Publications

Abstract not provided.

This article describes how features of event tree analysis and Monte Carlo-based discrete event simulation can be combined with concepts from object-oriented analysis to develop a new risk assessment methodology, with some of the best features of each. The resultant object-based event scenario tree (OBEST) methodology enables an analyst to rapidly construct realistic models for scenarios for which an a priori discovery of event ordering is either cumbersome or impossible. Each scenario produced by OBEST is automatically associated with a likelihood estimate because probabilistic branching is integral to the object model definition. The OBEST methodology is then applied to an aviation safety problem that considers mechanisms by which an aircraft might become involved in a runway incursion incident. The resulting OBEST model demonstrates how a close link between human reliability analysis and probabilistic risk assessment methods can provide important insights into aviation safety phenomenology.

Linear schemes applied to charged particle transport problems demonstrate high order accuracy but under certain conditions can also produce negative solutions. On the other hand, the recently developed nonlinear exponential discontinuous (ED) method has been shown to produce accurate strictly positive solutions, for positive sources, in neutral particle transport applications. We have applied this method to the solution, in space and energy, of the multispecies transport equations for relativistic heavy ions. The solution may be useful as a treatment planning tool for the irradiation of certain cancers using heavy ions. Collisions between projectile ions and atoms in the target medium can result in ion fragments different from the original species. The solution includes these projectile fragments. The primary ion and all fragments are treated using the straight ahead approximation under which the fragments continue on with the same velocity as the original projectile.

Heavy charged particles deposit much of their kinetic energy at very high rates in small volumes near the end of their range. This characteristic, coupled with the availability of modern particle accelerators, has sparked a revival of interest in the use of ions as a possible treatment tool for certain types of cancers. Collisions between projectile ions and atoms in the target medium can result in ion fragments that are different from the original projectile species. The energy deposition characteristics of these fragments differ from those of the projectile in a manner that allows them to travel beyond the range of the original particle. This can result in deposition of doses in healthy tissue beyond the tumor. The loss of projectiles due to the fragmentation process will also affect the dose deposited in the target tumor. An accurate dose calculation requires that these effects be taken into account. Monte Carlo calculations are expensive, time consuming, and can be limited in the number of ion species considered. Linear methods can yield high-order accuracy but can sometimes exhibit the undesirable characteristic of calculating negative fluxes. In order to bypass these difficulties, we have applied the recently developed exponential discontinuous (ED) finite- element method to a calculation of dose deposition by relativistic heavy ion projectiles and fragments. The ED method has been shown to yield strictly -- positive solutions for positive sources of neutral particles.