arXiv subjects

K. Morimoto

Publications and source records attributed to K. Morimoto.

14 recordsLinked to original sources

arXiv2021

Alpha-decay-correlated mass measurement of 206,207Ra using an $\alpha$-TOF detector equipped MRTOF-MS system

The atomic masses of the isotopes $^{206,207}$Ra have been measured via decay-correlated mass spectroscopy using a multi-reflection time-of-flight mass spectrograph equipped with an $\alpha$-TOF detector. The Ra isotopes were produced as fusion-evaporation products in the $^{51}$V+$^{159}$Tb reaction system and delivered by the gas-filled recoil ion separator GARIS-II at RIKEN. The $\alpha$-TOF detector provides for high-accuracy mass measurements by correlating time-of-flight signals with subsequent $\alpha$-decay events. The masses of $^{206}$Ra and $^{207g,m}$Ra were directly measured using a multi-reflection time-of-flight mass spectrograph equipped with an $\alpha$-TOF detector. A mass excess of ME = 3538(15) keV/c$^2$ and an excitation energy of E$_{\rm ex}$ = 552(42) keV were determined. The $\alpha$-decay branching ratio of $^{207m}$Ra, b$\alpha$ = 0.26(20), was directly determined from decay-correlated time-of-flight signals, and the reduced alpha width of $^{207m}$Ra was calculated to be $\delta^2$ = 50+62-41 keV from the branching ratio. The spin-parity of $^{207m}$Ra was confirmed to be $J^\pi$ = 13/2$^-$ from decay correlated mass measurement results.

nucl-ex↗

arXiv2020

First high-precision direct determination of the atomic mass of a superheavy nuclide

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of $^{257}$Db ($Z$=105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction $^{208}$Pb($^{51}$V, 2n)$^{257}$Db. The gas-filled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and some transfer products, prior to delivering the energetic beam of $^{257}$Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized $^{257}$Db$^{3+}$ ions were then transferred to a multi-reflection time-of-flight mass spectrograph for mass analysis. An alpha particle detector embedded in the ion time-of-flight detector allowed disambiguation of the rare $^{257}$Db$^{3+}$ time-of-flight detection events from background by means of correlation with characteristic $\alpha$-decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be $100\,063(231)_\textrm{stat}(132)_\textrm{sys}$~keV/c$^2$. Comparing to several mass models, we show the technique can be used to unambiguously determine the atomic number as $Z$=105 and should allow similar evaluations for heavier species in future work.

nucl-ex↗

arXiv2019

Development of an "$α$-ToF" detector for correlated measurement of atomic masses and decay properties

We have developed a novel detector, referred to as an "$α$-ToF detector", for correlated measurements of atomic masses and decay properties of low-yield, short-lived radioactive isotopes using a multi-reflection time-of-flight mass spectrograph. By correlating measured time-of-flight signals with decay events, it will be possible to suppress background events and obtain accurate, high-precision mass values even in cases of very low event rates. An offline test of the $α$-ToF detector has shown that the time-of-flight detection efficiency for 5.48~MeV $α$-rays is more than 90\% and yields a time resolution of 251.5(68)~ps and an energy resolution of 141.1(9)~keV. Using a two-dimensional spectrum of the correlated $α$-ray energy and time-of-flight, the $α$-rays from mixed $α$ sources could be fairly well resolved.

physics.ins-det↗

arXiv2018

First observation of the competing fission modes in the neutron-deficient sub-lead region

Fragment mass distributions from fission of excited compound nucleus $^{178}$Pt have been deduced from the measured fragment velocities. The $^{178}$Pt nucleus was created at the JAEA tandem facility in a complete fusion reaction $^{36}$Ar + $^{142}$Nd, at beam energies of 155, 170 and 180 MeV. The data are indicative of a mixture of the mass-asymmetric and mass-symmetric fission modes associated with higher and lower total kinetic energies of the fragments, respectively. The measured fragment yields are dominated by asymmetric mass splits, with the symmetric mode contributing at the level of $\approx1/3$. This constitutes the first observation of a multimodal fission in the sub-lead region. Most probable experimental fragment-mass split of the asymmetric mode, $A_{L}/A_{H}\approx 79/99$, is well reproduced by nuclear density functional theory using the UNEDF1-HFB and D1S potentials. The symmetric mode is associated by theory with very elongated fission fragments, which is consistent with the observed total kinetic energy/fragment mass correlation.

