Event Reconstruction Efficiency

The event reconstruction efficiency has been studied by implementing an algorithm which does the following:

1.   For each event, the ADC values for the 44 CsI crystals in the array are summed.
2.   For each event, the ADC values for each of the CsI crystal contained in the cluster defined by the cone from Sec. 6.1 are summed.
3. If the sums from steps 1 and 2 are both greater than or equal to 50 MeV, then the event has been successfully detected by the cluster.
4. If the sum from step 1 is greater than or equal to 50 MeV, but the sum from step 2 is less than 50 MeV, then the cluster has failed to detect the event.

Finally, the event reconstruction efficiency is defined as the number of events successfully detected by the cluster, divided by the total number of events detected by the whole array of CsI crystals.

The optimal cluster size for best event reconstruction efficiency can be inferred from a graph of efficiency plotted against , for simulated 70 MeV photons and positrons. Again, it is clear that the efficiency for approaches the ideal value which occurrs at .

  
Figure 6.5: Event reconstruction efficiency as a function of the cone opening angle , for simulated 70 MeV photons and positrons. The dotted horizontal lines represent the asymptotic values for 360 summing.

From the plots in Figs. 6.3 through 6.5, one can deduce that the energy resolution and event reconstruction efficiency approach ideal values for a cone opening angle of . By inspection of the graphs in Fig. 6.2, one can see that this corresponds to a cluster size of about 17 crystals, or one central crystal surrounded by two concentric rings of crystals. This arrangement can be seen in the two dimensional projection of the whole 240-element PIBETA calorimeter, shown in Fig. 6.6.

  
Figure 6.6: Two dimensional view of PIBETA calorimeter. One can see that the optimal cluster size of 17 crystals corresponds to one central crystal surrounded by two rings of neighboring crystals.