S-wave Resonances Research Articles

S-waveresonances in positron-lithium scattering

We have calculated three S-wave resonances in thepositron-lithium system using the stabilization method. For thetarget lithium atom, the interaction between the core and theoutermost active electron is taken care of by a model potential.Hylleraas-type wavefunctions are used to represent thecorrelation effects between the outer electron, the positron andthe core. The resonance energies and widths for two resonances thatare believed to be associated with and below the Ps (n = 2)threshold, and one resonance below the Ps (n = 3) threshold arereported.

Low lying S=−1 excited baryons and chiral symmetry

The s-wave meson-baryon interaction in the $S = -1$ sector is studied by means of coupled-channels, using the lowest-order chiral Lagrangian and the N/D method to implement unitarity. The loops are regularized using dimensional renormalization. In addition to the previously studied $\Lambda (1405)$, employing this chiral approach leads to the dynamical generation of two more s-wave hyperon resonances, the $\Lambda(1670)$ and $\Sigma(1620)$ states. We make comparisons with experimental data and look for poles in the complex plane obtaining the couplings of the resonances to the different final states. This allows us to identify the $\Lambda (1405)$ and the $\Lambda(1670)$ resonances with $\bar{K}N$ and $K\Xi$ quasibound states, respectively.

Differential cross sections for elastic and inelastic positronium–hydrogen-atom scattering

We report results of differential cross sections for elastic scattering, target-elastic Ps excitations and target-inelastic excitation of hydrogen in a five-state coupled-channel model allowing for Ps(1s)H(2s,2p) and Ps(2s,2p)H(1s) excitations using a recently proposed time-reversal-symmetric regularized electron-exchange model potential. The present model yields a singlet Ps-H S-wave resonance at 4.01 eV of width 0.15 eV and a P-wave resonance at 5.08 eV of width 0.004 eV. We also study the effect of the inclusion of the excited Ps and H states on the convergence of the coupled-channel scheme.

APS Honors Physics Contributions

APS Honors Physics Contributions

The astrophysically important3+state in18Neand the17F(p,γ)18Nestellar rate

Knowledge of the ${}^{17}\mathrm{F}(p,\ensuremath{\gamma}{)}^{18}\mathrm{Ne}$ reaction rate is important for understanding stellar explosions, but it was uncertain because the properties of an expected but previously unobserved ${3}^{+}$ state in ${}^{18}\mathrm{Ne}$ were not known. This state would provide a strong s-wave resonance for the ${}^{17}\mathrm{F}+p$ system and, depending on its excitation energy, could dominate the stellar reaction rate at temperatures above 0.2 GK. We have observed this missing ${3}^{+}$ state by measuring the ${}^{1}\mathrm{H}{(}^{17}\mathrm{F}{,p)}^{17}\mathrm{F}$ excitation function with a radioactive ${}^{17}\mathrm{F}$ beam at the ORNL Holifield Radioactive Ion Beam Facility (HRIBF). We find that the state lies at a center-of-mass energy of ${E}_{r}=599.8\ifmmode\pm\else\textpm\fi{}{1.5}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{2.0}_{\mathrm{sys}}$ keV ${(E}_{x}=4523.7\ifmmode\pm\else\textpm\fi{}2.9\mathrm{keV})$ and has a width of $\ensuremath{\Gamma}=18\ifmmode\pm\else\textpm\fi{}{2}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{1}_{\mathrm{sys}}\mathrm{keV}.$ The measured properties of the resonance are only consistent with a ${J}^{\ensuremath{\pi}}{=3}^{+}$ assignment.

Kinematically complete measurement of the1H(18F,p)18Fexcitation function for the astrophysically important 7.08-MeV state in19Ne

Knowledge of the astrophysical {sup 18}F(p,{alpha}){sup 15}O rate is important for understanding gamma-ray emission from novae and heavy-element production in x-ray bursts. A state with E{sub x}{approx_equal}7.08 MeV in {sup 19}Ne provides an s-wave resonance and, depending on its properties, could dominate the {sup 18}F(p,{alpha}){sup 15}O rate. By measuring a kinematically complete {sup 1}H({sup 18}F,p){sup 18}F excitation function with a radioactive {sup 18}F beam at the ORNL Holifield Radioactive Ion Beam Facility, we find that the {sup 19}Ne state lies at a center-of-mass energy of 665.3{+-}1.7 keV (E{sub x}=7077{+-}2 keV), has a total width of 38.5{+-}3.4 keV, and a proton partial-width of 15.8{+-}1.6 keV.

