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.     

β-delayed fission of 228Ac

The basic process of an exotic decay mode namely β-delayed fission is simply introduced. The progress status of the study in the world is essentialized. The observation of β-delayed fission of ²²⁸Ac is reported. The radium was radiochemically separated from natural thorium. Thin Ra sources in which ²²⁸Ac was got through ²²⁸Ra →β^－228Ac were prepared for observing fission fragments from β-delayed fission of ²²⁸Ac. They exposed to the mica fission track detectors, and measured by an HPGe γ-ray detector. The β-delayed fission events of ²²⁸Ac were observed and its β-delayed fission probability was found to be (5±2)×10^－12.     

Search for the doubly heavy baryon ■ decaying to ■

A first search for the ■decay is performed by the LHCb experiment with a data sample of proton-proton collisions, corresponding to an integrated luminosity of 9 fb^-1 recorded at centre-of-mass energies of 7, 8, and 13 TeV. Two peaking structures are seen with a local(global) significance of 4.3(2.8) and 4.1(2.4) standard deviations at masses of 6571 and 6694 MeV/c², respectively. Upper limits are set on the ■ baryon production cross-section times the branching fraction rela...     

Search for chiral bands in A ～ 110 neutron-rich nuclei

High spin states in A ～ 110 neutron-rich 106Mo, 110Ru and 112Ru nuclei have been reinvestigated by measuring the prompt γ-rays from the spontaneous fission of 252Cf. Two similar sets of bands are observed to high spins in each of three nuclei. Through analyzing of characters of the band structures, the chiral doublet bands are suggested in 106Mo, 110Ru and 112Ru.     

Total absorption spectroscopy of the β-delayed proton emitter 117Ba

The very neutron-deficient ¹¹⁷Ba nuclei were produced in ⁵⁸Ni-induced reactions on a ⁶³Cu target and selected for spectroscopic studies by using BaF⁺ molecules formed in the ion source of the GSI on-line mass separator. The -decay of ¹¹⁷Ba was investigated by means of the total absorption -ray spectrometer and a telescope for -delayed particle detection. In the analysis combining the -delayed -ray and proton data the energy window available for -delayed proton emission, the branching ratios for proton transitions to the ¹¹⁶Xe levels and the -feeding of the -ray and proton-emitting ¹¹⁷Cs states were determined. The -strength function for ¹¹⁷Ba derived from the measured -feeding distribution revealed the existence of a broad resonance structure at ¹¹⁷Cs excitation energy of about 4-5 MeV. The results of the -delayed proton studies and -strength measurements are confronted with theoretical predictions.     

Properties of the β-delayed proton decay of 146Ho

The proton-rich isotope 146Ho was produced via the fusion-evaporation reaction ^92Mo （^58Ni, 3p1n）. The β-delayed proton decay of 146Ho was studied by proton-γ coincidence measurements using a He-jet tape transport system. The γ-transitions in ^145Tb following the proton emissions were observed, and the β-delayed proton branching ratios to the final states in the grand-daughter nucleus ^145Tb were determined. According to the relative branching ratios, the ground-state spin of 146Ho has been proposed and the possible configuration discussed.     

Properties of the β-delayed proton decay of ~(146)Ho

The proton-rich isotope 146Ho was produced via the fusion-evaporation reaction 92Mo (58Ni, 3p1n). The β-delayed proton decay of 146Ho was studied by proton-γ coincidence measurements using a He-jet tape transport system. The γ-transitions in 145Tb following the proton emissions were observed, and the β-delayed proton branching ratios to the final states in the grand-daughter nucleus 145Tb were determined. According to the relative branching ratios, the ground-state spin of 146Ho has been proposed and the possible configuration discussed.     

The β-delayed one- and two-proton emission of 27S

In an experiment performed at the GANIL LISE3 facility, radioactive ²⁷S isotopes have been produced by projectile fragmentation of a 95 AMeV ³⁶Ar primary beam. After selection by means of the LISE3 separator, the isotope of interest was implanted in a silicon-detector telescope where its half-life ( T _1/2 = 15.5(15) ms) and its main decay branches were measured. Received: 19 October 2001 / Accepted: 5 December 2001     

Test of the heavy quark–light diquark approximation for baryons with a heavy quark

We check a commonly used approximation in which a baryon with a heavy quark is described as a heavy quark–light diquark system. The heavy quark influences the diquark internal motion reducing the average distance between the two light quarks. Besides, we show how the average distance between the heavy quark and any of the light quarks, and that between the heavy quark and the center of mass of the light diquark, are smaller than the distance between the two light quarks, which seems to contradict the heavy quark–light diquark picture. This latter result is in agreement with expectations from QCD sum rules and lattice QCD calculations. Our results also show that the diquark approximations produces larger masses than the ones obtained in a full calculation.     

