E. Justiniano

Energy and charge dependence of the rate of electron - ion recombination in cold magnetized plasmas

We have measured electron - ion recombination rates for bare ions of , , , and in a storage ring. For the multi-charged ions an unexpected energy dependence was found, showing a strong increase of the measured rates over the calculated radiative recombination rate for electron beam detuning energies below the electron beam transverse temperature. The measured enhanced rates increase approximately as with the charge state Z. A comparison of these rates with theoretical predictions for collisional - radiative recombination in the cold magnetized electron plasma, in particular three-body recombination including radiative de-excitation of electrons in Rydberg levels, is made.

Laser-Induced Recombination of D(+)

Laser induced recombination into the L shell (n = 2) of deuterium was investigated at the heavy-ion cooler storage ring CRYRING for two values of the electron density. The data shows the expected enhancement of radiative recombination, induced by the intense laser field applied, over the spontaneous case. Like in previous measurements, a strong intensity below the photoionization threshold was observed. Qualitatively, this gain is described by an average field induced threshold shift model. However, when reducing the electron density by roughly a factor of two, the effective field increased by an order of magnitude. This surprising result indicates that the precise origin of the observed fields is not simply related to the known field sources.

Determination of plasma trace elements in tumor-bearing animals by proton-induced X-ray emission spectroscopy.

Although altered levels of circulating essential trace elements are known to accompany malignant disease, the lack of sensitivity of conventional detection methods has generally limited their study to clinical conditions involving extensive disease (i.e., significant tumor burden). As such, the application of altered trace element levels as potential prognostic guides or as response indicators subsequent to treatment has been of limited use. During this study, proton-induced X-ray emission spectroscopy was evaluated as a tool to determine trace element imbalances in a murine tumor model. Using plasma from C57B1/6 mice bearing the syngeneic Lewis lung carcinoma (LLCa), levels of Fe, Cu, and Zn, as well as changes in the Cu/Zn ratio, were measured in animals carrying an increasing primary tumor burden. The plasma levels of Fe, Cu, and Zn were found to decrease significantly 7 d following implants of LLCa cells with no significant change observed in the Cu/Zn ratio. By d 21, however, an increase in the Cu/Zn ratio was found to accompany increased growth of the LLCa tumor; the plasma levels of Cu had returned to normal levels, whereas both the Fe and Zn plasma levels remained lowered. Collectively, the results suggest that although a net change in individual plasma trace element concentrations might not be accurately associated with tumor growth, a clear relationship was established between the Cu/Zn ratio and tumor size.

RECOMBINATION EXPERIMENTS AT CRYRING

Recent advances in studies of electron-ion recombination processes at low relative energies with the electron cooler of the heavy-ion storage ring CRYRING are shown. Through the use of an adiabatically expanded electron beam, collisions down to 10-4eV relative energies were measured with highly charged ions stored in the ring at around 15 MeV/amu energies. Examples of recombination measurements for bare ions of D+, He2+, N7+, Ne10+ and Si14+ are presented. Further on, results of an experiment measuring laser-induced recombination (LIR) into n=3 states of deuterium with polarized laser light are shown.

Using an OPO laser system for recombination studies in a storage ring: LIR to n=3 states of deuterium

We have investigated the use of a one-stage optical parametric oscillator (OPO) laser system for studies of laser induced radiative recombination (LIR) of stored ions. For the test case considered, LIR into n = 3 states of deuterium, we find that the energy resolution is limited by the broad linewidth of the OPO. Within this resolution, our LIR results are in good agreement with a theoretical average field induced threshold shift model.

Observation of Δn=1 dielectronic recombination resonances of boronlike argon

We report here an observation ofδn=1 dielectronic recombination resonances of boron-like argon in the energy region 140–195 eV. With the coolers electron beam as a target, a resolution of approximately 0.6 eV FWHM was obtained in the observed energy range. The energies of the doubly excited states were estimated with a Hartree-Fock calculation, which indicates that the observed resonances are from Ar13+(1s22s22p) to Ar12+(1s22s23l3l′) and Ar12+(1s22s2p3l3l′) transitions.

Formation of D- by double radiative recombination of D+

We have investigated double radiative recombination of D+ ions stored in CRYRING by searching for the formation of D- in the electron cooler. An upper limit for the double electron recombination rate coefficient and a ratio of this coefficient over the single electron radiative recombination rate coefficient of (3.8 ± 1.5) × 10-8 are obtained experimentally. Theoretical calculations of the rates and a discussion in comparison with double photoionization of H- are given.

Molecular effects in K-shell ionization by charged particles

Measurements of atomic inner-shell ionization cross sections for low-Z elements are complicated by small fluorescence yields and because elements do not occur naturally in atomic form. These conditions favor the measurement of Auger-electron yields to determine vacancy formation and they require the use of molecular gas targets. Because molecular structure has little influence on the energy levels of inner shells it is expected to have little effect on K-shell ionization probabilities. On the other hand, Auger-electron transitions involve molecular orbitals which introduce molecular effects in the spectra and could potentially influence yields. Variations in measured atomic K-shell ionization cross sections for different molecular targets have generally been found to be small (10–15%) and explained by simple geometric effects. Some recent measurements have, however, exhibited molecular effects as large as a factor of three which are beyond simple explanations. This has revived our interest in trying to further quantify the extent of molecular effects in K-shell ionization. We have employed a new method using well tested theoretical benchmarks to derive atomic cross sections from molecular targets to study K-shell ionization of carbon and fluorine in a number of fluorocarbon molecules excited by MeV protons and lithium ions. The atomic cross sections derived from these measurements are not found to exhibit effects of molecular structure within the experimental uncertainties of approximately 10% for protons and 25% for lithium ions.

Laser induced radiative recombination with polarized photons

Laser induced radiative recombination (LIR) to the n=3 states of deuterium was studied at the storage ring CRYRING. The aim of this work is to better understand the origin of the below threshold gain observed in such experiments. A beam of 23 MeV/amu D+ ions was stored and phase-space cooled in the storage ring. The rate of radiative recombination into the n=3 states of deuterium was enhanced over the spontaneous rate by overlaping linearly polarized laser radiation from a one-state optical parametric oscillator (OPO) system to the ion-electron interaction region in the cooler. To probe for directional effects in the below threshold gain we rotated the polarization direction of the laser with respect to the direction of an induced transverse electric field obtained by displacing the center of the electron beam from that of the ion beam.

Secondary Electron Production and Transport Induced by Fast Protons in Thin Foils

Monte Carlo (MC) simulation of charged particle track structure has become an important tool in radiation biology. MC calculations provide detailed spatial information on the production of radiation damage to DNA and other cellular structures. To test the physical parameters used in these models and to explore details of electron transport in condensed matter we have initiated a comparison of MC results to measured doubly differential yields of electrons emitted from thin foils by the passage of fast charged particles. Our initial studies focus on the calculation of differential electron yields resulting from passage of 1–2 MeV protons through thin carbon foils using the MC code PARTRAC developed at the Institute of Radiation Protection — GSF, Neuherberg, Germany. Preliminary calculations are in relatively good agreement with spectra measured for secondary electrons with energies larger than about 50 eV, however large variations are observed for smaller energies. The effects of changes in such parameters ...