Electromagnetic and weak transitions in light nuclei

Recent advances in the study of the p-d radiative and He weak capture processes by our group are presented and discussed. The trinucleon bound and scattering states have been obtained from variational calculations by expanding the corresponding wave functions in terms of correlated hyperspherical harmonic functions. The electromagnetic and weak transition currents include one- and two-body operators. The accuracy achieved in these calculations allows for interesting comparisons with experimental data.Received: 1 November 2002, Published online: 15 July 2003PACS: 21.45.+v Few-body systems - 23.40.-s Beta decay; double beta decay; electron and muon capture - 25.40.Lw Radiative capture     

Study of light proton-rich nuclei by complete kinematics measurements

Light proton-rich bound and unbound nuclei were produced by means of stripping reactions of secondary beams of ²⁰Mg and ¹⁸Ne. The decays of the unbound nuclei ^18,19Na have been measured by detecting their decay products ^17,18Ne and one proton and by performing an invariant-mass reconstruction. For ¹⁸Na, the present work is the first measurement of its decay. As the decay scheme of this nucleus could not be determined, two possible scenarios are proposed and discussed. In addition, the decay of excited states in ¹⁷Ne via two-proton emission was observed. The proton-proton angular distribution is isotropic for the first two-proton-emitting states, whereas higher-lying states seem to decay by a correlated two-proton emission, consistent with a ²He emission pattern for part of the decay strength.Received: 30 September 2003, Revised: 27 November 2003, Published online: 2 June 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 23.40.-s decay; double decay; electron and muon capture - 23.50. + z Decay by proton emissionM. Chartier: Present address: Physics Department, University of Liverpool, Liverpool, UK.M. Fallot: Present address: SUBATECH, Ecole des Mines, BP 20722, F-44307 Nantes Cedex, France.A. Shrivastava: Permanent address: Bhabha Atomic Research Center, Mumbai, India.Experiment performed at the GANIL facility, Caen, France.     

Beta decay of 50Ni

The very neutron-deficient isotope ⁵⁰Ni was produced in fragmentation reactions between a A MeV ⁵⁸Ni beam and a ⁹Be target. For the first time the decay of this nucleus was investigated, leading to the determination of the half-life as 12⁺³_-2 ms and the branching ratio for -delayed proton emission of ( )%.Received: 16 April 2003, Published online: 22 July 2003PACS: 21.10.Tg Properties of nuclei; nuclear energy levels: Lifetimes - 23.40.-s decay; double decay; electron and muon capture - 29.30.Ep Charged-particle spectroscopyR. Grzywacz: Present address : Physics Division, ORNL, Oak Ridge, TN 37831-6371, USA.I. Mukha: Present address : Instituut voor Kern- en Stralingsfysika, University of Leuven, B-3001 Leuven, Belgium.     

Muon Physics Possibilities at a Muon-Neutrino Factory

New intense proton accelerators with above GeV energies and MW beam power, such as they are discussed in connection with neutrino factories, appear to be excellently suited for feeding bright muon sources for low-energy muon science. Muon rates with several orders of magnitude increased flux compared to present facilities will become available. This will allow higher precision in experiments which were statistics limited so far such as searches for rare decays, muonium spectroscopy, muon capture, muon catalyzed fusion, muon decay studies and measurements muon moments and parameters. Novel and most important experiments will become possible. For example a permanent electric dipole moment (edm_μ) of a muon could be searched with by far unprecedented accuracy and with a physics potential well beyond the possibilities of present electron, neutron and nuclear edm searches. Investigations of short lived radioactive nuclei using muonic atom spectroscopy would become feasible. This revised version was published online in August 2006 with corrections to the Cover Date.     

