•In-flight particle identification of RI beams developed for BigRIPS separator.•Atomic number Z and mass-to-charge ratio A/Q are deduced by the TOF-Bρ-ΔE.•Precise determinations of Bρ and TOF with ...trajectory reconstruction and slew correction, respectively.•The achieved A/Q resolution is high enough to clearly identify the charge state.•Thorough removal of background events improves the reliability of identification.
We have developed a method for achieving excellent resolving power in in-flight particle identification of radioactive isotope (RI) beams at the BigRIPS fragment separator at the RIKEN Nishina Center RI Beam Factory (RIBF). In the BigRIPS separator, RI beams are identified by their atomic number Z and mass-to-charge ratio A/Q which are deduced from the measurements of time of flight (TOF), magnetic rigidity (Bρ) and energy loss (ΔE), and delivered as tagged RI beams to a variety of experiments including secondary reaction measurements. High A/Q resolution is an essential requirement for this scheme, because the charge state Q of RI beams has to be identified at RIBF energies such as 200–300MeV/nucleon. By precisely determining the Bρ and TOF values, we have achieved relative A/Q resolution as good as 0.034% (root-mean-square value). The achieved A/Q resolution is high enough to clearly identify the charge state Q in the Z versus A/Q particle identification plot, where fully-stripped and hydrogen-like peaks are very closely located. The precise Bρ determination is achieved by refined particle trajectory reconstruction, while a slew correction is performed to precisely determine the TOF value. Furthermore background events are thoroughly removed to improve reliability of the particle identification. In the present paper we present the details of the particle identification scheme in the BigRIPS separator. The isotope separation in the BigRIPS separator is also briefly introduced.
Unbound states in C17 were investigated via one-neutron removal from a C18 beam at an energy of 245 MeV/nucleon on a carbon target. The energy spectrum of C17, above the single-neutron decay ...threshold, was reconstructed using invariant mass spectroscopy from the measured momenta of the C16 fragment and neutron, and was found to exhibit resonances at Er=0.52(2), 0.77(2), 1.36(1), 1.91(1), 2.22(3) and 3.20(1) MeV. The resonance at Er=0.77(2) MeV Ex=1.51(3) MeV was provisionally assigned as the second 5/2+ state. The two resonances at Er=1.91(1) and 3.20(1) MeV Ex=2.65(2) and 3.94(2) MeV were identified, through comparison of the energies, cross sections and momentum distributions with shell-model and eikonal reaction calculations, as p-shell hole states with spin-parities 1/21− and 3/21−, respectively. A detailed comparison was made with the results obtained using a range of shell-model interactions. The YSOX shell-model Hamiltonian, the cross-shell part of which is based on the monopole-based universal interaction, was found to provide a very good description of the present results and those for the neighbouring odd-A carbon isotopes – in particular for the negative parity cross-shell states.
Structure of the neutron-rich N=86 isotope 140Xe, located northeast of a doubly-magic nucleus 132Sn, is investigated by β−γ spectroscopy. Two β-decay isomers in 140I are newly found in the study of ...two different β decays of 140I which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) β decay of 140Te at an active stopper. Half-lives of the β decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the β decay of the high-spin isomer and of the mixed β decays of the ground state and the low-spin isomer in 140I to 140Xe are constructed using the information on γ-ray coincidence relation and γ-ray intensity. Nuclear structures of the low-lying states in 140Xe and 140I are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)γ-band members of 2+ and 4+ states and the octupole collective 1− state are proposed in 140Xe. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.
•Large-area Parallel Plate Avalanche Counter (PPAC) developed for position measurement.•A specially developed delay line is utilized to obtain the position information.•Excellent performance in ...sensitivity uniformity as well as position measurement.•Double-PPAC structure enables to obtain high detection efficiency.•A signal transmission system using optical fiber cables has been developed for PPACs.