Level-density parameters in the back-shifted Fermi gas model

The parameters a and δeff appearing in the back-shifted Fermi gas model are determined for about 3000 nuclei on the basis of modern estimated experimental data and the proposed systematics. For 272 of these nuclei, the parameters are deduced from experimental data on the cumulative numbers of low-lying levels and on mean spacings between S-wave neutron resonances at the neutron binding energy in the nuclei. For 952 nuclei, the parameter δeff is calculated by using the cumulative numbers of low-lying levels and values of the parameter a that were obtained via an interpolation from the points corresponding to the aforementioned 272 nuclei. For the remaining nuclei, the parameters a and δeff are obtained on the basis of the proposed systematics. An expression is constructed for taking into account the damping of shell effects with increasing excitation energy of nuclei. The results are compared with those from other studies.

Semiempirical formula for describing neutron shell effects in the resonance-level density for s-wave resonances

A simple formula for the resonance-level density for s-wave resonances is found by fitting the assumed function of the number of neutrons to numerous experimental data. To describe shell effects, demonstrated as strong decreases in experimental data at the magic numbers of neutrons, the dependence of the assumed function on the “complexity” of a compound nucleus is introduced. The resulting function describes quite well the character of changes in the resonance-level density with the number of neutrons, including the regions of the magic numbers.

Measurement of neutron capture on50Tiat thermonuclear energies

At the Karlsruhe and T\"ubingen 3.75 MV Van de Graaff accelerators the thermonuclear ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}(5.8\mathrm{min})$ cross section was measured by the fast cyclic activation technique via the 320.852 and 928.65 keV $\ensuremath{\gamma}$-ray lines of the ${}^{51}\mathrm{Ti}$ decay. Metallic Ti samples of natural isotopic composition and samples of ${\mathrm{TiO}}_{2}$ enriched in ${}^{50}\mathrm{Ti}$ by 67.53% were irradiated between two gold foils which served as capture standards. The capture cross section was measured at the neutron energies of 25, 30, 52, and 145 keV, respectively. The direct capture cross section was determined to be $0.387\ifmmode\pm\else\textpm\fi{}0.011\mathrm{mb}$ at 30 keV. We found evidence for a bound state s-wave resonance with an estimated radiative width of 0.34 eV which destructively interferes with direct capture. The strength of a suggested s-wave resonance at 146.8 keV was determined. The present data served to calculate, in addition to the directly measured Maxwellian averaged capture cross sections at 25 and 52 keV, an improved stellar ${}^{50}\mathrm{Ti}(n,\ensuremath{\gamma}{)}^{51}\mathrm{Ti}$ rate in the thermonuclear energy region from 1 to 250 keV. The new stellar rate leads at low temperatures to much higher values than the previously recommended rate; e.g., at $kT=8\mathrm{keV}$ the increase amounts to about 50%. The new reaction rate therefore reduces the abundance of ${}^{50}\mathrm{Ti}$ due to s processing in asymptotic giant branch stars.

A rational approximation to Reich–Moore collision matrix of non-fissile nuclides

The cross sections of many important nuclides are represented in Reich–Moore (RM) formalism in the recent American Evaluated Nuclear Data file, ENDF/B-VI. Processing of cross sections with RM resonance parameters is much more difficult than the other multilevel formalisms such as MLBW and Adler–Adler. In this paper, we derive a rational approximation to the RM collision matrix in the vicinity of a resonance. This simplifies the cross section processing. The energy range of the validity of this approximation in the vicinity of a resonance is also derived. Choosing Ni 58 as an example, results of our approximation for a non-fissile nuclide are given for two typical s-wave resonances. Our rational approximation method is found to work with good accuracies in the vicinity of resonances.

