Target materials suitable for projection X-ray microscope observation of biological samples.

Trials for improving the contrast of projection X-ray images by finding better target materials than Ti (lambda K alpha: 2.75 A), which has been found to be suitable for many kinds of specimens, were carried out, considering the factors of melting point, thermal conductivity, mechanical strength, chemical stability, absorption of the X-ray, etc. Au, Ta, and Ge were found to be suitable, giving 5-10 A X-rays when low electron beam energies around 10 kV were used. In order to take advantage of the long wavelength X-rays of these targets, we tried to minimize the attenuation of the imaging X-rays in the air or to use a vacuum camera. Even in non-stained biological samples such as HeLa cells and lingual muscle section, their microstructures were visible with sufficient contrast.