Unfolding Charged Particle Energy Spectra From Space Radiation Instrument Measurements: Application of a Bayesian-based Technique to Derive Trapped Electron Spectra From Particle Telescope Measurements

Details Unfolding Charged Particle Energy Spectra From Space Radiation Instrument Measurements: Application of a Bayesian-based Technique to Derive Trapped Electron Spectra From Particle Telescope Measurements Unfolding Charged Particle Energy Spectra From Space Radiation Instrument Measurements: Application of a Bayesian-based Technique to Derive Trapped Electron Spectra From Particle Telescope Measurements

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm13a0340g&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM13A-0340

Mathematics

Logic

2774 Radiation Belts, 2794 Instruments And Techniques, 7984 Space Radiation Environment

Scientific paper

The Compact Environmental Anomaly Sensor (CEASE), a space radiation monitoring and warning instrument developed by Amptek, Inc. for the Air Force Research Laboratory, has for the past six years made measurements of the charged particle environment aboard spacecraft in low-Earth and geosynchronous orbits. The suit of detectors comprising CEASE includes a small 2-element coaxially-aligned charged particle telescope with a response threshold of ~0.055 MeV for electrons and ~1 MeV for protons. Each event detected by the telescope is analyzed by the instrument's internal processor and a counter incremented in one of 80 bins based on energy deposited in the two solid state detectors; 64 of the 80 bins correspond to events which satisfy the instrument's coincidence logic. Response functions for electrons and protons have been computed for each of the bins from Monte Carlo analyses using the MCNP and ITS code packages (Brautigam et al., AFRL-VS-HA-TR-2006-1030 (01 Mar 2006)). In principle these computed response functions can be used to unfold (i.e., deconvolve) the particle energy spectra from the binned data. In this initial study a Bayesian-based method (D'Agostini, NIM, A362 (1995) 487-98), used in analyzing data from high- energy physics experiments, is applied to measurements from the CEASE charged particle telescope aboard the TSX-5 spacecraft, operating in 410 x 1750 km/69° inclination orbit. To limit the complexity of this initial analysis, only measurements obtained when the spacecraft was in the outer electron belt are used, minimizing the effects of contamination caused by trapped protons. A total of 20 data channels, corresponding to 0.055-11 MeV incident electrons, are used as input to the unfolding algorithm. The Bayesian algorithm also requires input of an a priori spectral shape; a negative exponential in particle energy is used in this initial assessment. The resulting spectra evolve from iterative application of the Bayesian algorithm, using an appropriate metric to determine convergence on a reasonable solution. For this study, χ2, computed from the counts in each of the 20 data channels resulting from the unfolded spectrum and the measurements, is used as the metric. The resulting unfolded spectra converge after 3-6 iterations, and the shape and χ2 values remains stable through 25 iterations. In comparison to the initial exponential form, the unfolded spectra are softer in the region 0.8-1.3 MeV, and harder from 1.3-3.5 MeV. The unfolded spectra include a sharp drop-off below 0.8 MeV, an unrealistic feature resulting from inaccuracies in the modeled response functions for low-energy (0.055-0.8 MeV) electrons. The unfolding analysis is extended to quantify the method's sensitivity to the assumed a priori spectral shape and data bin size.

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