We have developed a position-sensitive Parallel Plate Avalanche Counter (PPAC), which has been used as a focal plane detector in the BigRIPS fragment separator and the subsequent RI-beam delivery lines at the RIKEN Nishina Center RI Beam Factory. The PPAC detector plays an important role not only in the tuning of the separator and delivery lines but also in the particle identification of rare isotope (RI) beams. The PPAC detector has a sensitive area of 240mm×150mm, and the position information is obtained by a delay-line readout method. Being called double PPAC, it is composed of two full PPACs, each measuring the particle locus in two dimensions. High detection efficiency has been made possible by the twofold measurement using the double PPAC detector. The sensitivity uniformity is also found to be excellent. The root-mean-square position resolution is measured to be 0.25mm using an α source, while the position linearity is as good as ±0.1mm for the detector size of 240mm. Characteristics, operating principles, specifications, performance and issues of the PPAC detector are presented, including its signal transmission system using optical fiber cables.
The β-decay scheme of 138Te and the level structure of 138I is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN ...Cluster Array campaigns. Secondary radioactive ions, including 138Te and 138Sb, were produced by the in-flight fission of a 238U beam with the energy of 345 MeV per nucleon. From the β decay of 138Te, the level scheme of 138I was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1+ state. This 1+ state can be interpreted as being associated with the π0h11/2⊗ν0h9/2 partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Details of the structure of 138I are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.
Neutron-rich Pm (Z=61) isotopes were studied by delayed γ-ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon 238U beam. A cluster-type Ge detector array, ...EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in 159Pm and 161Pm with half-lives of 4.97(12) μs and 0.79(4) μs, respectively. Level schemes for 159Pm and 161Pm were constructed in this study. The isomeric states of 159Pm and 161Pm could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν(7/2633⊗5/2523) and ν(7/2633⊗1/2521), respectively. They are analogous to the isomers that have been observed systematically in other even-mass N=98 and N=100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N-dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A=165, and provides new evidence supporting the existence of the deformed N=98 subshell gap in odd-mass nuclei for the first time.
Isomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon
238
U beam. Isomers ...reported in nuclei spanning a predicted prolate-oblate shape change boundary,
111
Zr (
E
=
283.1
keV;
τ
=
0.326
(
63
)
μ
s),
112
Nb (
E
=
44.2
keV;
τ
=
0.094
(
26
)
μ
s),
113
Nb (
E
=
135.4
keV;
τ
=
0.846
(
80
)
μ
s), and
115
Mo (
E
=
198.6
keV;
τ
=
63
(
4
)
μ
s), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from
γ
-ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for
N
>
70
in this region. In addition, isomers in
108
Nb,
109
Nb,
113
Tc,
117
Ru,
119
Ru,
120
Rh, and
122
Rh, not spanning the prolate-oblate transition discussed, are presented.
•We measured the production cross sections for a variety of radioactive isotopes.•They were produced from 124Xe, 48Ca, and 238U beams at 345MeV/nucleon using BigRIPS.•The measured production cross ...sections were compared with the EPAX formulae.•We discovered four new isotopes 85,86Ru and 81,82Mo produced by 124Xe+Be.•103Sb is particle unbound with an upper limit of 49ns for the half-life.
We have measured the production rates and production cross sections for a variety of radioactive isotopes which were produced from 124Xe, 48Ca, and 238U beams at an energy of 345MeV/nucleon using the BigRIPS separator at the RIKEN Nishina Center RI Beam Factory (RIBF). Proton-rich isotopes with atomic numbers Z=40–52 and neutron-rich isotopes with Z=5–16 were produced by projectile fragmentation of the 124Xe and 48Ca beam on Be targets, respectively. Neutron-rich isotopes with Z=20–59 were produced by in-flight fission of the 238U beam, in which both Be and Pb were used as production targets. The measured production rates and production cross sections were compared with those of the LISE++ calculations, and overall fairly good agreement has been obtained. Furthermore, in the measurements with the 124Xe beam, we have discovered four new isotopes on the proton-drip line, 85,86Ru and 81,82Mo, and obtained the clear evidence that 103Sb is particle unbound with an upper limit of 49ns for the half-life. The measurements of projectile-fragment momentum distributions have been also performed with the 124Xe beam, in which the low-momentum tails of the distributions have been measured for the first time at the energy of 345MeV/nucleon.
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