Data Analysis - Timing Windows and Reference Times - Burcu

The "very" first step in the data analysis is to determine two types of cuts and applying them on the raw data. One of them is the reference time cuts to pick the right reference time in the presence of multiple hits in the reference time TDC spectrum. The second one is the timing window cuts to select "good" ADC or TDC hit in a given channel for a detector.

What is a Reference Time and Why Do We Need It?

The CAEN 1190 TDC modules, that are currently being used in Hall C, have both low and high resolution internal clocks, 25 ns (40 Mhz) and 100 ps (10 GHz), respectively.

Once the experiment specific pre-triggers are formed, they are sent to the trigger supervisor (TS) where they are processed and, if they are accepted L1-accept (Level one accept) triggers are formed. L1-accept triggers are then sent to the FADCs and TDCs in the ROCs to initiate the readout. This is done via the low 25 ns resolution clock of the CAEN 1190 TDCs. This readout trigger could provide a very coarse "reference time" based on the module's 40 MHz (25 ns) internal clock. However, to achieve the full 100 ps resolution of the TDC, a high-resolution measurement to reference other signals against is needed. This is done by feeding one copy of the pre-trigger which is the "reference time" into one of the signal inputs on the TDC. With that, the signal inputs are all measured against a much higher resolution internal clock (10 GHz), and now the 'reference time' signal input value can be subtracted from any of the other signal inputs to get rid of the 25 ns jitter caused by the slow clock and recover timing value with 100 ps resolution.

The FADC reference time is the copy of the TDC reference time signal that is sent to a pulse shaper (RC circuit) to produce an analog pulse for the FADC.

The reference time cut sets a lower limit above which the first hit is selected as the reference time when there are multiple hits in the reference time TDC channel. If a reference time cut wasn't set, the analyzer would select the first hit in the TDC/ADC window as reference time. Bad reference time selection could lead to bad tracking which could cause a drop in the tracking efficiencies. Depending on the 3/4 hodoscope rates, this drop can be drastic. For example, given the drift chamber TDC readout window is about 2000 ns when the 3/4 rates are 100 kHz, 20% of the events could have a bad reference time which could lead that much of a drop in the tracking efficiencies.

Reference Time Cut Selection Procedure

To set the ADC/TDC reference times, one needs to look at the FADC pulse "raw" time and TDC "raw" time spectra and set the cuts just below the reference time signal. In each case, raw indicates no reference time subtraction, so we would expect to see 25 ns width/resolution due to the internal 40 MHz clock of the TDCs.

While all the detector responses, except for the drift chambers, are read by ADCs, the hodoscopes are measured with both ADCs and TDCs. The drift chambers responses are read by the TDCs. This requires to set one common ADC reference time cut for all the detectors and TDC reference time cuts separately for the hodoscopes and drift chambers, for EACH trigger type.

Reference time cut determination procedure needs to be revisited whenever there is a change in the trigger type.