Neutron halos in hypernuclei

Properties of single- Λ and double- Λ hypernuclei for even-N Ca isotopes ranging from the proton dripline to the neutron dripline are studied using the relativistic continuum Hartree-Bogoliubov theory with a zero-range pairing interaction. Compared with ordinary nuclei, the addition of one or two Λ-hyperons lowers the Fermi level. The predicted neutron dripline nuclei are, respectively, ⁷⁵ _ΛCa and ⁷⁶ _2ΛCa, as the additional attractive force provided by the Λ-N interaction shifts nuclei from outside to inside the dripline. Therefore, the last bound hypernuclei have two more neutrons than the corresponding ordinary nuclei. Based on the analysis of two-neutron separation energies, neutron single-particle energy levels, the contribution of continuum and nucleon density distribution, giant halo phenomena due to the pairing correlation, and the contribution from the continuum are suggested to exist in Ca hypernuclei similar to those that appear in ordinary Ca isotopes. Received: 21 October 2002 / Accepted: 11 January 2003 / Published online: 8 April 2003     

Equivalence between local Fermi gas and shell models in inclusive muon capture from nuclei

Motivated by recent studies of inclusive neutrino nucleus processes and muon capture within a correlated local Fermi gas model (LFG), we discuss the relevance of nuclear finite-size effects in these reactions at low energy, in particular for muon capture. To disentangle these effects from others coming from the reaction dynamics we employ here a simple uncorrelated shell model that embodies the typical finite-size content of the problem. The integrated decay widths of muon atoms calculated with this shell model are then compared for several nuclei with those obtained within the uncorrelated LFG, using in both models exactly the same theoretical ingredients and parameters. We find that the two predictions are in quite good agreement, within 1-7%, when the shell model density and the correct energy balance is used as input in the LFG calculation. The present study indicates that, despite the low excitation energies involved in the reaction, integrated inclusive observables, like the total muon capture width, are quite independent of the fine details of the nuclear wave functions. An erratum to this article is available at .     

Fission of muonic 232Th and 239Pu

The neutron emission is studied following the formation of muonic atoms of ²³²Th and ²³⁹Pu. Energy and time distributions are measured. Various processes which contribute to the measured spectra are considered. A collective resonance model of the muon capture is used to calculate the nuclear excitation function. The probability of the radiationless nuclear excitations and the influence of the presence of the bound atomic muon on the fission barrier are discussed. The existing data for the $\text{Γ}{}_{\mathrm{n}}\text{Γ}{}_{\mathrm{f}}$, are analysed. As a result of the analysis the rates of the prompt and delayed fission events (due to the radiationless mu-atomic transitions and the nuclear muon capture, respectively) are deduced from the experimental data to be 0.006/muon and 0.045/muon for ²³²Th and 0.10/muon and 0.49/muon for ²³⁹Pu, respectively. The increase of the fission barrier for muonic atoms is confirmed. The experimental neutron rates can be consistently explained only if it is assumed that in both nucleides the K_α radiationless transitions do not induce fission. The increase of the fission barrier for ²³⁹Pu is hence deduced to be not less than 1.2 MeV. The fate of the atomic muon after the nuclear fission is briefly discussed. Its influence on the interpretation of the present results is found to be small.     

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.     

The beta decay of neutron-deficient rhodium and ruthenium isotopes

The very neutron-deficient ^91,92,93Rh and ^90,91Ru nuclei were produced in ⁵⁸Ni(^36,40Ar, xn yp) fusion-evaporation reactions. Reaction products leaving the target were stopped in a gas cell and selectively laser ionised before mass separation using the LISOL setup. Data were collected in singles and coincidence formats to investigate -delayed and isomeric decays. Ground-state decays of ^91,93 Rh to excited states in the daughter nucleus were identified for the first time and the decay schemes of ^90,91 Ru were expanded. The low-spin isomeric decay of ⁹²Rh was identified for the first time and detail added to the decay scheme of the known high-spin state. Results are discussed in terms of comparison with shell model calculations. -delayed proton activity in the lighter ⁹⁰Rh and ⁸⁹Ru isotopes was also investigated and upper limits for this decay mode in these two nuclei have been included.Received: 26 November 2003, Revised: 12 January 2004, Published online: 10 August 2004PACS: 23.40.-s decay; double decay; electron and muon capture - 21.10.-k Properties of nuclei; nuclear energy levels - 21.10.Tg Lifetimes - 21.60.Cs Shell modelM. Górska: Present address: Gesellschaft für Schwerionenforschung, Planckstrasse 1, D-64291 Darmstadt, Germany.K. Van de Vel: Present address: VITO, IMS, Mol, Belgium.     

