Transcript 슬라이드 1

Understanding
134Cs
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. 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
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
Analysis and Results
This study is motivated to understand the properties of 134Cs radioactive source within
3. Gamma-ray Spectra of 134Cs
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.
134Cs
4+
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.
E [keV]
β-
27.3%
4+
2.50%
3+
70.1%
4+
0.033%
2+
0.10%
2+
1365 802 569
1969.87
1038 475 242
1643.28
795
1400.55
1168 563
1167.93
605
Figure 9. Background subtracted 605 Figure 10. Background subtracted
and 796 keV peaks
1401 keV
604.70
0+
Figure 11. Energy resolution curve
0.0
134Ba
Figure 1. Decay scheme of
4. Beta-ray Spectra of 134Cs
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.
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.
Y2L
Figure 12. Beta-ray spectrum of Max. 658
keV (Avg. 210 keV)
Figure 13. Beta-ray spectrum of Max. 89
keV (Avg. 23 keV)
5. Asymmetry Distributions of 134Cs
Asymmetry between the charge signals from two PMTs
Figure 2. Underground Laboratory
Figure 3. Array of CsI(Tl) crystals
660 keV from 137Cs
605 keV & 796 keV from
134Cs
660 keV from 137Cs
Analysis and Results
1. Energy Calibration
605 keV from 134Cs
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.
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 4. 59.54 keV from 241Am
Figure 5. 605 and 796 keV from 134Cs
Figure 6. Linearity of signal
area to energy
2. Measured Background Energy Spectra
Figure 6 and Figure 7 show the spectra of the energy deposited in one crystal, and
Figure 15. 1D asymmetry distributions
from 134Cs and 137Cs around
600 keV
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)
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 16. Decay curves of 134Cs in CsI(Tl) crystal
605 keV peak in multiple hit
(563 & 569 keV included)
796 keV peak in multiple
hit (802 keV included)
1401 keV peak in single hit
(pile-up of 605+796 keV)
662 keV
gamma from
Cs-137 (100%)
1401 keV
gamma from
Cs-134 (70%)
1970 keV
gamma from
Cs-134 (27%)
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
1365 keV peak in
multiple hit
known value. A more detailed study on 134Cs is in progress.
Figure 7. Energy spectrum in one crystal
Figure 8. Energy sum in all crystals