Transport of initial radiations in air over ground

A time-dependent system of classification is normally used to divide the ionizing radiation from a nuclear explosion into two groups: initial and residual. The initial radiation is the ionizing radiation emitted within the first minute following the detonation of the weapon, and the residual radiation is the ionizing radiation emitted later. Transport calculations for the prompt and delayed radiations were performed separately and then combined at fixed ground locations to obtain the total neutron and gamma-ray fluence, together with their energy and angular distributions. Modeling of the air-over-ground transport for the delayed radiation is a more difficult task. For example, such complex effects as the rapid early-time decay of the fission products, the rise of the fireball, and the blast-enhanced transport of the delayed radiation must be considered. The blast wave from the explosion enhances the transport of the delayed radiations by removing most of the air between the fireball and the shock front. This work uses transport modeling procedures and source term data developed in studies by Science Applications International Corporation (SAIC). In their studies, the delayed neutron and gamma-ray fluences were estimated by means of a one-dimensional ANISN calculation that takes into account the air density profile at a fixed location on the ground at discrete time intervals after the explosion. The effects of ground scattering were then computed separately by means of the VCS code. These transport modeling procedures were verified by applying them to weapon tests where time dependent gamma-ray measurements were made as a function of ground range. Many of the calculations and comparisons of this work involve the use of dosimetric parameter called kerma. Kerma equals the total kinetic energy of all the charged particles liberated by neutrons and gamma rays in a small volume of a given material divided by the mass of the material in that volume element