Enge hodoscope

Behind the honeycomb drift chamber, two sets of hodoscope counters are placed for trigger and timing measurement. Figure 60 shows a schematic view of the hodoscope. There are two identical sets of hodoscopes, EHOD1 and EHOD2, staggered with respect to each other by half a width of the segment. The estimated electron rate is 5 MHz at most. To keep the singles rate of each slat well below 1 MHz, the hodoscope is segmented into 25 plastic scintillator slats (BC420, $40^w\times120^l\times10^t$cm$^3$). Each of them is equipped with acrylic light guides and Hamamatsu H6612 3/4" phototubes on both ends.

The hodoscopes were tested at KEK-PS and the time difference between two counters was measured (figure 61). A Gaussian fit gave $\sigma = 99$ ps, and thus the timing resolution of one counter should be 70 ps (rms) assuming two counters have the same timing resolution.

All materials (plastic scintillator with light guide and PMT) have arrived at Jlab. Gluing and test with cosmic-ray are carried out at EEL building of Jlab.