Simplified Version of the Pulser Boards (Based on KAMLAND pulsers)

1. The next slide shows a simplified diagram of the pulser. A capacitor is charged up to perhaps 10V. Switches T1 and T2 close. Some of the capacitor current runs through the diode. Some current runs through the a resistor to ground. To monitor the process we probed both sides of the diode and sometimes compute the difference. Third slide is one simple model. I let both switches close simultaneously. The diode draws 100 mA for a few ns and then the voltage is below threshold. The capacitor discharges the rest of the way (pink) through the resistor. After 15 ns the voltage across the diode is zero.

2. Simplified Version of the Pulser Boards (Based on KAMLAND pulsers) FETs are switched on (SW1,SW2) to control the firing of the diode. 2 P5050 probes measure Voltage on diode.

3. MODEL: Switch 1 starts discharge through 100 Ohms R Switch 2 starts discharge through LED (constant current).

4. The following slide just compares the measured voltage between the capacitor and the diode with the simple model. The two are consistent. Never tried to measure the actual time sequence of the switches t1 and t2. So I am not sure when the switches actually close.

5. Shows the simple model and measurements using 1GHz and a 500 MHz probe.

6. The slide that shows the afterpulsing shows only the two voltages (not the difference). One sees that initially the voltage across the diode (pink to yellow difference). Both voltages and there difference drop to zero quickly. Then the capacitor recharges. As the capacitor charges up a voltage develops across the diode. I assume that when the voltage difference exceeds approaches threshold the diode fires but in a noisy not consistent way. Evidence for this is shown with low rate results (see ahead)

7. After pulse: Average over many WFMs

8. The next slide shows the prompt pulse and the after pulse at a lower rate. As we lowered the rate we found the after pulse was smaller and also wider. I don’t know why but it did seem that at low pulse rates we saw a good deal of single photon noise rather than a definite pulse. Thus I characterize it as an enhancement where the diode may be unstable in some way and tend to generate light for awhile during the charge up cycle. There was never any clear measured quantity that we could connect with the after pulsing.

9. Shows that the after-pulsing is an “enhancement rather than a single pulse for some pulser rates.

10. We decided to put a resistor across the LED so that the voltage difference during the recharge would be zero. The circuit below shows the changes we felt gave a better performance. The 2nd slide below shows the voltage at each probe (red/blue) and their difference in light blue. This a short time scale so you can see the voltage pulse that generates the light and pmt response in green. Notice now due to the resistor the voltage drop across the diode is normally zero whereas in previous slides it was typically a bit more than 2Volts. The 3rd slide below is an longer time scale with no after pulsing evident. Both slides show the same signals but the time scale has been changed.