nucl-ex↗

arXiv2018

New mass anchor points for neutron-deficient heavy nuclei from direct mass measurements of radium and actinium isotopes

The masses of the exotic isotopes $^{210-214}$Ac and $^{210-214}$Ra have been measured with a multi-reflection time-of-flight mass spectrograph. These isotopes were obtained in flight as fusion-evaporation products behind the gas-filled recoil ion separator GARIS-II at RIKEN. The new direct mass measurements serve as an independent and direct benchmark for existing $α$-$γ$ spectroscopy data in this mass region. Further, new mass anchor points are set for U and Np nuclei close to the $N=126$ shell closure for a future benchmark of the $Z=92$ sub-shell for neutron-deficient heavy isotopes. Our mass results are in general in good agreement with the previously indirectly-determined mass values. Together with the measurement data, reasons for possible mass ambiguities from decay-data links between ground states are discussed.

nucl-ex↗

arXiv2018

First direct mass measurements of nuclides around $Z=100$ with a Multireflection Time-of-Flight Mass Spectrograph

The masses of $^{246}$Es, $^{251}$Fm and the transfermium nuclei $^{249-252}$Md, and $^{254}$No, produced by hot- and cold-fusion reactions, in the vicinity of the deformed $N=152$ neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. The masses of $^{246}$Es and $^{249,250,252}$Md were measured for the first time. Using the masses of $^{249,250}$Md as anchor points for $α$ decay chains, the masses of heavier nuclei, up to $^{261}$Bh and $^{266}$Mt, were determined. These new masses were compared with theoretical global mass models and demonstrated to be in good agreement with macroscopic-microscopic models in this region. The empirical shell gap parameter $δ_{2n}$ derived from three isotopic masses was updated with the new masses and corroborate the existence of the deformed $N=152$ neutron shell closure for Md and Lr.

nucl-ex↗

arXiv2018

Atomic masses of intermediate-mass neutron-deficient nuclei with relative uncertainty down to 35-ppb via multireflection time-of-flight mass spectrograph

High-precision mass measurements of $^{63}$Cu, $^{64-66}$Zn, $^{65}$Ga, $^{65-67}$Ge, $^{67}$As, $^{78,81}$Br, $^{80}$Rb, and $^{79}$Sr were performed utilizing a multireflection time-of-flight mass spectrograph combined with the gas-filled recoil ion separator GARIS-II. In the case of $^{65}$Ga, a mass uncertainty of 2.1 keV, corresponding to a relative precision of $δm / m = 3.5\times10^{-8}$, was obtained and the mass value is in excellent agreement with the 2016 Atomic Mass Evaluation. For $^{67}$Ge and $^{81}$Br, where masses were previously deduced through indirect measurements, discrepancies with literature values were found. The feasibility of using this device for mass measurements of nuclides more neutron-deficient side, which have significant impact on the $rp$-process pathway, is discussed.

nucl-ex↗

arXiv2016

Footprints of hyperfine, spin-orbit, and decoherence effects in Pauli spin blockade

We detect in real time inter-dot tunneling events in a weakly coupled two electron double quantum dot in GaAs. At finite magnetic fields, we observe two characteristic tunneling times, T_d and T_b, belonging to, respectively, a direct and a blocked (spin-flip-assisted) tunneling. The latter corresponds to lifting of a Pauli spin blockade and the tunneling times ratio eta=T_b/T_d characterizes the blockade efficiency. We find pronounced changes in the behavior of eta upon increasing the magnetic field, with eta increasing, saturating and increasing again. We explain this behavior as due to the crossover of the dominant blockade lifting mechanism from the hyperfine to spin-orbit interactions and due to a change in the contribution of the charge decoherence.

cond-mat.mes-hall↗

arXiv2015

First Online Mass Measurements of Isobar Chains via Multi-Reflection Time-of-Flight Mass Spectrograph Coupled with GARIS-II

Using a mulit-reflection time-of-flight mass spectrograph (MRTOF-MS) located after a gas cell coupled with the gas-filled recoil ion separator GARIS-II, the masses of several heavy nuclei have been directly and precisely measured. The nuclei were produced via fusion-evaporation reactions and separated from projectile-like and target-like particles using GARIS-II before being stopped in a helium-filled gas cell. Time-of-flight spectra for three isobar chains, 205Fr-205Rn-205At-205Po, 206Fr-206Rn-206At and 201Rn-201At-201Po-201Bi, were observed. Precision atomic mass values were determined for 205,206Fr, 201At, and 201Po.

