collected to a single PMT. 1-(1-cos(28 deg.))/2 ~ 0.94 94% PMT Low BG, efficient light collection, and low cost 28 ° ° ° ° detector !!! photon come from left side
Detector design and things to be solved � Detector design to reach inverted hierarchy Water shield and ELT reduce PMTs gamma rays from rocks and PMTs. 6.5m 4m • Background from vessel and WLS 136 Xe • Internal background • Attenuation of scintillation lights 13m inside liquid xenon 5.3m • Dissolving contamination from Water vessel 13m • … We studied this method as one of our future plans.
Wave length R&D to achieve shifter good energy resolution 136 Xe Acrylic vessel 1. Measure photon yield of liquid Xe at room temperature. Small cell for liquid xenon at room temperature 2. Develop high efficiency wavelength shifter TPB doped polystyrene 3. Study ELT for collecting photon efficiently MC simulation and prototype
1. Measurement of light yield at room temperature. We produced liquid Xe at 1degree, 5.5MPa Piping for producing high pressure liquid Xe Cell Small bottle Xe gas Oxysorb
Cell Liq.Xe 16cc flange attached PMT Attached Window PMT 16mm
Measurement of light yield at room temperature. 70mm Outline Cell 1degree,5.5MPa Cool down liquid Xe temperature, Liq.Xe 85mm measured temperature dependence 16 -100degree,4.5MPa MgF 2 window Reduce liquid Xe pressure, light guide measured pressure dependence Compared 57 Co PMT -100degree,0.18MPa 122keV γ γ γ γ Well known scintillation yield 7.4kBq
Measurement of light yield at room temperature and high pressure . We measured and compared light yield of liquid Xe at 1degree and at -100degree. PMT Cell Reduce Xe and make liquid Xe Cool down -100degree,0.18MPa
Temperature dependence 57 Co 122keV γ γ γ γ 1 ℃ ℃ 5.5MPa -100 ℃ ℃ 4.5MPa ℃ ℃ ℃ ℃ counts/s counts/s 1 ℃ ℃ ℃ ℃ -100 ℃ ℃ ℃ ℃ 5.5 MPa 4.5 MPa p.e. p.e. 33.8 ± ± 0.6p.e. ± ± 39.8 ± ± 0.6p.e. ± ±
Temperature dependence (@~ 5MPa) Mean of photoelectron p.e. -100 ~ 1 ℃ ℃ ℃ ℃ 16%difference Cell temperature ℃ ℃ ℃ ℃
Pressure dependence (@-100 ℃ ) p.e. 0.18 ~ 4.5MPa at -100 ℃ ℃ ℃ ℃ 40 ± ± ± ± 2 p.e. constant within 4% Xe pressure (MPa)
Results for scintillation yield � Large light yield even at room temperature. � pressure dependence 0.18MPa. 0.98 ± ± ± ± 0.02 ± ± 0.04 ± ± 4.5MPa � temperature dependece Room temp. 0.84 ± ± 0.02 ± ± 0.06 ± ± ± ± -100degree
2. Wavelength shifter 0.5% TPB doped PS, 100 µ m We have developed TPB doped polystyrene (PS) as WLS. High wavelength shift efficiency was measured by D.N.Mckinsey et.al. only for 75nm and 48nm (NIMB 132 (1997) 351-358) We developed TPB doped in PS 0.5~4% and measured conversion efficiency for 175nm.
Wavelength shifter Xe Excimer lamp 165~179nm Beam splitter Monitor WLS PMT PMT
Result of conversion efficiency efficiency % 70 70 70 70 60 60 60 60 50 50 50 50 40 40 40 40 30 30 30 30 TPB 4% more than 50% 20 20 20 20 efficiency for ~ 175nm . 10 10 10 10 0 0 0 0 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5 TPB concentration %
3. Prototype ELT 250 250 800mm 145 polished stainless 300 steel Test photon collection efficiency. Start to measure photon 200mm collection efficiency. light source ( 136 Xe)
Simulation of prototype ELT Arrival position of photons on this plane Photon distribution Generate photon at the light source (on φ φ 54mm sphere ). φ φ Water and LXe absorption and ���������� ������� ������� ������� ��� ��� ��� scattering. Reflection 80% Photon collection efficiency 53% with a 8inch PMT is expected .
Current summary of R&D � The photon yield of liquid Xe at room temperature is large. � We developed TPB wave length shifter with an efficiency of 50%. � Prototype ELT is being constructed. Assuming ~ 50% of ELT photon collection efficiency Expected number of photoelectron at 0 νββ 35photon/keV × 2.5MeV × 0.5(wavelength shifter eff.) × 0.5(ELT photo ＝ 5400photoelectron collection eff.) × 0.25(PMT Q.E.) Energy resolution due to photon statistics is 1.4% . Meet our requirement.
800kg detector for Dark Matter search � Budget funded this year. � We are starting detail design of the detector. � Detector structure � Purification system � Cooling system � Electronics and etc….. � Excavation for the detector site at Kamioka mine will start soon. � Plan to finish the construction in two years and start measurement from 2009.
Structure of 800kg detector � liquid Xe 800kg (FV 100kg) � Total 812 PMTs � Photo coverage 67% � Diameter of LXe: 80cm � Diameter of FV: 40cm � PMTs immersed in LXe 80cm LXe (40cm FV)
BG estimation � PMT gamma background 8m � Reduced by self shielding Water water � External gamma γ γ γ γ μ μ μ μ Rock � Reduced by water shield � Neutron n � Reduced by water shield PMT � Internal gamma/beta � Reduced by distillation γ γ γ γ liquid Xe 8m Detail simulation predicts <10 -4 counts/day/kg/keV 80cm
Sensitivity of 800kg detector DM xsec for nucleon (cm 2 ) 10 -40 DAMA XMASS exposure 0.5 ton yr DAMA CRESTT CREST Eth = 5 keVee, 3 σ σ discovery σ σ Edelweiss Edelweiss T WARP ZEPLIN1 10 -42 CRESTTII ZEPLIN1 CDMSII ~ factor 100 CDMSII XENON10 ZEPLIN2 Edelweiss2 XENON100 improvements 10 -44 expected SuperCDMS XMASS SuperCDMS phase A 10 -46 phase A ~10 -45 cm 2 10 -10 10 1 10 2 10 3 WIMP mass (GeV) Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic
Summary � R&D for double beta decay has been performed. � Observed Light yield (35photon/keV) of liquid Xe at room temperature. We developed TPB wavelength shifter (50% conversion efficiency) and made prototype ELT. � We are going to start construction of 800kg detector for dark matter search which has sensitivity of 10 -45 cm 2 .
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