Derived Cross Sections Research Articles

Interference effects in cross sections calculated by photon difference methods

The photon difference type calculation transforms the photo-nuclear yield into the corresponding cross section. For a cross section containing very narrow resonances approximations in the transformation calculation can simulate interference like effects in the derived cross section, which can be caused by truncation errors and by the use of an incorrect bremsstrahlung spectrum.

Direct Reaction Inelastic Scattering to the 2? State of 16O Via a Nonlocal Interaction

A study is made of the effects of a purely nonlocal representation of the reaction mechanism in inelastic scattering. In particular, the inelastic scattering of 30 MeV protons and 65 MeV IX-particles leading to the 2- state of 8�88 MeV excitation in 160 are considered. The reaction mechanisms are represented by separable interactions between the projectile and target nucleon, and a simple spectroscopy for 160 is used in the plane wave Born approximation. The derived cross sections are compared with other calculations and the measured data. The asymmetry in proton inelastic scattering is also discussed.

Lyman-Alpha Emission Cross Sections for Collisions of 1–25 keV H+ and H with CO, CO2, CH4, and NH3

Cross sections are reported for emission of Lyman-α radiation in collisions of H+ and H with CO and CO2 in the 1–25-keV energy range and with CH4 and NH3 in the range 5–25 keV. In the experiments with CH4 and NH3 the Doppler-shifted Lyman α, emitted by H(2p) formed in electron capture by fast incident protons or collisional excitation of fast incoming hydrogen atoms, and the virtually unshifted Lyman α, emitted by the far slower H(2p) produced in dissociative excitation, were separated. An electric field was applied within the collision chamber, and the increase in Lyman-α intensity was used to derive cross sections for formation of H(2s). For CO and CO2 targets, H projectiles are more efficient than H+ in formation of H(2p), while at energies above 10 keV, H+ is more efficient than H in formation of H(2s). For both projectiles, the probability of formation of H(2p) is greater than or equal to the probability of forming H(2s). For CH4 and NH3 targets the projectile excitation cross sections are generally similar to those for CO and CO2, and the total projectile excitation cross section for formation of H(n=2) exceeds that for dissociative excitation in either H+ or H impact. For both projectiles formation of H(2p) in dissociative excitation is at least an order of magnitude more probable than formation of H(2s). Protons are found to be more efficient than hydrogen atoms in production of Lyman α via dissociative excitation. Relationships are presented between the projectile excitation cross section and the cross section for electron capture in H+ impact or stripping in H impact which are relatively independent of the identity of the target molecule.

Methoden zur Bestimmung von Wirkungsquerschnitten für Spinaustausch und Depolarisation indirekt optisch gepumpter Atome

Abstract Two methods are described to derive cross sections for spin exchange and spin depolarization of an indirectly pumped alkali vapor with zero nuclear spin. Either pulsed or sinusoidally modulated radiofrequency can be used to destroy the polarization of the indirectly pumped (secondary) system, while the intensity of the transmitted pumping light is observed. Cross sections follow from the indirectly produced depolarization of the primary system i. e. from the change in pumping light transmission or from the phase angle between the rf modulation and the intensity oscillations of the transmitted light. The quality of the two methods is tested by deriving cross sections for spin exchange between Cs and Rb and for depolarization of Rb in Argon buffer gas.

Polariton Theory of Raman Scattering. III. Effect of Exciton and Band Properties on the Frequency Dependence of the Cross Section

In the present work, a polariton approach is employed to derive cross sections for resonant Raman scattering mediated by dipole-forbidden and quadrupole-allowed excitons. The results are contrasted with those in the dipole-allowed case. We investigate differences in polariton dispersion and in the nature of the discrete-state and continuum contributions in the various cases, and demonstrate how these influence the resonance properties of the cross section. We find that, in general, continuum effects dominate in forbidden scattering, while continuum-discrete interference is strongest in allowed scattering. Considerably sharper onsets to resonance are predicted for the quadrupole case, as compared to the dipole. Resonance properties are shown to depend as well, often strongly, on the values of various crystalline parameters, such as the exciton effective mass and the frequency of the phonons involved in the scattering. The predictions are illustrated via detailed numerical computations for a hydrogenic exciton model, and parameters appropriate to insulators such as CdS.

High energy photonuclear experiments using beam hardening

The effect of beam hardening in high photon - difference type photonuclear experiments is calculated. With the gamma intensities available from linear accelerators an order of magnitude improvement in the accuracy of derived cross sections may be approached. Practical considerations are discussed and demonstrated by a numerical sample calculation.

