Soft X-ray contact microscopy using laser generated plasma sources
The ultimate objective of this project was to develop a small, transportable X-ray microscope which would be able to view a wide range of biological specimens without the need for any type of sample preparation at a resolution greater than that obtainable by conventional light microscopy (ie. about 250nm). Of the various possible implementations of X-ray microscopy currently being investigated, contact microscopy was chosen as being the most suitable for the development of such a small-scale instrument, while at the same time minimizing the effects on image quality of radiation damage to the biological specimen. The requirement for a high brightness pulsed X-ray source of less than 50ns duration for illumination of the specimen was met by the production of laser generated plasmas. These were formed by focusing a 2.2J KrF laser beam, of wavelength 248nm and duration 20ns, onto the surface of one of a number of different target materials. In order to obtain the large intensities required for the production of a sufficiently high temperature plasma, a doubly pre-ionized, discharge-pumped amplifier KrF laser was developed. This was seeded by a smaller oscillator laser by means of a coupled unstable resonator configuration. A number of different cavity arrangements were investigated and an output beam divergence of 2.5 times the diffraction limit was achieved. The plasmas generated by focusing the laser beam to an intensity of 1014W/cm2 onto carbon, titanium, molybdenum and tungsten targets were characterized as fully as was necessary for their use in the X-ray microscope. Preliminary investigations on the use of a grazing incidence ellipsoidal mirror to focus the emitted X-rays onto the specimen of the microscope were made and such an optical component was manufactured and tested. Finally, numerous images of a number of different biological specimens were made and resolutions of better than 100nm were achieved. Images were read out using a Park Scientific Instruments atomic force microscope, which enabled the entire microscopy process to be carried out in a single working day. The system is now in routine use and can produce more than ten images per session.