Nuclear capture of Σ− hyperons

The capture at rest of Σ^? hyperons in emulsion nuclei is studied through Monte-Carlo simulation. The Λ^o trapping probability following Σ^? capture is estimated and compared with the corresponding probability following K^? capture. The difference in the two probabilities is explained in terms of the dissimilar energy distributions of the Λ^o hyperons produced from the captures of Σ^? hyperons and K^? mesons. Information obtained on the features of the Σ^? capture star and on the characteristics of the hyperfragments produced from Σ^? captures is presented.     

Classical trajectory study of alignment effects in the capture process: He2+-Li1(2pΣ,2pΠ) collisions

A simulation method (CTMC) has been used to investigate the selectivity and alignment effects on the capture process in He²⁺-Li¹(2pΣ,2pΠ) collisions. The anomalous results of the experiment (∼ 50% error) as well as the AO calculation of Gieler et al. in case of capture into He II (n = 4) from the initial Li¹ (2pΣ) state are not found in the present work. Relative velocity and spatial overlap together mainly control the capture process in ion-atom collisions. The n-distribution of the final capture state is also presented.     

Disappearance of the hindrance ofE1-transition involving loosely bound nucleon

AlthoughE1-transition between levels below giant dipole resonance are almost always strongly hindered, there are several exceptions as observed in¹¹Be and¹³C. These fastE1-transitions are studied by adopting a simple “cutoff” model, in which the behavior of single particle wave functions only outside of the nucleus contribute. The results are compared with the particle-GDR coupling model. The reason why theE1-hindrance disappears is explained. We also investigate the direct capture of thermal neutron by⁴⁰Ca andN=82 nuclei, where similar situation is observed. Further, the implication of the direct capture in multiple neutron capture is discussed.     

Alignment and orientation of charge exchange excitation with polarized projectiles

The alignment and orientation of hydrogenlike excited atoms produced by electron capture in collisions of spin polarized bare atomic nuclei with atomic targets is described in terms of Stokes parameters. TheH(2p)^* excited state formed by spin polarized protons making charge exchange collisions on atomic targets is dealt with extensively for the purpose of illustration. The linear polarization and circular polarization of resulting Lyman-α when polarized protons make electron capture collision on atomic hydrogen target are plotted to compare observable aspects of different theories of electron capture.     

One-electron capture in collisions of fast ions with atoms

The properties of one-electron charge transfer between ions and atomsB ^Z++A →B ^(Z?1)+ A ⁺ are studied at relative velocities of the colliding particles higher than target electron velocities. Calculations of partial and total cross sections in collisions of protons and multiply-charged ions with neutral atoms are performed and compared with experimental data. The universal curve for the capture of the targetK- andL-electrons is given. In all cases at sufficiently high collision energies the electron capture from outer shells decreases and the capture of electrons from inner shells of the target atom becomes predominant.     

Electron capture and loss in electron swarm experiments

The effect of capture and subsequent loss of electrons upon their time distribution in time-of-flight electron swarm experiments has been investigated. It is found that if the mean release time of captured electrons is much smaller than both the drift time and the time width of the electron swarm distribution, the effective diffusion coefficientD′ is given approximately by whereD is the electron diffusion coefficient in the absence of capture,w is the drift velocity, γ is the inverse release time, β is the inverse capture time and ζ=ψ/β+γ It is shown that measurements ofw andD by the steady state Townsend method should be unaffected by temporary capture processes.     

The58Ni(12C,γ)70Se reaction

The⁵⁸Ni(¹²C,γ)⁷⁰Se capture reaction was studied with beam energies ofE _12C=30 to 42 MeV. The capture events were identified by means of the residual activity produced in the reaction. At a beam energy of 38 MeV the capture cross section has been determined to 1.5±0.7 μb. AtE _12C=30, 34 und 42 MeV we established upper limits of 0.3, 0.5 and 0.6 μb, respectively. The experimental results are compared with a statistical model calculation.     

Induced electron capture in field-assisted energetic atomic collisions

We present model considerations for the process of the electron capture in energetic nonrelativstic collisions of light atomic particles in the presence of a relatively weak low-frequency external electromagnetic field. The field is treated as an elliptically polarized quantum single-mode field. Establishing validity of the dipole approximation to the electron transfer (where the total momentum of all emitted or absorbed photons can be well above the typical inneratomic momenta of an electron in the initial and final states) and neglecting the Doppler and aberration effects, we give a fully nonrelativistic treatment for the field-assisted collisions and show that the capture cross section is invariant with respect to the Galilean transformations. The model consideration suggests that the field can substantially influence the capture dynamics and considerably change the capture cross section compared to the field-free collisions. This is especially the case if the “resonance” conditions nω≈±v ²/2 are satisfied, with nω being the energy transferred to or absorbed from the electromagnetic field and v the collision velocity.     

The19F(n, γ)20F reaction

The gamma-ray spectrum emitted following thermal neutron capture in¹⁹F has been studied with curved crystal and Ge(Li) spectrometers. From the 109 transitions assigned to²⁰F, 85 have been placed in a level scheme containing 26 levels. An average gammaray multiplicity of 2.8 gammas per neutron capture was observed. The neutron binding energy was found to be 6601.33(14) keV. The experimental level scheme is compared to rotational model predictions. In addition it is shown that the decay of the capture state is non-statistical and that there is a strong correlation between the strengths of excitation of levels by the (n, γ) and (d, p) reactions. Calculations of the partial cross-sections using the direct capture theory of Lane and Lynn give order of magnitude agreement with experiment.     