Resonance charge transfer between protons and excited hydrogen atoms II. Classical approximations

Symmetrical resonance charge transfer between a proton and an excited hydrogen atom is treated classically, the three constants of the classical motion of the electron in a static two-centre potential being chosen so as to ensure the closest possible match with the relevant quantal states. Approximate allowance is made for the quantal tunnel effect. The derived cross sections are in remarkably good agreement with the corresponding cross sections obtained from the quantal two-state approximation.

Notizen: Wirkungsquerschnitte für Gleichverteilung der 62P3/2-Zustände von Cäsium durch Stöße mit gesättigten Kohlenwasserstoffen

Additional cross sections for collisionally induced mixing of the Cs-62P3/2 sublevels have been measured from optical pumping with σ+ D2-light. Using the model of a random re­orientation of the electronic angular momentum during the collision the derived cross sections are: Cs-CH4 (206±40) Å2, Cs-C2H6 (303±60) Å2, Cs-C3H8 (370 ±70) Å2. The existence of a selection rule mJ ﹘ /→ ﹘mJ would in­crease these values by a factor of 1.5.

AUTOMATIC SEISMIC REFLECTION PICKING

A computational system is described that detects seismic reflections recorded digitally on magnetic tape, performs migration if desired, and plots the information in cross‐section form. The detection or “picking” philosophy is based on the selection of the time positions of either all peaks or all troughs in the set of traces which constitutes the seismic section of interest. A lateral continuity search “correlates” these peaks or troughs across the traces in order to define reflection segments. All reflection segments which span at least three consecutive traces are stored in digital format on magnetic tape. They are given a grade which is the product of their spatial span and mean amplitude. A time‐varying grading threshold is employed to limit the number of reflection segments which are output to an automatic plotter or carried for subsequent processing such as computer migration. Several examples of these automatically derived cross sections are shown and compared with the original variable‐area record sections. This system is readily extended to multiline or grid data. Thus it is feasible to generate and display automatically three‐dimensional interpretive aids such as isochrons and isopachs.

Die Spektren der Neutronen aus den Reaktionen C12(?,n)C11 und O16(?,n)O15

The ( gamma ,n)-reactions on carbon and oxygen were studied using the 30.5-Mev bremsstrahlung beam of the Heidelberg betatron. The photoneutrons were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of scintillations produced in the stilbene crystal by the recoil protons was analyzed to yield the incident neutron energy distribution. The gamma and electron background was strongly reduced through pulse shape discrimination. To test the apparatus the spectrum of the neutrons from a Po- alpha -Be source was measured. The energy spectrum of the photoneutrons from O/sup 16/ showed clearly two peaks at excitation energies of 22.4 and 24.4 Mev also seen in the photoproton spectra. The energy distribution of the neutrons from C/sup 12/( gamma ,n) also contained indication of structure already known from the proton spectra from C/sup 12/( gamma ,p). The derived cross sections of C/sup 12/( gamma ,n/sub 0/) and O/sup 16/( gamma ,n/sub 0/) coincided as well in their shape as in their absolute magnitude with the corresponding ( gamma ,p/sub 0/)-cross sections. In both C/sup 12/( gamma ,n) and O/sup 16/( gamma ,n)-reaction the neutrons seem to leave the final nucleus mainly in its ground state. (auth)

High-Energy Photoprotons from Silver

The energy distributions and absolute yields of photoprotons of energy greater than 10 MeV from silver have been measured for maximum bremsstrahlung energies between 16 and 32 MeV. The derived cross section for this component, which may be identified as arising from direct interactions, passes through a maximum at about 22 MeV, and the integrated cross section is found to be (36 ± 9) MeV mbn. This result is consistent with the idea that most of the emission arises from a 2p-2d transition at 22 MeV, and this interpretation is supported by the angular distribution measured at 30 MeV, which is strongly anisotropic.

Dissociative Attachment of Electrons in Iodine. III. Discussion

The absolute cross section, $\ensuremath{\sigma}(E)$, for electron attachment in iodine is determined from the measured values of the average cross section, $\overline{\ensuremath{\sigma}}({E}^{\ensuremath{'}})$, described in parts I and II of this series. This is accomplished by solution of the integral equation relating the average cross section, the actual cross section, and the energy distribution of the electrons. The derived cross section $\ensuremath{\sigma}(E)$ exhibits a much sharper energy dependence than that of $\overline{\ensuremath{\sigma}}({E}^{\ensuremath{'}})$ given in II, decreasing from a maximum value at zero electron energy to a half-value in \ensuremath{\sim}0.01 ev. The maximum cross section, $\ensuremath{\sigma}(0)\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, is orders of magnitude larger than the values obtained by previous investigators. It is proposed that the difference in values can be attributed to deficiencies in measurements involving electron distributions of appreciable energy spread.