nucl-ex↗

arXiv2014

Tuning the electrically evaluated electron Lande g factor in GaAs quantum dots and quantum wells of different well widths

We evaluate the Lande g factor of electrons in quantum dots (QDs) fabricated from GaAs quantum well (QW) structures of different well width. We first determine the Lande electron g factor of the QWs through resistive detection of electron spin resonance and compare it to the enhanced electron g factor determined from analysis of the magneto-transport. Next, we form laterally defined quantum dots using these quantum wells and extract the electron g factor from analysis of the cotunneling and Kondo effect within the quantum dots. We conclude that the Lande electron g factor of the quantum dot is primarily governed by the electron g factor of the quantum well suggesting that well width is an ideal design parameter for g-factor engineering QDs.

cond-mat.mes-hall↗

arXiv2014

Single Photoelectron Detection after Selective Excitation of Electron-Heavy Hole and Electron-Light Hole Pairs in Double Quantum Dots

We demonstrate the real-time detection of single photogenerated electrons in two different lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells having a thin or a thick AlGaAs barrier layer. The observed incident laser power and photon energy dependences of the photoelectron detection efficiency both indicate that the trapped photoelectrons are, for the thin barrier sample, predominantly photogenerated in the buffer layer followed by tunneling into one of the two dots, whereas for the thick barrier sample they are directly photogenerated in the well. For the latter, single photoelectron detection after selective excitation of the heavy and light hole state in the dot is well resolved. This ensures the applicability of our quantum well-based quantum dot systems for the coherent transfer from single photon polarization to single electron spin states.

cond-mat.mes-hall↗

arXiv2009

Decay Properties of $^{266}$Bh and $^{262}$Db Produced in the $^{248}$Cm + $^{23}$Na Reaction

Decay properties of an isotope $^{266}$Bh and its daughter nucleus $^{262}$Db produced by the $^{248}$Cm($^{23}$Na, 5\textit{n}) reaction were studied by using a gas-filled recoil separator coupled with a position-sensitive semiconductor detector. $^{266}$Bh was clearly identified from the correlation of the known nuclide, $^{262}$Db. The obtained decay properties of $^{266}$Bh and $^{262}$Db are consistent with those observed in the $^{278}$113 chain, which provided further confirmation of the discovery of $^{278}$113.

nucl-ex↗

arXiv2006

Hole Doping Effects on Spin-gapped Na2Cu2TeO6 via Topochemical Na Deficiency

We report the magnetic susceptibility and NMR studies of a spin-gapped layered compound Na2Cu2TeO6 (the spin gap $Δ\sim$ 250 K), the hole doping effect on the Cu2TeO6 plane via a topochemical Na deficiency by soft chemical treatment, and the static spin vacancy effect by nonmagnetic impurity Zn substitution for Cu. A finite Knight shift at the $^{125}$Te site was observed for pure Na2Cu2TeO6. The negative hyperfine coupling constant $^{125}A_{tr}$ is an evidence for the existence of a superexchange pathway of the Cu-O-Te-O-Cu bond. It turned out that both the Na deficiency and Zn impurities induce a Curie-type magnetism in the uniform spin susceptibility in an external magnetic field of 1 T, but only the Zn impurities enhance the low-temperature $^{23}$Na nuclear spin-lattice relaxation rate whereas the Na deficiency suppresses it. A spin glass behavior was observed for the Na-deficient samples but not for the Zn-substituted samples. The dynamics of the unpaired moments of the doped holes are different from that of the spin vacancy in the spin-gapped Cu2TeO6 planes.

cond-mat.str-el↗

arXiv2001

Measurement of parity-nonconserving rotation of neutron spin in the 0.734-eV p-wave resonance of $^{139}La$

The parity nonconserving spin rotation of neutrons in the 0.734-eV p-wave resonance of $^{139}La$ was measured with the neutron transmission method. Two optically polarized $^3He$ cells were used before and behind a a 5-cm long $^{139}La$ target as a polarizer and an analyzer of neutron spin. The rotation angle was carefully measured by flipping the direction of $^3He$ polarization in the polarizer in sequence. The peak-to-peak value of the spin rotation was found to be $ (7.4 \pm 1.1) \times 10^{-3} $ rad/cm which was consistent with the previous experiments. But the result was statisticallly improved. The s-p mixing model gives the weak matrix element as $xW = (1.71 \pm 0.25)$ meV. The value agrees well with the one deduced from the parity-nonconserving longitudinal asymmetry in the same resonance.

nucl-ex↗