Positron-annihilationS-parameter measurements in thermal equilibrium on pure and carbon-doped (50 and 750 at. ppm) a-iron are presented. It is shown that trapping of positrons in both monovacancies and carbon-vacancy pairs occurs, even far above the dissociation temperature of the vacancy pairs. Therefore, a three-state trapping model is used in the analysis of the measured S curves. The vacancy-formation enthalpy in both the paramagnetic and ferromagnetic state is deduced: It is found to be 1.79%0.10 eV in the paramagnetic state and 2.0+0.2 eV in the ferromagnetic state. These values are larger than those published so far. The activation enthalpy for vacancy migration obtained by combining the values cited above with recently published self-diffusion enthalpy values confirms the applicability of the one-interstitial model in a-iron.

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Various point defects in silicon are studied theoretically from the point view of positron annihilation spectroscopy. Properties of a positron trapped at a single vacancy, divacancy, vacancy-oxygen complexes (VO n ), and divacancy-oxygen complex are investigated. In addition to the positron lifetime and positron binding energy to defects, we also calculate the momentum distribution of annihilation photons ͑MDAP͒ for high momenta, which has been recently shown to be a useful quantity for defect identification in semiconductors. The influence of atomic relaxations around defects on positron properties is also examined. Mutual differences among the high momentum parts of the MDAP for various defects studied are mostly considerable, which can be used for the experimental defect determination. ͓S0163-1829͑98͒03039-2͔

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A miniaturized orthogonal time-of-flight mass spectrometer with an electron impact ionization ion source and a rf quadrupole ion guide has been developed. A mass resolving power of m/⌬m ϭ 5500 has been obtained in a 0.4 m instrument. The addition of helium at pressures of about 4.0 mtorr into the ion source showed collisional focusing taking place in the rf quadrupole. An automated gas chromatograph designed for air monitoring applications has been coupled to the time-of-flight mass analyzer and tested for the detection of simulants of chemical-warfare agents. A dvanced easily portable chemical sensor systems are essential for the on-site analytical measurements required for a variety of diverse applications such as environmental characterization and monitoring, law enforcement, and chemical emergency response. For such applications, mass spectrometry-based systems that combine laboratory performance with high portability and robustness are required. Time-of-flight mass spectrometers (TOF-MS) look promising for realization of these goals because of their simple designs and rapid delivery of full mass spectra. However, the fact that limited mass resolution m/⌬m (up to a few hundred) has been obtained with electron impact ionization has limited their applications for the analysis of volatile and semi-volatile chemical compounds.The major limitation comes from the initial velocity distribution of ions created by electron impact in the storage ion source [1], and particularly the inability to compensate for the turn-around time described early on by Wiley and McLaren [2]. The mass resolution of the TOF can be increased by the addition of an ion mirror [3]. An ion mirror images the McLaren space-time focus plane to the plane of the ion detector in such a manner that ions with differing energies (but the same mass) will arrive at the detector at the same time. The resolution of this type of instrument, when coupled to a gas chromatograph, is still limited because of a continuous gas load to the ion source located in the analyzer chamber. With this technology a mass resolution of about 700 was obtained on a 1.5 m long TOF instrument [4].An alternative approach consists in forming the ion beam in a direction which is perpendicular to the time-of-flight direction. The original velocity spread in the longitudinal direction no longer affects the arrival time of an ion packet on the detector. An orthogonal injection TOF was first proposed in 1964 [5] but has been reintroduced as a method for improving mass resolution [6,7]. A mass resolution of m/⌬m Ͼ 10,000 was demonstrated recently in a 1.2 m long instrument with an orthogonal electron impact ion source and ion mirror [8].In this paper we present a compact time-of-flight mass spectrometer with an orthogonal electron impact ion source. An rf quadrupole used as an ion focusing element allowed us to achieve high mass resolving power at elevated pressures in the ion source housing. To demonstrate the potential of orthogonal-extraction, time-of-flight mass spectrometer for air...