Photon Counters - Stanford Research Systems - #6

apparent that this threshold setting will count the majority of output pulses but will not count anode rings or amplifier noise.

The green LED is flashed at a 40 Hz rate and has a pulse width of about 1 ms. During this time, population integrates in the upper level of the laser transition. Spontaneous decays from the upper level are counted by the SR430 Multichannel

Scaler/Averager.

The multichannel scaler is triggered by the same pulse which flashes the LED. The bin width is 20.48 |J,s, and the record length is 1k bins. The records per accumulation is set to 100. Thus, each record takes approximately 21 ms of real time to acquire which is sufficient to measure the 3.5 ms lifetime of the excited state. The pulse rate of 40 Hz will not generate rate errors. A summary of the SR430 setup parameters is shown below.

After the SR430 Levels and Mode menus have been setup, data acquisition may begin. Pressing the [START] key starts the first record. Data accumulates on the screen until all 100 records have been acquired. When data acquisition is complete, the math menu may be used to fit an exponential curve to the data to measure the lifetime directly. Finally, the data curve is printed or plotted and stored to disk. A picture of an actual decay curve obtained with an SR430 is shown to the right, along with the exponential fit to the data. The decay time measured by the SR430 is 3.5 ms—quite close to the actual value.