Determination Of Total Cross Section Research Articles

23Na(α,p)26Mg Reaction Rate at Astrophysically Relevant Energies.

The production of 26Al in massive stars is sensitive to the 23Na(α,p)26Mg cross section. Recent experimental data suggest the currently recommended cross sections are underestimated by a factor of ∼40. We present here differential cross sections for the 23Na(α,p)26Mg reaction measured in the energy range E(c.m.)=1.7-2.5 MeV. Concurrent measurements of Rutherford scattering provide absolute normalizations that are independent of variations in target properties. Angular distributions are measured for both p0 and p1 permitting the determination of total cross sections. The results show no significant deviation from the statistical model calculations upon which the recommended rates are based. We therefore retain the previous recommendation without the increase in cross section and resulting stellar reaction rates by a factor of 40, impacting the 26Al yield from massive stars by more than a factor of 3.

Neutron transmission: A powerful technique for small accelerator-based neutron sources

Abstract The determination of total cross section of a material as a function of neutron energy by means of transmission experiments is rather easy on a pulsed neutron source. The spectrometer and associated instrumentation required may be described as relatively simple, and sound statistical data are achieved even using a small, pulsed, accelerator based neutron source. Despite its simplicity, detailed scientific and technological information can be extracted from such measurements, at the cost of fairly elaborate data analysis techniques. These features make this an attractive tool for neutron-based materials research within small research groups. Different physical properties of a material manifest over the different neutron energy ranges. At our group we have focused on the determination and analysis of the total cross section for thermal and sub-thermal neutrons, which is very sensitive to the geometric arrangement and movement of the atoms, over distances ranging from the’ first-neighbour scale’ up to the microstructural level or’ grain scale’.

Time-independent determination of total cross sections for molecular photodissociation

Working within a time-independent framework, we demonstrate that to obtain accurate values for total photodissociation cross sections, one need determine the photofragment wavefunction only within the Franck—Condon region. This considerably lightens the computational overhead. The present algorithm, similar in spirit to earlier work of Singer, Lee and Freed, represents the time-independent analogue of the particularly simple approach, due to Heller, which relates, in a time-dependent framework, the total photodissociation cross section to the Fourier transform of an autocorrelation function.

Remarks on the determination of total cross-sections in charged-particle scattering

The scattering of charged particles by a screened Coulomb potential plus a nuclear potential is reviewed. It is shown thata) the reaction cross-section is independent of the screening treatment,b) a precise knowledge of the screened Coulomb amplitude is necessary to obtain correctly up to 0° the nuclear amplitude both from elastic-scattering and transmission experiments,c) the sum-of-differences method to evaluate reaction cross-sections holds up to collision energies comparable with the height of the Coulomb barrier.

Determination of total cross sections for threshold excitation and ionization by electron impact

Total cross sections have been determined for electronic excitation and ionization processes in helium, carbon monoxide, nitrogen, oxygen, carbon dioxide, ethylene and benzene. The formation of transient negative ions greatly influences the behaviour of the cross sections as a function of the incident electron energy. In our apparatus we can determine the production of low-energy ions as well as the production of low-energy electrons resulting from an inelastic collision. Therefore, cross sections for excitation processes can be normalized on the well-known ionization cross sections. Assuming a value determined by Rapp and Englander-Golden 1) for the formation of positive ions from carbon monoxide as a standard, we find for the maximum in the cross section of the 2 3S excitation of helium 4 × 10 −18 cm 2 which is reached at about half a volt above threshold. Maier-Leibnitz 2) determined 5 × 10 −18 cm 2 for this maximum. Our results for the threshold excitation of molecules can not be compared with other measurements because the latter are not yet available. The maximum cross section for the formation of negative ions from O 2 near 6.5 eV is found to be 1.4 × 10 −18 cm 2. Rapp and Briglia 3) determined the same value for this process.

Total cross sections for fast neutrons by a time of flight method

A time of flight method for the determination of total cross sections for fast neutrons has been realized by detecting neutrons twice along their path by stilbene scintillation counters. The time interval between the pulses of the two counters is measured by a delayed coincidence circuit and determines the energy of the neutrons between the counters. The experimental arrangement and its properties with respect to efficiency, energy resolution and background are discussed. The time resolution was limited by the low efficiency of the coincidence circuit for resolving times smaller than 4 nanoseconds. The method has been tested by measuring with an average energy resolution of 20% the total cross section of sulphur between 400 and 1200 keV. The results are compared to earlier measurements and to the optical model. Further possibilities of the method are discussed.