Absolute Cross Section Measurements Research Articles

Absolute photofission cross section of 197Au, natPb, 209Bi, 232Th, 238U, and 235U nuclei by 69-MeV monochromatic and polarized photons.

Absolute cross-section measurements for the photofission reactions of $^{197}\mathrm{Au}$, $^{\mathrm{nat}}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, $^{238}\mathrm{U}$, and $^{235}\mathrm{U}$ nuclei have been performed at an incident photon energy of 69 MeV using monochromatic and polarized photon beams and dielectric fission-track detectors. Nuclear fissility values have been obtained and results are in agreement with those from other laboratories, although in some cases discrepancies are observed between one other. For nuclei in the region of the actinides the fissility result is \ensuremath{\gtrsim}0.4, while for Au, Pb, and Bi nuclei it only is \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}3}$--${10}^{\mathrm{\ensuremath{-}}2}$. Results have been interpreted in terms of the primary Levinger's quasideuteron nuclear photoabsorption followed by a mechanism of evaporation-fission competition for the excited nuclei. Shell effects have been taken into account, and they are clearly manifested when fissility is evaluated. The influence of photon polarization on photofission of $^{238}\mathrm{U}$ also has been investigated, and results have shown isotropy in the fragment azimuthal distribution.

Absolute total cross section measurements for intermediate-energy electron scattering. IV. Kr and Xe

For pt.III, see ibid., vol.20, no.19, p.5157-64 (1987). Absolute total cross section measurements with an overall experimental uncertainty below 5% were performed for Kr and Xe in the energy range from 80 eV to 4000 eV. The Xe measurements above 750 eV are the first available in the literature . In the overlapping energy range the present data are in good agreement with other experimental and semi-empirical results. Comparing the total cross sections for all noble gases it has been found that their energy dependencies do not follow the Born approximation even at the highest measured energies.

Absolute measurement of the differential cross section for the reaction 4He( gamma,p)3H from 63 to 71 MeV.

Results are presented from a $^{4}\mathrm{He}$ photodisintegration experiment performed with quasimonochromatic photons using a large solid-angle detector. The tagged-bremsstrahlung technique gave a reliable absolute normalization. In addition, an unambiguous separation of the proton-triton channel from other photodisintegration channels was achieved. A total cross section of 218\ifmmode\pm\else\textpm\fi{}8 \ensuremath{\mu}b was measured at a mean energy of 67\ifmmode\pm\else\textpm\fi{}4 MeV and Legendre coefficients up to ${\mathit{a}}_{5}$ were extracted from the angular distributions.

Absolute differential cross section measurements for proton-deuteron elastic scattering at 641.3 and 792.7 MeV.

The absolute p-d elastic-scattering differential cross sections were measured at 641.3 and 792.7 MeV beam energies over a range of c.m. angles from \ensuremath{\sim}35\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{\sim}115\ifmmode^\circ\else\textdegree\fi{} and \ensuremath{\sim}35\ifmmode^\circ\else\textdegree\fi{} to \ensuremath{\sim}140\ifmmode^\circ\else\textdegree\fi{}, respectively. The longitudinally polarized (L-type) proton beam produced by the Lamb-shift ion source at LAMPF was used. The beam intensity was measured to high accuracy (\ensuremath{\sim}0.1%) by a scintillator-beam particle-counting system designed and developed prior to the experiment. Typical uncertainties in the absolute cross sections were about 2--3% total, somewhat larger at back angles. The present results were compared with the existing measurements and the controversy about the previous data at 800 MeV was resolved. The present data can be fit with a relativistic multiple-scattering theory which uses off-mass-shell extrapolations of the nucleon-nucleon amplitudes suggested by the structure of derivative meson-nucleon couplings. Relativistic-impulse-approximation calculations do not fit these data at either energy.

Single electron capture for impact of (0.5-9 keV) C2+ on He, Ar and H2

Absolute total cross section measurements and state-selective translational energy spectroscopy have been performed for single electron capture by 0.5-9 keV C2+ from H2, He and Ar, for the C2+ ground state as well as the C2+ metastable state. The experimental results are compared with available data obtained with undefined metastable primary ion beam fractions. For the particular case of C2+-H2 collisions, experimental results could be explained by close-coupling calculations involving empirically derived coupling matrix elements and Franck-Condon factors for H2 ionization via electron capture.

Absolute cross section measurements for electron impact ionization of Ar7+

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar7+ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.

