Technical Papers and Presentations

For many decades Helium-3 (³He) and Argon gas filled detectors have been used for neutron and gamma detection. Detection requires the generation of electrons which can then be measured by electronic means. These electrons are generated either directly through gamma ionization of a gas or indirectly by a neutron being captured by ³He thereby generating highly energetic ions that will ionize the gas. These electrons are accelerated in an electric field and collide with gas atoms by elastic, excitation and ionization collisions. Since the initial number of electrons generated by the ions is not enough to be electronically detectable, a process named “gas multiplication” is used boost the signal. This is done by providing a sufficiently high electric field so that the electrons have enough energy to generate ionization collisions which generate more electrons, which in turn ionize other atoms in a cascade. The COMSOL Multiphysics^® simulation software's modules utilized to model these ions and electrons include the Charged Particle Tracing Module to track the charged particles and AC/DC Module to model the electric field. One issue that makes this modeling time consuming is the large number of collisions the electrons encounter since the gas pressures are typically around 1-10 atmospheres. Under these conditions the type of collision selected i.e. null collision or time stepped, along with the study time step parameters are critical to accurately model the process. To ensure an accurate representation, a very small time step can be specified, but this may make the model run time prohibitively long. Conversely too short and the model will not represent the underlying physics. What is needed is an understanding of how these basic particle tracing parameters will affect the model so the appropriate parameters can be selected to archive the desired result.