Ground-state Cross Sections Research Articles

Formation of antihydrogen in the ground state and n=2 level.

The cross sections of antihydrogen formation in the ground state and {ital n}=2 level by the impact of antiprotons on the ground state of positronium have been calculated under the framework of the eikonal approximation for incident energy of 30--1000 keV. The excited-state capture cross sections are quite appreciable and are even larger than the ground-state cross sections for impact energies {le}75 keV. The total eikonal cross sections ({sigma}={sigma}{sub 1{ital s}}+{sigma}{sub 2{ital s}}+{sigma}{sub 2{ital p}}) are always higher than the corresponding first-order Born approximation cross section throughout the present energy span.

Solvent Effects on C60 Excited State Cross Sections

AbstractA survey study of solvent effects on the linear and nonlinear absorption properties of C60 has been done. The nonlinear absorption measurements are done at 694 nm with a Q-switched Nd:YAG pumped dye laser. In conjunction with this study a three level rate equation system has been solved analytically and the model results used to extract effective excited state cross sections. A difference and a ratio are calculated from the effective excited state cross section and the ground state cross section. The analysis demonstrates that it is this difference and the initial transmission which determine the critical fluence for the onset of nonlinear absorption. The saturation behavior is determined by the ratio of the cross sections and the initial transmission. Based on these findings C60 is shown to have some of the highest reported nonlinear absorption properties at 694 nm. The effective excited state cross sections at 694 nm are reported for the solutions and correlations to the solvent types demonstrated.

Nonlinear Optical Properties of Tetrabenzporphyrins

AbstractOptical limiting in a solution of tetrabenzporphyrin (TBP) dissolved in tetrahydrofuran (THF) was measured in an f/5 optical focusing arrangement using 532 nm wavelength laser light with a 9 ns pulse duration. The performance of this solution was compared to a variety of other materials. In order to better understand the nonlinear behavior of this material, apertured and unapertured Z-scans were performed. From these measurements it was determined that the excited-state cross section and its ratio to ground-state cross section were large. The nonlinear refraction of this material is made up of contributions from thermal density change and population redistribution. Limiter performance was modeled in an f/64 limiter. The limiter behavior of a slightly modified compound and an additional solvent were measured.

Cold collisions of ground state 4He: Giant S-wave scattering cross sections.

We have measured integral elastic cross sections for ground state $^{4}\mathrm{\ensuremath{-}}^{4}$He scattering at collision energies from 1.35 to 0.5 K using a novel atomic beam apparatus built into a dilution refrigerator. Scattering is almost pure s wave, and is predicted to be enormous due to a possibly bound state near the continuum. The effective integral cross section ranges from 20 to 1000 A${\mathrm{\r{}}}^{2}$ as collision energy is reduced in our experiment. These cross sections are in agreement with the values predicted using the latest analytical potential of Aziz and Slaman.

Search for Gamow-Teller strength in the continuum via the 3He(n,p)3H reaction at 288 MeV.

Cross sections for the $^{3}\mathrm{He}$(n,p) reaction were measured at angles of 2.7\ifmmode^\circ\else\textdegree\fi{}, 7.5\ifmmode^\circ\else\textdegree\fi{}, 14.4\ifmmode^\circ\else\textdegree\fi{}, 28.2\ifmmode^\circ\else\textdegree\fi{}, and 34.9\ifmmode^\circ\else\textdegree\fi{} (c.m.) at an incident beam energy of 288 MeV. Outgoing protons were observed to energies corresponding to a Q value of -40 MeV. The reduced cross section for the Gamow-Teller component of the ground-state isobaric-analog transition was determined to be \ensuremath{\sigma}^=\ensuremath{\sigma}(q=0)/${\mathit{B}}_{\mathrm{GT}}$=6.9\ifmmode\pm\else\textpm\fi{}0.2 mb/sr. This value is significantly smaller than that observed for nuclei in the mass range 6\ensuremath{\le}A\ensuremath{\le}13. A multipole analysis of the data indicates that the cross section of the continuum up to a Q value of -20 MeV involves mainly \ensuremath{\Delta}L=1 spin dipole transitions. The analysis determines an upper limit of 0.06 units of Gamow-Teller strength in transitions to excited states between 7 and 16 MeV, but is consistent with no Gamow-Teller strength in this region. Distorted wave impulse approximation calculations using transition amplitudes from a large-scale shell model calculation provide a good fit to the measured ground-state cross section, but fail to account for the magnitude of the measured cross sections to the continuum below 30 MeV excitation.

