Ground-state Cross Sections Research Articles

Energy dependence of the 3He(3He, π+)6 Li reaction

The 3He(3He, π+)6 Li reaction has been studied as a function of energy using the 3He beam at SATURNE. Cross sections for the 6Li ground state (1+) and 2.18 MeV (3+) levels have been obtained at 350,420,500 and 600 MeV incident energies at angles θπ(lab) = 15° and 40°. These results are compared with two theoretical predictions.

A measurement of the cross section for electron impact ionisation of Al+

The crossed ion and electron beams technique has been used to make an absolute measurement of the cross section for the ionisation process e+Al+ to 2e+Al2+ at electron energies ranging from below threshold up to 750 eV. The compounded random and systematic errors are estimated to be about +or-5% at energies in excess of about 25 eV. The cross section for the ground-state ion is determined from the measured data using a small correction that allows for the presence of about 9% of metastable ions in the Al+ beam. Comparison of the present data with the predictions of Lotz shows that the latter work appreciably overestimates the ground-state cross section at energies above 35 eV. The scaled Born prediction of McGuire is in closer agreement but it also exceeds the measured cross section above 35 eV. The shape of the Coulomb-Born-exchange cross section of Moores is in fair agreement but the magnitude is lower than the present data.

Measured electron-impact ionization of Be-like ions:B+,C2+,N3+, andO4+

Electron-impact-ionization cross sections have been measured from threshold to about 20 times threshold for Be-like ions B/sup +/, C/sup 2 +/, N/sup 3 +/, and O/sup 4 +/ with the use of the crossed-beams technique. The ion beams consist of mixtures of ions in the 2s/sup 2/ /sup 1/S ground state and the 2s2p /sup 3/P/sub 0,1,2/ metastable states. For B/sup +/ and C/sup 2 +/ the metastable fractions could be changed, allowing estimates of ground-state cross sections, while for N/sup 3 +/ and O/sup 4 +/ the metastables appear to dominate the beams and the fractions could not be changed. The measured cross sections are compared with a variety of predictions. Distorted-wave calculations compare most favorably, but discrepancies up to 40% are found. The metastable-state--ground-state mixture complicates the comparisons for these beam experiments and is probably an issue for all environments where light (Z< or =15) Be-like ions occur.

Proton capture by 11B above the giant resonance

The cross sections for proton capture by 11B to the ground and excited states of 12C have been measured in the proton energy interval between 18 and 43 MeV. The ground-state cross section shows good agreement with theoretical calculations including correlations. Capture photons have also been observed to all the residual 1p-1h states of 12C having a dominant 1 p −1 3 2 hole: the corresponding cross sections systematically show a giant resonance whose energy increases with the increasing excitation energy of the “background” level. The resonances at 27.4, 31, 33.2, 37 and 43 MeV, seem to show observable interference effects.

The beta strength function of240Bk and the fission barrier of neutron-deficient actinides

The beta-delayed fission of 240Bk was studied following the reaction 241Am(3He,4n) at Elab=33 MeV. From the experiment the author obtains a half-life T12/=(4.2+or-0.8) min and a ground-state cross section of (28+or-16) mu b; using these data the beta-delayed fission probability is determined as (1.3-0.7+1.8)*10-5. The fission barrier parameters of 240Cm are obtained from an extrapolation of those for the adjacent 242Cm in accordance with the lifetimes. Assuming a Gaussian structure in the 240Bk beta strength function with a width of the order sigma =0.35 MeV, the centroid energy Ebeta =2.1+or-0.1 MeV is obtained from a model calculation. The model includes WKB resonant penetrability, double-delayed fission and microscopic level densities. Using published experimental data the fission barrier heights of 244,246,248Cf and 248,250Fm are estimated assuming the same strength functions. The consistency between the estimated barriers and known systematics is improved considerably with the inclusion of the strength function structure.

Atomic core polarisation influence on single and multiphoton processes in atomic caesium

The quantum defect method, generalised to the spin-orbit coupling scheme, has been used to calculate the oscillator strengths of the principal series for atomic caesium, taking into account the core polarisation produced by the valence electron. The good agreement obtained with respect to experimentally measured values has led the authors to carry out new calculations of generalised two-photon ionisation cross sections for the atomic caesium 6S ground state with linearly and circularly polarised incident radiation in the deep minimum region, related to destructive interferences, near the intermediate 7P resonance. A theoretical method previously developed was used; excellent agreement was obtained with respect to recent experimental data. The degree of spin polarisation of the ejected electrons has been calculated in the same region of incident photon energy.

