Collective effects induced by charge-exchange vibrational modes on 0− → 0+ and 2− → 0+ first-forbidden β-decay transitions

The influence of collective effects associated with spin-isospin-dependent modes upon unique first-forbidden β-decay transitions is studied. The ft values for the 0⁻ → 0⁺ and 2⁻ → 0⁺ transitions, in the mass region 70 ≤ A ≤ 214, are calculated and compared with the experimental data.     

A theoretical study of the beta decay transitions 0− → 2+ and 0− → 0+ of the Ho166 nucleus

The theoretical beta decay observables are compared with the experimental values for the 0^? → 2⁺ and 0^? → 2⁺ transitions. By considering a Ho¹⁶⁶ ground state configuration infered from (d, p) reaction experiments, and using all the matrix elements, satisfactory results are obtained for the 0^? → 2⁺ transition. For the 0^? → 2⁺ transition an important cancellation effect has been found which determines the failure of the ξ approximation and an energy dependent spectrum shape.     

The direct decay K+ → π+π0γ and its relation to other weak radiative decays

A model for weak nonleptonic transitions, previously shown to explain K₂⁰ → γγ and other weak radiative K-meson decays, is applied here to K⁺ → π⁺π⁰γ. It reproduces correctly the recently determined dominance of the magnetic transitions and allow to make firm predictions for the related K⁺ → π⁺γγ and K₂⁰ → π⁺π^?γ transitions.     

E0 transitions in 70Ge and shape-coexistence interpretation of even-mass Ge isotopes

The branching ratios of E0 and E2 transitions depopulating the 0⁺₂ and 0⁺₃ states in ⁷⁰Ge have been studied using conversion-electron and γ-ray spectroscopy, as well as a new internal-pair measurement technique. A value of 3.7 (2) ns has been obtained in a remeasurement of the half-life of the 0⁺₂ state. Two new E0 transitions, 0⁺₃ → 0⁺₂ and 0⁺₃ → 0⁺₁, have been observed. A comparison of the measured X(E0/E2), ρ²(E0) and B(E2) values for the excited 0⁺ states in ⁷⁰Ge with the corresponding ¹⁵⁰Sm data supports a recent shape-coexistence interpretation of the even-mass Ge isotopes.     

Electromagnetic decay of excited 0+ states in 64, 66, 68Zn

The E0 and E2 transitions depopulating the excited 0⁺ states in ^64,66,68Zn are investigated. Several methods of γ-ray, conversion-electron and internal-pair spectrometry are employed, including a new time measurement technique. A total of 5 E0 transitions are observed and the monopole strengths ρ²(0_i⁺ → 0_i⁺) for most of them are extracted. The results and the nature of the excited 0⁺ states are discussed in terms of several nuclear models.     

Auger electron emission spectra from foil and gas excited carbon beams

Auger electron emission spectra from 2 MeV C⁺ ion excited by collisions with thin carbon foils and Ne gas are presented. The similarity of qualitative features for the C⁺ → C (foil) and C⁺ → Ne spectra indicates the similarity of ionization mechanisms for beam foil and beam gas excitation. The spectra were normalized to the lowest lying Li-like quartet state (1s 2s 2p)⁴P⁰ in carbon by comparison with time delayed foil excited electron decay-in-flight spectra. Comparison to Hartree-Fock calculated transition energies indicates that transitions in three and four electron carbon ions dominate the prompt spectra.     

Lifetime measurements of excited levels in prompt fission products of 252Cf

Lifetimes in the range 10⁻¹¹ to 10⁻⁹ s of prompt γ rays emitted from the fission fragments of ²⁵²Cf were measured using a recoil distance method. A ²⁵²Cf source was deposited on a stretched Ni foil and placed in a plunger device, the recoil direction of the studied fragments being determined by the detection of the complementary fragment. The lifetime was determined by the change in the non-Doppler-shifted peak intensities of prompt γ rays as a function of the source-plunger distance. The 2⁺ → 0⁺ half-life in ¹¹²Pd and 4⁺ → 0⁺ halflives in ^{104, 106}Mo, ^108,110Ru and ¹⁴²Ba were determined for the first time. Several other measurements of longer lived 2⁺ → 0⁺ transitions were repeated as well. The experimental results are discussed within the framework of the rotational and the IBA-2 models.     

