Fine structure in the α decay of202Rn

The α decay of mass-separated²⁰²Rn is studied at the ISOLDE separator. Time sequential α-singles spectra together with α-X-t and α-e-t coincidence events are collected. Fine structure in the α decay is observed and feeding of a low-lying 0 ₂ ⁺ state at 816 keV in¹⁹⁸Po is evidenced. This 0⁺ state can be interpreted as the bandhead of an intruder-state based deformed band, coexisting with the spherical groundstate band. Mixing between normal and intruder states is discussed. A preliminary α-decay study of²⁰⁰Rn did not yet reveal any fine structure.     

Polarization bremsstrahlung in α decay

A mechanism of formation of electromagnetic radiation that accompanies α decay and is associated with the emission of photons by electrons of atomic shells due to the scattering of α particles by these atoms (polarization bremsstrahlung) is proposed. It is shown that, when the photon energy is no higher than the energy of K electrons of an atom, polarization bremsstrahlung makes a significant contribution to the bremsstrahlung in α decay.     

Investigation of the antineutrino angular distribution in experiments on the β decay of polarized neutrons

Since the emission of γ grays unavoidably accompanies β decay, the final state after the β decay of a neutron includes a photon along with a proton, an electron, and an antineutrino, i.e., four particles, rather than three. Therefore, when only the electron and proton momenta are detected and the γ-ray momentum is not detected in an experiment, the antineutrino momentum cannot be uniquely reconstructed, and only its mean value over a γ-ray momentum distribution determined from corresponding calculations can be considered. The γ grays are significant for finding the asymmetry parameter B of the antineutrino angular distribution from experiments on the β decay of polarized neutrons, where the electron momentum p directed along the x axis and the projection of the proton momentum P _x onto the x axis are detected, and the neutron polarization vector ξ is parallel or antiparallel to x. Since the γ rays are not detected in such experiments, the antineutrino kinematics are not uniquely specified by the observables p and P _x and can be reconstructed only on the average, so that the antineutrino momentum distribution averaged over a γ-ray momentum distribution is considered. Thus, the exact value of B cannot be obtained from these experiments, but the true value of B can be estimated on the average by considering the mean (most likely) value 〈B〉 and the dispersion (rms deviation) ΔB. The unavoidable uncertainty in the estimate of B amounts to several percent and is thus significant for present-day experiments, which are intended to obtain the value of B to a very high accuracy of ∼ (0.1–1)%. If electromagnetic interactions are taken into account, measurements of the electron and proton momentum distributions can also be used to obtain g _A, i.e., the axial β-decay amplitude, to high accuracy. Zh. éksp. Teor. Fiz. 116, 1505–1522 (November 1999)     

To the question of electron angular distribution at Μ-meson decay

The paper describes the measurement of the angular distribution of electrons at the decay of-mesons in a longitudinal magnetic field. The results of measurement indicate that electrons are emitted with preference not only in the direction parallel to the spin orientation of -meson but also in the antiparallel direction in disagreement with the angular distribution predicted by theory.

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The authors thank N. V. Rabin for irradiating the nuclear emulsions, Prof. Dr. V. Petrílka and M. Suk for mediating relations with the Joint Institute of Nuclear Research in Dubna and Dr. J. Pernegr for valuable discussions and remarks. Thanks also go to H. Koutová and H. Slámová for carefully scanning the nuclear emulsions.     

Quantum beats in the 3γ annihilation decay of Positronium observed by perturbed angular distribution

We have applied conventional Time Differential Perturbed Angular Correlation (TDPAC) method to observe the anisotropy oscillations in the 3γ annihilation decay of polarized Positronium in a weak magnetic field. The effect, as predicted theoretically and experimentally demonstrated by Barishevsky et al. [V.G. Barishevsky, O.N. Metelitsa, V.V. Tikhomirov, Oscillations of the positronium decay γ-quantum angular distribution in a magnetic field, J. Phys. B: At. Mol. Opt. Phys.22 (1989) 2835], is induced by the coherent admixture of the m = 0 states of ortho-Positronium (o-Ps) and para-Positronium (p-Ps) in interaction with the magnetic field.The following experimental characteristics are to be considered:

(i)

the oscillation frequency corresponds to the difference in energy of the Ps atom levels in magnetic field and is proportional with H²;

(ii)

in a fixed geometry the modulation depth (oscillations amplitude) depends on the mean positron polarization;

(iii)

privileged angles of the polarization vector, magnetic field and detectors are required for optimizing the observed oscillations amplitude.

