Level-1 Trigger

The Monte Carlo value of 150 KHz shown in Table 2 for the total charged flux within the angular range from $40^o$ to $60^o$ is about 1/3 of the total rate between $32^o$ and $90^o$ that is seen by the BSD. Thus there is a factor of 6 higher level-1 trigger rate with the BSD triple coincidence than was obtained in 1998 with the RPD. However this comparison is not fair because the RPD rate contains a significant reduction from raising the threshold above the minimum ionizing edge. This technique should also work for the BSD. In order to evaluate how effective this can be in reducing the level-1 rate, a sample of recoil protons in the BSD is needed. Knowing the pulse height spectrum of recoil protons, one can then adjust the threshold to optimize between rejection of minimum-ionizing background and efficient collection of the desired events.

A Monte Carlo sample of recoil protons was obtained by generating diffractive events of the type $\gamma p\rightarrow p \eta'$ and simulating the response of the BSD in Gradphi. Which reaction is used is immaterial, from the point of view of the proton spectrum, because it depends only on the mass of the photoproduced system and the diffractive slope. For this simulation I used a diffractive slope $b=8$ GeV$^{-2}$. The event generation takes into account the Fermi motion of the nucleon inside the $^9$Be nucleus. The pulse height spectrum of the recoil protons in the BSD is shown in Fig. 3. More precisely, the lowest energy loss in the three BSD layers is plotted in the figure. This is the parameter on which a cut is made to simulate raising the discriminator threshold in triple-coincidence logic. The corresponding least-of-three energy loss spectrum for the background is shown in Fig. 4.

Figure 3: Energy loss spectrum for recoil protons from the diffractive photoproduction reaction $^9$Be $(\gamma ,p\eta ')\,^8$X. All three layers are required in coincidence and only the lowest of the signals in the three BSD layers is entered in this histogram. This is a Monte Carlo spectrum.

Figure 4: Energy loss spectrum for background tracks in the BSD. All three layers are required in coincidence and only the lowest of the signals in the three BSD layers is entered in this histogram. This is a Monte Carlo spectrum.

Comparison of these two spectra shows that a significant fraction of the background may be rejected at a minimal cost in recoil proton detection efficiency simply by raising the pulse height threshold on the BSD. This is quantified in Fig. 5. Since the BSD rates are dominated by background, the red curve can be taken to be proportional to the total level 0 trigger rate. The normalization is that of Table 1. The blue curve represents the fractional loss in recoil proton detection efficiency as a function of BSD threshold. It is clear from this plot that a reduction of a factor of 6 in the level 0 rates can be obtained by raising the BSD threshold before reaching the end of the recoil proton efficiency plateau. This calculation indicates that by balancing the gains of the BSD counters and adjusting the threshold to twice the minimum ionizing edge, the trigger rates will be comparable to what was obtained with the RPD. A further safety factor of 2-3 in rate reduction can yet be obtained by further raising the threshold before significant efficiency losses are encountered.

Figure 5: Total BSD triple coincidence counts in Monte Carlo run 1999a vs discriminator threshold (red curve, left axis) and inefficiency for triggering on recoil protons from diffractive photoproduction (blue curve, right axis).

It should be kept in mind that the BSD recoil proton trigger is collecting 2.5 times more recoil protons than formerly did the RPD, simply because of the increased angular coverage. The laboratory angular distributions of the recoil protons and background tracks, respectively, are shown in Figs. 6-7. For contrast, the recoil proton angular distribution is shown in Fig. 8 for a hydrogen target. The peak position in Fig. 8 is sensitive to the mass of the photoproduced meson, the $\eta'$ in this case. This figure is presented to show the importance of the kinematic broadening due to the Fermi motion of the proton inside the nuclear target. The most important gains in recoil proton acceptance have been obtained at forward angles, where the background is also the highest. It appears fortuitous that the end of the scintillator barrel is located close to where the tradeoff between background and acceptance is turning over.

Figure 6: Lab polar angle distribution of recoil protons detected in the BSD during Monte Carlo simulation of the photoproduction reaction $^9$Be $(\gamma ,p\eta ')\,^8$X.

Figure 7: Lab polar angle distribution of triple coincidence triggers in the BSD during Monte Carlo simulation run 1999a. This distribution is dominantly low-energy electromagnetic background.

Figure 8: Lab polar angle distribution of recoil protons detected in the BSD during Monte Carlo simulation of the photoproduction reaction $(\gamma ,p\eta ')$ on a hydrogen target.