the RPD OR

Figure 3: RPD OR scaler rate vs. beam current measured at the beginning of the June run period.

Figure 4: RPD OR scaler rate vs. beam current measured at the end of the June run period.

The rates in the recoil proton detector are about two orders of magnitude smaller than in the tagger, whereas the time scale for the electronics is the same. Therefore all dead time effects in the circuits that produce the RPD OR are ignored. Logically the RPD OR is more complicated than the tagger OR: it is composed in the following way. First each of the 12 thick (E) counters is AND'ed with its corresponding thin (G) counter, then these 12 coincidence signals are combined together in an OR. The width of the coincidence gate is ignored (set to zero) in the model, and the rate is a linear function of the beam current.

$$S_{_R} = r_{_R}\,I$$ (4)

where $S_{_R}$ is the scaler monitoring the RPD OR signal and $r_{_R}$ is the proportionality constant between the RPD OR and the beam current.

The value of $r_R$ varied significantly during the run, depending in particular on the threshold and gains in the RPD counters, and to some extent on the quality of the beam tune. The difference between scans taken at the beginning of the June period and the end can be seen by comparing Fig. 3 with Fig. 4. The decrease in slope by a factor of 2.5 is mainly due to raising the RPD discriminator threshold. The deviation from a straight line during the high-current scan in Fig. 4 is due to changing beam conditions during that scan; CLAS emptied and refilled their target during this scan. The effect of the CLAS target is much more apparent in the UPV rate, shown below. The principal restriction on this part of the model is that the RPD rate must not exceed the level where electronics dead times begin to be important, around 5-10MHz in the OR. Given that this corresponds to $10^9$ tagged photons/s, this part of the model is free of restrictions.