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The isomer depletion of ^{93m}Mo was recently reported [Chiara et al., Nature (London) 554, 216 (2018)NATUAS0028-083610.1038/nature25483] as the first direct observation of nuclear excitation by electron capture (NEEC). However, the measured excitation probability of 1.0(3)% is far beyond the theoretical expectation. In order to understand the inconsistency between theory and experiment, we produce the ^{93m}Mo nuclei using the ^{12}C(^{86}Kr,5n) reaction at a beam energy of 559 MeV and transport the reaction residues to a detection station far away from the target area employing a secondary beam line. The isomer depletion is expected to occur during the slowdown process of the ions in the stopping material. In such a low γ-ray background environment, the signature of isomer depletion is not observed, and an upper limit of 2×10^{-5} is estimated for the excitation probability. This is consistent with the theoretical expectation. Our findings shed doubt on the previously reported NEEC phenomenon and highlight the necessity and feasibility of further experimental investigations for reexamining the isomer depletion under low γ-ray background.

Effects of the prolate deformation of 238 U on fusion were studied in the reaction 30 Si + 238 U at bombarding energies close to the Coulomb barrier. The fission (capture) cross sections were measured at the JAEA tandem accelerator to see the enhancement of the cross sections in the subbarrier energy due to the lower Coulomb barrier in the collisions of projectile at the polar sides of 238 U. In order to obtain the direct evidence for complete fusion, evaporation residue cross sections were measured at UNILAC of GSI. At the subbarrier energy of Ec.m. = 133.0 MeV, where only polar collisions to 238 U occur, we measured three spontaneously fissioning nuclei which we assigned to the isotope 264 Sg. The obtained cross section (10 +10 – 6 ) pb was smaller than the calculation that the system captured at the polar collisions results in complete fusion, suggesting the competition between fusion and quasifission. At the above barrier energy of Ec.m. = 144.0 MeV, where collisions on the equatorial side of 238 U start to contribute, we observed α-decay chains starting from 263 Sg and the cross section was determined to be (67 +67 ) pb. The data was reproduced by the calculation that the system captured from the equatorial side of 238 U results in complete fusion.

A new isotope of Db has been produced via the reaction Am(Ne, 4n) at Elab = 118 MeV, and its half-life of 0.51± 0.16 s and α-decay energy of 9.47 MeV were determined in this work. The americium targets with thickness ranged from 0.7 to 1.0 mg/cm were prepared by molecular plating method through a single electrodeposition cycle. The reaction products were transported and collected using a helium-jet technique and a rotating wheel apparatus. The α decays of both the products and their daughter nuclides were detected by a series of Si(Au) surface barrier detectors arranged ingeniously. The identification of Db was performed based on the genetic relationship between the new activity and the known Lr established by α-recoiled milking measurement. The Qα value of Db derived from the present experiment is in good agreement with theoretical prediction.