Muon capture rates in Se and Cd isotopes

Energy and time spectra of γ rays following nuclear capture of negative muons in natural Se and Cd and isotopically enriched ⁷⁶Se and ¹⁰⁶Cd targets have been measured with HPGe detectors. Total muon lifetimes in Se and Cd isotopes and partial μ capture rates to excited levels of ⁷⁶As and ¹⁰⁶Ag are obtained. These results are necessary for calculation of nuclear matrix elements of the 2β decay of ⁷⁶Ge and ¹⁰⁶Cd respectively.     

Correlation between muonic levels and nuclear structure in muonic atoms

A method that deals with the nucleons and the muon unitedly is employed to investigate the muonic lead, with which the correlation between the muon and nucleus can be studied distinctly. A “kink” appears in the muonic isotope shift at a neutron magic number where the nuclear shell structure plays a key role. This behavior may have very important implications for the experimentally probing the shell structure of the nuclei far away from the β-stable line. We investigate the variations of the nuclear structure due to the interaction with the muon in the muonic atom and find that the nuclear structure remains basically unaltered. Therefore, the muon is a clean and reliable probe for studying the nuclear structure. In addition, a correction that the muon-induced slight change in the proton density distribution in turn shifts the muonic levels is investigated. This correction to muonic level is as important as the Lamb shift and high order vacuum polarization correction, but is larger than anomalous magnetic moment and electron shielding correction.     

Chiral invariant phase space event generator

The Geant4 CHIPS model simulates both decay and nuclear capture of negative muons. In hadron level models the muon is captured by a proton, p(μ,ν_μ)n , and the neutron transfers to the nucleus only 5MeV, which is not enough to split a nucleon from the nucleus, while the measured spectra of emitted nucleons reach 80MeV. In CHIPS, which considers asymptotically free quark-partons, the muon can be captured by a quark, u(μ,ν_μ)d , and transfers 52MeV to the nucleus. This capture mechanism fits the main part of the nucleon spectra, while the high-energy part of the spectra is not fitted. A precise fit of the high-energy part of the nucleon spectra can be made if the muon decay μ → dˉν_μ is taken into account.     

The Muon Science Facility at the JKJ Project

The muon science facility is one of the experimental arenas of the JKJ project, which was recently approved for construction in a period from 2001 to 2006, as well as neutron science, particle and nuclear physics, neutrino physics and nuclear transmutation science. The muon science experimental area is planned to be located in the integrated building of the facility for the materials and life science study. One muon target will be installed upstream of the neutron target in a period of phase 1. The beam line and facility are designed to allow the later installation of a 2nd muon target in a more upstream location. The detailed design for electricity, cooling water, primary proton beam line, one muon target and secondary beam lines (a superconducting solenoid decay muon channel, a dedicated surface muon channel, and an ultra slow muon channel) is underway. In the symposium, a latest status of the muon science facility at JKJ project will be reported. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study of the double-beta-decay of 100 <A$ \leq 150$ nuclei within the QRPA,RQRPA and the SRQRPA formalisms

We have used a self-consistent version of the BCS + RQRPA method for a systematic study of the two-neutrino double-beta-decay of nuclei with . The comparison with other approaches, namely the QRPA and the RQRPA, shows that inclusion of the quasiparticle correlations at the BCS level reduces the ground-state correlations in the particle-particle channel of the proton-neutron interaction. This in turn results in a systematic reduction of the double-beta-decay matrix elements. The effect of the extension of the formalism on the Ikeda sum rule has been discussed.Received: 28 July 2003, Published online: 5 February 2004PACS: 21.60.Jz Hartree-Fock and random-phase approximations - 23.40.Bw Weak-interaction and lepton (including neutrino) aspects - 23.40.Hc Relation with nuclear matrix elements and nuclear structure - 27.60. + j      

