May 2002
ASD Threshold ranges for CEM and PEM

Here is where I estimate the region of threshold ranges for the ASD for the CEM and PEM parts of the EMTiming system. I am producing numbers for V_Max of the pulse into the ASD. The numbers are the following*:

CEM using splitter:

(1 count/2.90 MeV) * (5.758fC/count) * (0.002mV/fC) * (1000 MeV/GeV) * (0.15 split/input) = 0.596 mV (at the peak)

==> 0.6mV/GeV. 


PEM using dynode out: 

(1 count/3.68 MeV) * (5.758fC/count) * (0.002mV/fC) * (1000 MeV/GeV) * (0.7 dynode/anode) = 2.19 mV/GeV

==> 2.2 mV/GeV


Discussion: My feeling is that we want to have about 50% efficiency at around 5 GeV (real physics isn't done below 7 GeV, and we want to reject underlying event from making the TDC's for late arriving cosmics). So the design should center on the average of the two systems for 5 GeV. This is 7 mV. The question is how big a spread to we need? How much can we handle? The PEM/CEM ratio is 3.7 which is fairly big. I would guess we want to have the ability to trigger from 0.5 GeV to 20 GeV (really 1 GeV to 10 GeV with a factor of two safety margin). So this is 0.3mV -> 12mV for the CEM and 1.1mV -> 44mV for a total range of 0.3mV -> 44mV.  Is this possible? It sounds like a lot.

I should say that the current 2mV thresholds would give turn-ons at 3GeV in the CEM and 1GeV in the PEM so our baseline system looks good for testing. 


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* The numbers used are:

1) Assuming 50 ns wide pulses into 50 Ohms which gives a conversion factor (assuming a triangular pulse) of 0.002mV/fC. 

2) CEM splitter secondary to primary ratio is measured at about 15%. 

3) The other conversion factors are taken from http://www-b0.fnal.gov:8000/ace2help/calorimeter/faqfiles/faq_admem_parameters.html

4) PHA has a 3 mV average threshold and 50% efficiency at 1.25 GeV (taken from ObjectMon). Using 3.31 MeV/count we find a dynode/anode ratio of about 70%.