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

2 1. Various signal rates in large LXe detector kg detector design 3. Gamma backgrounds in 800 kg detector 4. Neutron backgrounds 5. Possible calibration sources Outline

3 Introduction Dark matter Double beta Solar neutrino  What’s XMASS Xenon MASSive detector for solar neutrino (pp/ 7 Be) Xenon neutrino MASS detector (  decay) Xenon detector for Weakly Interacting MASSive Particles (DM search) Multi purpose low-background experiment with liq. Xe

4 Why Liquid Xenon(LXe) ? HeNeArXe Z  Largest self- shielding effect Radiat. L. (cm) Ionization(eV)  Lowest threshold Scint. Eff. (phtons/keV) Density  Compact detector B.P. (K)  Cooling easiest Scint. (nm)  No need for WLS In Air (%)5X X X10 -6 Relatively expensive Purification by distillation+ No longlived radioactive isotope Longest half-life : 127 Xe(36.4 days) Little internal background Inert gas(liquids) ZEPLIN 1(Single), ZEPLIN2,3(Double), CLEAN : Ne, WARP:Ar

5 Signals expected with natural LXe , T 1/2 >2.4x10 21 yr WIMP (SI:10 -7 pb, M W =100 GeV) 7 Be pp Current direct limit 51 Cr Source 1-2m distant Solar Energy resolution is not applied. Recently  T 1/2 >1.0x10 22 yr

6 All volume(23ton) 20cm wall cut 30cm wall cut (10ton FV) BG normalized by mass 1MeV02MeV3MeV Large self-shield effect External  ray from U/Th-chain  Key idea: self-shielding effect for low energy events  tracking MC from external to Xenon Blue : γ tracking Pink : whole liquid xenon Deep pink : fiducial volume Background are widely reduced in < 500keV low energy region U-chain gamma rays

7 Basic performances have been already confirmed using 100 kg prototype detector  Status of 800 kg 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 1PMT 12 pentagons / 60 triangles pentakisdodecahedron  Structure of 800 kg detector -- tried to optimize the photocathode coverage. 5 triangles make pentagon 34cm 31cm Hexagonal PMT 10 PMTs / triangle surface

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

10  Event reconstruction(Simulation) Generated R = 31cm E = 10keV  = 2.3 cm Reconstructed position [cm] Events 5 keV 10 keV 50 keV 100 keV 500 keV1 MeV Fiducial volume Distance from the center [cm]  (reconstructed) [cm] Position resolution Boundary of fiducial volume 10 keV ~ 3.2 cm 5 keV ~ 5.3 cm

11 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 5keV ~ 1MeV Distance from the center [cm] R_reconstructed(cm) Vertex reconstructed

12 Achieved (prototype detector) Goal (800kg detector)  ray from PMTs ~ cpd/kg/keV cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) Liquid Xenon 238 U = (33±7)× g/g 1× g/g → Remove by filter 232 Th < 23× g/g (90% C.L.) 2× g/g → Remove by filter (Only upper limit) Kr = 3.3±1.1 ppt 1 ppt → Achieve by 2 purification pass 1/100 1/33 1/12 1/3  800kg BG study

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

14  Water shield for ambient  and fast neutron wa water Liq. Xe Generation point of  or neutron Necessary shielding was estimated for the estimation of the size of the new excavation MC geometry Configuration of the estimation 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

15   attenuation PMT BG level  attenuation by water shield Distance from LXe [cm] Detected/generated*surface [cm 2 ] More than 200cm water is Needed to reduce the BG to the PMT BG level Initial energy spectrum from the rock Deposit energy spectrum (200cm)

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

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

Rn(Thoron) source for position calibration Xe Gas In Longest T 1/2 Since the chamber volume is small, most of the Rn gas will enter to Lxe chamber before decay. If we flow 220Rn+Xe gas for an hour, then within 10 minutes, most will be 212 Pb. Q=2.25MeV  will give the position resolution of beta events.

19 G4 simulation For 10 5 beta-alpha coincidence events below E(beta)<100 keV during 10 hours data taking, the activity of 228 Th should be ~ 20 kBq. Commercilly available. Whole volume of LXe can be studied. Beta energy spectrum

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

21 Summary Multi-purpose ultra low background experiment with large mass liquid Xe.(ton scale) 800 kg detector: Dark Matter Search 10 2 improvement of sensitivity above existing experiments is expected Design of 800 kg detector is under progress. BG estimation Shielding New excavation