The normalized difference spectrum function (R(t)) obtained from time spectra measured in vacuum and in different gaseous atmospheres (Ar, H₂, N₂) have the oscillations amplitude constant and we conclude that the Ps atoms are not fully thermalized over a time interval of about 400 ns.The R(t) functions obtained for o-Ps annihilation decays, in dry air or Ar-O mixture, have the oscillations amplitude time dependent due, probably, to the paramagnetism of the Oxygen molecules.     

Half-lives and fine structure for the α decay of deformed even-even nuclei

The α-decay properties of well-deformed even-even nuclei are systematically calculated within the multichannel cluster model (MCCM). Instead of working in the WKB framework, the quasibound solution to the coupled Schro¨dinger equation is presented with outgoing wave boundary conditions, and the coupling potential is taken into full account in terms of the general quantum theories. The calculated α-decay half-lives are found to agree well with the experimental data with a mean factor of less than 2. The fine...     

Effects of shell correction on α decay properties

The shell correction effects on the α decay properties of heavy and superheavy nuclei have been studied in a macroscopic-microscopic manner. The macroscopic part is constructed from the generalized liquid drop model(GLDM), whereas the microscopic part, namely, the shell correction energy, brings about certain effects on the potential barriers and half-lives under a WKB approximation, which is emphasized in this work. The results show that the shell effects play a significant role in the estimation of the α decay half-lives within the actinide region.Predictions of the α decay half-lives are then generated for superheavy nuclei, which will provide useful information for future experiments.     

On the possibility of accelerating α decay: To the problem of disposing of radioactive nuclear waste

A number of experimental tests of the recent information on the possibility of accelerating α decay by implanting α-emitting isotopes into metals with their further cooling to a low temperature were performed using the nuclear orientation technique. The measurement data on half-lives for ²⁵³Es in Fe at 4 K and 50 mK, ²²⁴Rn, ²²⁵Ra, and ²²⁷Ac in Fe at temperatures down to 25 mK, and ²²¹Fr in nonmetal (Si) and metal (Au) at 4 K and ∼20 mK are reviewed. In all cases, no change in half-life was found within an accuracy of 1%.     

Excitation of strontium atom in electron–atom collisions

The excitation of ³D levels of strontium atom by slow monoenergetic electrons has been studied experimentally. Thirty six excitation cross-sections were measured at 30-eV electron energy. Optical excitation functions for most of the transitions were recorded in the 0–200-eV electron-energy range. The excitation cross-section as a function of the principal quantum number has been found to correspond to a power law for all ³D series.     

Estimating the entropy of liquids from atom–atom radial distribution functions: silica,beryllium fluoride and water

Molecular dynamics simulations of water, liquid beryllium fluoride and silica melt are used to study the accuracy with which the entropy of ionic and molecular liquids can be estimated from atom–atom radial distribution function data. The pair correlation entropy is demonstrated to be sufficiently accurate that the density–temperature regime of anomalous behaviour as well as the strength of the entropy anomaly can be predicted reliably for both ionic melts as well as different rigid-body pair potentials for water. Errors in the total thermodynamic entropy for ionic melts due to the pair correlation approximation are of the order of 10% or less for most state points, but can be significantly larger in the anomalous regime at very low temperatures. In the case of water, the rigid-body constraints result in larger errors in the pair correlation approximation, between 20 and 30%, for most state points. Comparison of the excess entropy, S _e, of ionic melts with the pair correlation entropy, S ₂, shows that the temperature dependence of S _e is well described by T ^?2/5 scaling across both the normal and anomalous regimes, unlike in the case of S ₂. The residual multiparticle entropy, ΔS = S _e ? S ₂, shows a strong negative correlation with tetrahedral order in the anomalous regime.     