Ground state andS-wave autodissociating resonant states of positronium hydride

The fully correlated calculations in Hylleraas coordinates for the nonrelativistic ground-state energy and the first three S-wave resonances of positronium hydride are reported. For the resonances, the positions and widths are determined using the method of complex-coordinate rotation. Our results significantly improve previous work. For example, the ground-state energy is about five parts in 10 in accuracy, the lowest variational upper bound reported to date. A comparison with other previous work is made. @S1050-2947~99!04204-3#

The deformation effect and time-reversal violation in neutron resonances

We describe how the deformation effect in nuclear spin aligned can be used to identify small d-wave admixtures in predominantly s-wave resonances. Using standard angular momentum penetrabilities, we estimate deformation effect cross sections for resonances below 500 eV. If d-wave admixtures are identified in a number of s-wave resonances, then a test for a parity-conserving, time-reversal violating term in the neutron-nucleus forward scattering amplitude is possible.

The spectral function of the rho meson in nuclear matter

We calculate the modification of a rho meson in nuclear matter through its coupling to resonance-hole states. Starting from a recently proposed model, we include all four star resonances up to 1.9 GeV. In contrast to previous works, we include not only resonances that couple to the rho in a relative p-wave, but also those that couple to an s-wave state. In addition, we solve the equation for the rho spectral function self-consistently. We find that s-wave resonances affect the in medium spectral function of the rho strongly. In the transverse channel the rho meson is, especially at non-zero momentum, completely washed out and can in the presence of nuclear matter no longer be viewed as a resonant excitation of the vacuum. Instead, our model shows a continuum of possible excitations with the quantum numbers of a transversely polarized rho. In the longitudinal channel, however, the rho retains its resonant character in our calculation. As a consequence of the self-consistent treatment we also find a strong enhancement of the widths of the included nucleon resonances in medium.

Implications of a Subthreshold Resonance for Stellar Beryllium Depletion

Abundance measurements of the light elements lithium, beryllium, and boron are playing an increasingly important role in the study of stellar physics. Because these elements are easily destroyed in stars at temperatures (2 - 4) × 106 K, the abundances in the surface convective zone are diagnostics of the star's internal workings. Standard stellar models cannot explain depletion patterns observed in low-mass stars, and so do not account for all the relevant physical processes. These processes have important implications for stellar evolution and primordial lithium production in big bang nucleosynthesis. Because beryllium is destroyed at slightly higher temperatures than lithium, observations of both light elements can differentiate between the various proposed depletion mechanisms. Unfortunately, the reaction rate for the main destruction channel 9Be(p, α)6Li is uncertain. A level in the compound nucleus 10B is only 25.7 keV below the reaction's energetic threshold. The angular momentum and parity of this level are not well known; current estimates indicate that the resonance entrance channel is either s- or d-wave. We show that an s-wave resonance can easily increase the reaction rate by an order of magnitude at temperatures T ≈ 4 × 106 K. Observations of M < M☉ stars can constrain the strength of the resonance, as can experimental measurements at laboratory energies lower than 30 keV.

R-Matrix Analysis of the 240Pu Neutron Cross Sections in the Thermal to 5700-eV Energy Range

A Bayesian R-matrix-fitting code called SAMMY is used in a new analysis of the 240Pu neutron cross sections in the 0.02- to 5700-eV energy range. This work aims to resolve the discrepancies revealed by the JEF2 validation studies. A set of experimental data, suitable for the analysis, is determined from preliminary SAMMY analyses of the available experimental data treated individually. Finally, the sequential analysis of the selected transmission, total, and fission cross-section measurements gives an accurate set of resonance parameters. For the 1.056-eV resonance, the analysis of a recent transmission measurement gives parameters very close to those adopted in ENDF/B-VI. In the energy range above 200 eV, the average capture cross section calculated from the present evaluation is in agreement with ENDF/B-VI but is 25% lower than the values of JEF-2 and JENDL-3. A rigorous study of the statistical properties of the resonance parameters is done during this work. Although 158 new resonances are identified by the careful examination of the experimental fission cross-section and transmission data, the value of the average spacing above 2750 eV (D0 = 16.10 eV) shows a 25% loss of resonances compared with the lower energy region (D0 = 12.06 eV). Finally, for the s-wave resonance parameters, this work recommends the following average values: resonance spacing, D0 = 12.06 ± 0.60 eV; strength function, S0 = (1.032 ± 0.071) × 10−4 eV; and capture width, .

