Could it be a baud rate mismatch? I already backed off from the 80 MHz system clock. Regenerating SOPC system files and BSP to make sure those match... Nope, that wasn't it.
After spending a while stepping through the code in the debugger, I realized that I was trying to print the new DAC_LEVELS message before the serial port was even open! So of course it was crashing. DERP... OK, that is fixed now.
Collecting new data. Wrote Scilab script to look for coincidences. Found only 3 coincidences within +/- 30 ns:
Saturday at 7:56 pm: (Coincidence within +10 ns.)
Sat Aug 27 19:55:49 2011 + 109 ms: < PULSE,1,39642,1806343255288,1,(0,8)
Sat Aug 27 19:55:49 2011 + 110 ms: < PULSE,2,99774,1806343255289,1,(0,3)
Saturday at 9:31 pm: (Coincidence within +/- 5 ns.)
Sat Aug 27 21:31:14 2011 + 285 ms: < PULSE,1,64706,2951362942182,3,(0,(1,(1,7),4),13)
Sat Aug 27 21:31:14 2011 + 286 ms: < PULSE,2,163209,2951362942182,5,(0,(1,(0,(1,(0,6),2),3),4),2)
Total width 95 ns.
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Saturday at 9:33 pm: (Coincidence within +10 ns.)
Sat Aug 27 21:33:39 2011 + 949 ms: < PULSE,1,65347,2980500374764,2,(0,(1,7),4)
Sat Aug 27 21:33:39 2011 + 950 ms: < PULSE,2,164778,2980500374765,5,(0,(1,(0,(0,(1,5),2),3),3),3)
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314937 total pulses
Sat Aug 27 17:25:17 2011
Sat Aug 27 23:05:18 2011
5 hours, 40 minutes = 340 minutes = 20,400 secs.
average pulse rate: 15.44 pulses/sec.
PMT 1: 89,389 pulses ( 4.38 pulses/sec.)
PMT 2: 225,548 pulses (11.06 pulses/sec.)
So, looking at the arrival times, the pulses arrived at the PMTs at the same time within 5 ns, or at PMT1 (which was in the more Southerly location) about 5 ns before PMT2 (in the more Northerly location).
It is interesting that two coincidences arrived within 2 minutes of each other (at about 9:32 pm) that both had similar features: Crossing 2-3 thresholds (550-700 mV) on PMT1, and 5 thresholds (1 V) on PMT2. Perhaps some activity from the center of our galaxy (which was in that general direction)?
Here's what was in the Southern sky as seen from our location at that time:
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| Southern sky as viewed from Tallahassee at 9:32 pm on Sat. Aug. 27th, when we saw two candidate showers coming from slightly South of the zenith. |
Anyway, here also is a plot of total pulse rate from both PMTs over the course of Saturday's 5-hour run:
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| Pulse rates from two test PMTs between 5:25 pm and 11:05 pm on 8/27/11. Threshold for pulse detection was 400 mV. Horizontal axis is minutes, vertical axis is number of pulses detected in that minute. |
It looks like the pulse rate was fairly constant, apart from white-noise looking fluctuations, which isn't so interesting, but at least we have some real data now!
We can estimate the probability of getting the 3 coincidences we saw by chance. Since the average pulse rate on PMT 2 was 11.06 pulses/sec, the expected number of pulses per nanosecond is 11.06e-9. There were 89,389 pulses on PMT 1 over the run, so the total number of expected pulses on PMT 2 occurring within +/- 5 nanoseconds of pulses on PMT 1 over the course of the run if there were no showers would be 10*(11.06e-9)*89,389 = 0.00988, less than 0.01. Instead we saw 3 such pulses.
We can use the Poisson distribution to estimate the probability of these coincidences occurring by chance. f(k;lambda) = lambda^k exp(-lambda) / k!. Here, lambda = 0.00988 (the expected number of occurrences) and k=3 so k!=6. The calculation gives 1.59e-7. Let's see, how do we convert that to the number of sigmas? I think there is some simple logarithmic approximation... Anyway, it is more than 6 sigmas, at least...
I think tomorrow I should turn up the voltage on PMT1 (and if necessary, turn down the voltage on PMT2) until both PMTs produce above-threshold pulses at approximately the same average rate. Then, next time we get a coincidence, I can check to see if the matching pulses are consistently the same size as well. This will be a good test as to whether the coincidences are indeed from particle shower pancakes passing through. That seems consistent with the data for the 3 coincidences above, except that the PMT1 pulses are consistently smaller (perhaps due to a different scintillator material).
I should also try turning the thresholds back down to 350 mV (or maybe even 300 mV) to increase the pulse rate and thus (hopefully) the frequency of coincidences. However, if the resulting pulse rate is too high, I may have to do some other tweaks to the firmware to get the data rate manageable again.
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