10-100 MeV Gamma-Ray Astronomy with GLAST

The purpose of this paper is to report on alternatives to the baseline GLAST-type gamma-ray pair telescopes that may provide enhance performance from 10 to 100 MeV while preserving the targeted performance above 100 MeV. At the low end of the energy range the single event angular resolution (point-spread-function or PSF) pair telescopes rests with the instrument's ability to reconstruct the original directions of the electron and positron pair particles. The electron and positron multiple Coulomb scattering in the converter layer where the pair interaction occurs (the vertex) limits this resolution at low energies. We have investigated several alternatives that utilize the silicon strip detectors as both the pair converter material and tracker. These include removing the lead converter material from a single vertical section of the instrument and using only short track pair events originating in the bottom silicon layers where the particle tracks won't traverse any lead material. Finally, we have considered uniformly spaced double-sided silicon strip detectors in a single vertical section. Each alternative provides a significant improvement in angular resolution. A comparison was made on the basis of the relative statistical sensitivities of the different alternatives and the distinguishability of nearby discrete sources. In some cases the full instrument was compared with a single vertical section. Because of the large effective area of the full baseline instrument below 100 MeV none of the alternatives could match its sensitivity. However, by sacrificing some statistical sensitivity the prospects for significant measurements below 100 MeV with the GLAST instrument can be realized. The authors wish to acknowledge the support of NASA Contract NAS5-98173.