0+ states and E0 and E2 transition rates in even Sn nuclei

0⁺ states and their depopulating E0 and E2 transitions have been studied in ^112–124Sn. Several methods of γ-ray and electron spectrometry have been employed, including special coincidence techniques for lifetime, conversion electron and double Coulomb excitation measurements. For ^114–118Sn the E2 transition probabilities from the first excited 0⁺states (0₂⁺) are about 20 W.u., which is compatible with a vibrational two-phonon character. Also the E0 transition probabilities from these states are within a factor of two from the vibrational values. For the second excited 0⁺ states (0₃⁺) the corresponding E2 and E0 transition probabilities are considerably smaller. A total of 12 E0 transitions have been observed. The 0₃⁺ → 0₂⁺ transition is observed in ^114–120Sn and has in ¹¹⁶Sn a reduced transition probability 1–2 orders of magnitude larger than those of the groundstate E0 transitions, which indicates that the 0₃⁺ and 0₂⁺ states are strong mixtures of components with different 〈r²〉. The 0⁺ states in ¹¹⁶Sn are discussed as possible rotational band heads associated with a deformation β₂ ≈ 0.2. The Coulomb excitation cross section of the 0₂⁺ state is found to be sensitive to an interference term including matrix elements with the 2₂⁺ state. The relative sign of this term has been determined.     

The nature of M1 admixtures in intra-band transitions in the γ-vibrational band

The M1 admixtures in the 74.6 keV (3⁺ → 2⁺), 122.8 KeV (5⁺ → 4⁺) and 146.3keV (6⁺ → 5⁺) transitions in the γ-vibrational band of ¹⁶⁸Er have been determined to be 33 ± 1, 29 ± 2 and 22 ± 6 percent from measurements of L subshell conversion line intensity ratios. It is shown that the M1 admixtures in the intra-band transitions in the γ-band are a general and suprisingly constant feature in deformed rare-earth nuclei. This suggests that they are the result of a collective effect. The theory of Greiner, which takes the diferent deformation of the proton and nuetron cores into account, is found to be able to give a quantitative explanation of the observed M1 admixtures.     

Feasibility study of laser cooling of InF molecule

By employing ab initio quantum chemistry method, we investigate the feasibility of laser cooling InF molecule. Four low-lying electronic states (X¹Σ⁺, C¹Π, ³Π and 2³Π) of InF have been calculated using the multi-reference configuration interaction (MRCI) method. The spin-orbit coupling effects are also taken into account in the electronic structure computation at the MRCI level. The highly diagonal Franck-Condon factors for C¹Π → X¹Σ⁺ transitions are estimated. The radiative lifetime of the C¹Π (v′ = 0) state is about 2.22 ns, which is found to be enough short for rapid laser cooling. Though the cooling wavelength of InF is located in the short-wavelength ultraviolet light (UVC), a frequency quadrupled Ti: sapphire laser (189–235 nm) could be capable of generating laser transition wavelength of InF. Furthermore, the C¹Π → X¹Σ⁺ transitions perhaps can be followed by the B³Π₁ → X¹Σ⁺_0⁺ transitions to attain a lower Doppler temperature. Meanwhile, for achieving quasi-closed transition cycle of InF molecule, we investigate the hyperfine structure of the lowest state X¹Σ⁺. Overall, the present results indicate the possibility of laser cooling InF molecules.     

Techniques to Treat the Continuum Applied to Electromagnetic Transitions in 8Be

Bremsstrahlung emission in collisions between charged nuclei is equivalent to nuclear gamma decay between continuum states. The way the continuum spectrum can be treated is not unique, and efficiency and accuracy of cross section calculations depend on the chosen method. In this work we describe, relate, and compare three different methods in practical calculations of inelastic cross sections, that is, by (i) treating the initial and final states as pure continuum states on the real energy axis, (ii) discretizing the continuum states on the real energy axis with a box boundary condition, and (iii) complex rotation of the hamiltonian (complex scaling method). The electric quadrupole transitions, 2⁺ → 0⁺ and 4⁺ → 2⁺, in α + α scattering are taken as an illustration.     

A study of the de-excitation of primary fission fragments from the neutron-induced fission of 235U

Two separate three-dimensional experiments have been performed in which the energies of coincident fragment pairs and γ-rays or internal conversion electrons, emitted within ≈ 1.6 nsec of the thermal-neutron-induced fission of ²³⁵U, were recorded event by event. The fragment kinetic energies were used for mass identification. The self-consistency of the values of electron energy, γ-ray energy and fragment charge, and its agreement with X-ray selection data, were used to identify the atomic numbers of the fragments. The analysis of the γ-ray and conversion electron spectra resulted in the assignment of many transitions to new isotopes as well as improvement in or confirmation of many assignments from the ²⁵²Cf spontaneous fission data. Limited information on the multipolarities of the transitions in even nuclei is presented. The relative yield of electrons per fragment indicates softness to deformation in the mass region 100–110. Data are presented supporting the assignment of a 193 keV transition as the 2⁺ → 0⁺ transition in ⁹⁸Sr. An examination of the 2⁺ → 0⁺ level systematics of neighbouring even nuclei suggests a transition from vibrational to rotational behaviour in the light fragments between neutron numbers 58 and 60.     

