암흑물질 탐색을 위한 CsI 섬광검출기의 특성시험

- CsI 결정제작과정의 Cs 화합물의 방사능 측정 -

KIMS (Korean Invisible Mass Search) Collaboration

김태연 , 양명수 , 최진명 , 이현수 , 양혜영 , 원효식 , 박현서 , 원은일 , 박일흥 , 김선기 ( 서울대 ),강운구 , 김영덕 ( 세종대 ),권영준 , 김홍주 , 김상열 , 황명진 , 이종호 ( 연세대 ),한인식 , 김지훈 ( 이화여대 ),유인태 , 조일성 , 노승훈 , 최동혁 ( 성균관대 ),최성렬 ( 전북대 ),고병원 ( 과기대 ),서은숙 , 이무현 (Univ. of Maryland)J.Li(Institute of High Energy Physics, Beijing),H.Wong(Acanemia Cinica, Taipei),M.Z.Wang(National Taiwan University, Taipei)

1) 나선형 은하의 회전속도

2) Clusters of galaxies - Virial mass - Giant arcs

암흑물질에 대한 관측증거암흑물질탐색 실험

Elastic Scattering of WIMP off a nucleus in the crystal

Energy loss by ionization and lattice vibration

Cs

I

- Supersymetry particle - Lightest stable particle - Neutral particle - Mass : 10 -1000 GeV - Density : 0.3 GeV/cm3

- Speed : 230 km/sRecoiled nucleus

되튄 원자핵의 ( 이온화 ) 에너지 손실을 검출

WIMP

WIMP

직접 검출 원리

WIMP Mass (the Lightest Supersymmetry Particle) ~ 10 - 1000 GeVSpeed ~ 270 km/s

Density ~ 0.3 GeV/cm3

Cross section is very small : less than 10-41 cm2

Recoil energy of nucleus is also very small : less than100 keV

Background(gamma-rays, neutrons, X-rays, cosmic rays) is very large

• Detector has large target mass and long-term stability• Detector can measure tiny energy ~ few keV• Detector have to be made of pure material• Detector have to be located underground

검출기 요구조건

KIMS’s detector KIMS’s expected limit curve ( 1 year run )

10-2

10-1

1

10

0 100 200 300 400 500 600 700 800 900 1000Energy(keV)

Cou

nt/

(keV

kg

day)

87Rb = 1 ppb134Cs = 30 pCi137Cs = 0.3 pCi

87Rb

137Cs

134Cs

Total

Power plant

Homyung Mt.( 虎鳴山 )

Reservoir

Laboratory

Access tunnel(1.4km)350mPukhan River( 北漢江 )

KIMS 지하 실험실

0

50

100

150

200

250

300

350

400

250 500 750 1000 1250 1500 1750 2000 2250 2500Energy(keV)

Cou

nt/

(keV

kg

day)

10-2

10-1

1

10

0 100 200 300 400 500 600 700 800 900 1000Energy(keV)

Cou

nt/(

keV

kg

day)

진행중인 테스트

Neutron beam test

MC

PMT test

PSD

Muon flux

Crystal

: Budker Institute of Nuclear Physics (Ukraine)

: Shanghai Institute of Ceramics (China)

: Crismatec (France)

Background energy spectrum of CsI(Tl) crystal

Production of CsI powder

CsOH solution(50%)

Evaporation

Conversion to CsOH solution

Purification of Cesium salt solution

Digestion

Waste 2

Waste 1

Quartz

Pollucite Ore

Carbonization

Cs2CO3 Solution

Filtration

CsHCO3 +HX

Pure technical Cs Salts

CsOH solution + HX

High Purity Cs Salts

Cs Metal

Cs 화합물의 방사능 측정

시료 : CsI powder(A,B,C,D) 검출기 : HPGe (GMX, 23 %) CsOH CsNO3

Process water(A) 18 Mohm water 원광 (pollucite)

Spectrum 분석 I- Background spectrum

테프론 삽입

133Ba

137Cs and 60Co

30.6(X-ray)30.9(X-ray)53.1(X-rsy)79.680.9276.4302.8356.0383.8

31.8(X-ray)32.2(X-ray)36.5(X-ray)661.61173.21332.5

Spectrum 분석 II - Background 원천 Ratio of the peak area w/ teflon to w/o teflon

(calibration sources)

Ratio of the peak area w/ teflon to w/o teflon(background)

Spectrum 분석 III

weightYEFFTwtBqMDA

***

*66.471.2)/(

L.A.Currie, Anal. Chem. 40, 586(1968)

TsLLD

*66.471.2)(/

Lower Limit of Detection

LLD = 1.9 x 10-4 /s( T = 350,000 s )

LLD = 2.8 x 10-4 /s( T = 930,000 s)

Spectrum 분석 IV- Spectrum of pollucite

----------------------------------------------------------------- EGS4 GEANT3 GEANT4 ----------------------------------------------------------------- Coding Mortran( F77) Fortran C++ EM energy range Low to high High( low) Low, High Interaction EM only EM, HAD EM,HAD, n, decay Test Long term Long term Ongoing Geometry setup Terrible Easy Easy Interaction update Diffcult Difficult Easy Visualization 3D 2D 3D

Absolute detection efficiency - MC simulation I

Random event generation at Marinelli beaker where CsI powder is filled. 661keV gamma( 667 keV), 1MeV electron with 1M , 100M events. Check energy deposition in CsI or Ge. G3 by Y.D.Kim, G4 by H.J.Kim and EGS4 by T.Y.Kim and J.H.Kim

