Intensity Limitations in a Point Source of X-Rays

Abstract

When the diameter of the focal spot of electrons on an X-ray target is reduced to the order of a micron, the conditions of heat dissipation become so favourable that a greatly increased specific loading is possible. The use of an electron lens to obtain such a small spot introduces a limitation on the rate of supply of energy, as its high spherical aberration requires insertion of a very small aperture, the size of which must be reduced pari passu with the size of spot desired. For magnetic lenses of usual design the rate of energy supply is then proportional to the radius of the spot. Comparison with the rate of heat dissipation shows that, for spots of the order of a micron, the limitation on X-ray intensity is not the thermal properties of the target, as in tubes with large focus, but the emission obtainable from the cathode. Specific loadings of the order of 107 watts cm-2 can be tolerated in a spot of 1 micron; the corresponding emission current density would be of the order of 100 amp cm-2.