ALBERT

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: 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 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.

Notizen: The article is devoted to the problems of heating and cooling of highly charged ions in EBIS. The loss rates for ions with the Boltzmann distribution function of energies for different ionic species are determined. The results suggest the use of the lightest possible ions for ion cooling. Continuous flow of light ions is proposed for cooling. The necessary parameters of light ion flow are defined in the article.

Notizen: For the injection of a low-current (100 μA) high-brightness ion beam into an EBIS for subsequent ionization to very high-charge states, we have constructed a bucket-type ion source with the following specific features: a) microwave discharge plasma generator for long lifetime, b) isolated end flanges to allow for versatile ion species production by sputtering, c) SmCo magnets in 90° easy-axis rotation for the magnetic cusp fields, and d) large diameter for reduction of gas load to the UHV beamline of the EBIS. The extraction system has been optimized for maximum brilliance at an emission current density of about 2 mA/cm2. The extraction electrode consists of a spent washer of the sputtered material and of the long-lasting complex shaped main part. The design provides almost unchanged extraction optics until the washer is sputtered away.

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: The kinetics of deuterium doping of and removal from polycrystalline (Ba,Sr)TiO3 (BST) thin films during annealing were investigated using secondary ion mass spectrometry depth profiling, and the data were correlated to changes in the electrical behavior of the films. Results for deuterium doping of exposed BST films on a Pt bottom electrode layer are consistent with incorporation of deuterium interstitial defects at the BST/Pt interface and "upward" diffusion toward the film surface. The incorporation kinetics of deuterium in Pt/BST/Pt capacitors are more complex and are greatly enhanced by the presence of the Pt top electrode. Removal of deuterium from D2/N2-exposed Pt/BST/Pt specimens during oxygen recovery anneals appears to be limited by the rate of an interfacial reaction at low temperatures (200–250 °C). The pre-D2 exposure leakage current properties of the BST capacitors were found to be largely recovered when the deuterium concentration in the films was reduced to ∼1019 cm−3 during post-D2 oxygen recovery anneals. Recovery annealing in vacuum, although it removed deuterium from the films, was found to result in an increase in leakage current density for annealing temperatures greater than 300 °C. These results suggest that introduction of large amounts of positive space charge into the BST films has a pronounced effect on the electrical properties of the Pt/BST interface. © 2001 American Institute of Physics.

Notizen: 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.

Notizen: 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.