Ground-state photoneutron reactions inN15

Photoneutron angular distributions were measured by time-of-flight techniques for the reaction $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})^{14}\mathrm{N}$ 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 ${J}^{\ensuremath{\pi}}={\frac{3}{2}}^{+}$, $T=\frac{1}{2}$ states in $^{15}\mathrm{N}$ 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 ${p}_{\frac{1}{2}}\ensuremath{\rightarrow}{d}_{\frac{3}{2}}$ transition is consistent with an observed value for the $\frac{{a}_{2}}{{a}_{0}}$ coefficient of -0.7\ifmmode\pm\else\textpm\fi{}0.2. The ($\ensuremath{\gamma}, {n}_{0}$) 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 composition with a theoretical prediction based on a shell model calculation using a residual interaction with a Soper mixture of exchange forces.NUCLEAR REACTIONS $^{15}\mathrm{N}(\ensuremath{\gamma}, {n}_{0})$, ${E}_{\ensuremath{\gamma}}=15\ensuremath{-}25$ MeV; measured differential cross sections as function of angle; extracted Legendre coefficients; estimated $\frac{s}{d}$ matrix-element ratio in dipole approximation.

Sensitivity of tungsten neutron cross sections to target band mixing andβ6deformation

It is shown that insertion of a deformation parameter ${\ensuremath{\beta}}_{6}$ in coupled channels calculations may be necessary for proper predictions of the neutron total cross sections (${\ensuremath{\sigma}}_{T}$) at incident energies below \ensuremath{\sim}700 keV. This is illustrated in the present work with the ${\ensuremath{\sigma}}_{T}$ measurements of Whalen et al. on the $^{182,184,186}\mathrm{W}$ isotopes. In a previous report it has been established that these data could not be well reproduced using coupled channel calculations involving only ${\ensuremath{\beta}}_{2}$ and ${\ensuremath{\beta}}_{4}$ deformations. It is shown presently that this discrepancy can be removed if, provided that self-shielding corrections to these measurements are made, a small deformation parameter ${\ensuremath{\beta}}_{6}$ is considered in coupled channel analyses. As the W isotopes are lying at the rotational edge of the $A=190\ensuremath{-}200$ transitional mass region, it is also emphasized that band mixing effects on the model predictions might be of some importance. These effects have been investigated for $^{182,184,186}\mathrm{W}$ as well as for $^{183}\mathrm{W}$ using realistic collective wave functions.NUCLEAR REACTIONS Neutron total and scattering cross sections. Tungsten isotopes. Statistical model calculations. Realistic collective models. Coupled channels analyses. Deduced ${\ensuremath{\beta}}_{6}$ deformation parameters. Calculated inelastic cross sections for ground state and vibrational band levels.

Ground-state photoionization cross sections for lithiumlike carbon, nitrogen, and oxygen

Ground-state photoionization cross sections for lithiumlike carbon, nitrogen, and oxygen

Near-resonant charge transfer in collisions of N+and O+ions with inert gases

Total charge-transfer cross sections for ground-state and metastable N+ and O+ ions colliding with inert-gas atoms are investigated experimentally and calculated theoretically. The experimental cross sections of O+ and previously measured N+ cross sections are presented together with results compiled from the literature in a large energy range. Theoretical calculations are performed within the framework of the Smirnov (1973) and Demkov theories (1978). In most cases a two-state approximation is valid, providing overall good agreement between experimental and theoretical results.

Ground-state (p, t) cross sections forsd-shell nuclei

Ground-state ($p, t$) cross sections measured for five $\mathrm{sd}$-shell nuclei are, with one exception, explained well by the shell-model calculations of Chung and Wildenthal. The deviation in the case of the $^{18}\mathrm{O}(p, t)^{16}\mathrm{O}$ reaction is quantitatively explained by taking account of the core-excited components in $^{16}\mathrm{O}$ and $^{18}\mathrm{O}$.

