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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. Solar neutrino. What’s XMASS. Multi purpose low-background experiment with liq. Xe. X enon MASS ive detector for solar neutrino ( pp/ 7 Be )

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XMASS, Status of 800 kg detector design

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  1. XMASS, Status of 800 kg detector design Ko Abe for the XMASS collaboration Kamioka Observatory, ICRR, University of Tokyo

  2. 1. Introduction Solar neutrino • 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) Dark matter Double beta

  3. 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)

  4. pp & 7Be solar n Target for 800kg : Dark Matter search g tracking MC from external to Xenon External g ray BG: 20cm fiducial volume, 100kg U-chain gamma rays 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,) • Dark matter search • With liquid xenon ~1ton, reduce BG below 100 keV to 10-4/day/keV/kg by self shielding. • Search the signal from dark matter in low energy region.

  5. Expected sensitivities XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 10-4 • Large improvements will be expected Two order higher than experimental results so far. DAMA CRESTT Edelweiss ZEPLIN1 10-6 CRESTTII CDMSII Cross section to nucleon [pb] ZEPLIN2 Edelweiss2 XENON100 10-8 XMASS 800kg SuperCDMS phase A 10-10 Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic ~10-45 cm2

  6. 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

  7. 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.

  8. Resolution of event reconstruction and BG estimation from MC • In current design, performance of detector was estimated using Geant4 MC. • Resolution of position. • BG from PMTs • Resolution • Using signals from the PMTs, vertex position is calculated so as to maximize likelyhood. • At boundary of fiducial volume 10keV ~3cm 5keV ~5cm Fiducial volume 12 10 8 DR_reconstructed(cm) 5keV 6 4 10keV 50keV 2 100keV 500keV 1MeV 0 5 10 15 20 25 30 35 40 R(cm)

  9. Background from PMT 238U dru(day-1kg-1keV-1) All volume 5cm self shield 40cm from center 10cm shield 35cm • 1.8 x 10-3 Bq/PMT • <100keV • 5cm shield ~10-3 dru • 10cm shield ~10-4dru • 20cm shield ~10-5 dru 20cm shield 25cm 10-5 10-4 10-3 10-2 10-1 1 Reconstructed Energy(keV)

  10. Background from PMT 60Co All volume 5cm self shield 39.5cm from center dru(day-1kg-1keV-1) 10cm shield 34.5cm 20cm shield 24.5cm • 5.5 x 10-3 Bq/PMT • <100keV same level as 238U • We can achieve 10-5 dru level 10-5 10-4 10-3 10-2 10-1 1 Reconstructed Energy(keV)

  11. Design of 800 kg Detector 2 Water shield for ambient g and fast neutron • Ambient background g and neutron is another large background source. • To reduce these background, use thick water shield. • Estimated how thick shield is needed with simple simulation. Generation point of g or neutron wa Configuration of the estimation Liq. Xe • 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 water MC geometry

  12. g attenuation g attenuation by water shield 104 103 102 More than 200cm water is needed to reduce the BG to the PMT BG level Detected/generated*surface [cm2] 10 1 PMT BG level 10-1 10-2 0 100 200 300 Thickness of shield [cm]

  13. Reach points of fast neutron Reach points before thermalized Generation:107 water: 200cm, energy: 10MeV • Fast n flux @Kamioka mine: • (1.15+0.12) x10-5 /cm2/sec - • Assuming all neutron’s energies are 10 MeV very conservatively water Z [cm] Liq. Xe < 2 x 10-4 counts/day/kg 200cm of water is enough to reduce the fast neutron X [cm]

  14. 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 5keV ~5cm at boundary of fiducial volume • Background from PMT • 238U, 60Co ~10-5 dru insidefiducial volume • Water shield for ambient g and fast neutron • 200cm shield is enough

  15. All volume 5cm shield 10cm shield 20cm shield • 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. Energy (keV) • 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 photon PMT PMT

  16. New excavation @Kamioka mine New excavation for XMASS and other underground experiment will be made soon ~5 m ~20 m ~15 m

  17. Initial energy spectrum from the rock • g attenuation 104 g attenuation by water shield 103 Deposit energy spectrum (200cm) 102 Detected/generated*surface [cm2] 10 1 PMT BG level 10-1 More than 200cm water is needed to reduce the BG to the PMT BG level 10-2 0 100 200 300 Thickness Distance [cm]

  18. Key idea of XMASS: self-shielding effect for low energy events External g ray from U/Th-chain g tracking MC from external to Xenon All volume 20cm wall cut 30cm wall cut (10ton FV) U-chain gamma rays BG normalized by mass Large self-shield effect Large self-shield effect 0 1MeV 2MeV 3MeV Blue : g tracking Pink : whole liquid xenon Deep pink : fiducial volume Background are widely reduced in < 500keV low energy region

  19. Expected sensitivities XMASS FV 0.5 ton year Eth = 5 keVee~25 p.e., 3s discovery w/o any pulse shape info. 10-4 106 • Large improvements will be expected Two order higher than experimental results so far. DAMA CRESTT 104 Edelweiss Edelweiss Al2O3 ZEPLIN1 10-6 Tokyo LiF 102 CRESTTII CDMSII Modane NaI Cross section to nucleon [pb] CRESST ZEPLIN2 Edelweiss2 XENON100 1 UKDMC NaI 10-8 XMASS(Ann. Mod.) NAIAD 10-2 XMASS 800kg SuperCDMS phase A XMASS(Sepc.) 10-10 10-4 Plots except for XMASS: http://dmtools.berkeley.edu Gaitskell & Mandic

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