Handbook of Modern Coating Technologies

Principles of positron beam generation

Within a few years, the HF low-energy positron beam has become a powerful tool for spectroscopic analysis of defects based on the measurement of positron lifetime in the
near-surface layer [140,157,158]. Positrons emitted from a radioactive source thermalize in a thin layer of single-crystalline tungsten and are re-emitted with efficiency of ~2 X 10_4 and energy close to 3 eV. The re-emitted positrons make up a continuous beam. It is compressed to a 100-ps pulsed beam near the target using special RF (HF) devices. Timing signals for measurement of lifetime are generated by one of the annihilation photons, while the respec¬tive synchronizing signals come from the RF system. The positron beam diameter is ~4 mm. The first operation of such systems was in 1988. It was possible to measure the lifetime depending on the positron energy (penetration depth). After modifications of the system made in 1993, the quality of lifetime spectrum measurements was improved (by decreasing the contribution from background signals, as well as the level of backscattering signals) and measurement intervals were increased up to 20 ns to achieve the best filling factor [159]. Moreover, such a system permitted measurements at variable temperature.
The system has a chamber for in situ annealing of a single-crystalline tungsten foil and a prebuncher to compress the continuous beam to pulses of a 1.7-ns half-width at the 
half-height by applying sawtooth voltage to the drift tube (for details see article [157]). The system has been further improved recently (Fig. 5—15). The distance between the positron source and the detector was increased to reduce the level of background signals, and a chopper was installed at the beamline behind the device. The new, accelerator and main 50-MHz buncher installed at the drift tube were subsequently removed from the target, which allowed reducing the positron backscattering effects due to greater volume in front of the sample chamber.
The built-in Wien filter prevents the return to the surface of positrons backscattered at the angles of less than 90 degrees. Differential pumping ensures a vacuum down to 10_1° mbar around the target. Furthermore the recently introduced system for the substitu¬tion of samples makes it possible to study a few without disturbing the vacuum conditions. The new temperature control system allows the sample temperature to be varied from 10 to 600K. The horizontal position of the sample chamber permits studying liquid alloys and metals. The system has one more advantage over conventional methods, besides the possi¬bility of measuring positron lifetimes as a function of the energy (penetration depth). Specifically it is free of limitations on positron intensities due to accidental coincidences. One of the timing signals comes from the HF system, and the final coincidence level is equivalent to the detector count rate for annihilation photons (integrated rate) with this tim¬ing signal. In the case of a highly intense main positron source, it is possible to increase the count rate almost without limit.