MODELING DETECTOR RESPONSE FOR NEUTRON DEPTH PROFILING

In Neutron Depth Profiling (NDP), inferences about the concentration p rofile of an element in a material are based on the energy spectrum of charged particles emitted due to specific nuclear reactions. The dete ctor response function relates the depth of emission to the expected e nergy spectrum of the emitted particles. Here, the detector response f unction is modeled for arbitrary source and detector geometries based on a model for the stopping power of the material, energy straggling, multiple scattering and random detector measurement error. At the NIST Cold Neutron Research Facility, a NDP spectrum was collected for a di amond-like carbon (DLC) sample doped with boron. A vertical slit was p laced in front of the detector for collimation. Based on the computed detector response function, a model for the depth profile of boron is fit to the observed NDP spectrum. The contribution of straggling to ov erall variability was increased by multiplying the Bohr Model predicti on by a ramp factor. The adjustable parameter in the ramp was selected to give the best agreement between the fitted profile and the expecte d shape of the profile. The expected shape is determined from experime ntal process control measurements.