Novel gamma-ray telescope schemes are under development so as to bridge the 0.1-100 MeV sensitivity
gap of gamma-ray astronomy (Compton, pair creation), (silicon wafer stacks, emulsions,
gas detectors). The lower average density with respect, e.g. to the tungsten/silicon active target of
the Fermi-LAT makes square-meter effective area telescopes voluminous objects, for which the
photon energy measurement by conventional means (calorimeter, magnetic spectrometer, transition
radiation detector) is a challenge for the mass budget.
We present an optimal measurement of track momentum by the multiple measurement of the
angular deflections induced by multiple scattering in the active target itself, using a Bayesian
analysis of the filtering innovations of a series of Kalman filters applied to the track.