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Galactic positrons in the interstellar medium (ISM) annihilate to produce a narrow emission line at 511 keV. The profile of the annihilation line contains information about the physical conditions and processes of the ISM. The Line Shape Integral (LSI) method, a new statistical technique we have developed, directly compares theoretical models to observations, with superior sensitivity to previous methods. The LSI method is applied to high resolution observations towards the Galactic Center (GC) and two theoretical composite line profiles of Guessoum et al. (1991), i.e. (1) positrons penetrate and annihilate in all of the ISM and (2) they are excluded from atomic and molecular clouds. Both models are individually excluded, but an optimum combination of the models is allowed. We give two possible interpretations: (1) Positrons propagate freely in the ISM and observations towards the GC reflect a depletion of atomic and/or molecular clouds (A/MC) of $54\pm18\%$ relative to the models or (2) Magnetic fields exclude positrons from A/MC with an efficiency of $54\pm18\%$. In case (2) low energy cosmic rays would also be excluded with this efficiency.
\noindent Guessoum, N., Ramaty, R., \& Lingenfelter, R.E. 1991 ApJ, 378, 170
