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Status and background considerations of XMASS experiment

Yeongduk KimSejong University

for the XMASS collaboration

LRT2006Oct. 3, 2006

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1. Various signal rates in large LXe detector2. 800kg detector design3. Gamma backgrounds in 800 kg detector4. Neutron backgrounds5. Possible calibration sources

Outline

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Introduction

Dark matter

Double beta

Solar neutrino

What’s XMASS

Xenon MASSive detector for solar neutrino (pp/7Be) Xenon neutrino MASS detector ( decay) Xenon detector for Weakly Interacting MASSive Particles (DM search)

Multi purpose low-background experiment with liq. Xe

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Why Liquid Xenon(LXe) ?

He Ne Ar XeZ 2 10 18 54 Largest self-shieldi

ng effectRadiat. L. (cm) 756 24 14 2.4Ionization(eV) 24.6 21.6 15.8 12.1

Lowest thresholdScint. Eff. (phtons/keV)

15 40 42

Density 0.13 1.2 1.4 3.0 Compact detectorB.P. (K) 4.2 27.1 87.3 165.0 Cooling easiestScint. (nm) 73 80 128 174 No need for WLSIn Air (%) 5X10-4 1.8X10-3 0.93 9X10-6 Relatively expensive

Purification by distillation+ No longlived radioactive isotope Longest half-life : 127Xe(36.4 days)

Little internal background

Inert gas(liquids)

ZEPLIN 1(Single), ZEPLIN2,3(Double), CLEAN : Ne, WARP:Ar

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Signals expected with natural LXe

, T1/2>2.4x1021 yr

WIMP (SI:10-7 pb, MW=100 GeV)

7Be

pp

B10101

Current direct limit

51Cr Source(1MCi) @ 1-2m distant

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Solar Energy resolution is not applied.

RecentlyT1/2>1.0x1022 yr

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All volume(23ton)20cm wall cut30cm wall cut (10ton FV)

BG

nor

mal

ized

by

mas

s

1MeV0 2MeV 3MeV

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 trackingPink : whole liquid xenonDeep pink : fiducial volume

Background are widely reduced in < 500keV low energy region

U-chain gamma rays

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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

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1PMT

12 pentagons / 60 triangles pentakisdodecahedron

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

5 triangles make pentagon

34cm

31cm

31cm

12

34

5

67

8910

Hexagonal PMT

10 PMTs / triangle surface

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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

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Event reconstruction(Simulation)

60

50

40

30

20

10

030 3426 3822

GeneratedR = 31cmE = 10keV

= 2.3 cm

Reconstructed position [cm]

Eve

nts

5 keV10 keV

50 keV100 keV500 keV1 MeV

Fiducial volume

2010 300 40

8

10

64

2

0

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Distance from the center [cm]

(r

econ

stru

cted

) [cm

]

Position resolution Boundary of fiducial volume

10 keV ~ 3.2 cm 5 keV ~ 5.3 cm

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• 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

0 5 10 15 20 25 30 35 40 45

50

45

40

35

30

25

20

15

10

5

50Distance from the center [cm]

R_reconstructed(cm) Vertex reconstructed

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Achieved (prototype detector) Goal (800kg detector)

ray from PMTs ~ 10-2 cpd/kg/keV 10-4 cpd/kg/keV → Increase volume for self shielding → Decrease radioactive impurities in PMTs (~1/10)

Liquid Xenon238U = (33±7)×10-14 g/g 1×10-14 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 ppt 1 ppt → Achieve by 2 purification pass

1/100

1/33

1/12

1/3

800kg BG study

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Estimation of BG from PMTs

Energy [keV]

Cou

nts/

keV

/day

/kg

All volumeR<39.5cm

R<34.5cmR<24.5cm

All volumeR<39.5cm

R<34.5cmR<24.5cm

Statistics: 2.1 days• U-chain• 1/10 lower BG PMT than present R8778

No event is found below100keV after fiducial cut (R<24.5cm)

(Now, more statisticsis accumulating)

< 1×10-4 cpd/kg/keVcan be achieved

238U : 1.8×10-2 Bq/PMT232Th : 6.9×10-3 Bq/PMT40K : 1.4×10-1 Bq/PMT

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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

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attenuation

PMT BG level

attenuation by water shield

0 100 200 300Distance from LXe [cm]

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104

Det

ecte

d/ge

nera

ted*

surfa

ce [c

m2 ]

103

102

1

10-1

10-2 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)

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water: 200cm, n: 10MeV

Liq. Xe

water

No event is found fromthe generated neutron

of 105

< 2×10-2 counts/day/kg

fast neutron attenuation

• Fast n flux @Kamioka mine: (1.15±0.12) ×10-5 /cm2/sec

• Assuming all the energies are 10 MeV conservatively

~200cm water is enoughto reduce the BG to the PMT BG level

BG caused by thermal neutronIs now under estimation

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0.9mm

~7mm

• stainless steel 316L tube

welding

tube

cap

Source wire

resin

spacer

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.

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220Rn(Thoron) source for position calibration 228 224 220 216 212

90 88 86 84 82

(1.9y) (3.7d) (56s) (0.15s) (10h) Th Ra Rn Po Pb

212 208 20883 81 82

212 20884 82

(36%)(61m) (3m) (stable)

(64%) (0.3 s) (stable)

Bi Tl Pb

Po Pb

Xe Gas InLongest T1/2 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 212Pb.

Q=2.25MeV

( ) ( )V V

will give the position resolution of beta events.

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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

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800kg chamber

Water shield tank

Experiment areaexcavation

20m15m

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.

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Summary Multi-purpose ultra low background 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