Shell-model study of partial muon-capture rates in light nuclei

The nuclear shell model is used to study ordinary muon capture of light nuclei in the p, sd and p-sd shell-model spaces. Several well-established two-body interactions are applied to calculate the involved nuclear matrix elements and the emerging results are compared with each other. The resulting theoretical partial muon-capture rates are compared with experimental data and their stability against different model spaces and interactions studied. The effects of the induced-pseudoscalar strength, C_p, on the capture rates is discussed. The relation between the allowed partial muon capture rates and the Gamow-Teller strength function is stressed.     

Weak interaction rates for Kr and Sr waiting-point nuclei under rp-process conditions

Weak interaction rates are studied in neutron deficient Kr and Sr waiting-point isotopes in ranges of densities and temperatures relevant for the rp process. The nuclear structure is described within a microscopic model (deformed QRPA) that reproduces not only the half-lives but also the Gamow–Teller strength distributions recently measured. The various sensitivities of the decay rates to both density and temperature are discussed. Continuum electron capture is shown to contribute significantly to the weak rates at rp-process conditions.     

Properties of nuclear levels excited by neutron capture γ-rays from cobalt

Neutron capture γ-rays from cobalt have been used to photoexcite nuclear levels in the 5–8 MeV region. The decay properties of the 7491 keV level in ⁵⁵Mn and the 6877 keV level in ¹⁴²Nd were studied in detail. Total and partial radiative widths of nuclear levels in several isotopes were determined using nuclear self-absorption, temperature variation, and absolute scattering cross section measurements. The total radiative widths were found to be of the same magnitude as those of unbound levels populated in neutron resonances. The spins and parities of some resonance levels were determined by carrying out angular distribution and polarization measurements, respectively.     

Corrections to the fluxes of a neutrino factory

To reach their physical goals, future neutrino factories using muon decay aim at an overall flux precision of or better. We analytically study the QED radiative corrections to the neutrino differential distributions from muon decay. Kinematic uncertainties due to the divergence of the muon beam are considered as well. The resulting corrections to the neutrino flux turn out to be of order , safely below the required precision. Received: 10 September 2002 / Revised version: 4 April 2003 / Published online: 2 June 2003 RID="a" ID="a" e-mail: alicia@delta.ft.uam.es RID="b" ID="b" e-mail: mena@delta.ft.uam.es     

Structure of the isovector dipole resonance in neutron-rich <Superscript>60</Superscript><Emphasis Type="Italic">Ca</Emphasis> nuclei and direct decay from pygmy resonance

The structure of the isovector dipole resonance in neutron-rich calcium isotope, ⁶⁰Ca, has been investigated by implementing a careful treatment of the differences of neutron and proton radii in the continuum random phase approximation (RPA). The calculations have taken into account the current estimates of the neutron skin. The estimates of the escape widths for direct neutron decay from the pygmy-dipole resonance (PDR) were shown rather wide, implicating a strong coupling to the continuum. The width of the giant-dipole resonance (GDR) was evaluated, bringing on a detailed discussion about its microscopic structure.Received: 22 September 2003, Revised: 9 February 2004, Published online: 7 September 2004PACS: 21.10.Pc Single-particle levels and strength functions - 21.60.-n Nuclear structure models and methods - 24.30.Cz Giant resonances - 24.30.Gd Other resonances     

μ－ CAPTURE IN NUCLEI

Using the many-body field theory the problems of μ^－ captures in nuclei are systematically studied.Without ajustable parameter,calculations are performed for the total nuclear capture rates of muon for nuclei ranging from light to heavy ones.It shows that the renormalization due to the spin-isospin-dependent strong interaction and the muon binding in the μ^－ atom are essential for a thorough understanding of this type of weak interaction processes.