Handbook of Modern Coating Technologies

Scanning positron microscope

The reduction of the generated pulsed positron beam from few millimeters to few microns in diameter makes it applicable for the scanning positron microscope (SPM). It takes advan¬tage of a positron beam of a micron-scale diameter with variable energy. The SPM combines a conventional SEM and a pulsed positron microprobe with the pulse duration ~100 ps and the spot of size 1 pm.
Visualization with an electron beam provides a conventional image of the surface. The areas of interest can be selected on such image in a nondestructive mode for subsequent non-destructive study and analysis with the aid of a positron microprobe [167—170]. This method is based on the results of positron beam research and recent developments in the field of parti-cle optics. Systems with positron and electron beams differ in the density of fluxes of their sources. For instance, LaB6 source of electrons has flux density that is 16 orders of magnitude greater than that of a monoenergetic positron beam. However, the use of an intense source of positrons improves this ratio by several orders [171]. Therefore during the formation of the 
probe for a scanning positron microscope it is important to maintain the positron flux as high as possible compared with the SEM. This means that scanning positron microscopy has to apply re-moderation. Furthermore the SPM development must focus mainly on the sufficient time and spatial resolutions, effective utilization of positron radiation, sufficient stability of 
functioning mechanical and electrical systems, low magnetic background, and an appropriate size of the specimen chamber. The general layout of the setup is presented in Fig. 5—17A.
This system operates in an ultrahigh vacuum of 10_9 to 10_10mbar. Specimens with dimensions of up to 20 X 20 X 3 mm3 are inserted into the chamber through the use of a manipulator. Any region of a specimen can be positioned with the manipulator within the positron- and electron-scanned area of at least 600 X 600 mm2 without detriment to spatial resolution. The positron source moderator assembly coupled with the first accelerator pro¬duces a continuous positron beam of 20-eV kinetic energy. This beam is injected into the drift tube to which a sawtooth signal with a repetition rate of 50 MHz is applied. Positrons in each 20-ns segment of the continuous beam are in this way compressed into bunches ~2 ns wide. The compression in the drift tube is much more efficacious than the usual beam split¬ting, where the losses amount to 30% compared with ~10%. Then positrons leave the drift space and the sinusoidal buncher (resonator) accelerates them up to 800 eV. The first buncher is designed by analogy with that used earlier in Ref. [155]. The beam noise damper positioned in front of the buncher eliminates the time-uncorrelated background. The first buncher increases the width of a pulse from 2 to 200 ps with the help of a re-moderator comprising tungsten crystal cooled in a cryostat to the liquid-nitrogen temperature (80K). The cooling leads to a threefold decrease in the energy spread of re-moderated (re-emitted) positrons, which finally reduce to 40% at the spot diameter. Positron experiments planned for the near future have the objective to decrease the transverse energy spread of positrons by decreasing the temperature of W crystal up to the temperature of liquid-helium.
Optical system of the remoderator block consists of a combination of electric and mag¬netic lenses focusing the input parallel positron beam onto the moderator and guiding the reemitted moderated positrons parallel to each other with E = 200 eV [155,172]. To separate the input and remoderated beams and guide the lagging one into the optical column, the system contains a toroidal reflector [155,172]. An accelerator, which ensure an implantation energy to be in the range 0.5—30 keV, the deflecting coils, the probe-forming magnetic lenses, and a sample chamber are components of the optical column.
At first in the accelerator input, the pulsing beam reaches the sample chamber, particu¬larly a big Faraday cage and the probe-forming lenses, which create magnetic field. Outside the vacuum chamber there are placed toroidal windings of reflector for the magnetic field governing the beam for scanning. Due to the large variety of measuring techniques using a positron beam, the sample chamber in the SPM should be designed differently compared with that in the conventional SEM. The high-count rate of annihilation photons is achieved by placing a large-area detector very close to the specimen chamber. On the other hand, the half space of the chamber forepart must be free of material to suppress possible distortions of the lifetime spectra by annihilation radiation resulting from backscattered positrons anni¬hilating at the wall of the vacuum chamber. The necessary design requirements are met by a side-gap single-pole lens placed behind the specimen chamber just outside of the vacuum chamber with the radiation detector inside the central field pole piece [155].
A BaF2 crystal scintillation counter is fitted directly on a Valvo-XP-2020Q photomulti¬meter used as the detector of 511-keV positrons. The size of the pulsed beam spot in the remoderator decreases to 20 pm. The phase-space density of the positrons re-emitted from this spot exceeds that in one of the first moderators by a factor of 3 X 104. Up to now, the typical gain in phase-space density of a single remoderation stage has been only 20 [135,155], meaning that in the design of this SPM a single remoderation stage replaces the three conventional remoderation stages necessary in other methods [135,155,168], thereby reducing the primary source intensity by a factor of 25. This progress in beam quality is due to the well-balanced positron beam transport system, the superior properties of the single¬pole lens, and the application of time bunching. It increases the transfer coefficient in the phase space by a factor of 50.
An electron beam that goes through the same optical column as a beam of positrons pro-vides an image that can be used as a readily available important reference for the chosen region to be further scanned by the positron beam. This approach makes it possible to pre¬cisely focus and adjust the optical column. An electron-beam imaging of a gold mesh is pre¬sented in Fig. 5—17B.
Actual beam formation (bunching) is 70% and re-moderation efficiency is 23%. The beam of positrons has spatial resolution of ~1 pm. This value is slightly higher than for the electron beam. For practical applications, a preliminary positron source is needed to ensure an acceptable measurement time. A typical lifetime spectrum contains as high as 106 counts. It is planned to use 1-Ci58Co as the preliminary source of positrons;given the aforemen¬tioned efficiency, it is expected to have 5:8 X 105 positrons per second at the target, and 2 X 104 s-1 at the annihilation photon detector. For a highly intense positron source, such as the one installed in the reactor [68,155], the rise may be as high as three orders of magnitude.
Worthy of note is the most successful application of a proton beam microprobe (the PIXE method) and a positron beam microprobe for the analysis of segregation and diffusion pro-cesses at micrograin boundaries [173,174].
The authors of Refs. [173,174] implanted Al1, C1, and Ti1 ions into polished a-Fe speci¬mens at a fluence of 2 X 1017 or 5x1017 cm-2 and analyzed impurity segregation regions, grain boundaries, and regions of vacancy clusters with interstitial carbon atoms (2.3 atoms); in other words, defect complexes (a vacancy plus a few interstitial C atoms) were examined. The same a-Fe specimens irradiated by high-current electron beams with a dif¬ferent number of pulses were used in paper [174] to analyze the segregation of C atoms and diffusion of Fe vacancies. Interstitial C atoms bunched together in high-carbon regions, where subsequent irradiation with a high-current electron beam caused crater for¬mation resulting in the considerable alteration of the surface relief for the polished a-Fe specimens [173,174].
The scanning positron microscope and the pulsed positron beam described in this sec¬tion are the first such currently operating systems. Both are designed not only to demon¬strate phenomena (principles) but also to be used in various practical applications. These developments open up new prospects for micro- (and smaller)-scale near-surface research of special interest for solid state physics, materials science, and investigations of nanomater¬ials, first and foremost nanocomposite, superhard, and thermally stable coatings [175].