Momentum transport cross sections for excited and ground state potassium with rare gases by light-induced diffusive pulling

The authors have used the light-induced diffusive pulling (LDP) method to measure the ground state momentum transport cross sections and the relative difference of the excited and ground state cross sections with respect to the ground state for potassium with several inert gas atoms. For the LDP measurements, the excited state cross section is a weighted average of the potassium 42P3/2 and 42P1/2 fine structure levels, which are mixed by collisions with the inert gas atoms. The authors have also used a light-induced drift (LID) experiment to measure the ratio of the differences in the excited and ground state cross sections for the individual fine structure levels ( Delta sigma /J=3/2): Delta sigma (J=ff) for potassium-Ne and potassium-Ar. For the case of potassium-Ar, the authors have combined this ratio with the results from the LDP measurements to obtain absolute transport cross sections for the individual 42S1/2, 42P1/2 and 42P3/2 levels.

Ground-state Gamow-Teller strength in 64Ni(n,p)64Co cross sections at 90-240 MeV.

Cross sections have been measured for the reaction $^{64}\mathrm{Ni}$(n,p${)}^{64}$Co at laboratory angles between 0\ifmmode^\circ\else\textdegree\fi{} and 10\ifmmode^\circ\else\textdegree\fi{} for incident neutron energies from 90 to 240 MeV. The ground-state cross sections together with the ${\mathrm{\ensuremath{\beta}}}^{\mathrm{\ensuremath{-}}}$ decay ft value for the transition $^{64}\mathrm{Co}$(g.s.)${\ensuremath{\rightarrow}}^{64}$Ni(g.s.) are used to normalize the q=0 differential cross section in units of mb/sr per unit Gamow-Teller (GT) strength. This is the first absolute measurement of the A(n,p) unit cross section for a nucleus in the (fp) shell, and it may be used to calibrate the GT strength measured in other (n,p) reactions of similar mass nuclei. Since the (${\mathit{e}}^{\mathrm{\ensuremath{-}}}$,${\ensuremath{\nu}}_{\mathit{e}}$) channel involves the same nuclear matrix element as the (n,p) channel, knowledge of GT strength in these nuclei is important for supernova modeling codes which depend on knowledge of ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ capture rates of (fp)-shell nuclei to determine parameters of stellar core collapse.

The photoionization of nitrogen-like Al6+ from its ground state

The R-matrix method has been applied to calculate photoionization cross sections for the 2s22p3 4S0 ground state of the N-like Al6+ ion. To test the accuracy of results, calculations are carried out in the LS coupling scheme with two sets of target states and results were obtained in both the length and velocity formulations. In the first, the six target states, 3Pe, 5S0, 3D0, 3P0, 3S0 and 2p4 3Pe, are constructed by using 1s, 2s and 2p atomic orbitals. In the second set these states are represented by extensive configuration interaction using 3s, 3p and 3d correlation orbitals in addition to the spectroscopic ones. The partial and total cross sections are obtained. The cross sections are dominated by Rydberg series of resonances converging onto higher thresholds of Al7+. Some prominent resonances have been analysed and their positions and widths are listed.

The photoionization of the 4S0 ground state of N-like Cr17+ ion

Photoionization cross sections for the 2s22p3 4So ground state of the nitrogen-like Cr17+ ion have been obtained using the R-matrix method. Calculations are carried out in the LS coupling scheme with two sets of target states. In the first, the six target states, 3Pe, 5So, 3Do, 3Po, 3So and 2p4 3Pe, are constructed by using 1s, 2s and 2p atomic orbitals. In the second set these are represented by the extensive configuration interaction using 3s, 3p and 3d correlation orbitals. The initial bound and the final continuum state wavefunctions are represented consistently in terms of the R-matrix basis functions. The partial and total cross sections are obtained. The cross sections are dominated by Rydberg series of resonances converging onto higher thresholds. Some of these resonances have been analysed and their positions and widths are listed.