High Resolution Laser Techniques Applied to Reaction Dynamics

The applications of high resolution laser techniques to crossed-beam experiments which are examined in this paper concern: (a) the reactivity of atomic/molecular excited states and the influence of light polarization and (b) the measurement of absolute and differential cross sections by use of the laserinduced fluorescence technique performed in a coherent saturation regime.Examples are provided by the crossed-beam study of the Cs(7P) + H2→CsH + H photochemical reaction, where two C.W. single-mode tunable lasers are used to excite Cs atoms and to probe CsH product molecules. It is shown how the fine and hyperfine structures of the Cs atom are involved in the reaction mechanism (harpooning). The measurement of the total cross section (≈4.10−16cm2) gives an indication of the overall efficiency of the reactive process (Cs atoms in the 7P state) with respect to the quenching ones (Cs atoms in 6S, 6P, 5D and 7S states). Finally, the differential cross section shows a marked forward “peaking” of CsH products which narrows with collision energy but which is rather independent of the product rotation.

Absorption spectrum and solar photodissociation of gaseous nitrous acid in the actinic ‐ wavelength region

The absorption cross section of gaseous nitrous acid (HONO) in the actinic wavelength region (λ≥290 nm) of the solar radiation is mapped through laser photodissociation experiments. HONO is photodissociated by a tunable ultraviolet beam, and the OH product is monitored through fluorescence induced by a frequency‐doubled dye laser. The absorption of HONO is mapped by scanning the photolysis wavelength. Since this technique yields relative cross‐sections, we calibrate our measurements with previous measurements of absolute cross‐section at 354 nm (because there is reasonable agreement among previous measurements at this wavelength). The present experimental approach is insensitive to the presence of NO2, which apparently contributed to inaccuracies in some of the previous conventional measurements on HONO absorption.

Absolute small-angle electron excitation cross sections for the resonant transition in sodium.

We present measurements of absolute differential cross sections for the scattering of 10-eV electrons by ground-state sodium, with no calibration needed. Results for 3{ital S}{r arrow}3{ital P} have been obtained in the angular range 1{degree}--20{degree}. From these results the total cross section is estimated to be 23{times}10{sup {minus}16}{plus minus}30% cm{sup 2}. Comparisons with computational results and other (normalized) measurements are made.

Band oscillator strengths of the (2, 1)–(12, 1) Schumann‐Runge bands of O2 from absolute absorption cross‐section measurements at room temperature

The absolute absorption cross sections of the (2, 1)–(12, 1) Schumann‐Runge bands of O2 at room temperature have been obtained from measurements of the total absolute absorption cross sections arising from transitions from the ground‐state levels with υ″ = 0 and 1 by subtraction of the υ″ = 0 contributions, which have been synthesized from our previous determinations of the band oscillator strengths, line center positions and predissociation line widths of the (υ′, 0) Schumann‐Runge bands of O2. The resultant absolute absorption cross sections of the (2, 1)–(12, 1) bands have been integrated numerically to obtain the band oscillator strengths. Our experimental band oscillator strengths are somewhat lower than the experimental values of Lewis et al.(1986) but are in good agreement with the recently calculated values of Friedman (1990).

Excited-state absorption of Co2+ in MgF2 and KZnF3

We report on measurements of excited-state absorption spectra and absolute cross-sections in Co2+-doped KZnF3 and MgF2 at room temperature in the near-infrared domain. The data are compared with the predictions of the single-configuration co-ordinate model.

Final-state interactions and quasi-free mechanism in the3He(d,3Hp)p reaction

A measurement of absolute differential cross sections of the3He(d,3Hp)p reaction atEd=23.083 MeV shows the importance of the quasi-free reaction (QFR) mechanism. The observation of a prominent QFR peak indicates that complicated multiple-scattering processes may not be so important as to prevent a simple phenomenological analysis based on two-body interactions. In this respect we isolate final-state-interaction (FSI) effects related toS andP wave3H-p interactions. In particular, the FSI, associated with the4He level at 25.5 MeV of excitation energy, plays a dominant role.

Direct and charge-transfer scattering of keV-energy H+ and He+ projectiles from rare-gas atoms to obtain small-angle absolute differential cross sections.

