On the properties of the element 106 isotopes produced in the reactions Pb+54Cr

In⁵⁴Cr ion bombardments of targets prepared from enriched lead isotopes the spontaneous fission probability for the isotopes of element 106 formed in the reactions Pb(⁵⁴Cr, 1,2n)106 was investigated. The spontaneous fission half-life of the doubly-even isotope²⁶⁰106 has been shown to beT _SF?5 ms. The half-lives of neighbouring oddmass isotopes have also been estimated:T _SF(²⁵⁹106)?0.1 s andT _SF(²⁶¹106)?0.4 s. The results of the experiments are discussed in comparison with the latest theoretical calculations.     

Determination of the fission barrier of232Pu from β-delayed fission and the problem of the first barrier

A delayed fission activity with a half-life of (55±7) s and a production cross section of (5±1)nb has been observed in the irradiation of²³⁷Np with 104 MeVα particles. It is tentatively assigned to the electron capture decay of²³²Am followed by fission of²³²Pu With a probability of (1.3 _?0.8 ⁺⁴ )· 10^?2 A barrier height of (5.3±0.4) MeV is extracted for²³²Pu exceeding theoretical barrier heights by about 2MeV. Systematic deviations between experimental and theoretical barrier heights previously observed in particular for U and Th isotopes are now also established for neutron deficient Pu isotopes indicating severe defects in the procedure of calculating static fission barriers.     

The decay of the neutron deficient plutonium isotopes 232, 233 and 234

The plutonium isotopes²³²Pu,²³³Pu and²³⁴Pu were produced by (α,xn) reactions with²³³U. After a chemical separation their decay was studied by γ-ray and α-spectros-copy. For²³³Pu twenty-seven γ-ray energies and intensities were measured and a decay scheme is proposed. An additional α-transition was detected for²³²Pu. The half-lives were determined for all three isotopes.     

Calculations of the β-decay half-lives of neutron-deficient nuclei

In this work,β~+/EC decays of some medium-mass nuclei are investigated within the extended quasiparticle random-phase approximation(QRPA),where neutron-neutron,proton-proton and neutron-proton(np) pairing correlations are taken into consideration in the specialized Hartree-Fock-Bogoliubov(HFB) transformation.In addition to the pairing interaction,the Br¨uckner G-matrix obtained with the charge-dependent Bonn nucleon-nucleon force is used for the residual particle-particle and particle-hole interactions.Calculations are performed for even-even proton-rich isotopes ranging from Z =24 to Z =34.It is found that the np pairing interaction plays a significant role inβ-decay for some nuclei far from stability.Compared with other theoretical calculations,our calculations show good agreement with the available experimental data.Predictions of β-decay half-lives for some very neutron-deficient nuclei are made for reference.     

A systematic study of α-decay in superheavy nuclei using a Hartree-Fock-Bogoliubov model

Predictions on the alpha decay and spontaneous fission of the superheavy nuclei in the range of Z = 106–118 have been systematically done within the Hartree-Fock-Bogoliubov model and different Skyrme type interactions. A comparison of the results of the current study with experimental values showed good agreement. This study was extended to unknown isotopes in the range of Z = 116–118. The calculated alpha decay half-lives are in the range of ${10}^{-5}$ to 10² s which lie within measurable values. To identify the long-lived isotopes in the mass region, spontaneous fission half-lives were performed using a semi-empirical formula. The results revealed that the isotopes ^{276,275,274,272,268}110, ^276,275,273111, ^282,280,279112, ^281,280,279113, ^284,283114, ²⁸⁶115, ²⁸⁹116, ^{296,295,292,291}117, ^297,296118, and ^295,293,292118 survived fission and had alpha decay mode as the predominant mode of decay which could be synthesized in the laboratory.     

Beta-decay half-lives of neutron rich Cu and Ni isotopes produced by thermal fission of235U and239Pu

The half-lives of very neutron rich isotopes of Ni and Cu have been measured. The isotopes are produced in very asymmetric thermal fission of²³⁵U and²³⁹Pu at the I.L.L. high flux reactor. They are separated by means of the Lohengrin spectrometer. They are identified with aΔE-E ionization chamber and implanted in one of the 8 Si planar detectors where theβ-particles are also detected. The time correlations between the implantation and the detection ofβ-particles provide the half-life. The values obtained are compared to current theoretical predictions.     