Absolute detection efficiency - MC simulation II

Deposited energy in Ge detector (gamma-ray of 662 keV)

Deposited energy in Ge detector (beta-ray of 1 MeV)

Deposited energy in CsI powder (gamma-ray of 662 keV)

(662keV gamma) 1M evt 100M (X-G4l)/G4l

--------------------------------------------------------------------------------

G4-low : Ege>0.0 : 6.69 % 6.688 % 0.0 %

Ge full peak : 1.62 % 1.617 % 0.0 %

G4-high : Ege>0.0 : 6.71 % 6.691 % 0.0 %

Ge full peak : 1.59 % 1.596 % -1.3 %

G3 : Ege>0.0 : 6.63 % x -0.9 %

Ge full peak : 1.58 % x -2.2 %

EGS4 : Ege>0.0 : x 6.581 % -1.6 %

Ge full peak : x 1.644 % 1.7 %

EGS4 : Ege>0.0 : 6.69 % x 0.0 %

Ge full peak : 1.64 % x 1.4 %

--------------------------------------------------------------------------------

Systematic error caused by different model +- 2%

Absolute detection efficiency - MC simulation III

Absolute detection efficiency - MC simulation IV

Sample Energy Efficiency (keV) ( % )CsI poweder 662 1.56 +- 0.0138CsOH 662 1.12 +- 0.0163CsNO3 662 1.58 +- 0.0137H2O(1L) 662 0.27 +- 0.0331h2O(450mL) 662 1.19 +- 0.0158

CsI poweder 604 1.71 +- 0.0132CsOH 604 1.17 +- 0.0159CsNO3 604 1.67 +- 0.0133H2O(1L) 604 0.32 +- 0.0304H2O(450mL) 604 1.28 +- 0.0152

CsI poweder 795 1.40 +- 0.0146CsOH 795 0.98 +- 0.0174CsNO3 795 1.37 +- 0.0147H2O(1L) 795 0.24 +- 0.0347H2O(450mL) 795 1.02 +- 0.0171

Calculated detection efficiency curve

Radioactivity of the samples Crystal

Bq

Bq/kg pCi/kg Bq/kg

662 9.51E- 04 0.01620.

6.41E- 02 1.73E+00 6.41E- 02605 9.91E- 04 0.0173

0.

5.45E- 02 1.47E+00 5.45E- 02795 7.80E- 04 0.0143

0.

5.93E- 02 1.60E+00 5.93E- 02662 1.22E- 03 0.0162

0.

9.00E- 02 2.43E+00 9.00E- 02605 9.19E- 04 0.0173

0.

5.52E- 02 1.49E+00 5.52E- 02795 5.48E- 04 0.0143

0.

4.54E- 02 1.23E+00 4.54E- 02662 4.68E- 04 0.0162

0.

2.86E- 02 7.74E- 01 2.86E- 02605 6.11E- 04 0.0173

0.

3.05E- 02 8.24E- 01 3.05E- 02795 4.88E- 04 0.0143

0.

3.36E- 02 9.09E- 01 3.36E- 02662 3.79E- 04 0.0162 0

.2.46E- 02 6.65E- 01 2.46E- 02

605 5.25E- 04 0.01730.

2.78E- 02 7.52E- 01 2.78E- 02795 4.93E- 04 0.0143

0.

3.61E- 02 9.76E- 01 3.61E- 02

662 3.67E- 04 0.01120.

2.79E- 02 7.53E- 01 3.22E- 02605 5.49E- 04 0.0118

0.

3.44E- 02 9.31E- 01 3.97E- 02795 3.58E- 04 0.00987

0.

3.06E- 02 8.28E- 01 3.53E- 02662 4.17E- 04 0.0159 2.62E- 02 7.08E- 01 1.96E- 02605 8.51E- 04 0.0168 4.40E- 02 1.19E+00 3.30E- 02795 6.71E- 04 0.0137 4.86E- 02 1.31E+00 3.64E- 02

662 2.04E- 04 0.002736 5.62E- 02 1.52E+00605 3.31E- 05 0.003228795 4.64E- 05 0.002484662 2.61E- 04 0.01193 3.17E- 02 8.56E- 01605 4.15E- 05 0.01288795 1.42E- 05 0.01026

662 1.95E- 04605 0.00E+00795 0.00E+00

Pollucite

18Mohm water

Process water

less than LLD

less than LLD

less than LLD

CsNO3

CsOH

Energy(keV)

Powder D

Sample

Powder A

Powder B

Powder C

Sample Eff.Count rate(/sec)

결 론

- 현재 상용화 되어 있는 CsI 결정으로는 기존실험들의 검출기 sensitivity 와 견줄 수 없슴 .- 적극적으로 CsI 결정 제작과정에 관여해야 할 것임 .

- 현재 상용화 되는 CsI 결정의 137Cs 내부 방사능은 0.025 - 0.1 Bq/kg 보다 큼 ( 결정생성 전단계의 CsI powder 측정결과 ).- 원광 (pollucite ore) 에서의 137Cs 내부 방사능 오염양은 현재의 측정장치로는 효과적으로 측정하기 힘듬 . - 137Cs 내부 방사능을 0.001 Bq/kg(0.027 pCi/kg) 이하로 줄이기 위해서 노력하고 있슴 .