ALBERT

Notizen: 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.

Notizen: Single GaAs/AlxGa1−xAs quantum wells, grown by molecular beam epitaxy with growth interruptions at each interface, are investigated using low-temperature photoluminescence. The three clearly resolved photoluminescence peaks are attributed to discrete monolayer thicknesses of the well. The splitting of the peaks is investigated for several hundred points across a 2 in. wafer. The negligible variation of the peak splitting is consistent with abrupt interfaces in the growth direction, atomically smooth interfaces, and discrete thicknesses of the quantum well with changes of only integer multiples of monolayers.

Notizen: Vacuum scanning tunneling microscopy has been used to investigate the hydrogen-terminated Si(111) surfaces obtained upon dissolution of the native oxide in HF and NH4F solutions. Whereas etching in aqueous HF acid produces an atomically rough surface, comparable treatment in NH4F results in atomically flat surfaces. These atomically flat surfaces are extremely well ordered and exhibit terraces which extend thousands of angstroms.

Notizen: 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.

Notizen: 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.

Notizen: igun is a program for the simulation of positive ion extraction from plasmas. It is based on the well known program egun for the calculation of electron and ion trajectories in electron guns and lenses. The mathematical treatment of the plasma sheath is based on a simple analytical model, which provides a numerically stable calculation of the sheath potentials. In contrast to other ion extraction programs, igun is able to determine the extracted ion current in succeeding cycles of iteration by itself. However, it is also possible to set values of current, plasma density, or ion current density. Either axisymmetric or rectangular coordinates can be used, including axisymmetric or transverse magnetic fields.

Notizen: The space-charge compensation of electron beams by hot ions with a Boltzmann energy distribution has been investigated by solving Poisson's equation. A unique relation is obtained between the degree of central compensation and the ion temperature: At high temperatures (as compared to the potential depression inside the beam) the compensation is low and the ions stay inside the electron beam only to a minor part, while at low ion temperatures, the beam is almost fully compensated. The potential difference between the beam axis and the surrounding tube is reduced by compensation, but when measured in terms of the temperature of compensating ions, it increases. This shall better confine the ions to the electron beam, the more complete the compensation. Increasing with the square of their charge state, trapped ions are heated by the ionizing beam electrons through small angle Coulomb collisions. For different ions with different charge states, the almost compensated electron beam acts as an energy filter, where the hotter ions are lost radially or axially. Therefore highly charged ions, which became "hot'' by waiting for stepwise ionization, can be cooled by lower charged ones, which still are "cold.'' This phenomenon has been called "evaporative cooling'' in electron beam ion sources (EBIS) and traps (EBIT) and space-charge compensation of the electron beam is the driving physic for it.

Notizen: 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).

Notizen: The electron beam ion source (EBIS) is an intense source of bare nuclei of light elements (C,N,O,Ne) for short pulses. A fast extraction time of 4 μs allows advantageous single turn injection into a medically dedicated synchrotron. Since the EBIS can provide bare nuclei of all light ions, it combines favorably with a 1.8 m long radio-frequency-quadrupole accelerator (RFQ) as the only LINAC needed for an injection energy of 1 MeV/u. The required intensity of 108 ions per pulse in 1 s is easily exceeded by a factor of 10 to compensate for beam losses. The technology of an EBIS with an electron current of 165 mA at 3 keV and 50 A/cm2 is well established and has proven to be very reliable for synchrotron injection over many years in Dubna, Saclay, and Stockholm. The low duty factor (10−4) of the RFQ, where 250 kW pulses of rf power will be applied for 25 μs, eliminates the need for cooling. Therefore a four-rod structure may be put on ceramic stands, biased with a negative high voltage to avoid high voltage on the source. The combination of an EBIS with a short RFQ may be considered as the least complex and most reliable injector solution for filling a cancer therapy synchrotron with fully stripped light ions.

Notizen: 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.