A search for isomeric γ-decays among fission fragments from 345 MeV/nucleon 238 U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed γ-rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 μs following the implantation. We identified a total of 54 microsecond isomers with half-lives of ~ 0. on the obtained spectroscopic information and the systematics in neighboring nuclei. Nature of the nuclear isomerism is discussed in relation to evolution of nuclear structure.KEYWORDS: Nuclear reactions Be( 238 U, x) and Pb( 238 U, x) E = 345 MeV/nucleon, in-flight fission, fission fragments, in-flight RI beam separator, short-lived isomers, new isomers, half-life, γ-ray relative intensity, γγ coincidence, proposed level schemes DOI: PACS number(s): 23.35.+g, 23.20.Lv, 29.38.Db _____________________ *
A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identified by using the superconducting in-flight separator BigRIPS. We observed 45 new neutron-rich isotopes: Since the pioneering production of radioactive isotope (RI) beams in the 1980s, 1) studies of exotic nuclei far from stability have been attracting much attention. Neutron-rich exotic nuclei are of particular interest, because new phenomena such as neutron halos, neutron skins, and modifications of shell structure have been discovered.2-5) Furthermore these neutron-rich nuclei are important in relation to astrophysical interests, 6) because many of them play a role in the astrophysical r-process. 7) To make further advances in nuclear science and nuclear astrophysics, it is essential to expand the region of accessible exotic nuclei towards the neutron dripline. In-flight fission of a uranium beam is known to be an excellent mechanism for this purpose, having large production cross sections for neutron-rich exotic nuclei. became operational, in which the superconducting in-flight separator BigRIPS 10,11) has been used for the production of RI beams. The BigRIPS separator is designed as a two-stage separator with large acceptance, so that excellent features of in-flight fission can be exploited. In May 2007, right after the commissioning of the BigRIPS separator, we performed an experiment to search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam, aiming to expand the LETTERS Ã
A high-spin isomer in 145 Sm was discovered by using Inverse kinematic reactions, 20 Ne e Xe,a7n) 145 Sm and 16 0 e 36 Xe,7n) 145 Sm. The half life was determined to be 0.96 J.LSec. Sixty-five 1-rays were identified by the /')'-coincidence measurements to belong to the isomer decay. The low-lying level scheme of 145 Sm was established in detail by the in-beam 1-ray measurements using the 139 La e 0 B,4n) 145 Sm reaction. A complex decay scheme of this isomer was constructed by using the data obtained from the 136 Xe induced reactions, combining the informations of low-lying states mentioned above. The excitation energy of this isomer was determined to be 8.8 MeV. The /')'-coincidence measurement using the 138 Ba (13 C,6n) 145 Sm reaction was also performed. Based on this information, the level scheme above the high-spin isomer was extended up to the state at 14.6 MeV. A 1-ray angular distribution measurement using the same reaction with pulsed beam was carried out and was used to assign a spin value of each level. Low-lying states in 145 Sm were interpreted to originate from a single neutron coupled to the 144 Sm core excitation. Experimental yrast states were compared with a calculation of a deformed independent particle model (DIPM). A configuration of the high-spin isomer was deduced by the DIPM calculation to be { 7r h ll /2 2 v (f 7 /2 h 9/2 i 13/2) } 49/2+ .
A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out in the commissioning experiment of the next-generation in-flight radioactive isotope beam separator BigRIPS at the RI Beam Factory at the RIKEN Nishina Center. Two neutron-rich palladium isotopes 125 Pd and 126 Pd were observed for the first time, which demonstrates the great potential of the RIKEN RI beam factory. 2) at the RIKEN Nishina Center that promises to advance the study of exotic nuclei to a great extent. The new superconducting in-flight RI beam separator BigRIPS is a major experimental device at the RIBF for RI beam production based on the in-flight separation technique and for research with exotic nuclei.3,4) Thanks to its large acceptances, not only projectile fragmentation of various heavy-ion beams but also in-flight fission of fissile beams 5) can be efficiently used as a production reaction in the BigRIPS separator. Figure 1 shows a schematic layout of the BigRIPS separator along with the IRC and SRC cyclotrons 1) and the ZeroDegree spectrometer.3) The cyclotrons at the RIBF can accelerate all heavy ions up to approximately 400 MeV/nucleon, including very heavy elements such as uranium, with the goal of reaching an intensity of 1 pmA (6 Â 10 12 particles/s). 1)In-flight fission of fissile beams, such as a 238 U beam, is known as an excellent mechanism for producing a wide range of neutron-rich exotic nuclei far from stability. This was well demonstrated by experiments at GSI, in which more than a hundred new isotopes were identified in a single experiment.6,7) The BigRIPS separator has been designed with large acceptances, to take advantage of the high production cross sections of neutron-rich isotopes from these reactions. The full angular acceptances of 80 mrad (horizontal) and 100 mrad (vertical), and the full momentum acceptance of 6% allows efficient RI beam production with in-flight fission, in which fission fragments are produced with large spreads in both angle and momentum. Due to its large acceptance, the BigRIPS separator has an approximately 50% efficiency for the collection of these fission fragments.Another important feature of the BigRIPS separator is its two-stage structure, which allows delivery of tagged RI beams, or use as a two-stage separator. In the tagging mode, which may also be called a separator-spectrometer mode, the first stage is used to produce and separate RI beams with a wedge energy degrader, while the second works as a spectrometer to analyze and identify those RI beams. The momentum resolution of the second stage has been designed to be high enough to identify RI beams without measuring their total kinetic energies, even though they are produced in several charge states in our energy domain. In the two-stage LETTERS Ã
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