Ground-state photoneutron angular distribution for 14N

Photoneutron angular distributions were measured for the reaction 14N(γ, n 0) 13N using neutron time-of-flight techniques over the region of excitation energy from 17 to 26 MeV. Coefficients of a Legendre polynomial fitted to the data are interpreted in terms of a simple single-particle nuclear model. For reactions to the ground state in 13N it appears that most of the photon absorption in the giant resonance region studied forms J π = 2 − states in 14N which decay by d-wave neutron emission. Some evidence was found for the existence of J π = 0 − strength at about 22.5 MeV (the peak of the cross section). The results are in general consistent with an identification of the (γ, n 0) process with the promotion of a p 1 2 neutron to the s-d shell. Little or no E2 or M1 absorption was observed. Comparison with photoproton measurements shows some evidence for a small amount of isospin T = 0 mixing also near 22.5 MeV. Comparison with calculations of the ground-state cross section shows reasonable agreement if the theoretical results are displaced upwards in energy by about 1 MeV.

Systematics of ground-state (t,p) cross sections in the 2s-1d shell

Ground-state (t,p) cross sections have been measured for seven sd-shell nuclei, using enriched gas targets in a rotating cell. Results have been compared with DWBA, using amplitudes from a recent shell-model calculation. Most of the observed features are reproduced by the calculations. Deviations at both ends of the shell can be understood in terms of out-of-shell configuration mixing.

The ESR spectrum and linewidth measurements of atomic iodinea)

The experimental spectrum from ground-state 2P3/2 atomic iodine is compared with a theoretical calculation using a best-fit g value equal to 1.333980±0.000006 and previously determined nuclear and electronic spin interaction constants. A rate coefficient for iodine-atom wall recombination (γ?10−3) is reported. Broadening coefficients and hyperfine interaction relaxation cross sections for the iodine-atom ground state with He, O2, and HI are determined from the line broadening of the MJ=3/2→1/2 and MJ=1/2→−1/2 transitions of I and compared with theory.

Positive-pion photoproduction onC12near threshold

The ..pi../sup +/ photoproduction on /sup 12/C has been measured relative to the proton from 0--20 MeV above threshold. Total cross sections from 0--12 MeV, summed over states in /sup 12/B have been extracted with accuracies between 5% and 8% relative to the proton photoproduction cross section. Distorted wave impulse approximation calculations have been performed and show good agreement with the data. The yield from 0--3 MeV above threshold is well described by a one parameter fit for the ground state cross section using only the sigma-arrow-right x epsilon-arrow-right term of the Hamiltonian.

A simple model for multiphoton molecular absorption

A simple model is provided for the multiphoton absorption process in a gas based on the discrete level, quasicontinuum picture. The model is an extension of one used to describe saturation of a laser medium. Numerical solutions for the fractional energy absorbed illustrate a number of features observed in experiments. These include a wide range of behavior for the fractional energy absorbed vs fluence, including a positive slope. A second feature is the relatively large value for the fractional energy absorbed, even when the ground-state cross section is small.

