KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Status of the KIMS dark matter search experiment with CsI(Tl) crystals

Hyun Su Lee

Seoul National University

(For KIMS Collaboration)

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

KIMS experiment

• Purpose – WIMP Dark Matter Search • Required

Cross section of WIMP is too small : less than 10-41 cm2

As possible as low background environments is neededDeep underground experimental hall to avoid cosmic ray and induced backgroundShielding with pure materials to avoid environment background from radioisotope Developed low background CsI(Tl) detector

WIMP interact with nucleus of target Improve PSD capability to extract nuclear recoil from data (mainly electron recoil) Increasing photo-electron yields

Recoil Energy is very smallLow energy threshold

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Korea Middleland Power Co.Yangyang Pumped Storage Power Plant

Yangyang Underground Laboratory

Minimum depth : 700 m / Access to the lab by car (~2km)

(Upper Dam)

(Lower Dam)

(Power Plant)

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Shielding for External background reduction

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

CsI(Tl) Crystal detector

CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) 3” PMT (9269QA) Quartz window, RbCs photo cathode 4~6 Photo-electron/keV DAQ 500MHz Home Made FADC 5 photo-electron within 2μsec trigger condition

total 32μsec window

3 crystal modules 3 crystal modules about 6cpd level about 6cpd level

was already madewas already made

Best available Crystal at MarketBest available Crystal at Market

Powder SelectionPowder Selection

Cs137 Reduction Using Pure waterCs137 Reduction Using Pure water

Rb87 Reduction by Re-crystallizationRb87 Reduction by Re-crystallization

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

DATA

• Data for WIMP search (826 kg days data are used to this analysis) SET1 (July 2004 ~ Sep. 2004)

0406 Crystal (8x8x23 cm3) – 237kg day (w/o quartz block – 5.5PE/keV)

SET3 (Feb. 2005~May. 2005) 0501A Crystal(8x8x30cm3) – 202kg day (w/o quartz block – 4.5PE/keV) 0501B Crystal(8x8x30cm3) – 386kg day (w quartz block – 3.7PE/keV)

SET6 (Aug. 2005~Oct. 2005) Low Temperature Running (~0oC) with 3 crystals (~6cpd level) Now we are taking Compton events 700~900kg days data (Analysis is on going)

• MC data using Geant4 simulation • Calibration data

Neutron data from Neutron calibration facility Reference distribution for recoil events - equivalent to 2000 kg days of underground data

Am241(59.5keV) Energy Calibration

137Cs ( 661.6 keV ) Compton data Reference distribution for γ events equivalent to 3600 kg days of underground data Cut efficiency calculation

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

46o

CsI

BC501a

BC

501

a

BC501a LSC

90o

17.4

o

n

Am/Be

Tag γ(4.4MeV)to measure TOF and energy of neutrons

Calibration of Neutron

Neutron Calibration Facility at Neutron Calibration Facility at SNUSNU

Compton events of WIMP search crystal was taken with Cs137 source in the underground lab

Neutron events of small size crystal was taken with neutron source at SNU

Compton of small size crystal was taken for comparison

Compton at Compton at SNUSNU

Neutron at SNUNeutron at SNU

4-5 keV4-5 keV

Compton at undergroundCompton at underground

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Event Selection to reject PMT related backgrounds

Th

e b

igg

est

clu

ste

r ch

arg

e

Th

e b

igg

est

clu

ste

r ch

arg

e

(PE)

(PE)

Th

e b

igg

est

clu

ste

r ch

arg

e

Th

e b

igg

est

clu

ste

r ch

arg

e

(PE)

(PE)

Mean

ch

arg

e o

f clu

ste

r

Mean

ch

arg

e o

f clu

ste

r

(PE)

(PE)

Cs137 Compton DataCs137 Compton Data

Mean

ch

arg

e o

f clu

ste

r

Mean

ch

arg

e o

f clu

ste

r

(PE)

(PE)

PMT BackgroundPMT Background

Energy(keVEnergy(keV))

Energy(keVEnergy(keV))

Energy(keVEnergy(keV))

Energy(keVEnergy(keV))

Equivalent to ~167 kg Equivalent to ~167 kg days underground datadays underground data

~less than 0.01CPD~less than 0.01CPD

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Cut Efficiency

Compton at undergroundCompton at underground

Compton at SNUCompton at SNU

Neutron at SNUNeutron at SNU

The difference of efficiencies is taken into account as systematic error.

Really we conservatively Really we conservatively calculate efficiencycalculate efficiency

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Extraction of Nuclear recoil events

3 ~4 keV

10~11 keV9~10 keV8~9 keV

7~8 keV6~7 keV5~6 keV

4~5 keV

Background DataBackground Data

Gamma Gamma Calibration Calibration

Neutron Neutron CalibrationCalibration

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Nuclear Recoil Rate

241Am energy distribution solid line for MC and dotted points for data

Simulated energy distributions of WIMP for different masses 20 GeV,50 GeV,100 GeV,1 TeV

Log mean time distribution of background events are fitted to distributions of Compton events and neutron events

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Limit curve Dark matter density at the solar system

D = 0.3 GeV c-2 cm -3

Use annual average parameters V0 = 220 km s-1, VE = 232 km s-1, VEsc = 650 km s-1

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Low Temperature(~0 oC) Operation

• To Avoid PMT related backgrounds

• Photo-electron yield increasing

• MT increasing• We had 2 month data

Now we are taking Compton events

Number of PENumber of PE Log(MT)Log(MT)

Normal OperationNormal Operation

Low Temp. OperationLow Temp. Operation

Normal OperationNormal Operation

Low Temp. OperationLow Temp. Operation

Background Spectrum without Background Spectrum without cutscuts

Energy(keV)Energy(keV)

CP

DC

PD

Fe55 CalibrationFe55 Calibration

Improve PSD Improve PSD powderpowder

Decrease Decrease backgroundbackground

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

New Crystal arrived

• We made further Cs137 reduced powder than now using crystals ~2cpd reduction is expected Small size crystal(0504) was made and tested

• At last month, two full size(8.7kg) crystal was arrived Cs137 amounts was measured with HPGe detector

Similar with expectation (~2cpd reduction will be possible)

• One more crystal was ready in beijing• 200kg powder(same quality) will be sent

2crystals/1month can be possible

Arrived date

Name B.G. Level Size PE Yield Company

2004/06 0406 6CPD 8x8x23 cm3 6.7 kg 5.5/keV Shanghai

2005/01 0501A 6CPD 8x8x30 cm3 8.7 kg 4.5/keV Shanghai

2005/01 0501B 6CPD 8x8x30 cm3 8.7 kg 4.5/keV Shanghai

2005/04 0504 ~4CPD 5x5x23 cm3 2.6 kg 6.8/keV Beijing

2005/10 0510A ~4CPD 8x8x30 cm3 8.7 kg ? Beijing

2005/10 0510B ~4CPD 8x8x30 cm3 8.7 kg ? Beijing

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Summary & Prospects

• 826 kg days data with 3 modules of 6cpd crystals was analyzed We can draw competitive limit to other

crystal detector experiments

• Low Temperature running was done Analysis is on going

• New crystal was arrived Cs137 was reduced

2~4cpd level Increase of light output

ProspectProspect

Increasing of PSD power Increasing of PSD power can improve sensitivitycan improve sensitivity

KIMS 826 kg d

ay

KIMS 826 kg d

ay

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

KPS meeting @ Chonbuk University 2005/10/22

HYUNSU LEE

Crystal Dependence of Mean Time

20~25 PE events selected20~25 PE events selected

04060406

0501A0501A

0501B0501B