ALBERT

Notes: The effect of the ambient conditions in the growth chamber of the molecular beam epitaxy machine during the growth of GaAs/Al0.35Ga0.65As structures was investigated. Both growth-interrupted (120 s at each heterointerface) and uninterrupted surfaces and interfaces were evaluated using a growth temperature of 580 °C. Two ambient conditions were studied: (a) ∼1×10−10 Torr O2; and (b) ultrahigh vacuum (UHV, ∼5×10−11 Torr, with no intentional introduction of contaminants). A striking difference was observed in both the 1.7 K photoluminescence (PL) spectra of single quantum well (SQW) structures and UHV scanning tunneling microscopy (STM) of surfaces, which were grown under ambient condition (a) as opposed to (b). When consecutive growth-interrupted SQW samples were grown with different well widths (25 and 28 A(ring)) under condition (a), the emission energy splitting into several peaks was observed, indicating discrete thicknesses of the well. However, the peak energies shifted as the laser spot was scanned across each sample. Additionally, the peak energy shifted from sample to sample for the same nominal well width.On the other hand, when SQW samples were grown under condition (b), no variation in the emission energy was observed as the laser was scanned across the sample, or from sample to sample for a given well width. Furthermore, the PL observations are supported by UHV-STM results. UHV-STM images indicated a very rough surface with large islands containing small terraces on top (a bimodal distribution) for condition (a). Conversely, when samples were grown under condition (b), only large islands were observed. For growth interrupted GaAs surfaces, 400 A(ring)×600 A(ring) islands were observed, and for Al0.35Ga0.65As, they were 150 A(ring)×400 A(ring), with a one-monolayer step in between islands. These data are consistent with abrupt interfaces with only a single-mode distribution for growth-interrupted surfaces. On the other hand, UHV-STM images of uninterrupted GaAs surfaces grown under condition (b) showed islands that were 40–60 A(ring) across. Photoluminesce spectra of a similarly grown SQW sample showed only a single broad emission line, consistent with an interface configuration of many steps which are smaller than the exciton diameter. The results show that interface roughness is sensitive to background O2.

Notes: The switching of second-harmonic generation (SHG) at a C60 single-crystal surface has been observed in a pump-and-probe experiment. The SHG signal from a picosecond 1.17 eV laser pulse is suppressed by one order of magnitude upon illuminating the crystal surface with a 3.49 eV pump pulse. The nonlinear optical response is faster than 45 ps and persists for longer than 20 ns. SHG suppression to 1/e occurs at pump densities as low as 2.8 μJ/cm2. We suggest that nonlocalized excited electronic states determine the change in the nonlinear optical behavior. © 1996 American Institute of Physics.

Notes: We have determined densities of negative hydrogen ions in a discharge by a laser detachment technique. We measured the electron density, the electron temperature, and the positive ion density using a Langmuir probe. We also performed extraction measurements. Combination of H− density measurements and extraction measurements yields information about the H− drift velocity. It was found that the velocity scaled with the square root of the electron temperature. All measurements were done as a function of discharge voltage, discharge current, and gas pressure. The densities are compatible with a semiquantitative model in which H− is produced by dissociative attachment of plasma electrons to vibrationally excited molecules and destroyed by wall collisions at very low pressure and collisions with H atoms, positive ions and/or hot thermal electrons at higher pressure.

Notes: The Frankfurt MEDEBIS has been constructed to study the techniques of an electron beam ion source (EBIS) designed as an injector for a synchrotron dedicated to cancer therapy. The MEDEBIS uses a normal conducting solenoid of 0.8 T and a trap length of 0.25 m. The aims are to inject fully stripped light ions like C6+ and O8+ in a single turn into a synchrotron for the most simple and reliable operation. Therefore fast ion extraction in less than 4 μs has been examined, both theoretically and experimentally. The source complexity has been reduced by iron shielding of an electron gun and collector without any bucking coils. The electron gun operates with partially immersed flow focusing and provides a maximum current density of about 70 A/cm2. With a second anode the beam perveance is variable. The ion extraction time is reduced by a novel electrostatic structure providing a pulsed extraction field along the axis of the whole ion trap. Measurements of extracted ion pulses and the total number of extracted charges are presented. At confinement times of about 200 ms bare nuclei are obtained with maximum abundance in the charge spectra in good agreement with design data and Lotz cross sections. © 1996 American Institute of Physics.

Notes: A multipassage magnetic spectrometer has been constructed which allows selection of a specific high charge state from the extracted ion pulse of our electron beam ion source (EBIS) and to reinject it in the emptied source, now using the electron beam of meanwhile changed energy as a target of free electrons for the study of ionization, dielectronic recombination (DR), or radiative electron capture (REC). The spectrometer consists of a H-magnet with round pole pieces and 4 identical arrangements of achromatic lenses and mirrors under 90° forming a versatile ion switchyard with the possibility of recirculation. The chromaticity of the recirculating transport system and the dispersion of this spectrometer are adjustable by electric potentials on suitably placed electrostatic deflectors inside of the magnetic field. For the injection of ions from an external ion source into the EBIS and to the beam lines, the mirror potentials are pulsed accordingly.

