The focal dimension of the X-ray source with microbeam is in proportion to the size of the e-beam focused on the target surface. The finite difference method is utilized to calculate the distribution of the electrical-field intensity. The track of the electron-beam is traced by using the Runge-Kutta method within the whole symmetrical system. As a result, the diameter of the electron beam focus is 0.6-1.0μm while the total high brightness emission of the LaB₆ crystal cathode is 0.06-0.3mA.

A model for the fluorescence escape efficiency of CsI:Na(CsI:Tl) phosphor is proposed,and the converting factor of CsI:Na(CsI:Tl) to X-rays is discussed in detail.Calculated results show that the fluorescence escape efficiency,as well as the converting factor,strongly depends on the substrate reflectivity and 1/(σL)(σ is the absorption coefficient of CsI:Na(CsI:Tl) to fluorescence, and L is the phosphor layer thickness.).For a phosphor,1/(σL) should be above 10,and 30~40 is even better.In point of converting factor and spatial resolution,the reflection model is superior to the transmission model.The maximum converting factor can be obtained by optimizing the phosphor layer thickness.

The interaction processes of bremsstrahlung X rays with CsI:Na scintillator of different thickness(200~400 μm) are first simulated by means of Monte Carlo analysis. The energy spectra of exit photons from CsI:Na are presented, which are used as the incident spectra on iron, the constituents of the inner electrodes and shell of intensifiers. The backscattering properties of X rays in intensifiers are analyzed. The potential producing bremsstrahlung X rays is 20~120 kV. Results show that with the increase of X ray tube potential the number of backscattered photons gets bigger and their mean energies increase rapidly.