Simulation of angular distribution of molecular flux at the exit of narrow tubes and slits

In order to develop a gas-sheet target, the angular distribution of molecular flux at the exit of narrow tubes and slits was simulated by employing the Monte Carlo method. In the case of the longer tubes, the greater part of the flux is collimated near the polar angle. However, when the ratio of length to diameter, L/d (L: length, d: diameter), is greater than 10, angular distribution is almost independent of the L/d value. Even at the L/d value of 100, the fraction of outgoing molecules with polar angles less than 0.01 rad is less 0.5%. The molecules emitted from the slit can be shaped into a sheet. To do so, the following two conditions need to be satisfied: L >> b and a >> b. The letters a, b, and L represent the lengths of the long and short sides, and the depth of the slit, respectively. With the parameter set of L : a : b=100 : 50 : 0.1, the fraction of outgoing molecules collimated within the azimuth of 0.01 rad becomes 25% of all molecules. (author)