Measurements of absolute differential cross sections for direct and charge-transfer scattering of ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ by the rare gases have been made. Direct scattering cross sections are reported at 1.5 keV for collisions of ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ with Ne, Ar, Kr, and Xe over the laboratory angular range 0.03\ifmmode^\circ\else\textdegree\fi{}--1.0\ifmmode^\circ\else\textdegree\fi{}. Charge-transfer cross sections are reported for the laboratory scattering angles below 1.0\ifmmode^\circ\else\textdegree\fi{} for ${\mathrm{H}}^{+}$-Ar, Kr at 0.5, 1.5, and 5.0 keV, for ${\mathrm{H}}^{+}$-Xe at 5.0 keV, and for ${\mathrm{He}}^{+}$-Ne at 1.5 keV. The data exhibit a wealth of structure over the experimental angular range, and cross sections have been integrated over angle to provide absolute integral cross sections.

Low-energy electron capture by N3+, N4+, and N5+ from hydrogen atoms using merged beams.

Measurements of absolute total cross sections for single-electron capture in collisions of ${\mathrm{N}}^{3+}$, ${\mathrm{N}}^{4+}$, and ${\mathrm{N}}^{5+}$ ions with ground-state atomic hydrogen are reported in the energy region 0.9--1400 eV/amu. Good agreement is obtained with available theoretical calculations and with previous experimental results at the higher energies. None of the systems studied shows a clear signature of the induced-dipole attractive force in enhancing the cross section.

Electron-impact double-ionization cross sections for Xe8+

Absolute cross-section measurements are presented for the double ionization of ${\mathrm{Xe}}^{8+}$ by electron impact from below threshold to 1500 eV. A search for direct double ionization indicates that this process is small or negligible compared to the single-electron processes which contribute to double ionization. The presence of a high percentage of metastable target ions in the interaction region of this experiment significantly affects the interpretation of the measured cross sections and indicates the potential importance of selected metastable ions in plasmas.

Total absolute cross section measurements for electron scattering on NH3, OCS and N2O

The absolute total electron scattering cross sections for NH3, OCS and N2O were measured using a linear transmission technique. The presented results cover the range of impact energy from 1 to 80 eV for NH3 and from 40 to 100 eV for OCS and N2O. The most prominent feature of the cross section for NH3 is the very broad hump centred at about 10 eV with a maximum value of 23.5*10-20 m2. Above 10 eV the present results on NH3 are in excellent agreement with recent calculations of Jain (1988).

Merged-beams measurements of electron-capture cross sections for O5++H at electron-volt energies.

Absolute total cross-section measurements are presented for electron capture in O/sup 5+/+H(D) collisions over the energy range 0.9--800 eV/amu. These are the first experimental data to be reported using a new multicharged-ion--atom merged-beams apparatus developed at Oak Ridge National Laboratory. The experimental method and the cross-section measurements are described in detail. At energies below 2 eV/amu, the data show a cross-section enhancement which may be attributed to the ion-induced dipole attraction between the reactants. Agreement with recent unpublished quantum-mechanical calculations ranges from fair at the lower energies to excellent at the higher energies.

SINGLE ELECTRON CAPTURE INTO EXCITED STATES IN COLLISIONS OF Ar2+ WITH Li AND Na

Absolute measurements of emission cross sections have been performed for the lines from Arll excited states in Ar2++Li and Ar2++Na collision. The energy of incedent ions ranged from 40 keV to 300 keV. The dependence of the emission cross sections on the energy defect △E is discussed. We find that the processes with intermediate △E(>0) have larger cross sections. We also find that the excited states correlated to the observed lines belong to the high J quantum numbers in an electron configuration.

Absolute cross-section measurements of the direct charge transfer of He+ in neon in the energy range 0.5-5 keV.

Measurement of the differential cross sections of the electron capture of He/sup +/ in Ne in the energy range 0.5--5 keV has been made. From these the total cross sections were calculated by integration. The present values are compared with those from previous measurements and with a recent diabatic calculation, with which good agreement has been found.

Absolute cross sections for photoionization of laser-excited BaI states measured on a thermionic diode.

We describe a method that allows the measurement of absolute photoionization cross sections with a simple experimental setup. In a barium oven designed as a thermionic diode, ions are produced by use of a two-step pulsed-laser excitation and ionization combined with optogalvanic detection. The variation of the photoionization cross section of the excited BaI 6s6p /sup 1/P/sup 0//sub 1/ state is measured versus wavelength. At threshold the cross section is (5 +- 2) x 10/sup -20/ m/sup 2/. In the experiment strong evidence for the existence of Ba/sub 2//sup +/ dimers was found.