[en] The charge states of divacancies induced by 5 MeV self-implantation of doped silicon were investigated by positron annihilation methods. For low doping concentrations, results were found to be in agreement with the predictions of Fermi statistics. For the case of heavily boron-doped silicon (n_B=1times10¹⁹cm^-3) an anomalous single-negative divacancy charge state was detected. We attribute this to the introduction of new levels in the band gap, due to the capture of boron by divacancies, resulting in a boron-divacancy complex. Detailed analysis of positron annihilation spectra suggests that the boron does not reside on a nearest-neighbor site to the divacancy. Isothermal annealing experiments yield activation energy of 0.9±0.1eV for migration of this defect. [copyright] 2001 American Institute of Physics

[en] Purpose: The aim of the current study is to investigate the effect of machine output variation on the delivery of the RapidArc verification plans. Methods: Three verification plans were generated using Eclipse™ treatment planning system (V11.031) with plan normalization value 100.0%. These plans were delivered on the linear accelerators using ArcCHECK− device, with machine output 1.000 cGy/MU at calibration point. These planned and delivered dose distributions were used as reference plans. Additional plans were created in Eclipse− with normalization values ranging 92.80%–102% to mimic the machine output ranging 1.072cGy/MU-0.980cGy/MU, at the calibration point. These plans were compared against the reference plans using gamma indices (3%, 3mm) and (2%, 2mm). Calculated gammas were studied for its dependence on machine output. Plans were considered passed if 90% of the points satisfy the defined gamma criteria. Results: The gamma index (3%, 3mm) was insensitive to output fluctuation within the output tolerance level (2% of calibration), and showed failures, when the machine output exceeds ≥3%. Gamma (2%, 2mm) was found to be more sensitive to the output variation compared to the gamma (3%, 3mm), and showed failures, when output exceeds ≥1.7%. The variation of the gamma indices with output variability also showed dependence upon the plan parameters (e.g. MLC movement and gantry rotation). The variation of the percentage points passing gamma criteria with output variation followed a non-linear decrease beyond the output tolerance level. Conclusion: Data from the limited plans and output conditions showed that gamma (2%, 2mm) is more sensitive to the output fluctuations compared to Gamma (3%,3mm). Work under progress, including detail data from a large number of plans and a wide range of output conditions, may be able to conclude the quantitative dependence of gammas on machine output, and hence the effect on the quality of delivered rapid arc plans.

[en] Ferric and magnesium stearate Langmuir-Blodgett films deposited on silica glass were studied by variable-energy positron beam. Positrons signal the presence of the film even after deposition of only 3 monolayers. A signal with 3 and 9% higher S parameters values are characteristic for the ferric and magnesium stearate films, respectively. In addition, positrons were found to be sensitive to the chemical structure of the layers. The results suggest that positron beams could be promising probes of the thickness, composition, and adhesion of Langmuir-Blodgett films

[en] This work investigated surface material modifications in high-purity beryllium foils resulting from 1-keV deuterium ion implantation into specimens for which the anneal temperatures and implantation temperatures were varied. Defects in unimplanted and in deuterium-implanted beryllium were characterized principally by positron-beam depth-profile analyses. Depth-profiles of the defect distributions in the specimens were made by stepping the energy of the positron beam from 0.055 keV to 40 keV, accompanied by measurements of the Doppler-broadened annihilation radiation line shape at each positron energy. These analyses identified a varying defect structure in beryllium, dependent on the previous anneal history of the material and on the temperature of the material during implantation with energetic deuterium ions. For specimens implanted at room temperature with 1-keV/D ions, the beam-induced defect structure had a profile that was peaked near the mean. range of the implanting deuterium and that extended beyond the implantation zone. Isochronal step-thermal anneal experiments revealed that deuterium was released from these defects at a temperature of about 400K, indicative of shallow traps, and that the defect structure annealed at temperatures above 623K. The beam-induced vacancy-defect complexes were estimated to be 1-nm voids for 1-keV/D implantation into Be at room temperature. For beryllium implanted at temperatures of 723K with 1-keV/D ions, these measurements revealed that the beam-induced defect structure was much broader and extended far beyond the implantation zone. Isochronal stepthermal anneal experiments for these specimens revealed a more stable defect structure, with the onset of defect annealing at 775K. There was no evidence of deuterium release at lower temperatures, indicating that for beryllium implanted at elevated temperatures, deuterium is retained in deep traps

[en] The rate of defect accumulation due to ion irradiation is discussed. Silicon was implanted with 11.5 MeV Au^-6 ions, to fluences ranging from 2 x 10⁹ to 1 x 10¹¹ cm^-2. We find the measured defect concentrations C can be related to ion fluence φ via C = A φⁿ, with A∼1600 and n∼1.0. The linear dependence of C on φ is consistent with expectations for the rate of accumulation of defects in the limit of low ion fluences, and is in contrast to previously reported values of n<1, which were obtained using higher fluences, for which significant overlap between individual damage cascades is expected. (orig.)