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Status and background considerations of XMASS experiment

Status and background considerations of XMASS experiment. Yeongduk Kim Sejong University for the XMASS collaboration. LRT 2006 Oct . 3, 200 6. Outline. Various signal rates in large LXe detector 800kg detector design Gamma backgrounds in 800 kg detector Neutron backgrounds

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Status and background considerations of XMASS experiment

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  1. Status and background considerations of XMASS experiment Yeongduk Kim Sejong University for the XMASS collaboration LRT2006 Oct. 3, 2006

  2. Outline • Various signal rates in large LXe detector • 800kg detector design • Gamma backgrounds in 800 kg detector • Neutron backgrounds • Possible calibration sources

  3. 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

  4. Why Liquid Xenon(LXe) ? Inert gas(liquids) • Purification by distillation+ No longlived radioactive isotope • Longest half-life : 127Xe(36.4 days) • Little internal background ZEPLIN 1(Single), ZEPLIN2,3(Double), CLEAN : Ne, WARP:Ar

  5. 51Cr Source (1MCi) @ 1-2m distant Solar Signals expected with natural LXe WIMP (SI:10-7 pb, MW=100 GeV) Current direct limit 2n2b, T1/2>2.4x1021 yr pp Recently, T1/2>1.0x1022 yr 7Be Energy resolution is not applied.

  6. Key idea: self-shielding effect for low energy events External g ray from U/Th-chain g tracking MC from external to Xenon All volume(23ton) 20cm wall cut 30cm wall cut (10ton FV) U-chain gamma rays Large self-shield effect BG normalized by mass 0 1MeV 2MeV 3MeV Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume Background are widely reduced in < 500keV low energy region

  7. Status of 800 kg detector • Basic performances have been already confirmed using 100 kg prototype detector • Vertex and energy reconstruction by fitter • Self shielding power • BG level • Detector design is under progress using MC • Structure and PMT arrangement (812 PMTs) • Event reconstruction • BG estimation • New excavation will be done soon • Necessary size of shielding around the chamber

  8. Structure of 800 kg detector -- tried to optimize the photocathode coverage. 1PMT 31cm 6 2 7 34cm 1 3 5 8 4 9 10 31cm 12 pentagons / 60 triangles pentakisdodecahedron Hexagonal PMT 5 triangles make pentagon 10 PMTs/ triangle surface

  9. Total 812 hexagonalPMTs immersed into liq. Xe • ~70% photo-coverage • Radius to inner face ~44cm Each rim of a PMT overlaps to maximize coverage

  10. 60 Generated R = 31cm E = 10keV 12 50 10 Fiducial volume 40 8 5 keV 30 6 Events s (reconstructed) [cm] 10 keV s = 2.3 cm 4 20 50 keV 2 100 keV 10 1 MeV 500 keV 0 0 10 20 30 40 0 22 26 30 34 38 Distance from the center [cm] Reconstructed position [cm] • Event reconstruction(Simulation) Boundary of fiducial volume • Position resolution 10 keV ~ 3.2 cm 5 keV ~ 5.3 cm

  11. R_reconstructed(cm) • Vertex reconstructed 50 45 5keV ~ 1MeV 40 • Up to <~40cm, events are well reconstructed with position resolution of ~2~3cm • Out of 42cm, grid whose most similar distribution is selected because of no grid data • In the 40cm~44cm region, reconstructed events are concentrated around 42cm, but they are not mistaken for those occurred in the center • No wall effect 35 30 25 20 15 10 5 50 0 5 10 15 20 25 30 35 40 45 Distance from the center [cm]

  12. 800kg BG study Achieved(prototype detector) Goal(800kg detector) • g ray from PMTs ~ 10-2 cpd/kg/keV10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) • Liquid Xenon 238U = (33±7)×10-14 g/g1×10-14 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 ppt1 ppt → Achieve by 2 purification pass 1/100 1/33 1/12 1/3

  13. Estimation of g BG from PMTs Statistics: 2.1 days • U-chain • 1/10 lower BG PMT • than present R8778 All volume R<34.5cm R<39.5cm R<24.5cm 238U : 1.8×10-2 Bq/PMT 232Th : 6.9×10-3 Bq/PMT 40K : 1.4×10-1 Bq/PMT Counts/keV/day/kg No event is found below 100keV after fiducial cut (R<24.5cm) All volume R<34.5cm R<39.5cm R<24.5cm < 1×10-4 cpd/kg/keV can be achieved (Now, more statistics is accumulating) Energy [keV]

  14. Water shield for ambient g and fast neutron Necessary shielding was estimated for the estimation of the size of the new excavation Generation point of g or neutron Configuration of the estimation wa • Put 80cm diameter liquid Xe ball • Assume several size of water shield 50, 100, 150, and 200cm thickness • Assume copper vessel (2cm thickness) for liquid Xe Liq. Xe water MC geometry

  15. 104 g attenuation by water shield 103 102 Detected/generated*surface [cm2] 10 1 PMT BG level 10-1 10-2 0 100 200 300 Distance from LXe [cm] Initial energy spectrum from the rock • g attenuation Deposit energy spectrum (200cm) More than 200cm water is Needed to reduce the BG to the PMT BG level

  16. fast neutron attenuation • Fast n flux @Kamioka mine: • (1.15±0.12) ×10-5 /cm2/sec water: 200cm, n: 10MeV • Assuming all the energies are • 10 MeV conservatively water < 2×10-2 counts/day/kg Liq. Xe ~200cm water is enough to reduce the BG to the PMT BG level No event is found from the generated neutron of 105 BG caused by thermal neutron Is now under estimation

  17. Source wire welding cap spacer ~7mm tube resin 0.9mm • stainless steel 316L tube 57Co microsource for internal calibration Active resin 57Co source For lower energy and position calibration, need X-ray source. Electro deposition of I-125 on 20 micrometer metal wire is planned.

  18. 220Rn(Thoron) source for position calibration Xe Gas In Longest T1/2 Since the chamber volume is small, most of the Rn gas will enter to Lxe chamber before decay. Q=2.25MeV If we flow 220Rn+Xe gas for an hour, then within 10 minutes, most will be 212Pb.  will give the position resolution of beta events.

  19. G4 simulation • For 105 beta-alpha coincidence events below E(beta)<100 keV during 10 hours data taking, the activity of 228Th should be ~ 20 kBq. • Commercilly available. • Whole volume of LXe can be studied. Beta energy spectrum

  20. New excavation will be made in kamioka mine. ※Tentative design 20m 15m 8m Neutron/γ are reduced >250cm water shield -fast neutron : 1/10000 for 2m shield -<500keV γ : 1/10 for 50cm shield 9m 15m Water shield tank Experiment area excavation 800kg chamber Other similar scale experiment such as DBD will be housed.

  21. Summary • Multi-purpose ultra lowbackground experiment with large mass liquid Xe.(ton scale) • 800 kg detector: Dark Matter Search 102 improvement of sensitivity above existing experiments is expected • Design of 800 kg detector is under progress. • BG estimation • Shielding • New excavation

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