The Large Area Telescope (LAT) instrument is designed to determine the energy and direction of gamma ray photons that arrive at the detector. The detector is a stack of conversion and detection layers. The gamma ray photon is converted into an electron/positron pair in one of the conversion layers, and these particles cascade through the detector. The electron and positron are themselves subject to secondary scattering, and also produce further electrons and photons . All these particles trigger the detector strips, which read out the position that a particle crossed the detector layer. The particles finally interact with the calorimeter, which gives an estimate of the energy of the event. The anticoincidence detector eliminates detections due to charged particles (charged particles trigger the anticoincidence detector and the detector strips, whereas gamma rays trigger only the detector strips). The main output from the detector is the set of positions of the microstrip detectors that were triggered, together with the response of the calorimeter. It is from this set of positions and the calorimeter response that the energy and direction of the photon must be determined.
We present work-in-progress on analysing the event data using sequential importance sampling. We represent the distribution by a set of samples, distributed as our current belief in the direction and energy of the photon, and at each level we propagate the samples down the stack by drawing from a set of importance sampling distributions that focus the particles on the positions of the hits at the next level. We then resample the particles, using as the weighting the ratio of the the scattering distribution defined by the physics of the instrument to the importance sampling distribution.
We will present results of applying this approach to a simplified representation of the GLAST detector.
Time and Place: Wed., Feb. 25, at 4 pm in 4610 Engr. Hall. *** NOTE SPECIAL TIME ***
SYSTEMS SEMINAR WEB PAGE: http://www.cae.wisc.edu/~gubner/seminar/