Numerical simulation of argon–methane gas filled proportional counters

A new Monte Carlo simulation code has been developed in order to examine the nature of single electron-induced avalanching in cylindrical single wire proportional counters. The twin effects of high reduced field gradients and reduced scatter of electrons away from the anode, which occur in the near-anode region, cause the first ionisation coefficient to depart from its value at the equivalent uniform field. The simulations also suggest that better energy resolution is obtained for counters of lower radii for the same mean gain. This is because of the higher reduced field gradient at a given value of the reduced field in the early stages of the avalanche development.