Large charmless yield inB decays and inclusiveB decay puzzles

In recent studies of inclusiveB decays, it has been suggested that eitherB mesons decay much more copiously to final states with no open charm than currently assumed, orB(D ⁰ → K^?π⁺) has to be reduced significantly. This note takes the experimentalB(D ⁰ → K^?π⁺) at its face value and estimatesB(b → no open charm) using complementary methods: one accounts for thec quark inb → c transitions, the other accounts for the c quark inb → ccs transitions. Through cancellation of errors, the average gives our best estimate ofB(b → no open charm), and the difference measures the consistency. The results of the methods are consistent with each other, strongly suggesting a much enhancedB(b → no open charm). This observation indicates that non-perturbative QCD effects are probably causing a sizable fraction of theb → ccs transitions to be seen as charmlessb → s processes, contrary to smaller traditional expectations. This mechanism has generally been overlooked and may explain the existing experimental data within the framework of the standard model. We then briefly discuss implications on baryon production governed byb → ccs processes, rare hadronicB decays and CP violation studies.     

Multipion production in photon-photon collisions

The magnitudes of the matrix elements for γγ→2π⁺2π^?, γγ→π⁺π^?2π⁰, γγ→2π⁺2π^?π⁰, γγ→2π⁺2π^?2π⁰, γγ→3π⁺3π^?3π⁺ and γγ→3π⁺3π^?π⁰are deduced from experimental data as a function of the invariant massW of the system (from 1.0 to 3.0 GeV). It is shown that the data are consistent with a universal energy dependence of the reduced channel amplitudes. A statistical model is shown to describe the peaks in the energy dependence of multipion production in γγ collisions quite well without introducing explicit intermediate resonances.     

Structure of even-even actinides in the interacting boson model and the N = 152 subshell closure

We show that a wide range of deformed actinides can be described in terms of an interacting boson model hamiltonian with three parameters, two of them [including the coefficient of the only SU(3) symmetry breaking term] remaining almost constant over the whole region. In addition to ground γ₁ and β₁ spectra, B(E2:0_g⁺ → 2_g⁺) values are well reproduced with no extra adjustable parameters for nuclei with 136⩽N⩽146, while for nuclei beyond N = 146 an effective boson number has to be considered in order to fit the observed in the B(E2:0⁺_g → 2⁺_g) values, which is due to the presensce of a subshell closure at N = 152. The sensitive dependence of the B(E2:0_g⁺→2_g⁺) values on the effective boson numbers is emphasized. β₁ → ground and β₁ → ground transitions are fitted by breaking the SU(3) symmetry of the E2 transition operator.     

Electromagnetic transition probability from the ground to the first excited 2+ state in138ce

The ratio of the B(E2) values for the 0 ₁ ⁺ → 2 ₁ ⁺ transitions in¹³⁸Ce and¹⁴²Ce was measured by Coulomb excitation with α-particles. From the known value of the transition probability in¹⁴²Ce it results: B(E2,¹³⁸Ce, 0 ₁ ⁺ → 2 ₁ ⁺ =0.45 +/?0.03 e² b ²     

Semi-leptonic weak and electromagnetic interactions in nuclei:: Parity violations in electron scattering and weak neutral currents

A general result for the difference in differential cross sections for electron scattering between any two nuclear levels with incident longitudinally polarized right- and left-handed electrons is derived. This difference must involve the parity-violating weak interaction at least linearly and can be used to study weak neutral currents as pointed out by Feinberg. A V — A structure for the weak neutral currents is assumed with a semi-leptonic current-current interaction, and the electromagnetic interaction is treated in the one-photon-exchange approximation. The result is expressed in terms of the same set of reduced matrix elements of the multipoles of the nuclear currents which govern all electromagnetic and weak transitions between these levels. A previously developed unified analysis of semi-leptonic weak and electromagnetic interactions in nuclei which determines one-body transition densities, including their spin and spatial dependences, through electron scattering provides nuclear transitions to serve as known analyzers in testing the structure of this part of the weak interaction. Examples are given using Weinberg's model of the weak neutral currents. Feinberg's result for elastic scattering from spin-0 nuclei is rederived and two new examples using previously determined one-body densities are discussed : the 1⁺0 → 0⁺1 (3.56 MeV) transition in ⁶Li and the 0⁺0 → 1⁺ 1(15.1 MeV) transition in ¹²C.