R. W. McCullough

Damage to plasmid DNA induced by low energy carbon ions

The damage induced in supercoiled plasmid DNA molecules by 1-6 keV carbon ions has been investigated as a function of ion exposure, energy and charge state. The production of short linear fragments through multiple double strand breaks has been demonstrated and exponential exposure responses for each of the topoisomers have been found. The cross section for the loss of supercoiling was calculated to be (2.2 +/- 0.5) x 10(-14) cm(2) for 2 keV C(+) ions. For singly charged carbon ions, increased damage was observed with increasing ion energy. In the case of 2 keV doubly charged ions, the damage was greater than for singly charged ions of the same energy. These observations demonstrate that ion induced damage is a function of both the kinetic and potential energies of the ion.

Atomic nitrogen production in a high efficiency microwave plasma source

A high‐resolution spectroscopic study of a high efficiency nitrogen atom beam source, which provides dissociation fractions of up to 0.67 and is suitable for the treatment and doping of materials, has been used to identify and study atomic and molecular emissions from the source plasma. The ratio of the intensity of selected atomic and molecular emission lines has been compared with the N/N2 concentrations in the beam measured using a quadrupole mass spectrometer. Our observations show that spectroscopic measurements can be used to determine the N atom content of the beam. This technique is likely to be valuable in applications where nitrogen beams are used for the growth and treatment of materials where other means of characterization are precluded.

Charge exchange emission from solar wind helium ions

Charge exchange X-ray and far-ultraviolet (FUV) aurorae can provide detailed insight into the interaction between solar system plasmas. Using the two complementary experimental techniques of photon emission spectroscopy and translation energy spectroscopy, we have studied state-selective charge exchange in collisions between fully ionized helium and target gasses characteristic of cometary and planetary atmospheres (H2O, CO2, CO, and CH4). The experiments were performed at velocities typical for the solar wind (200-1500 km s-1). Data sets are produced that can be used for modeling the interaction of solar wind alpha particles with cometary and planetary atmospheres. These data sets are used to demonstrate the diagnostic potential of helium line emission. Existing Extreme Ultraviolet Explorer (EUVE) observations of comets Hyakutake and Hale-Bopp are analyzed in terms of solar wind and coma characteristics. The case of Hale-Bopp illustrates well the dependence of the helium line emission to the collision velocity. For Hale-Bopp, our model requires low velocities in the interaction zone. We interpret this as the effect of severe post-bow shock cooling in this extraordinary large comet.

DETERMINATION OF THE ATOMIC NITROGEN FLUX FROM A RADIO FREQUENCY PLASMA NITRIDE SOURCE FOR MOLECULAR BEAM EPITAXY SYSTEMS

Direct measurements of both the dissociation fraction and the atom flux have been carried out for the first time to characterize a radio frequency (rf) atomic nitrogen source. The measurements have been made over a range of source pressures and rf powers. Dissociation fractions of up to 0.4 and atomic nitrogen fluxes of up to 0.85×1018 atom s−1 were obtained. In addition these results have been correlated with spectroscopic measurements of the ratios of atomic and molecular optical emissions from the source plasma in order to provide a way of estimating in situ the atomic flux.

A new microwave discharge source for reactive atom beams

A new atom beam source has been developed in which two special slotted line radiators have been used to couple 2.45 GHz microwave power to a simple pyrex glass discharge tube with a relatively large plasma volume. For hydrogen, oxygen and chlorine dissociation, fractions of up to about 0.95, 0.54 and 0.95, respectively, have been obtained with corresponding atom beam densities at the exit of 4*1013, 3.5*1012 and 1.3*1012 cm-3. Atom beam fluxes of up to 0.34, 0.016 and 0.012 SCCM have been obtained for H, O and Cl respectively. The source is simple, cheap to construct, and has proved stable and reliable in operation.

