Efficient Simulation of Secondary Fluorescence via NIST DTSA-II Monte Carlo
Secondary fluorescence, the final term in the familiar matrix correction triumvirate Z·A·F, is the most challenging for Monte Carlo models to simulate. In fact, only two implementations of Monte Carlo models commonly used to simulate electron probe x-ray spectra can calculate secondary fluorescence - PENEPMA [Llovet, 2006] and NIST DTSA-II (discussed herein). These two models share many physical models but there are some important differences in the way each implements x-ray emission including secondary fluorescence. PENEPMA is based on PENELOPE [Salvat, 2006], a general purpose software package for simulation of both relativistic and sub- relativistic electron / positron interactions with matter. On the other hand, NIST DTSA-II was designed exclusively for simulation of sub-relativistic electron generated x-ray spectra. These different intended audiences lead to different implementation decisions. NIST DTSA-II makes certain computational optimizations that aren't available for general purpose code. These optimizations primarily have to do with how x-rays are accumulated by the simulated detector. In PENEPMA, many x-rays are generated for each one detected (much like in the real world) whereas in NIST DTSA-II fractional x-rays are generated each time an electron scatters and a fraction of each fractional x-ray is detected. Since x-ray generation is rare and the collection solid angle of typical detectors are measured in tens of milliradians, these optimizations help NIST DTSA-II to be orders-of-magnitude more computationally efficient while retaining detector position sensitivity. The result is simulations that execute in minutes rather than hours and can model differences that result from detector position. Both PENEPMA and NIST DTSA-II are able of handling complex sample geometries and we will demonstrate that both are of similar accuracy when modeling experimental secondary fluorescence data from the literature.