Unified approach to pseudoscalar meson photoproductions off nucleons in the quark model

An unified approach to the pseudo scalar meson ($\pi, \eta$, and $K$) photoproduction off nucleons are presented. It begins with the low energy QCD Lagrangian, and the resonances in the s- and u- channels are treated in the framework of the quark model The duality hypothesis is imposed to limit the number of the t-channel exchanges. The CGLN amplitudes for each reaction are evaluated, which include both proton and neutron targets. The important role by the S-wave resonances in the second resonance region is discussed, it is particularly important for the $K, \eta$ and $\eta^\prime$ photoproductions.

Quantum Proximity Resonances.

It is well known that at long wavelengths $\ensuremath{\lambda}$ an s-wave scatterer can have a scattering cross section $\ensuremath{\sigma}$ on the order of ${\ensuremath{\lambda}}^{2}$, much larger than its physical size, as measured by the range of its potential. Very interesting phenomena can arise when two or more identical scatterers are placed close together, well within one wavelength. We show that, for a pair of identical scatterers, an extremely narrow p-wave ``proximity'' resonance develops from a broader s-wave resonance of the individual scatterers. A new s-wave resonance of the pair also appears. The relation of these proximity resonances (so called because they appear when the scatterers are close together) to the Thomas and Efimov effects is discussed.

Improved analysis of parity violation at neutron p-wave resonances of 238U based on resonance spin assignments.

In this journal the TRIPLE Collaboration published its first result on parity violation ~PV! at p-wave resonances of 238 U in a transmission experiment with polarized low energy ~10‐300 eV! neutrons using a neutron time-of-flight beam from the LANSCE facility @1#. In this work PV could be studied at many, that is 16, p-wave resonances. This experiment is therefore a considerable improvement with respect to pioneering experiments of this kind at Dubna in which PV has been studied at one or two resonances per isotope below 20 eV @2#. In the zero-spin nucleus 238 U the 1/2 2 p-wave resonances can be admixed with the 1/2 1 s-wave resonances due to PV weak nuclear interactions, and thus, may show PV effects. Strong enhancements of these PV effects occur due to ~i! the proximity of s and p resonances, and ~ii! by admixing a ~strong! s-wave resonance or several s-wave resonances into a weak p-wave resonance (G n @G n ). Although one might expect parity mixing between 3/2 2 and 3/2 1 compound nuclear levels of 239 U, there are no 3/2 1 neutron resonances in the reaction of low energy neutrons with 238 Ut o make parity violation manifest at the 3/2 2 p-wave resonances. The weak p-wave resonances can be distinguished easily from the strong s-wave resonances in the considered low energy range of the 238 U1n reaction; however, their spins ~1/2 and 3/2! are not known without further information.

Highly doubly excited S states of the helium atom

We calculate and analyse S-wave resonances of helium up to an energy of -0.02 au applying the complex rotation technique. Rydberg series converging to the hydrogenic thresholds of the He+ ion are analysed by quantum defect theory. For moderately excited inner electrons the series converging to different thresholds begin to overlap resulting in perturbed Rydberg series. We find that approximate quantum numbers as well as propensity rules governing the decay of the resonances and the perturbation scheme of the Rydberg series prevail. With increasing excitation of the inner electron however, only series whose states tend to extreme interelectronic angles (cos theta )=+or-1 remain regular.

Intermediate structure and the shape isomer in 233Th

With a neutron-capture experiment the s-wave resonances in 233Th between 50 eV and 4.2 keV were investigated for delayed γ-γ coincidences. At least two intermediate structures are found in the distribution of the relative delayed γ-yield R at about 1 and 2 keV incident neutron energy confirmed by statistical tests. We interpret them as being due to the population of a shape-isomeric state via class-II states admixed to the resonances. From the derived class-II spacing DII and the gross shape of the structures the isomeric ground-state energy EII as well as the coupling width Γ↓ may then be estimated. These results are compatible with the decay of a shape isomer with a half life between 1 and 100 ns. They also would suggest that the height of the inner potential barrier EA is less than or of the order of the neutron binding energy, and that the inner barrier is relatively thin.