User:Boleszek/Notebook/Physics 307l, Junior Lab, Boleszek/2008/09/29

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Speed of Light

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Initial setup

We find the apparatus already set up according to the procedure, nonetheless we check to make sure that everything is correctly connected. We turn on the oscilloscope and make sure it is set to DC coupling to correctly recieve the signals from the TAC and the annode. We turn on the LED power supply and set it to 150V. We turn on the High Voltage, starting at 0V and stepping it up until we get a nice sized signal on the scope. Now we fool around with voltage levels and different triggers to get the best signal.

We were able to adjust the frequency of the LED light by watching the CH1 signal fq indicator on the scope while adjusting the LED power supply voltage. We set it to about 11kHz.

Trigger sourse was set to CH1 falling edge with DC coupling.

The annode stopping signal has the shape of a sharp cliff that ends in a point and curves back up. The TAC signal seems to try to approximate a square wave.

We decide to set the stopping signal to -500mv.

We check the voltage rise on the TAC signal for all delays.

In order to stabilize our measurements we go to the acquire menu and set the average function to its highest value (128).


  1. Tektronix TDS 1002 two channel digital storage oscilloscope
  2. Custom buit PMT
  3. Custom built LED source
  4. Bertan Ascociates inc. Model 315 DC Power supply (high voltage PMT power supply)
  5. Canberra NSEC DELAY
  6. EG&G ORTEC Model 567 TAC
  7. Harrison Laboratories Model 6207A DC power supply (LED power supply)


Since our time delay signal will be displayed to us as a voltage signal we must calibrate our system so that we can relate voltage amplidude to seconds. We do this by measuring the displayed voltage (using the measure function on the scope) for each successive delay value.

We take two sets of calibration data:

Delay (nsec) Set1(V) Set2(V)
0 5.72 5.68
.5 5.84 5.76
1 5.92 5.84
2 6.24 6.16
4 6.64 6.56
6 7.28 7.28
9 7.49 7.52