Analysis and Results3.Gamma-ray Spectra of 134Cs

The background subtracted spectra of 605 keV

and 796 keV in multiple hit events, and their coincidence summing peak

1401 keV in single hit events.

4.Beta-ray Spectra of 134Cs

The spectra of beta-rays with the maximum

energies of 658.0 keV (70.23%) and 88.6 keV (27.28%) were obtained on

the basis of corresponding gamma transitions.

5.Asymmetry Distributions of 134Cs

Asymmetry between the charge signals from two PMTs

6.Decay Curve and Calculated Half-life of 134Cs

Measured half-life 2.108 ± 0.3958 Y

Published half-life 2.067 ± 0.0005 Y

(R. H. Martin et al. Nucl. Instr. And Meth. In Physics.

Res. A 390 (1997) 267-273)

Analysis and Results1.Energy Calibration

The energy is calibrated using 59.54 keV

gamma-ray peak from 241Am standard radioactive source, and 605 keV

and 796 keV gamma-ray peaks from 134Cs internal background source.

2.Measured Background Energy Spectra

Figure 6 and Figure 7 show the spectra of the

energy deposited in one crystal, and the sum of the energy deposited in

all 12 crystals, respectively. 605 keV and 796 keV peaks are suppressed

in energy sum spectrum from the multiple hit events, because they are

appeared as their coincidence summing peak at 1401 keV.

Figure 12. Beta-ray spectrum of Max. 658 keV (Avg. 210 keV)

Figure 13. Beta-ray spectrum of Max. 89 keV (Avg. 23 keV)

IntroductionThis study is motivated to understand the properties of 134Cs radioactive

source within the CsI(Tl) crystal. 134Cs decays to a 134Ba by beta emission,

followed by several gamma-ray transitions as shown in Figure 1. The

most dominant gamma-rays are 605 keV and 796 keV (thick red arrows)

with intensities of 97.62% and 85.53%, respectively.

Understanding 134Cs Background in CsI(Tl) CrystalsJ. K. Lee, S. K. Kim, H. C. Bhang, S. L. Olsen, S. S. Myung, M. J. Lee, S. C. Kim, J. H. Choi, J. H. Lee, S. J. Lee, S. Ryu, I. S. Seong, K. W. Kim,

Y. D. Kim1, W. G. Kang1, J. I. Lee1, H. J. Kim2, J. H. So2, Y. J. Kwon3, M. J. Hwang3, I. S. Hahn4, Q. Yue5, J. Li5, Y. J. Li 5

Seoul National University, 1Sejong University, 2Kyungpook National University, 3Yonsei University, 4Ewha Womans University, 5Tsinghua University

AbstractThe purpose of this study is to estimate the 134Cs contamination in CsI(Tl) crystals used in the KIMS experiment. 134Cs is one of the major internal

background sources in the CsI(Tl) detectors for WIMP search. To understand 134Cs background, the gamma and beta spectra of 134Cs have been studied

based on the data taken 12 CsI(Tl) crystals at Yangyang Underground Laboratory. Although 134Cs produces a complex gamma-ray spectrum, our

coincidence tagging technique with 4x3 CsI(Tl) array detectors makes it possible to determine the energy and relative intensity of each gamma-ray

peak. Our preliminary results will be reported.

ConclusionTo estimate 134Cs contamination of CsI(Tl) crystals, gamma and beta

spectra of 134Cs have been studied. And the half life of 134Cs is also

measured and compared with the known value. A more detailed study on

134Cs is in progress.

134Cs

134Ba

1365

802 569

1038 475 242

795

1168 563

605

4+

4+

3+

4+

2+

2+

0+

1969.87

1643.28

1400.55

1167.93

604.70

0.0

E [keV]β-

27.3%

2.50%

70.1%

0.033%

0.10%

Figure 1. Decay scheme of 134Cs

1401 keV gamma from Cs-134 (70%)

1970 keV gamma from Cs-134 (27%)

662 keV gamma from Cs-137 (100%)

1401 keV peak in single hit (pile-up of 605+796 keV)

1365 keV peak in multiple hit

605 keV peak in multiple hit (563 & 569 keV included)

796 keV peak in multiple hit (802 keV included)

Figure 8. Energy sum in all crystalsFigure 7. Energy spectrum in one crystal

Figure 9. Background subtracted 605 and 796 keV peaks

Figure 10. Background subtracted 1401 keV

Figure 16. Decay curves of 134Cs in CsI(Tl) crystal

Figure 11. Energy resolution curve

Figure 14. 2D asymmetry vs. energy (top) and 1D energy spectra of 3 different asymmetry regions (bottom) for single hit events (left) and multiple hit events (right), respectively

Figure 15. 1D asymmetry distributions from 134Cs and 137Cs around 600 keV

Experimental SetupExtreme low-background experiment operated in underground

laboratory (Y2L)

4 x 3 array of 12 CsI(Tl) crystals with 8 x 8 x 30 cm3 dimensions

Each crystal is coupled to 2 PMTs.

Figure 2. Underground Laboratory Figure 3. Array of CsI(Tl) crystals

Y2L

Figure 4. 59.54 keV from 241Am

Figure 5. 605 and 796 keV from 134Cs

Figure 6. Linearity of signal area to energy

605 keV from 134Cs

660 keV from 137Cs660 keV from

137Cs605 keV & 796 keV from 134Cs