Satoshi Takeuchi

Evidence for a new nuclear /`magic number/' from the level structure of 54Ca

Atomic nuclei are finite quantum systems composed of two distinct types of fermion—protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called ‘magic numbers’ are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus 54Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of 54Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

Identification of New Isotopes 125 Pd and 126 Pd produced by In-flight Fission of 345 MeV/nucleon 238 U: First Results from the RIKEN RI Beam Factory

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.

DALI2: A NaI(Tl) detector array for measurements of

A NaI(Tl) detector array called DALI2 (Detector Array for Lo w Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (R I) beams. It consists typically of 186 NaI(Tl) scintillator s covering polar angles from∼15◦ to ∼160◦ with an average angular resolution of 6 ◦ in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift correcti ons that result in, for example, 10% energy resolution and 20 % full-energy photopeak e fficiency for 1-MeVγ rays emitted from fast-moving nuclei (velocities of v/c ≃ 0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams wi th velocities ofv/c = 0.3 − 0.6 provided by the RIKEN RI Beam Factory.

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We report on the measurement of the first 2(+) and 4(+) states of (66)Cr and (70,72)Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p,2p) reactions at incident energies of 260u2009u2009MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN, using the DALI 2γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2(1)(+) and 4(1)(+) energies as a function of the neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified Lenzi-Nowacki-Poves-Sieja (LNPS) interaction in the pfg(9/2)d(5/2) valence space reproduce the observations. Interpretation within the shell model shows an extension of the island of inversion at N=40 for more neutron-rich isotopes towards N=50.

rays from fast nuclei

Abstract It is shown that a charged particle can be trapped in a magnetic neutral sheet produced by the combination of a static magnetic field B 0 and an electromagnetic wave propagating perpendicularly to B 0 with its magnetic field component parallel to B 0 . The neutral sheet as well as the trapped particle moves with the phase velocity of the wave and the trapped particle feels a constant force. Once the particle is trapped it never detraps and the acceleration continues unlimited. Possible ways of creating such a slow electromagnetic wave which can interact with the particles are briefly discussed.

Extension of the N = 40 Island of Inversion towards N =50: Spectroscopy of 66Cr,70,72Fe

Colorectal cancer (CRC) is a significant cause of cancer-related morbidity and mortality all over the world. Improvements of cytotoxic and biologic agents have prolonged the survival in metastatic CRC (mCRC), with a median overall survival of approximately 2 years and more in the past two decades. The biologic agents that have proven clinical benefits in mCRC mainly target vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFR). In particular, bevacizumab targeting VEGF and cetuximab and panitumumab targeting EGFR have demonstrated significant survival benefits in combination with cytotoxic chemotherapy in the first-line, second-line, or salvage setting. Aflibercept, ramucirumab, and regorafenib are also used in second-line or salvage therapy. Recent retrospective analyses have shown that KRAS or NRAS mutations were negative predictive markers for anti-EGFR therapy. Based on the evidence from large randomized clinical trials, personalized therapy is necessary for patients with mCRC according to their tumor biology and characteristics. The aim of this paper was to summarize the results of the major randomized clinical trials and highlight the benefits of the molecular targeted agents in patients with mCRC.

Unlimited acceleration of a charged particle by an electromagnetic wave with a purely transverse electric field

It is analytically shown that the longitudinal electric field of a Gaussian laser beam in vacuum accelerates an electron to an ultra-relativistic energy. The most favorable electron is accelerated in a length of twice the Rayleigh range. It is found that the ultimate energy increment of the electron with a single laser beam is given by the product of transverse field intensity and the beam waist. Also stability of the electron orbit and the trapping condition are discussed. The gain can be of the order of 100 MeV per single stage and then a multi-stage acceleration enables TeV-order-acceleration in a length of a few kilometers with the present state of the art.

Role of targeted therapy in metastatic colorectal cancer

The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.

Electron Acceleration by Longitudinal Electric Field of a Gaussian Laser Beam

Excited states in 1 2 Be and 1 4 Be were studied by using an exotic beam of 1 4 Be at 75 A MeV. A new excited state in 1 2 Be was observed at 11.8 MeV. The angular-correlation analysis showed that the spin for the new level is tentatively assigned to be 0. According to the energy-spin systematics, the observed state may be a member of the rotational band of the 6 He- 6 He cluster structure. Three excited states in 1 4 Be were also found at 10.8 MeV, 11.6 MeV, and 15.5 MeV.

Collectivity evolution in the neutron-rich Pd isotopes toward the N=82 shell closure

We measured fragmentation cross sections produced using the primary beam of 86 Kr at 64 MeV/nucleon on 9 Be and 181 Ta targets. The cross sections were obtained by integrating the momentum distributions of isotopes with 25 ≤ Z < 36 measured using the RIPS fragment separator at RIKEN. The cross-section ratios obtained with the 181 Ta and 9 Be targets depend on the fragment masses, contrary to the simple geometrical models. We compared the extracted cross sections to EPAX; an empirical parametrization of fragmentation cross sections. Predictions from current EPAX parametrization severely overestimate the production cross sections of very neutron-rich isotopes. Attempts to obtain another set of EPAX parameters specific to the reaction studied here to extrapolate the neutron-rich nuclei more accurately have not been very successful, suggesting that accurate predictions of production cross sections of nuclei far from the valley of stability require information of nuclear properties that are not present in EPAX.