Pion double charge exchange on 42,44,48Ca for 300

Measurements of cross sections for pion-induced double-charge-exchange reactions to the double-isobaric-analog state for $^{42,44,48}\mathrm{Ca}$ at incident pion energies of 300--550 MeV at ${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$=5\ifmmode^\circ\else\textdegree\fi{} are reported, together with upper limits for cross sections for transitions to the residual ground states for $^{44,48}\mathrm{Ca}$. The data are fitted with a two-amplitude model which gives expressions for cross sections for both transitions. The fit overpredicts the ground-state cross sections, indicating a failure of the model in this energy range.

Pion double charge exchange on T=2 nuclei in the Delta 3/2,3/2 resonance region.

We present cross sections for the reactions $^{44}\mathrm{Ca}$${,}^{56}$Fe(${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$) leading to the residual ground states and double-isobaric-analog states (DIAS) for incident pion energies from 100 to 300 MeV. Data include both angular distributions and 5\ifmmode^\circ\else\textdegree\fi{} excitation functions. Ground-state cross sections were also obtained from the $^{12}\mathrm{C}$ and $^{16}\mathrm{O}$ contaminants in the $^{44}\mathrm{Ca}$ target. The DIAS data are compared to known systematics and to theoretical predictions. Some features of the data are reproduced by the theoretical predictions, but major discrepancies exist.

Electron scattering from vibrationally excited CO2 in the energy range of the 2 Pi u shape resonance.

We measured the total electron scattering cross section for ${\mathrm{CO}}_{2}$ in a time-of-flight gas-target transmission experiment at gas temperatures of 250 and 520 K in the energy range 0.5--9 eV. From measurements at both temperatures we determined the cross section for molecules in vibrationally excited states ${\ensuremath{\sigma}}_{\mathrm{exc}}$ and the ground-state cross section ${\ensuremath{\sigma}}_{0}$. $sigma sub roman exc--- exceeds ${\ensuremath{\sigma}}_{0}$ at all energies. A qualitative theoretical interpretation of this shift is possible on the basis of the calculated potentials from the ${\mathrm{CO}}_{2}^{\mathrm{\ensuremath{-}}}$ compound state formed in the resonance scattering.

Photoneutron angular distribution of 19F

Photoneutron time-of-flight spectra from the reaction 19F(γ, n 0) 18F were measured between 48° and 139° using 10 m flight paths over the excitation energy range from 15–25 MeV. The measured values of the normalized Legendre a 1 and a 3 coefficients are very small or close to zero over the energy region studied, indicating dominance of E1 absorption in this region. A simple modeldependent analysis of the a 2 coefficient showed that the likely reaction mechanisms are mainly s → p and d → p single-particle transitions of channel spin 1 2 . A comparison of the present angleintegrated ground-state cross section with the (γ, n tot) work of Veyssière et al. indicates that decays to excited states in 18F are much preferred (typically by a factor of 5) over the ground-state channel. The 19F(γ, n 0) cross section shows reasonable agreement in structure and magnitude with the 19F(γ, p 0) cross section of Kerkhove et al. as well as with the 18O(γ, n 0) data of Jury et al. (although some discrepancies are seen at 16 MeV and above 23 MeV).

Coexistence in the even zinc isotopes.

A generalized two-state coexistence model developed to describe the 0/sup +//sup '/ to ground state cross-section ratios in the (p,t) and (t,p) reactions on even-even nuclei is applied to the zinc isotopes. We demonstrate that two-state mixing wave functions for the ground state and excited 0/sup +/' state in /sup A/Zn can quantitatively account for all sigma(0/sup +//sup '/)/sigma (g.s.) ratios in Zn(p,t) and Zn(t,p). Finally, we couple these 0/sup +/ wave functions with a set of two-state mixing 2/sup +/ wave functions to describe the E2 transition-rate data in /sup 68/Zn.

Double charge exchange to the double isobaric analog state at T pi ~292 MeV.