On the accuracy of valence–shell computations for heavy and super–heavy elements

Recent atomic computations on the (super–) heavy elements have raised the expectation that their low–lying excitation and ionization energies can be calculated with an accuracy of a few hundredth of an eV and, hence, that such computations might help in the identification of new lines. For most many–electron atoms, however, the higher–order relativistic and quantum electrodynamical (QED) effects are included so far only in a rather approximate form. Using different model computations for the neutral and weakly ionized ytterbium (Z = 70) and nobelium atoms (Z = 102), it is shown here that QED effects alone may lead to an uncertainty of 20–50 meV for the excitation energies of all super–heavy elements, and that even for highly–correlated wave functions the theoretical predictions are presently not more accurate than about 0.1 eV. Moreover, in order to support forthcoming spectroscopic measurements on the elements beyond Z = 100, detailed computations have been carried out for the two low–lying 1S0 - 1,3P1o excitation energies of nobelium by using systematically enlarged multiconfiguration Dirac–Fock wave functions.     

β-decay studies of the neutron-rich 18,21N isotopes

The β-decay studies of neutron-rich ^18,21N isotopes have been performed using β-n, β-γ, and β-n-γ coincidence methods. The ^18,21N ions were produced by the fragmentation of the ²²Ne and ²⁶Mg beams, respectively, on a thick beryllium target. The time of flight of the emitted neutrons following the β-decay of ^18,21N was measured by a neutron detector system with wide energy detection range and low-energy detection threshold. In addition, several clover germanium detectors were used to detect the β-delayed γ-rays. The half-lives of the β-decays of ¹⁸N and ²¹N were determined to be (619±2) ms and (82.9±7.5) ms, respectively. Several new β-delayed neutron groups were observed with a total branching ratio of (6.98±1.46)% and (90.5±4.2)% for ¹⁸N and ²¹N, respectively. The level schemes of ¹⁸O and ²¹O were deduced. The experimental Gamow-Teller β-decay strengths of ¹⁸N and ²¹N to these levels were compared with the shell model calculations.     

New Results of β-delayed α Decay of 20Na

The β-delayed α decay of ²⁰Na was restudied using neon target bombarded by 25 and 35.5 MeV protons.In this experiment a newly developed particle-telescope system including a gas ionization chamber as a thin ΔE detector was used. Two new peaks,one at 0.695 MeV and other at 0.907 MeV, were observed for the first time in the charged particle spectra. Based on particle identification, branching ratio and life time measurement,it is believed that these new peaks result from β-delayed low energy αdecay of ²⁰Na.The decay scheme and the low-lying level scheme of 20Na were thus revised.Its possible astrophysical implications were discussed.     

Corrections to Kramers’ formula for the fission rate of excited nuclei

A Kramers?? formulas with corrections of the first and second infinitesimal orders, R _F and R _S , are derived from the integral Kramers?? formula. In these corrections, we consider higher derivatives of the potential and the distance between the saddle and scission points in R _F . The rates R _F and R _S are compared with the results of dynamic simulations R _D . It is shown that R _F and R _I agree with R _D equally well. The calculations are performed for different forms of the potential. Although the corrections are derived for the overdamping mode they can be used for the case of medium friction.     

Optical polarization of neutron-rich sodium isotopes and β-NMR measurements of quadrupole moments

The nuclear quadrupole moments of neutron-rich sodium isotopes are being investigated with the help of in-beam polarization by optical pumping in combination with-NMR techniques. First measurements have yielded the quadrupole splittings of NMR signals in the lattice of LiNbO₃ for the isotopes²⁶Na,²⁷Na and²⁸Na. Interaction constants and ratios of the electric quadrupole moments are derived. In view of future experiments,-decay asymmetries for the sequence of isotopes up to theN=20 neutron shell closure,^26–31Na, have been measured.     

Search for exotic decay modes of the ϒ(1S)

A search for two exotic decay modes of the ϒ(1S) meson has been made using the ARGUS detector at the e⁺e⁻ storage ring DORIS II at DESY. The first was ϒ(1S) decays into weakly interacting particles, for which an upper limit of 2.3% at the 90% CL on the branching ratio is found. The second mode was ϒ(1S)→γa, where a is a neutral particle with mass less than 1.5GeV/c², that either decays inside the detector or is sufficiently long-lived to escape detection. For short-lived particles a decaying into an e⁺e⁻ pair, upper limits on the BR(ϒ(1S)→γa) of 3.1 × 10⁻ at the 90% CL are found. The upper limit without restriction on the lifetime of a is found to be 1.3 × 10⁻³. Limits for other decay modes are also given.     

Search for the doubly charmed baryon ■

A search for the doubly charmed baryon Ξ_(cc)~+ is performed through its decay to the Λ_c~+K~-π~+ final state, using proton-proton collision data collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 Te V. The data correspond to a total integrated luminosity of 9 fb~(-1). No significant signal is observed in the mass range from 3.4 to 3.8 Ge V/c~2. Upper limits are set at 95% credibility level on the ratio of the Ξ_(cc)~+ production cross-section times the branching fraction to that of Λ_c~+ and Ξ_(cc)~(++) baryons. The limits are determined as functions of the Ξ_(cc)~+ mass for different lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and the transverse momentum range from 4 to 15 GeV/c.     

Bounds for the limiting variance of the “heavy particle” inR 1

We consider a one-dimensional system consisting of a tagged particle of massM surrounded by a gas of unit-mass hard-point particles in thermal equilibrium. Denoting byQ _t the displacement of the tagged particle, we give lower and upper bounds — independent ofM — for. It results from the proof that the correct nontrivial norming ofQ _t — if any — is .Work supported by the Central Research Found of the Hungarian Academy of Sciences (Grant No. 476/82)