D. S. Gemmell

Collisional effects in the passage of fast molecular ions through thin foils

Abstract Some recent developments in studies on the interactions of fast molecular-ion beams with thin foil targets are discussed.

Structure effects seen in the dissociation of 3.5-MeV beams of CO+2 and N2O+ in thin foils☆

Abstract We show examples of energy spectra obtained for ionic fragments that are emitted in the forward direction when 3.5-MeV beams of CO + 2 and N 2 O + dissociate in thin (≈ 150 A) carbon foils. The spectra demonstrate in a novel way the structures of the two species of incident molecular ions.

The (d, n) reaction on 24Mg and 28Si

Abstract The reactions 24 Mg(d, n) 25 Al and 28 Si(d, n) 29 P were studied. The J -dependent effects similar to those observed in (d, p) reactions were seen. Pronounced differences in neutron distributions resulting from I p = 2 transitions to the 1.36( 3 2 + ) and 1.96( 5 2 + ) MeV states in 29 P and to the ground state ( 5 2 + ) and 0.95 MeV state ( 3 2 + ) in 25 Al were noted. These differences were observed at incident deuteron energies of 7.0, 9.0 and 10.0 MeV. Absolute differential cross sections were determined for transitions to the states at 0, 0.45, 0.95, 1.79, 2.48, 2.68 and 3.06 MeV in 25 Al. Relative cross sections were determined for the states at 0, 1.36, 1.96 and 3.43 MeV in 29 P. The data were analysed by use of DWBA calculations and spectroscopic factors obtained for most of the above transitions. The results are compared with structure in the mirror nuclei 25 Mg and 29 Si.

Diminished stopping power for fast nitrogen and oxygen diclusters in carbon

Measurements of stopping powers for several diatomic ions are presented and compared to values derived from well known wake models. While independently determined wake forces can be superposed to calculate correctly the stopping forces on light fast ions, such as H2+, the method appears to be invalid for slower, more highly charged N2+ ions. To include non-linear terms in the interaction wake charge distributions must be determined for pairs of ionic cores. To avoid the complexities of a proper quantum mechanical treatment a semi-classical calculation of free electron scattering by two screened ionic cores was used to determine the stopping forces. While a quantum mechanical calculation is needed to obtain a definitive result the semi-classical calculation provides qualitative agreement with experiment for 3 MeV N2+ ions in carbon.

Stopping Power for Fast Nitrogen and Oxygen Diclusters in Carbon

Measurements of stopping powers for several diatomic ions are presented and compared to values derived from well known wake models. While independently determined wake forces can be superposed to calculate correctly the stopping forces on light fast ions, such as H2+, the method appears to be invalid for slower, more highly charged N2+ ions. To include non-linear terms in the interaction wake charge distributions must be determined for pairs of ionic cores. To avoid the complexities of a proper quantum mechanical treatment a semi-classical calculation of free electron scattering by two screened ionic cores was used to determine the stopping forces. While a quantum mechanical calculation is needed to obtain a definitive result the semi-classical calculation provides qualitative agreement with experiment for 3 MeV N2+ ions in carbon.

Experimental confirmation of the Jahn–Teller distortion of CH4+

We report measured energy and angular distributions for H+ and Cn+ (n=2,3,4) fragments resulting from the collisional dissociation of 194 keV/amu Chn+ (n=0–4) in thin carbon foils. From the systematic trends of these ’’Coulomb explosion’’ spectra, one can obtain qualitative information on the structures of these species. In particular, the series displays a narrowing of the carbon angular and energy widths as protons are symmetrically added around a central carbon atom and provide a ’’focusing’’ effect. Because of the Jahn–Teller distortion, the widths measured for CH4+ are dramatically increased. (AIP)

Apparatus for measurements on the fragmentation of MeV molecular‐ion beams

An apparatus for the study of the fragmentation of MeV molecular ions is described. The system permits high‐resolution measurement of the joint distribution in energy and angle for fragment ions of selected charge and mass. Two movable detectors allow the study of spatial and temporal correlations among up to three fragments resulting from a single dissociation event. All experimental parameters are monitored, controlled, displayed, and recorded by a computer system that uses a linked network of four processors. Real‐time, computer‐generated color graphics are employed to give a visual rendition of the relation between the detector positions and the trajectories of outgoing fragments.

The preparation of thin self-supporting Boron films

Abstract A technique for the preparation of thin (20–400 μg/cm 2 ) self-supporting Boron films by an electron bombardment method is described. The technique differs from those used previously in that it uses a water-cooled anode and a new method of removing the evaporated film from the backing.

Measurement of the distributions of internuclear separations in 3.0 MeV H2+ and 3.63 MeV HeH+ beams☆

Abstract Angular distributions of charged dissociation fragments are measured for 3.0 MeV H2+ and 3.63 MeV HeH+ ions incident on ∼ 160 A carbon targets. By using the reflection method for a pure Coulomb potential we unfold from these data the distributions of internuclear separations for each molecular-ion species prior to dissociation. These results are insensitive to ion-source conditions. For H2+ this distribution, while ∼2 times wider than a pure ground vibrational state population, is markedly different from the Franck-Condon distribution that has been previously assumed by other authors with similar rf and duo-plasmatron ion sources. For HeH+ the distribution is slightly broader (∼1.5 times) than that expected for a pure ground state population. From the data, we are able to extract the initial vibrational state population in the incident beam.

AN ON-LINE COMPUTER SYSTEM IN USE WITH LOW-ENERGY NUCLEAR PHYSICS EXPERIMENTS.

Abstract A description is given of a computing system used on-line with experiments performed at the 4 MeV Van de Graaff and the tandem Van de Graaff accelerators at Argonne National Laboratory. The requirements for the system are listed and the hardware and software obtained to fulfill these requirements are described. Some typical applications of the system are given.