Notes: We have measured the efficiency (tracks per incident neutron) of pure CR-39 for detecting DD and DT neutrons. Neutrons having average energies of 2.9 MeV (DD) and 14.8 MeV (DT) were produced by a 200-keV electrostatic accelerator and the neutron yields were measured using the associated particle counting technique. All CR-39 samples irradiated by DD or DT neutrons were etched for 2 h in a 70°, 6.25-N(underbar) NaOH bath. For bare CR-39, the efficiencies for detecting 2.9- and 14.8-MeV neutrons were found to be (1.3±0.4)×10−4 and (5.0±1.8)×10−5, respectively. We also investigated using CR-39 and polyimide as proton radiators. For detecting 2.9-MeV neutrons, the radiators had no significant effect on efficiency; but for detecting 14.8-MeV neutrons the polyimide radiator increased the efficiency to (7.8±2.8)×10−5.

Notes: The Frankfurt superconducting electron beam ion source (EBIS) is under reconstruction for higher ion yield by employing an immersed gun with a 2-mm-diam cathode for a 3 A electron beam. The trap electrode construction is as simple as possible to avoid any rf production. The ion extraction will be similar to the one applied in our MEDEBIS, using tapered electrodes to create a high axial extraction gradient. Research was continued on the use of oscillating electrons to reduce the power requirements of the beam, investigating the formation of a virtual cathode by decelerating the beam at full magnetic field strength while operating the gun under immersed flow conditions with adjustable compression. The MEDEBIS has proven its quality and reliability for the application as an injector for a medically dedicated synchrotron. To improve its yield of bare nuclei, better vacuum conditions are provided by drilling out the inner windings of the warm solenoid to allow for the installation of higher vacuum conductance. After the successful presentation of our XEBIST principle that delivers highly charged ions as Ar18+ and Ba46+ we have now shown its application for the production of singly charged ions as an injector of metallic ions. Finally, an EBIS/T with an internal Penning trap has been constructed to prepare selected ion species and charge states inside the ionization region. This new device will allow the study of electron–ion interactions in well-defined initial and final charge states, i.e., to be able to distinguish between single and multiple step ionization. If the homogeneity of the magnetic field in the trap region will reach 10−9, the determination of binding energies of highly charged ions may be better than a 1 eV accuracy. © 1998 American Institute of Physics.

Notes: Fundamental questions of electron beam ion sources (EBIS) are studied using three different setups. Cryogenic classical EBIS: The limit of ion–ion cooling in EBIS devices operating near the space-charge neutralization limit has been studied experimentally. To investigate different degrees of compensation of the electron beam by highly charged ions, a hemispherical analyzer has been constructed including a novel deceleration optic for high resolution. Using its energy definition we tested different modes of ion transportation in our multipassage magnetic spectrometer, which can be operated chromatically as well as extremely dispersively: up to 200 passages could be obtained. EBIS without magnetic field: First results on ion production in the short trap at the very dense crossover of an electron beam (XEBIS) confined only inertially without the use of magnetic fields are presented. Normal conducting short EBIS: Using a normal conducting field of 0.8 T of 25-cm length an EBIS is under construction to study extremely short ion extraction of approximately 4 μs for single turn injection into a medical synchrotron for cancer therapy (MEDEBIS).

Notes: The 7th symposium on EBIS and EBIT and their applications has been organized by M. Kleinod and R. Becker in Gelnhausen, Germany the week before ICIS'97. It was combined with a workshop on ion sources for hadron colliders being reported on by J. Alessi. Former EBIS symposia which have taken place are: 1977 at GSI, Darmstadt, Germany; 1981 in Saclay and Orsay, France; 1985 at Cornell University, Ithaca, New York; 1988 at Brookhaven National Lab (BNL), Brookhaven, New York; 1991 at JINR, Dubna, Russia; and 1994 at MSI, Stockholm, Sweden. The next one will be organized in the year 2000 by Krsto Prelec, BNL and Martin Stöckli, Manhattan, Kansas. © 1998 American Institute of Physics.

Notes: The simulation of volume produced negative ions from a plasma is by far more complicated than the extraction of positive ions, while in experiments the only difficulty seemes to be connected with the power of the electrons, which are extracted at the same time. The reason for this complication in simple minded simulations is the infinite space charge, which builds up in the turning point of the positive ions in the extraction aperture for the negative ions. Smearing out the energy of the positive ions seems to help, however, this is mostly not justified by experiments, showing a low ion energy, especially in the region between the magnetic filter and the extraction hole. This difficulty may be overcome by using experience from virtual cathode formation in magnetically focused, decelerated electron beams. The decelerated electrons behave similarly to the reflected positive ions and are forming a virtual cathode in the reflection zone. From the analysis of the electron deceleration experiment, a simple power law is deduced to describe the decreasing electron current by the local potential. In turn, this power law may also be applied to the positive ion current, resulting in simulations without space charge singularity, even in the case of monoenergetic ions. As a first step towards the numerical simulation of negative ion extraction, a linear model has been made, using this power law. The transition from a Boltzmann distribution for the plasma electrons to a truncated one for the extracted beam electrons is considered as well, parallel to Langmuir's treatment of a thermal diode for electrons. © 1998 American Institute of Physics.