Measurements of High-energy Excited States and γ -rays of Fission Products with a 4π Clover Detector Y. Shima, 1, * Y. Kojima, 2 H. Hayashi, 3 A. Taniguchi, 4 and M. Shibata 2 1 Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 2 Radioisotope Research Center, Nagoya University, Nagoya 464-8602, Japan 3 Institute of Health Biosciences, The University of Tokushima, Tokushima 770-8509, Japan 4 Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japan Gamma-rays in the β-decay of 147 La and 145 Ba were measured using a 4π clover detector to identify high-energy excited levels and γ-rays. In order to determine γ-ray intensities, an efficiency calibration was carried out using single and multiple γ-ray emitters. Applying appropriate coinci- dence summing corrections, the peak efficiency was experimentally determined from 50 to 3200 keV with 3% accuracy. Through analyses of sum peaks and cascade relations of γ-rays, we newly iden- tified 170 levels between 924 and 3568 keV, and more than 930 γ-rays in the decay of 147 La, and 70 levels between 973 and 3703 keV, and 250 γ-rays in the decay of 145 Ba. I. INTRODUCTION Decay schemes and associated decay data are essential information for studies on nuclear physics and engineer- ing, for example, decay heat evaluations. However, decay schemes of fission products with large β-decay energies are scarce in the high-energy region because it is difficult to measure the γ -rays depopulating the high-energy lev- els. In this case, β-branching ratios will be incorrectly de- duced. In particular, the β-branching ratio to the ground state can be overestimated when calculated as the differ- ence between the total number of β-decays and the β- branching ratios to the excited levels. This problem is related to the pandemonium effect [1]. Therefore, mea- surements of high-energy γ -rays and their intensities are important to determine precise β-branching ratios. Total absorption γ -ray spectrometers composed of large NaI detectors have been utilized to determine the β-branching ratios of fission products [2, 3]. However, detailed decay schemes are not proposed because such spectrometers have poor energy resolution. On the other hand, HPGe detectors with good energy resolution have been also utilized for construction of decay schemes, but the efficiencies of such detectors are usually lower than that of the NaI spectrometers. A 4π clover detector com- posed of four large Ge crystals has high efficiency and good energy resolution. Using this detector, we aim both to identify high-energy levels and to determine the γ -ray intensities of fission products. * Corresponding author: shima.yosuke@g.mbox.nagoya-u.ac.jp We have studied the decay schemes of short-lived fis- sion products around a mass number of 150 using the on-line isotope separator of the Kyoto University Reactor (KUR-ISOL) [4]. In this work, we measured the γ -rays in the decay of 147 La and 145 Ba with the 4π clover detec- tor. The β-decay energy of 147 La was reported to be 5335 keV [5], but the excited levels in the daughter nuclide of 147 Ce have been reported only up to 924.3 keV [6]. The β-decay energy of 145 Ba was also reported to be 5319 keV [5] and the excited levels in 145 La have been reported up to 2566.4 keV [6]. However, no excited level was identi- fied between 1000 and 2500 keV. This means that miss- ing high-energy levels and γ -rays most probably exist in 147 Ce and 145 La. In order to determine the high-energy γ -ray intensities, we also deduced detection efficiencies from 50 to 3200 keV. Coincidence summing corrections using Monte-Carlo calculations were applied to determine the peak efficiencies. II. 4π CLOVER DETECTOR A4π clover detector is composed of four large Ge crys- tals each of which is 80 mm in diameter and 90 mm in length. The detector has a cylindrical hole of 15 mm in diameter along the central axis. The inner window of this hole is made of 0.4-mm-thick aluminum. Radioac- tive sources are placed at the center of the hole, where the solid angle subtended by the four Ge crystals is 98%. An energy signal from each Ge crystal is recorded in list mode together with the corresponding time informa- tion. The list data were converted to various spectra through off-line sorting. For example, we can obtain a Available online at www.sciencedirect.com Nuclear Data Sheets 120 (2014) 30–32 0090-3752/© 2014 Elsevier Inc. All rights reserved. www.elsevier.com/locate/nds http://dx.doi.org/10.1016/j.nds.2014.06.133