the xmass 800kg experiment jing liu ipmu, univ. of tokyo taup2011, munich xmass collaboration:...

The XMASS 800kg Experiment

Jing LIUIPMU, Univ. of TokyoTAUP2011, Munich

XMASS collaboration:Kamioka Observatory, ICRR, Univ. of Tokyo ：

Y. Suzuki, M. Nakahata, S. Moriyama, M. Yamashita, Y. Kishimoto,Y. Koshio, A. Takeda, K. Abe, H. Sekiya, H. Ogawa, K. Kobayashi,K. Hiraide, A. Shinozaki, S. Hirano, D. Umemoto, O. Takachio, K. Hieda

IPMU, University of Tokyo ： K. Martens, J.LiuKobe University: Y. Takeuchi, K. Otsuka, K. Hosokawa, A. MurataTokai University: K. Nishijima, D. Motoki, F. KusabaGifu University ： S. TasakaYokohama National University ： S. Nakamura, I. Murayama, K. FujiiMiyagi University of Education ： Y. FukudaSTEL, Nagoya University ： Y. Itow, K. Masuda, H. Uchida, Y. Nishitani, H. TakiyaSejong University ： Y.D. KimKRISS: Y.H. Kim, M.K. Lee, K. B. Lee, J.S. Lee

The XMASS Experiment

XMASS 800kg Jing LIU @ TAUP2011 2

Xenon MASSive detector for Solar neutrino (pp/7Be)Xenon neutrino MASS detector (double beta decay)

Xenon detector for weakly interacting MASSive Particles

~100 kg LXeprototype

~800 kg LXedirect dark matter search

~26 ton LXeMulti-purpose

LXe (Liquid Xenon) surrounded by PMTs (Photomultiplier Tubes) recording scintillation lights generated by nuclear or electronic recoils in LXe

PMT

LXe inside

Jing LIU @ TAUP2011


XMASS 800kg 3







PMT

LXe inside

, n, ,… or ?

PMT

LXe

scintillation

Jing LIU @ TAUP2011


XMASS 800kg 4







PMT

LXe inside PMT

LXe









PMT

LXe inside

Jing LIU @ TAUP2011


XMASS 800kg 6







PMT

LXe inside

Jing LIU @ TAUP2011


XMASS 800kg 7







PMT

LXe inside

Jing LIU @ TAUP2011 8

Structure of XMASS 800kg detector

XMASS 800kg

PMT


Structure of XMASS 800kg detector

XMASS 800kg

PMT

Pentakis dodecahedron

~10 PMTs in one triangle642 PMTs in total

~0.8 meter

PMT photo-cathodes cover ~62% inner surface


Reconstruct interaction point from PMT hit pattern

XMASS 800kg

Colored photo cathodes indicating number of p.e. (photoelectrons) recorded by PMTs

Interaction point (vertex) can be reconstructed from the PMT hit pattern.


LXe self-shielding

XMASS 800kg

Simulation: g into LXe

waterLXe

E [keV]

Att

enu

atio

n le

ng

th o

f

[cm

]


Where is it?

XMASS 800kg

1000 m rock overburden(2700 m water equiv.):Muon: 6.0x10-8 /cm-2/s/srNeutron: 1.2x10-6/cm-2/s

360m above the sea

Horizontal access:15 minutes drive from office,too easy to get in!No excuse to avoid 24-hour shift

Kamioka underground observatory

Jing LIU @ TAUP2011 13XMASS 800kg

New experimental hall, Aug. 2008

15 meter15

met

er20 meter


water tank, Nov. 2008

10 m x 10 m


Frame of clean room in water tank, Mar. 2008


PMT mounting in clean room, Dec. 2009


PMT mounting finished, Feb. 2010


Water filling, Sep. 2010


Commissioning started, Nov. 2010


Scintillation light yield :: calibration system

XMASS 800kg

Top PMT can be pulled out

Source changed here

Z position of source is controlled by a motor on top at <1 mm accuracy

x

y

z

57Co, 241Am, 109Cd, 55Fe, 137Cs

~F 4mm

~F 0.15mm for 57Co


Scintillation light yield: 15.9 1.2 p.e./keV (57Co at center)

XMASS 800kg

DataMC

122 keV

136 keV59.3 keV (W)

Number of photoelectrons

Arbi

trar

y un

it

Position & energy resolution (122keV g from 57Co)

DataReconstructed vertices for various source positions Position resolution (RMS):

• 1.4 cm @ z = 0 cm• 1.0 cm @ z = 20 cm

Reconstructed energy [keV]

Arbi

trar

y un

it

DataMC

122 keV

136 keV59.3 keV (W)

RMS ~4%


y [cm]

z [cm]

DataMC

Arbi

trar

y un

it


Background under control?

