2. Modified Block Detectors Two important modifications have been made to the basic design of the block detector. The first is to use proportionately larger PMTs positioned so they overlap portions of adjacent blocks (Fig. 18-16). Thus each block still is monitored by four PMTs, but each PMT also monitors the corners of four different blocks. This approach, known as quadrant sharing, reduces the total number of PMTs required for the array by approximately a factor of four as compared with the basic block design described in the preceding section. (The actual reduction is slightly smaller, as a result of edge effects.)