Trigger Processing

The bulk of the hardware shown on the Signal Derivation page is dedicated to processing the trigger signal. This page will give an overview of the trigger processing subsystems that create the final trigger generated by the Wiener CC-USB CAMAC module.

Figure 1 from the Signal Derivation page is shown here for convenience.

Fig. 1 Diagram of the GEARS system

Busy DAQ

The GEARS DAQ system takes a set amount of time (~40 us) to process an event. During this time the DAQ is considered “busy”, meaning it should not be interrupted by triggers received during active signal processing. The time interval during which the GEARS DAQ is considered as "busy" is comprised of the time it takes the ORTEC 671 Amplifier to shape the linear signal plus the time it takes the ORTEC AD114 and the Wiener CC-USB to digitize and store the event energy and timing information.

The above implies that there are events that can not be processed by the GEARS DAQ. However, missed events can be accounted for, if the total number of triggers received by the DAQ and the total number of triggers processed by the DAQ is known. It should be observed that the total number of triggers can be registered by scalers of the DAQ with minimal losses even while an event’s energy is being processed. The correction for the missed events is known as deadtime correction. The GEARS DAQ is designed to facilitate the deadtime correction.

The derivation of the trigger starts with the output of the CF discriminator being sent to the first section of a LeCroy 429A Fan-in/Fan-out for splitting of the signal, where it is split to three locations:

  1. Channel 2 on not-inhibited LeCroy 2551 Scaler modified to accommodate a 48-bit counter. This counter, since there is no inhibit line connected, counts the total number of triggers.
  2. Channel 2 on inhibited LeCroy 2551 Scaler modified to accommodate a 48-bit counter. This scaler has an inhibit line connected. The inhibit line is activated when the DAQ is busy. Consequently, the scaler counts live triggers only.
  3. The first section of a Phillips Scientific 755 Quad Four-fold Logic Unit which is vetoed by an external signal indicating that the GEARS DAQ is busy.

Three outputs of the first section of a Phillips Scientific 755 Quad Four-fold Logic Unit are used for

  1. generating part of the GEARS DAQ busy signal related to CAMAC readout, this signal is generated using Delay Gate Generator provided by the CC-USB CAMAC controller,
  2. counting live triggers using channel 4 of the un-inhibited LeCroy 2441 Scaler modified to accommodate a 48-bit counter, this scaler counts the number of gates received by the CC-USB Delay Gate Generator,
  3. generating a gate for pileup flagging using the LeCroy 2323 Dual Gate Generator.

The CC-USB, based on the event trigger input mentioned in point 1 above, generates a busy signal that is sent to one of two inputs on the fourth section of the LeCroy 429A, which acts as a 2-input OR gate in this scenario. The other input to the OR gate comes from the BUSY signal output on the ORTEC 671 Amplifier (after being passed through a LeCroy 688AL Level Adapter to convert the TTL logic pulse to a NIM logic pulse). A logical OR of these two signals is the total GEARS DAQ busy signal which is pulsed in when either the ORTEC 671 Amplifier is busy or the CC-USB is busy.

The GEARS DAQ busy signal described above is used to inhibit the first section of a Phillips Scientific 755 Quad Four-fold Logic Unit. Consequently, the logic described above blocks any triggers that occur when the DAQ is busy from generating the final trigger for the AC114 ADC readout.

The GEARS DAQ busy signal is also used to inhibit the LeCroy 2551 inhibited Scaler. As a result of inhibiting the trigger counts in the inhibited scaler are only those that were accepted during the live time of the system. In addition, inhibiting of the LeCroy 2551 Scaler provides means for measurement of the live time for data collection during a run using a clock, as described below.

Time Stamping

Channel 0 on both, not-inhibited and inhibited LeCroy 2551 Scalers is modified to accommodate a 48-bit counter and counts NIM pulses generated by a 10 MHz Ovenized Oscillator that acts as a clock. Un-inhibited scaler counts the total number of clock cycles generated during the run time. The value readout from channel 0 of the not-inhibited scaler is used as a time stamp for an event. The inhibited scaler counts the number of clock cycles generated during the live time. For a given event the ratio of the clock cycles from inhibited to not-inhibited scalers can be used to estimate the fraction of the time the GEARS DAQ was active prior to the recording of the event.

Pileup Flagging

Pileup occurs when multiple gamma rays interact with the HPGe crystal in rapid succession, resulting in a distorted preamplifier output. This distortion leads to false energy readings by the DAQ, and as such these events need to be flagged so they can be dealt with in data analysis.

In the GEARS DAQ detecting pileup is done using a Kinetic Systems 3615 Hex Scaler counting the number of pileup event triggers during user-controlled inspection window. The process begins with the generation of an inspection window, which is a user set period of time in which pileup can be detected and the event can subsequently be flagged. The inspection window is started by a valid trigger and is generated by the LeCroy 2323 Programmable Dual Gate Generator. The pile-up inspection window gate is passed to the input of the third section of the LeCroy 429A Logic Fan-in/Fan-out. The 429A then passes the logic pulse on to the third section of the Phillips Scientific 755 Quad Four-fold Logic Unit, and its complement to the inhibit input of a Kinetic Systems 3615 Hex Scaler. The purpose of this hex scaler is to count the number of pileup event triggers during the inspection window, and as such the scaler should be inhibited at all other times, hence the complement of the inspection window acting as the inhibit signal. Note: Due to the logic described below, the inhibit signal is technically redundant, however for the sake of potential user error the inhibit remains connected.

The third section of the Phillips Scientific 755 Quad Four-fold Logic Unit acts as a 2-input AND gate, where the inputs are the inspection window (as described above) and a direct connection to the CF discriminator. This means that should a trigger be received in the duration of the inspection window, a pulse is generated that indicates a pileup event. The output of the AND gate is then connected to channel 1 of the Kinetic Systems 3615 Hex Scaler, where this hex scaler is read and cleared on every digitized event. Therefore, for every event, there is a data field that indicates if pileup occurred, and if so how many pileups were detected.

Final Trigger Generation

The final trigger signal is generated by the Wiener CC-USB CAMAC module, whereupon a successful live trigger input, an output trigger is generated and sent through a LeCroy 688AL Level Adapter to the external trigger input of the ORTEC AD114 ADC. Note: The conversion of logic from NIM to TTL is necessary since the TTL trigger input requirements of the ORTEC AD114 are not met by the output of the CC-USB Delay Gate Generator.