Understanding 134 Cs Background in CsI(Tl) Crystals

1. 605 keV peak in multiple hit (563 & 569 keV included) 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. 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 796 keV peak in multiple hit (802 keV included) 1401 keV peak in single hit (pile-up of 605+796 keV) 1365 keV peak in multiple hit Abstract The 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. Introduction This 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. • Analysis and Results • 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. • 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. • Asymmetry Distributions of 134Cs • Asymmetry between the charge signals from two PMTs • 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) Understanding 134Cs Background in CsI(Tl) Crystals 134Cs E [keV] β- 4+ 1365 802 569 4+ 27.3% 1969.87 475 242 1038 3+ 1643.28 2.50% Figure 9. Background subtracted 605 and 796 keV peaks Figure 10. Background subtracted 1401 keV Figure 11. Energy resolution curve 795 4+ 70.1% 1400.55 563 1168 1167.93 0.033% 2+ 605 0.10% 2+ 604.70 0.0 0+ • 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 cm3 dimensions • Each crystal is coupled to 2 PMTs. Figure 1. Decay scheme of 134Cs 134Ba Y2L Figure 13. Beta-ray spectrum of Max. 89 keV (Avg. 23 keV) Figure 12. Beta-ray spectrum of Max. 658 keV (Avg. 210 keV) dd Figure 2. Underground Laboratory Figure 3. Array of CsI(Tl) crystals 660 keV from 137Cs • Analysis and Results • 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. • 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. 660 keV from 137Cs 605 keV & 796 keV from 134Cs 605 keV from 134Cs 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 Figure 4. 59.54 keV from 241Am Figure 5. 605 and 796 keV from 134Cs Figure 6. Linearity of signal area to energy Figure 16. Decay curves of 134Cs in CsI(Tl) crystal Conclusion To 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. 1401 keV gamma from Cs-134 (70%) 662 keV gamma from Cs-137 (100%) 1970 keV gamma from Cs-134 (27%) Figure 7. Energy spectrum in one crystal Figure 8. Energy sum in all crystals