To study non-linear relations between field emission current density and temperature at top surface of a cathode microprotrusion, thermal effects on cathode microprotrusion are investigated numerically at various applied electric fields. Enhancement factor of electric field at top surface of microprotrusion is different as ratio of top radius to bottom radius is changed. It leads to obvious difference in temperature distribution in microprotrusion when temperature at microprotrusion peak reaches melting point of cathode material. At a given applied electric field, the smaller the ratio of top radius to bottom radius of microprotrusion, the shorter the time delay of explosive emission. Time delay of explosive emission increases exponentially with decrease of applied electric field.

A mixed Monte Carlo algorithm for transport with electrons of energy from 50 eV to 1 GeV is described which combines detailed simulation and condensed simulation.Through adjusting scattering angle and energy loss factors,times of single collisions are decided.In the process,single collisions are simulated with detailed method,and multiple scatterings among single collisions are simulated with condensed method.Results and efficiency of calculation affected by factors mentioned above are discussed.Backscattering on solids induced by incident electrons is simulated.Calculated backscattering coefficients and energy distribution agree with theoretical and experimental results well.

The atmosphere is a kind of non-uniform media,whose density drops rapidly with the increase of height.In order to simulate transport of gamma ray in high-altitude atmosphere,the media is divided into many uniform layers with different densities.To eliminate error,the number of layers should be as many as possible,which results in a large amount of computation.We propose a method without layering.It samples mass distance to replace step length of particles.The results agree well with those calculated directly by MCNP code.The method increases speed of computation considerably.

Backscattering coefficients of low energy electron beams(from 100 eV to(10 000) eV) impinging on solids(Au,Si and Al) are simulated with detailed Monte Carlo code in which all interaction experienced by single electron are simulated in chronological succession.Mott cross-section and atomic generalized-oscillator-strength model within Born approximation are used to calculate elastic and inelastic scatterings in solid,respectively.Compared with various Monte Carlo codes it is concluded that detailed Monte Carlo code provided better agreement with experimental results at electron energies less than 10 keV.