The analytical capabilities of the ion beam facility at the department of physical electronics FNSPE have been extended to allow lateral mapping of surface elemental concentrations. The method of a fixed beam collimator and a moving sample stage was chosen.

The new, easily removable target chamber was constructed. It consists of two parts. The analysing chamber on top includes an ion beam collimator, an exit window for a Si(Li) detector, a CCD camera, an LED for illumination and a He gas inlet. The exit window is a 50 um Kapton foil, which also prevents backscattered protons to reach the detector. The analysed point of a sample is surrounded by a suppression electrode. When electrically conductive samples in high vacuum are analysed, the negative bias on this electrode returns secondary electrons to the sample. In case of non-conductive samples and those that cannot be placed in high vacuum, e.g., biological samples, the chamber is filled with He gas at approximately 10 Pa .

The lower part contains the target holder chamber with 93 mm inside diameter. The sample stage is positioned by two stepper motors within the 20x20 mm range and controlled by a computer. There are also two 25 KF-flange ports for pumping, vacuum and beam current measurements. The target holder chamber was designed and manufactured by Delong Instruments in Brno.

This project also included a complete design, construction, and testing of an on-demand beam deflection system. Signal from a spectroscopic amplifier is fed into an electronic circuit, which controls a high voltage switching transistor. The fast high voltage pulse forms an electric field between two plates, which are placed inside the beam line upstream of the target chamber. This setup assures that the proton beam is deflected from the target during the time, it takes to process a single X-ray event. A photon detected during this time would be lost. The on-demand beam deflector also decreases the radiation damage of the sample, which is invaluable in particular in case of biological samples.

The initial tests revealed a low X-ray background of the chamber, and verified the functionality of the on-demand beam deflection system, which significantly decreased the intensity of the pile-up peaks. The first applications of the Prague Proton Milliprobe included analyses of industrial, archaeological, and geological samples, e.g., copper-nickel bronze weld sample, surface sample of metal artefacts, and samples of rocks. The analyses were carried out with a 0.2 mm diameter beam.

References

This research has been supported by grant Grant Agency of Czech republic No. 202/98/0922.

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