We have investigated the basic operation of a composite
detector comprising of elements arranged in the shape of an U-shaped
rectangular well. Considering an isotropic scattering of gamma-rays
and partial energy absorptions in up to four detector modules,
expressions for the addback factor and the peak-to-total ratio have
been obtained in terms of only one probability amplitude. We have
compared the performance of two U-shaped detectors having different
geometries and observed negligible gain in addback due to the longer
arms. For completeness, comparisons have been made with composite
detectors like the two element stacked detector and the two level
pyramidal detector, both being embedded inside the U-shaped
detector. Our pen-on-paper approach could be used to understand the
operation of modern arrays having detector elements arranged in
various sophisticated ways.