Yield ratio for the two 241Pu fission isomers in the 242(γ, n) reaction

The half-lives and yield ratios for the two ²⁴¹Pu fission isomers have been measured in the ²⁴²Pu(γ, n) reaction. The observed half-life for the long-lived isomer $\text{(T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 20.5 ± 2.2 μs)}$ is in agreement with previous data, and the existence of a short-lived 34 ± 7 ns isomer in ²⁴¹Pu could be confirmed. The measured yield ratios are Y_iso/Y_pr = (9.2 ± 0.8) × 10^?5 and Y_iso/Y_pr = (3.7 ± 0.7) × 10^?5, respectively. From a statistical model analysis of the isomeric fission yield ratio, Y^long_iso/Y^short_iso = 2.5 ± 0.6, a spin assignment for the two isomers is attempted. Possible spin combinations are compared with single-particle shell-model calculations and with available spectroscopic data for the other even-odd Pu isotopes.     

The study of prompt and delayed muon induced fission

The ratios of prompt to delayed fission yields for the isotopes²³³U,²³⁴U,²³⁵U,²³⁶U,²³⁸U,²³⁷Np,²⁴²Pu, and²⁴⁴Pu and the fission probabilities relative to each other have been investigated experimentally. Using the value of the total fission probability for²³⁷Np the absolute probabilities for prompt and delayed fission have been determined. The fission probabilities per muon captureP _fc have been derived for all the isotopes and compared with an evaluation based on excitation functions from theory.     

Bestimmung der Triplett-Potentialdaten für das K2-Stoßmolekül

Short lived Ru isotopes produced in thermal fission of²³⁵U have been chemically separated from the other fission products. Successiveγ-ray spectra were taken by means of Ge(Li) detectors. The observedγ-lines have been assigned to the various Ru isotopes (A=107 to 110) or to their daughter elements on the basis of their half-lives, intensity ratios and known level properties. Partial decay schemes are discussed and relative fission yields are given.¹¹¹Ru has been identified (T ₁₂: 1.5±0.3 s).     

The study of prompt and delayed muon induced fission

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture ratesΛ _c are 1.392(4)·10⁷/s for²³⁷Np, 1.290(7)·10⁷/s for²⁴²Pu and 1.240(7)·10⁷/s for²⁴⁴Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement ofΛ _c for²³⁹Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.     

Half-lives,neutron emission probabilities and fission yields of neutron-rich rubidium isotopes in the mass regionA=96 toA=100

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of²³⁵U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms¹⁰⁰Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP _n of⁹⁹Rb and of the fission yields of⁹⁷Rb,⁹⁸Rb, and⁹⁹Rb were performed.     

Introducing the fission–fusion reaction process: using a?laser-accelerated Th beam to produce neutron-rich nuclei towards the N=126 waiting point of the r-process

We propose to produce neutron-rich nuclei in the range of the astrophysical r-process (the rapid neutron-capture process) around the waiting point N=126 (Kratz et al. in Prog. Part. Nucl. Phys. 59:147, 2007; Arnould et al. in Phys. Rep. 450:97, 2007; Panov and Janka in Astron. Astrophys. 494:829, 2009) by fissioning a dense laser-accelerated thorium ion bunch in a thorium target (covered by a polyethylene layer, CH₂), where the light fission fragments of the beam fuse with the light fission fragments of the target. Using the ‘hole-boring’ (HB) mode of laser radiation pressure acceleration (RPA) (Robinson et al. in Plasma Phys. Control. Fusion 51:024004, 2009; Henig et al. in Phys. Rev. Lett. 103:245003, 2009; Tajima et al. in Rev. Accel. Sci. Technol. 2:221, 2009) using a high-intensity, short pulse laser, bunches of ²³²Th with solid-state density can be generated very efficiently from a Th layer (ca. 560 nm thick), placed beneath a deuterated polyethylene foil (CD₂ with ca. 520 nm), both forming the production target. Th ions laser-accelerated to about 7 MeV/u will pass through a thin CH₂ layer placed in front of a thicker second Th foil (both forming the reaction target) closely behind the production target and disintegrate into light and heavy fission fragments. In addition, light ions (d,C) from the CD₂ production target will be accelerated as well to about 7 MeV/u, also inducing the fission process of ²³²Th in the second Th layer. The laser-accelerated ion bunches with solid-state density, which are about 10¹⁴ times more dense than classically accelerated ion bunches, allow for a high probability that generated fission products can fuse again when the fragments from the thorium beam strike the Th layer of the reaction target.     

Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei ²³²Th, ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L _m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is L _m ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from ²³⁶U and ²³⁸U photofission in the low-energy region is confirmed.     

Time features of delayed neutrons and partial emissive-fission cross sections for the neutron-induced fission of 232Th nuclei in the energy range 3.2–17.9 MeV

The energy dependence of the relative abundances of delayed neutrons and the energy dependence of the half-lives of their precursors in the neutron-induced fission of ²³²Th nuclei in the energy range 3.2–17.9 MeV were measured for the first time. A systematics of the time features of delayed neutrons is developed. This systematics makes it possible to estimate the half-life of delayed-neutron precursors as a function of the nucleonic composition of fissile nuclei by using a single parameter set for all nuclides. The energy dependence of the partial cross sections for emissive fission in the reaction ²³²Th(n, f) was analyzed on the basis of data obtained for the relative abundances of delayed neutrons and the aforementioned half-lives and on the basis of the created systematics of the time features of delayed neutrons. It was shown experimentally for the first time that the decrease in the cross section after the reaction threshold in the fission of ²³²Th nuclei (it has a pronounced first-chance plateau) is not an exclusion among the already studied uranium, plutonium, and curium isotopes and complies with theoretical predictions obtained for the respective nuclei with allowance for shell, superfluid, and collective effects in the nuclear-level density and with allowance for preequilibrium neutron emission     

Deformation of nuclei close to the two-neutron drip line in Mg region

We present Hartree-Fock-Bogoliubov (HFB) calculations of the ground states of even Mg isotopes. A Skyrme force is used in the mean-field channel and a density-dependent zero-range force in the pairing channel. Our study shows that the ground states of^36,38,40Mg are strongly deformed with significantly different deformations for the neutrons and protons. Our study supports the disappearance of theN=28 shell gap in the Mg isotopes.     

Deformation of nuclei close to the two-neutron drip line in the Mg region

We present Hartree-Fock-Bogoliubov (HFB) calculations of the ground states of even Mg isotopes. A Skyrme force is used in the mean-field channel and a density-dependent zero-range force in the pairing channel. ⁴⁰Mg and ²⁰Mg are predicted to be at the two-neutron and two-proton drip lines respectively. A detailed study of the quadrupole deformation properties of all the isotopes shows that the ground states of ^36,38,40Mg are strongly deformed with significantly different deformations for the neutrons and protons. Our study supports the disappearance of the N = 28 shell gap in the Mg and Si isotopes.     

Evidence for quasi-fission in40Ar+208Pb collisions near the coulomb-barrier

Fission-fragment angular distributions were measured in the reaction of⁴⁰Ar with²⁰⁸Pb near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around θ=90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission (‘quasi-fission’) competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extra-push model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4<E _xx <9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of⁴⁰Ar with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed.     

An all-silicon linear chain NMR quantum computer

An updated version of our all-silicon quantum computing scheme [T.D. Ladd, J.R. Goldman, F. Yamaguchi, Y. Yamamoto, E. Abe, K.M. Itoh, Phys. Rev. Lett. 89 (2002) 017901. [3]] and the experimental progress towards its realization are discussed. We emphasize the importance of revisiting a wide range of isotope effects which have been explored over the past several decades for the construction of solid-state silicon quantum computers. Using RF decoupling techniques [T.D. Ladd, D. Maryenko, Y. Yamamoto, E. Abe, K.M. Itoh, Phys. Rev. B. 71 (2005) 014401] phase decoherence times T₂=25 s of ²⁹Si nuclear spins in single-crystal Si have been obtained at room temperature. We show that a linear chain of ²⁹Si stable isotopes with nuclear spin I=1/2 embedded in a spin free ²⁸Si stable isotope matrix can form an ideal building block for solid-state quantum information processors, especially, in the form of a quantum memory which requires a large number of operations within T₂ for the continuous error correction.