• External– Comic ray: underground, muon veto– Ambient gamma & neutron: water shielding– PMT radiation: LXe self-shielding

• Internal– Rn: material screening, clean room filled with Rn

free air– Kr: distillation

XMASS 800kg


Xe

water

X [cm]y

[cm

]

Ambient and n: pure water tank, ~10 meter

XMASS 800kg

Pure water tank (large enough for 26 ton LXe)equipped with 20 inch PMTs on the wall as

• active muon veto and • passive ambient and n shielding

20 inch PMTs

Wat

er ta

nks

LXe sphere

107 neutrons, simulation

g << g from PMT, n<<10-4/d/kg


LXe

copp

er c

ryos

tat

Calib

ratio

n pi

pe

g

g n

n

PMT radiation: Ultra low background PMTs

XMASS 800kg

Neutron: <1.2x10-5 dru @5-10 keV


PMT & PMT holder radiation: LXe self-shielding

XMASS 800kg

Energy [keV]C

oun

ts [

dru

]

Simulation: g into LXe

fiducial volume: r<20cm, 100 kg LXe

BG/PMT [mBq]

U chain 0.70 0.28

Th chain 1.51 0.3140K < 5.1060Co 2.92 0.16


85Kr (Qb=687keV) : distillation

XMASS 800kg

Kr

LXeintake

LXeoutlet

Gas Kroutlet

K. Abe et al. for XMASS collab., Astropart. Phys. 31 (2009) 290

Kr can be boiled out from LXe 0.1 ppm ~1ppt (~1 ton in 10 days)


Kr concentration

XMASS 800kg

Xenon samplebg sample 1bg sample 2Kr concentration: < 2.7 ppt (90% C.L.)

Measured by gas chromatography + API mass spectrometer


222Rn

XMASS 800kg

p0 * exp(-t/t) + p1,t: decay constant

Time difference [s]

1st event (214Bi b)2nd event (214Po a)

Tail due tosaturation

214Po decays with 164 ms half life.

It can be identified by time coincidence between two consecutive events:

1. 214Bi b decays into 214Po2. 214Po a decays into 210Pb

x103


Even

ts

8.20.5 mBq

Even

ts


220Rn

XMASS 800kg

p0 * exp(-t/t) + p1,t: decay constant

216Po decays with 140 ms half life

Time difference [ms]

Even

ts

Even

ts

x103


1st event2nd event

<0.28 mBq (90%C.L.)


Target sensitivity of WIMP-nucleon XS (spin independent)

XMASS 800kg

XENON100

CDMSII

XMASS 1yr

Expected energy spectrum assuming

• 1 year exposure• flat background (10-4 dru)• s c = 10-44 cm2

• MWIMP = 50 GeV

• Leff = 0.2

Black: signal + backgroundRed: background (10-4dru)


Conclusion

• The XMASS 800kg detector is a single phase LXe scintillation detector

• Construction of the 800kg detector finished last winter• Commissioning runs are on going to confirm the

detector performance and low background properties– Energy resolution and vertex resolution were as expected.

~1cm position resolution and ~4% energy resolution for 122 keV g.

– Radon and Kr background are close to the target values.

• The physics results are on the way

XMASS 800kg


Thanks!

PMT holer made of

OFHC copper

3 Steps in reconstruction• Step1

– Search the map grid where likelihood becomes smallest.

• Step2– By linear interpolation, calculate likelihood at finer “interpolated grid” in

Cartesian coordinate.– 1.5cm interval, 5x5x5 finer grid points are evaluated.

Result grid of step1

1.5cm

å -=PMT

pe

GL )

(pe+1)

)exp(Log()Log(

mm

• Ln likelihood is calculated from the expected pe and observed pe. Using gamma distribution.

• Treatment of saturated PMT, cumulative probability of gamma distribution.

z

x

y

Integral spectrum

Assumption 1x10^-44 cm^2100 kg X 1 year exposure

WIMP Mass [GeV]

50 100 1000

No. of Events 26 37 6

25 keVr(5keVee) energy threshold100kg x 1 year

the xmass 800kg experiment jing liu ipmu, univ. of tokyo taup2011, munich xmass collaboration:...

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