Li7andBe7production in theα+αreaction

Differential cross sections for the $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ reaction leading to the ground and 478-keV states of $^{7}\mathrm{Li}$ have been measured at 11 energies between 39.0 and 49.5 MeV and used to determine integral cross sections for $^{7}\mathrm{Li}$ production in $^{4}\mathrm{He}(\ensuremath{\alpha},p)$. Comparison of the ground state cross sections with $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data using the principle of detailed balance indicates good agreement with the most recent $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ measurements but not with some older results. Cross sections for $^{4}\mathrm{He}(\ensuremath{\alpha},d)^{6}\mathrm{Li}$ (ground state), extracted at three energies, are consistent with the most recent $^{6}\mathrm{Li}(d,\ensuremath{\alpha})$ measurements. Cross sections for $^{7}\mathrm{Be}$ production via $^{4}\mathrm{He}(\ensuremath{\alpha},n)^{7}\mathrm{Be}$ were determined at nine energies between 39.4 and 47.4 MeV by direct collection of the $^{7}\mathrm{Be}$ and detection of the $\ensuremath{\gamma}$ rays following its decay. These cross sections are essentially identical to the $^{7}\mathrm{Li}$ production cross sections above 43 MeV but are smaller below this energy because of threshold effects. The $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ reactions are involved in several mechanisms which have been proposed to explain the universal abundance of $^{7}\mathrm{Li}$, and calculations of these processes have assumed cross sections larger than our data. We therefore confirm earlier conclusions that $^{7}\mathrm{Li}$ is unlikely to be produced by galactic cosmic-ray spallation alone, but the status of the other mechanisms remains unclear. Peaks in the $^{4}\mathrm{He}(\ensuremath{\alpha},p)$ and $^{4}\mathrm{He}(\ensuremath{\alpha},n)$ excitation functions are discussed in terms of resonances in $^{8}\mathrm{Be}$. Parameters determined by Kumar and Barker in a fit to $^{7}\mathrm{Li}(p,\ensuremath{\alpha})$ data are inadequate to explain our $^{4}\mathrm{He}(\ensuremath{\alpha},p)^{7}\mathrm{Li}$ (478 keV) cross sections.NUCLEAR REACTIONS $^{4}\mathrm{He}(\ensuremath{\alpha},p)$, $E=39.0\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta},{E}_{p})$, $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},n)$, $E=39.4\ensuremath{-}47.4$ MeV; measured $\ensuremath{\sigma}(E)$. $^{4}\mathrm{He}(\ensuremath{\alpha},d)$, $E=46.7\ensuremath{-}49.5$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\theta})$, $\ensuremath{\sigma}(E)$. Astrophysical implications for $^{7}\mathrm{Li}$ production considered. Discussion of $^{8}\mathrm{Be}$ levels in $R$-matrix formalism.

High-lying pairing resonances

Pairing vibrations based on the excitation of pairs of particles and holes across major shells are predicted at an excitation energy of about 70 A 1 3 MeV and carrying a cross section which is 20%–100% the ground state cross section.

Photoneutron cross sections in 7Li

Photoneutron energy spectra from 7Li have been measured by time-of-flight methods, for bremsstrahlung end-point energies increasing in 2 MeV steps from 13 to 25 MeV. The ground-state and approximate first-excited-state differential cross sections at 90° have been obtained from 8.5 to 23 MeV. No pronounced fine structure has been observed. The measured branching ratio to the first excited state falls from an average value of 0.70 between 10.3 and 14.5 MeV to an average of 0.29 between 14.5 and 18 MeV, and rises again to an average of 0.38 between 18 and 23 MeV. This behaviour can be explained by a crude theoretical model in which 1p → 2s and 1p → 1d single particle transitions dominate below 18 MeV. The calculation predicts a branching ratio of 0.50 near threshold, falling to 0.23 at higher energies, in reasonable agreement with the experiment. The integrated value of the ground-state cross section up to 23 MeV is about (38.7 ± 3.9) MeV mb, while that for the first excited state is about (17.2 ± 3.4) MeV mb. Together they account for 39% of the exchange-augmented dipole sum of 7Li.

Cross sections for theLi7(p,n)Be7reaction between 4.2 and 26 MeV

Using pulsed-beam neutron time-of-flight spectrometry relative differential cross sections for /sup 7/Li(p, n/sub 0/)/sup 7/Be(g.s.) and /sup 7/Li(p, n/sub 1/)/sup 7/Be* (0.431 MeV) have been measured over the laboratory angular range 3.5degree to 159degree and from an incident bombarding energy of 4.2 to 26 MeV. These cross sections have been integrated over angle and at 5 MeV the resulting integral was normalized to the absolute measurement of Macklin and Gibbons in order to obtain absolute differential and total cross sections for each neutron group. At energies above 6 MeV the 3.5degree cross section for each group decreases monotonically, reaching a minimum at about 12 MeV. Both cross sections then increase, the ground-state cross section approaching a center-of-mass value of 15 mb/sr at 26 MeV. The forward-angle ratio of n/sub 1/ to n/sub 0/ neutrons remains around 30% from 8 to 26 MeV, obtaining a maximum of about 35% at 26 MeV. The integrated cross sections, on the other hand, both decrease from 6 MeV to the maximum energy. Neutrons from states in /sup 7/Be at 4.55, 6.51, 7.19, and possibly 10.79 MeV as well as those from three-body breakup were also observed in this experiment. (AIP)