Spin precession of Po in Ni detected by In-beam conversion electrons and the magnetic moment of the 15− isomer in204Po

The magnetic moment of the 11 ns, 15^? isomer in²⁰⁴Po and the internal magnetic field of Po in Ni were determined in an in-beam recoil implantation experiment. Time-differential perturbed angular distributions of conversion electrons were measured in an iron-free orange spectrometer. The magnetic hyperfine field of ¦H(PoNi)¦=555(22) kOe is in agreement with the systematic trend for the 6p elements. Theg-factorg(15^?) =0.41(2) is discussed, together with theB (E2, 15^?→13^?). within the shell model.     

Calculation of α decay half-lives for superheavy elements using the double folding model＊

α decay half-lives of some new synthesized superheavy elements, possibly synthesized superheavy elements and decay products are calculated theoretically within the WKB approximation by using microscopic α-nucleus interaction potentials. These nuclear potentials between the α particle and daughter nuclei are obtained by using the double folding integral of the matter density distribution of the α particle and daughter nuclei with a density-dependent effective nucleon-nucleon interaction, in which the zero-range exchange term is supplemented. The calculated α decay half-lives are compared with those of the different models and experimental data. It is shown that the present calculation successfully provides the half-lives of the observed decays for some new superheavy elements and therefore gives reliable predictions for α decay of possibly synthesized superheavy elements in future experiments.     

Analyzing Affect of Image Charge in Space Charge Effect

There is an increasing requirement of high injection current and highly charged ion beams for accelerators at many laboratories, such as CERN, GSI, GANIL and IMP, with the development of super-conducting ECR source in recent.years. In this case, the space charge effect becomes a major concern when the beam current is as high as tens of mA. In fact, the faradic field induced by the image charges will be come into the metallic surfaces while the beams are transported in a vacuum tube or in between two plates. In order to ensure studying the space charge effect in reason, it is necessary to investigate the effect from such a field.     

Possible α decay chains from isotopes of superheavy element 120

We investigate the α decay half-lives of even-Z superheavy nuclei up to Z = 120 by the modified two-potential approach. Based on the density-dependent cluster model, the α daughter potential is obtained from the double folding integral, taking into account the effect of nuclear deformation. A reasonable formula related with the fissility parameter and the fission barrier height, is applied to evaluate the spontaneous fission half-lives of the studied nuclei. After the performance of the competition between α decay and spontaneous fission, possible α decay chains are proposed in pursuit of being useful for the future experiment.     

The α stable distribution model in ocean ambient noise

In view of the non-Gaussian ocean ambient noise,the a stable distribution is applied to the statistical modeling.Firstly,the correspondence between the model characteristic exponent and the sample kurtosis is established based on the similarity in physical meaning of the two.And the kurtosis of the sum of two independent random variables is deduced,and this can be used to explain the kurtosis characteristic of interfered background noise.Secondly,some numerical simulations are made to model samples with higher and lower kurtosis than three,the results show good adaptability in leptokurtic distribution.Finally,three typical samples are taken under quiet environment,air gun interference and ship interference environment.Their Gaussianities are judged by the quantile-quantile plot.And then the four parameters of the model are estimated by the quantile based method and characteristic function based method jointly of the three categories.The statistical law of the characteristic exponent is emphatically analyzed:in the quiet environment,the characteristic exponent is close to 2.0,which is the value of Gaussian model;but for the air gun data,it is between 1.2 and 1.7,which means leptokurtic;and under the interference of the sailing ship,the value of a is related to the ship speed and the distance from the ship source to the receiving location,the more significant the ship impact is,the smaller the characteristic exponent is,which is consistent with the variation of kurtosis of disturbing noise in physical meaning.The results of the model verification show that the a stable distribution behaves well under the above three kinds of trial conditions,especially for the air gun environment which is far superior to the normal distribution.