Nuclear γ-rays produced by π mesons

Energies and yields of nuclearγ-rays accompanyingπ-mesic X-ray spectra have been measured. Theγ-lines arise either in nuclei of the atomic number of the target, or in nuclei of smaller atomic numbers. The first kind ofγ-rays are due mainly to inelastically scattered neutrons produced by pion capture in another target nucleus (secondary process), whilst the second kind are due to ordinary nuclear capture (primary process).     

K-Einfang der Silberisotope Ag110 und Ag108

K-electron capture of Ag¹¹⁰ has been observed for the first time using scintillation spectrometers in anticoincidence technique. The ratio of electron capture toβ ^?-decay was determined to (0.3±0.06)%.K-electron capture of Ag¹⁰⁸ has been remeasured with better accuracy, yielding a relative frequency of (1.73±0.12)% for the capture transition to the ground state of Pd¹⁰⁸.     

Statistical theory of nuclear neutrino capture: (II). Inclusion of first-forbidden transitions

The statistical theory of nuclear neutrino capture is extended to include first-forbidden transitions. A comparison with the theory of Bahcall and Frautschi is made. It is found that the present theory predicts neutrino capture cross sections which are smaller than those of Bahcall and Frautschi by a factor 2–3 for neutrino energies less than 50 MeV when first-forbidden transitions are dominant. Calculation of the cross section is made for the process in which ³⁷Cl nuclei capture electron neutrinos that are emitted in muon decay. The present calculation gives a cross section which is around one half of that of Donnelly and Haxton. Finally the contributions of the highly excited states in ³⁷Ar to the neutrino capture cross section are evaluated. It is shown that the contributions from the highly excited states (E > 6.02 MeV) to the neutrino capture cross section amount to 60% for E_v = 50 MeV.     

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on ⁵⁶Fe and ⁶⁰Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.     

Emission of neutrons following muon capture in heavy nuclei

The energy and multiplicity distribution of neutrons following muon capture in heavy nuclei is investigated. High energy neutrons (E > 10 MeV) are shown to be emitted directly by the fundamental capture process.     

Capture of ultracold neutrons in thin absorbing films

The dependence of the ultracold neutron capture cross section in targets with a thickness smaller than the neutron wavelength is calculated in the time-dependent quantum theory. It is shown that, for low velocities of neutrons, their capture cross section σ_c～v, i.e., tends to zero as the neutron velocity v tends to zero.     

The influence of the neutron-neutron off-energy-shell interaction on the differential μ−d capture rates

The effects of the neutron-neutron off-energy-shell interaction in muon capture by deuteron give ríse to uncertainties in the total and differential capture rates. It is shown that the uncertainties in the differential spectra can be reduced by accurate measurements of the total rates. Valuable information ona _n can therefore be expected from aμ ^? d kinematically complete experiment.     

Dependence of Coulomb capture ratio and x-ray intensities on the mesonic-particle energy spectrum

The dependence of the per-atom capture ratioA(Z₁, Z₂) and the x-ray intensity pattern on the spectral flux densityn(W) of slow mesonic particles in the target is calculated in closed form with a semiclassical theory. The intensity pattern turns out to vary only slightly withn(W) whereas the capture ratio shows a more pronounced dependence. Numerical results are presented.     

Neutron strength functions of nuclei in the deformed region

P-wave neutron strength functions of 21 nuclei, in the rare-earth and deformed region 138 <A < 202, have been extracted from the average capture cross sections measured using SbBe photoneutrons and activation technique. S-wave neutron contribution is computed using the s-wave resonance parameters available in literature and subtracted out from the measured total capture cross section to yield the p-wave neutron capture contribution from which the p-wave neutron strength functions are extracted. Present results are found to be in general agreement with the values reported in literature. The experimental p-wave neutron strength functions are also compared with the theoretical predictions based on the different versions of the optical model potential, and qualitative agreement is observed with the deformed optical model theory of Buck and Perey. Strong evidence for shell structure effects is also noticed.     

The reaction 23Na(n, γ)24Na studied with unpolarized and polarized thermal neutrons

The γ-radiation produced by thermal neutron capture in a natural Na sample has been investigated. Of the 158 γ-rays ascribed to the ²³Na(n, γ)²⁴Na reaction, 143 have been placed in a ²⁴Na decay scheme accounting for 100(2) % of the total primary strength. The reaction Q-value amounts to 6959.42 ± 0.08 keV. The data resulted in spin assignments for four and spin restrictions for six levels.The circular polarization of 14 γ-rays from the capture of polarized neutrons has been measured. The contribution of the J^π = 2⁺ channel in thermal capture was determined to be below 5 %.     

Analytical model for the magnetophoretic capture of magnetic microspheres in microfluidic devices

Magnetic microspheres are used as mobile substrates in micro-total-analysis systems (μTAS), since the particles can be selectively functionalized to attach different bioconjugates and can be precisely manipulated using external magnetic field gradients. A large number of MEMS-based bio-analytical devices employ magnetophoretic separation as an important step during their operation. An analytical technique is proposed in this paper that describes the magnetophoretic transport of magnetic microspheres under an imposed magnetic field when there is a pressure-driven or electroosmotic flow through a microchannel. Successful magnetophoretic capture occurs if the strength of the field-inducing magnetic dipole exceeds a critical value, or if the particles are larger than a critical size. The magnetophoretic separator performance is characterized in terms of capture efficiency. The analysis shows that the capture efficiency is a function of two independent non-dimensional parameters, λ and γ that in turn involve all the physical design and operating parameters of the microfluidic separator, e.g., the dipole strength, particle size and susceptibility, fluid viscosity and velocity, channel height, and the separation of the dipole. Parametric plots of capture efficiency as function of λ and γ helps in choosing the right design and operation parameter of a practical microfluidic separator for a target level of performance.