Abstract
We present some of the preflight tests carried out with the MASCO balloon-borne imaging gamma ray experiment in order to test the detector system and associated electronics employed by the telescope. The detector system is composed by a 41 cm-diameter, 5 cm-thick NaI(Tl) crystal surrounded by plastic scintillators on the top and on the sides, for shielding. A similar NaI(Tl) is used at the bottom with the same purpose. The imaging device capability is provided by an extended 19×19 MURA-based coded mask placed 305 cm away from the detector plane which is mounted in such a way that it becomes an antimask with a 90 degree rotation. The tests determined the position sensitive detector diameter to be approximately 24 cm and showed that it is possible to do imaging in a fully coded circular (14.2 degree-diameter) field of view with a positioning power of 4.5 arcminutes for a 5 σ source. The measured energy resolution was about 10% at 662 keV and the spatial resolution was approximately 10 mm at 100 keV. We have made a series of laboratory images using a 320 mCi 137Cs radioactive source to test the effectiveness of the mask-antimask subtraction technique and obtained a 60% improvement in signal-to-noise ratio of the images. Temperature tests of the on board electronics were carried out and the results of the peak detector circuits tests are presented.
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