1. Analysis and Results 3.Gamma-ray Spectra of 134 Cs 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 134 Cs 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 134 Cs Asymmetry between the charge signals from two PMTs 6.Decay Curve and Calculated Half-life of 134 Cs Measured half-life2.108 ± 0.3958 Y Published half-life2.067 ± 0.0005 Y (R. H. Martin et al. Nucl. Instr. And Meth. In Physics. Res. A 390 (1997) 267-273) dd Analysis and Results 1.Energy Calibration The energy is calibrated using 59.54 keV gamma-ray peak from 241 Am standard radioactive source, and 605 keV and 796 keV gamma-ray peaks from 134 Cs 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) Introduction This study is motivated to understand the properties of 134 Cs radioactive source within the CsI(Tl) crystal. 134 Cs decays to a 134 Ba 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 134 Cs Background in CsI(Tl) Crystals J. 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. Kim 1, W. G. Kang 1, J. I. Lee 1, H. J. Kim 2, J. H. So 2, Y. J. Kwon 3, M. J. Hwang 3, I. S. Hahn 4, Q. Yue 5, J. Li 5, Y. J. Li 5 Seoul National University, 1 Sejong University, 2 Kyungpook National University, 3 Yonsei University, 4 Ewha Womans University, 5 Tsinghua University Abstract The purpose of this study is to estimate the 134 Cs contamination in CsI(Tl) crystals used in the KIMS experiment. 134 Cs is one of the major internal background sources in the CsI(Tl) detectors for WIMP search. To understand 134 Cs background, the gamma and beta spectra of 134 Cs have been studied based on the data taken 12 CsI(Tl) crystals at Yangyang Underground Laboratory. Although 134 Cs 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. Conclusion To estimate 134 Cs contamination of CsI(Tl) crystals, gamma and beta spectra of 134 Cs have been studied. And the half life of 134 Cs is also measured and compared with the known value. A more detailed study on 134 Cs is in progress. 134 Cs 134 Ba 1365802569 1038 475242 795 1168 563 605 4+4+ 4+4+ 3+3+ 4+4+ 2+2+ 2+2+ 0+0+ 1969.87 1643.28 1400.55 1167.93 604.70 0.0 E [keV] β-β- 27.3 % 2.50 % 70.1 % 0.033 % 0.10% Figure 1. Decay scheme of 134 Cs 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 134 Cs 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 134 Cs and 137 Cs around 600 keV Experimental Setup  Extreme low-background experiment operated in underground laboratory (Y2L)  4 x 3 array of 12 CsI(Tl) crystals with 8 x 8 x 30 cm 3 dimensions  Each crystal is coupled to 2 PMTs. Figure 2. Underground LaboratoryFigure 3. Array of CsI(Tl) crystals Y2L Figure 4. 59.54 keV from 241 AmFigure 5. 605 and 796 keV from 134 CsFigure 6. Linearity of signal area to energy 605 keV from 134 Cs 660 keV from 137 Cs 605 keV & 796 keV from 134 Cs