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A monovacancy defect is observed by positron-lifetime and Doppler-broadening experiments in semiinsulating GaAs after photoquenching the ELI defects. The monovacancy exhibits the same thermal and optical recoveries as the metastable state of EL2 and its concentration is proportional to that of the ELI defect. It is concluded that the metastable state of the ELI defect involves the monovacancy. The results indicate that the monovacancy or its immediate surroundings is negatively charged.

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The feasibility and first results of a near sensor digitalization of fluxgate signals (digital fluxgate magnetometer) are presented. Applying the usual magnetometer electronics we have substituted the analogue section by a digital processing unit (ow). The 2f , signal is digitized at its second harmonic and mean values are online phase-sensitively calculated. Based on this development we present a completely redesigned magnetometer experiment for applications on planetary surfaces. Sensor and electronics including serial interface have to be in one housing, its weight is less than 150 g and the number of electrical connections is limited to four (power lines + serial link). Applications for the digital magnetometer on Earth are also discussed.

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Experimental and theoretical investigations of positron trapping by disk shaped Guinier-Preston zones as appearing in AlCu alloys are performed. Using positron lifetime and angular correlation S-parameter measurements the decomposition phenomena are studied in an AI-1.9 at% Cu alloy. The energy and the wave function of a positron within a cylindrical shaped trap is calculated using Ritz's variational principle. Although a weak localization of the positrons by multilayer Guinier-Preston zones (GP zones) is to be expected from theory, the experimental spectra are dominated by positron annihilation in vacancies surrounded by Cu atoms. Above 200 "C positron trapping by semicoherent and incoherent precipitates is found.Es werden experimentelle und theoretische Untersuchungen des Positroneneinfangs durch scheibenformige Guinier-Preston-Zonen, wie sie in A1Cu-Legierungen auftreten, vorgenommen. Am Beispiel einer Al-1,9 at% Cu-Legierung werden die Entmischungsvorgange mittels Positronen-Lebensdauersowie Winkelkorrelations-S-Parameter-Messungen untersucht. Die Energie und die Wellenfunktion cines Positrons in einer zylinderformigen Haftstelle werden mit Hilfe des Ritzschen Variationsverfahrens berechnet. Obwohl ein Positroneneinfang durch die mehrschichtigen Guinier-Preston-Zonen theoretisch moglich ist, werden die Spektren durch Annihilationsakte in Leerstellen in kupferreicher Umgebung bestimmt. Oberhalb 200 "C wird ein Positroneneinfang durch semikoharente und inkoharente Ausscheidungen beobachtet.

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