Measurements of cross sections for the (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) reaction to the double isobaric analog state on targets of $^{30}\mathrm{Si}$, $^{34}\mathrm{S}$, $^{44}\mathrm{Ca}$, $^{50}\mathrm{Ti}$, $^{51}\mathrm{V}$, $^{52}\mathrm{Cr}$, and $^{58}\mathrm{Ni}$ at ${\ensuremath{\theta}}_{\mathrm{lab}=5}$\ifmmode^\circ\else\textdegree\fi{} and an incident pion kinetic energy of \ensuremath{\sim}292 MeV are presented. We also present limits for the cross section to the residual ground state for the four T>1 targets. The data are compared with a phenomenological two-amplitude model and with a two-amplitude model that uses seniority-zero shell-model wave functions. The latter model provides expressions for both ground state and double isobaric analog state cross sections.

Ground state reorientation effects in elastic scattering cross sections

Elastic scattering cross sections are shown to be quite sensitive to ground state reorientation amplitudes and cross sections. These effects are dramatically evident when comparing scattering from even-even nuclei with that from high-spin odd- A nuclei, and may affect choices of potentials for odd versus even nuclei. Neutron scattering data from 235U and 238U are analyzed via coupled channels methods. The diffraction minima in elastic scattering from 235U are found to be dominated by reorientation cross sections. The majority of this reorientation is shown to follow from second order virtual nuclear excitations rather than proceeding through amplitudes for direct ground state reorientation.

Optical pulse compressor composed of saturable and reverse saturable absorbers

It is shown that an optical pulse compressor can be made of a first stage composed of a saturable absorber to reduce the leading edge of the pulse and a second stage composed of a reverse saturable absorber (a substance whose excited-state cross section is larger than its ground-state cross section) to reduce the trailing edge of the pulse. Criteria are given for choosing appropriate cross sections, concentrations, lifetimes, and optical paths, for the saturable and reverse saturable absorber. A numerical example is presented.

A dependence of GS Se(t,p), cross sections

Absolute ground-state cross sections for ASe(t,p) are observed to increase about a factor of 2.5 from 82Se to 76Se. They exceed the (1g9/2)n expectation by a factor of twenty but are in reasonable agreement with a simple model allowing excitations in 1f5/2, 2p3/2, 2p1/2 and 1g9/2 orbitals.

Ground state photoneutron reactions in 17O.

Photoneutron angular distributions and differential cross sections have been measured for the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},${\mathrm{n}}_{0}$${)}^{16}$O over the excitation energy region from 10 to 24 MeV. The angle-integrated ground state cross section was obtained and compared to the total photoneutron cross section. The comparison indicates that the ground state channel dominates the pygmy resonance region below ${E}_{\ensuremath{\gamma}}$=16 MeV but contributes very little (<10%) to the giant resonance region (near 22 MeV). Legendre coefficients extracted from the angular distribution data suggest that electric dipole transitions make up nearly all of the absorption strength in the region studied except in narrow regions near 10.8, 15.1, 17.3, and 22.3 MeV, where small but significantly nonzero values of the ${a}_{1}$ coefficient are observed. Values of the ${a}_{2}$ coefficient, interpreted in an E1 approximation, suggest a relatively pure single-particle composition of many of the states observed in the pygmy resonance region from 10 to 16 MeV.

Ground-state photoneutron reactions in 14C.

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction /sup 15/N(..gamma..,n/sub 0/) /sup 14/N over the region of excitation energy from 15 to 25 MeV. Ground state cross sections were obtained by stepping the bremsstrahlung end point over the energy region of interest in 2 MeV intervals. By fitting the spectral data to a series of Legendre polynomials, angular distribution coefficients were extracted and interpreted on the basis of a simple single particle model. It appears that a large fraction of the photoabsorption strength leading to decays via the ground state channel is due to the formation of J/sup ..pi../ = (3/2)/sup +/, T = (1/2) states in /sup 15/N which decay by d-wave neutron emission. The data support an approximation of purely electric dipole absorption in the region measured. Some small amount of s-wave neutron emission interfering with the dominant p/sub 1/2/..-->..d/sub 3/2/ transition is consistent with an observed value for the a/sub 2//a/sub 0/ coefficient of -0.7 +- 0.2. The (..gamma..,n/sub 0/) cross section integrated between threshold and 30 MeV is estimated to represent about one-third of the total strength in the neutron channel. A state identified at 17.3 MeV is consistent in energy and compositionmore » with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.« less