Last Update:01/19/2005 10:28:29 PM   by Erik Johnson

LED TOF PMT Gain Monitoring

The LED gain monitor system is used to identify any drifts in the gain of each tube separately. A pulse is generated using two gate generators in a loop, which is sent out to the tunnel to the LED circuits. The circuits send a pulse to one of four LED’s in a box mounted on the back of each TOF wall section. A pin diode inside the box measures and drift in light intensity of the LED, whose signal is sent into a fastamp and then into an ADC channel. A bundle of fibers grouped together inside the box as well direct light to the face of each tube.

For a quick check of the electronic setting, go to the common run mode settings table. 

The LED circuit requires +120V DC to operate. The voltage is supplied from within the in the counting house. There are 2 dual high voltage units in a NIM bin above the Lecroy high voltage mainframe. These units should only be on during an LED run only. 120V should not be exceeded because the voltage can damage the LED and/or the pin diode not to mention the tubes.

The trigger for the LED system is first sent through a gate generator to delay the signal properly and then into the 4-fold logic unit. Channel C needs to be unmasked, as the other channels are masked. Like with the other running modes, the output gets routed into the trigger electronics the same way.   The LED pulser circuit needs a drive pulse to generate a strobe on the LED.  To generate this strobe, the delay/gate generator to the left of the 4-fold logic unit needs to be triggered.  The start button on the top generate needs to be pressed.  Continue to re-press the button until the trigger LED on the gate generator remains illuminated.

The light measured in each photo-anode in each PMT is different, and some of the signals are too low to be useful. This isn’t a concern since at least two anodes need to fire to track the history of the tube’s gain because any lose in the gain should be seen over the tube uniformly. Furthermore, not all of the channels produce at least a 500mV signal, and therefore, the discriminators could be set to a lower absolute level.  Setting the timing discriminators to 50mV is sufficient in obtaining signal in the TDC’s if it is necessary to have TDC signal in the run.  If there is no reason to have timing information in the run, do not change the discriminator thresholds for it may lead to taking physics data using the wrong threshold levels.

The DAQ run type is TOFLED. 

Out of all the diagnostic tests, this test is the most important. There has to be at least one LED test per week regardless. The discriminator thresholds should be documented in the run logbook along with the supply voltage to the LEDs . The run should have at least 10,000 events.

Warning!!!  These steps have not been tested to work at the moment!

Step by Step

  1. Make sure there is no beam in the ring for at least 10 minutes.
  2. Start gate generator strobe.  The gate generator strobe is running when the LEDs on the unit remain illuminated.
  3. Enable channel C in the upper module of the 4-fold logic unit.
  4. Disable channel D if it isn't already disabled.
  5. Turn on the LED voltage to 120V.
  6. Following the DAQ instructions on stopping and starting a run, start a TOFLED run by typing rc_start(RUN_TOFLED+WRITING) in the RunControl Window.
  7. Collect 10,000 events.  This takes about 7 minutes at 25Hz.
  8. Stop DAQ after at least 10000 events.
  9. Turn off the LED voltage.
  10. Stop gate generator strobe.
  11. Disable channel C in the upper module of the 4-fold logic unit.
  12. Enable channel D in the 4-fold logic unit.

IT IS NECESSARY THAT THE LED SUPPLY VOLTAGES ARE RAMPED DOWN TO ZERO, OTHERWISE THERE WILL BE FALSE SIGNALS PRODUCED IN THE TOF TUBES!