Electrical characteristics of nitrogen incorporated hydrogenated amorphous carbon

Nitrogen incorporation into hydrogenated amorphous carbon (a-C:H) films has recently attracted a wide range of interest due to its contribution in reducing film stress and improving field emission properties. In this work we characterize the electrical properties of nitrogen containing a-C:H films. The a-C:H films were prepared by plasma enhanced chemical vapor deposition in an acetylene (C2H2) environment with a range of bias voltages. Nitrogen incorporation was achieved by exposing the films to an atomic nitrogen flux from a rf plasma with up to 40% dissociation and atomic nitrogen fluxes of up to 0.85×1018 atoms s−1. Raman results indicate that the doping process is accompanied by some structural changes seen by the G-band peak shifts. X-ray photoelectron spectroscopy spectra suggest that the dopant levels exceed those previously reported. Capacitance probe and I–V techniques showed a decrease in contact potential difference and density of states for doped films, indicating a rise in the Fermi level.

Study of the emission characteristics of a rf plasma source for atomic oxygen: Measurements of atom, ion, and electron fluxes

A 13.56 MHz rf plasma atom source with an alumina discharge tube was characterized for use with oxygen. For a rf input power of 500 W the maximum degree of dissociation was found to be 70% at an indicated source pressure of 0.20 Torr, and a dissociation of approximately 40% was obtained at an indicated source pressure of 0.60 Torr. Corresponding atom fluxes of 6.4×1019 atoms cm−2 s−1 and 1.66×1020 atoms cm−2 s−1, respectively were calculated at the source exit from the measured degree of dissociation and gas throughput. The energy distributions of the charged particle emissions from the source were also measured. It was found that the source produced positive ions with an average energy of 278 eV and electrons with an average energy of 8 eV. The ion and electron spatial distributions were also measured. The electron and positive ion currents were measured as a function of source pressure. Plasma emission spectra in the range 500–1000 nm were monitored at two different source pressures corresponding to max...

DNA damage by low-energy ions.

Ion-beam irradiation provides a promising treatment for some types of cancer. This promise is due mainly to the selective deposition of energy into a relatively small volume (the Bragg peak), thus reducing damage to healthy tissue. Recent observations that electrons with energies below the ionization potential of DNA can cause covalent damage to the bases and backbone have led to investigations into the ability of low-energy (<1 keV x Da(-1)) ion beams to damage double-stranded DNA. It has been clearly demonstrated that these low-energy ions induce a mixture of single- and double-strand breaks to dried DNA in vacuo. These effects depend upon the number of ions incident upon the DNA, the kinetic energy of the ions and on their charge state. This DNA damage may be important, as all radiotherapies will result in the production of low-energy secondary ions as radiation passes through tissues. Currently, their effects are neglected in treatment planning, and thus more work is required to quantify and understand DNA damage by low-energy ions.

Survival of Deinococcus radiodurans Against Laboratory-Simulated Solar Wind Charged Particles

In this experimental study, cells of the radiation-resistant bacterium Deinococcus radiodurans were exposed to several different sources of radiation chosen to replicate the charged particles found in the solar wind. Naked cells or cells mixed with dust grains (basalt or sandstone) differing in elemental composition were exposed to electrons, protons, and ions to determine the probability of cell survival after irradiation. Doses necessary to reduce the viability of cell population to 10% (LD(10)) were determined under different experimental conditions. The results of this study indicate that low-energy particle radiation (2-4 keV), typically present in the slow component of the solar wind, had no effect on dehydrated cells, even if exposed at fluences only reached in more than 1000 years at Sun-Earth distance (1 AU). Higher-energy ions (200 keV) found in solar flares would inactivate 90% of exposed cells after several events in less than 1 year at 1 AU. When mixed with dust grains, LD(10) increases about 10-fold. These results show that, compared to the highly deleterious effects of UV radiation, solar wind charged particles are relatively benign, and organisms protected under grains from UV radiation would also be protected from the charged particles considered in this study.

Importance of dissociative excitation by slow He2+ ions in one-electron capture collisions with H2

Translational energy spectroscopy has been used to study one-electron capture by 0.5-2.0 keV amu-1 He2+ ions in collision with H2. The main excited state product channels have been identified and the relative cross sections determined. These measurements provide the first direct evidence of the relative importance of dissociative excitation channels in electron capture (with He+ formed in the ground state and H atoms mainly in the n=2 states) which dominate the entire energy range. Non-dissociative electron capture into the n=2 and n=3 states of He+, which is also observed, increases from 1% of the total electron capture cross section at 0.5 keV amu-1 to about 25% at 2 keV amu-1.