Status and background considerations of XMASS experiment Yeongduk

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Status and background considerations of XMASS experiment Yeongduk Kim Sejong University for the XMASS

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

Outline 1. Various signal rates in large LXe detector 2. 800 kg detector design

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

Introduction Ø What’s XMASS Multi purpose low-background experiment with liq. Xe l Xenon MASSive

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

Why Liquid Xenon(LXe) ? Inert gas(liquids) He Ne Ar Xe 2 10 18 54

Why Liquid Xenon(LXe) ? Inert gas(liquids) He Ne Ar Xe 2 10 18 54 Radiat. L. (cm) 756 24 14 2. 4 Ionization(e. V) 24. 6 21. 6 15. 8 12. 1 15 40 42 Lowest threshold Compact detector Z Scint. Eff. (phtons/ke. V) Largest selfshielding effect Density 0. 13 1. 2 1. 4 3. 0 B. P. (K) 4. 2 27. 1 87. 3 165. 0 Cooling easiest Scint. (nm) 73 80 128 174 No need for WLS 5 X 10 -4 1. 8 X 10 -3 0. 93 9 X 10 -6 In Air (%) Relatively expensive Purification by distillation+ No longlived radioactive isotope Longest half-life : 127 Xe(36. 4 days) Little internal background ZEPLIN 1(Single), ZEPLIN 2, 3(Double), CLEAN : Ne, WARP: Ar 4

Signals expected with natural LXe WIMP (SI: 10 -7 pb, MW=100 Ge. V) 51

Signals expected with natural LXe WIMP (SI: 10 -7 pb, MW=100 Ge. V) 51 Cr Source (1 MCi) @ 1 -2 m distant Current direct limit 2 n 2 b, T 1/2>2. 4 x 1021 yr Recently, T 1/2>1. 0 x 1022 yr pp 7 Be Solar Energy resolution is not applied. 5

Ø Key idea: self-shielding effect for low energy events U-chain gamma rays Blue :

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

Ø Status of 800 kg detector l Basic performances have been already confirmed using

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

Ø Structure of 800 kg detector -- tried to optimize the photocathode coverage. 12

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

l Total 812 hexagonal PMTs immersed into liq. Xe l ~70% photo-coverage l Radius

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

Ø Event reconstruction(Simulation) Boundary of fiducial volume l Position resolution 10 ke. V ~

Ø Event reconstruction(Simulation) Boundary of fiducial volume l Position resolution 10 ke. V ~ 3. 2 cm 5 ke. V ~ 5. 3 cm s (reconstructed) [cm] 60 Generated R = 31 cm 50 E = 10 ke. V Events 40 30 20 s = 2. 3 cm 10 0 22 26 30 34 38 Reconstructed position [cm] 12 10 Fiducial volume 8 5 ke. V 6 10 ke. V 4 50 ke. V 2 0 100 ke. V 500 ke. V 1 Me. V 0 10 20 30 40 Distance from the center [cm] 10

R_reconstructed(cm) 50 l Vertex reconstructed 45 40 35 30 25 20 15 10 5

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

Ø 800 kg BG study Achieved (prototype detector) Goal (800 kg detector) l g

Ø 800 kg BG study Achieved (prototype detector) Goal (800 kg detector) l g ray from PMTs ~ 10 -2 cpd/kg/ke. V 1/100 10 -4 cpd/kg/ke. V → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10) l Liquid Xenon 238 U = (33± 7)× 10 -14 g/g → Remove by filter 232 Th < 23× 10 -14 g/g (90% C. L. ) → Remove by filter (Only upper limit) Kr = 3. 3± 1. 1 ppt → Achieve by 2 purification pass 1/33 1× 10 -14 g/g 1/12 2× 10 -14 g/g 1/3 1 ppt 12

Ø Estimation of g BG from PMTs Statistics: 2. 1 days Counts/ke. V/day/kg All

Ø Estimation of g BG from PMTs Statistics: 2. 1 days Counts/ke. V/day/kg All volume R<39. 5 cm R<34. 5 cm R<24. 5 cm • U-chain • 1/10 lower BG PMT than present R 8778 238 U : 1. 8× 10 -2 Bq/PMT 232 Th : 6. 9× 10 -3 Bq/PMT 40 K : 1. 4× 10 -1 Bq/PMT All volume R<39. 5 cm R<34. 5 cm R<24. 5 cm No event is found below 100 ke. V after fiducial cut (R<24. 5 cm) < 1× 10 -4 cpd/kg/ke. V can be achieved Energy [ke. V] (Now, more statistics is accumulating) 13

Ø Water shield for ambient g and fast neutron Necessary shielding was estimated for

Ø 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 wa Liq. Xe water Configuration of the estimation l Put 80 cm diameter liquid Xe ball l Assume several size of water shield 50, 100, 150, and 200 cm thickness l Assume copper vessel (2 cm thickness) for liquid Xe MC geometry 14

Detected/generated*surface [cm 2] Ø g attenuation Initial energy spectrum from the rock 104 g

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

Ø fast neutron attenuation water: 200 cm, n: 10 Me. V • Fast n

Ø fast neutron attenuation water: 200 cm, n: 10 Me. V • Fast n flux @Kamioka mine: (1. 15± 0. 12) × 10 -5 /cm 2/sec • Assuming all the energies are 10 Me. V conservatively water < 2× 10 -2 counts/day/kg Liq. Xe No event is found from the generated neutron of 105 ~200 cm water is enough to reduce the BG to the PMT BG level BG caused by thermal neutron Is now under estimation 16

57 Co microsource for internal calibration Sourc e wire welding cap space r resi

57 Co microsource for internal calibration Sourc e wire welding cap space r resi n ~7 m m Active resin tub e 0. 9 mm • stainless steel 316 L tube 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. 17

220 Rn(Thoron) source for position calibration Xe Gas In Longest T 1/2 Since the

220 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. Q=2. 25 Me. V will give the position resolution of beta events. If we flow 220 Rn+Xe gas for an hour, then within 10 minutes, most will be 212 Pb. 18

 • For 105 beta-alpha coincidence events below E(beta)<100 ke. V during 10 hours

• For 105 beta-alpha coincidence events below E(beta)<100 ke. V during 10 hours data taking, the activity of 228 Th should be ~ 20 k. Bq. • Commercilly available. • Whole volume of LXe can be studied. G 4 simulation 19 Beta energy spectrum

New excavation will be made in kamioka mine. ※Tentative design 15 m 20 m

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

Summary l Multi-purpose ultra low background experiment with large mass liquid Xe. (ton scale)

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