We took 8 showers in 17 hours.

Last night's overnight run crashed at about 8 am this morning (about an hour and a half after sunrise).  At least we got data that might give us a night/day contrast in the event rate.

Adjusting PMT 1 voltage to try to match PMT 1's event rate.  At 1,400 V the PMT1 rate is still slightly less (about 17% lower) than PMT2's rate (at 1,200 V).  Above 1,450 V, PMT1 stops producing pulses.  Right at 1,450 V, PMT1's rate is about 6% higher than PMT2's rate.  At 1,430 V, PMT1's rate is still 10% higher than PMT2's.  At 1,410 V, PMT1's rate is about 2% less than PMT2's - about the same within the margin of error.

Of course, this is all dependent on the scintillators' type, and their positioning (due to any shielding and/or radiation sources in their immediate environment).  Just now, during that test, PMT1 was sitting behind my keyboard.  Let me put it back in the window sill and check again.  Now the PMT1 pulse rate is 25% higher than PMT2!  Let's go back down to 1,350V.  Now the PMT1 rate is about 3% lower than PMT2.  I think that's about as close as it's worth trying to get for now.  Once we're using the paddles, this kind of calibration will be more meaningful, since at least we'll know we have two scintillators that are the same.

I'm going to let the current run (with the almost-matching pulse rates) continue for a bit.  Oh wait, is it worth trying to lower the threshold?  We have to be careful, because if we lower it too much, the pulse rate will be too high and could crash the system.

Aha, I wonder if the crashing problem is due to these occasional errors I'm seeing filling up the STDOUT buffer when the nios-terminal is not attached:

ERROR: icdp_handle_have_data(): The HAVE_DATA flag is not up!

These are probably due to the fact that the handler possibly processes multiple pulses per HAVE_DATA event, which could result in another HAVE_DATA interrupt coming in for a pulse that was already handled.  Also, we do extra HAVE_DATA handling whenever there is a BUF_FULL event.

I changed these things.  This will lower our peak performance a little (since every pulse will now trigger a separate interrupt) but it is a little bit cleaner.  If we have throughput problems later, consider changing this back (or else turning the system clock speed back up to 80 MHz again).

OK, we got eight coincidences in the overnight data set:

(#1) 4:33 pm.  Neither North nor South:

Tue Aug 30 16:33:27 2011 + 211 ms: < PULSE,1,20748,871599293573,2,(0,(1,6),4)
Tue Aug 30 16:33:27 2011 + 212 ms: < PULSE,2,47445,871599293573,4,(0,(0,(0,(1,8),3),2),2)

(#2) 5:42 pm.  From South by 5 ns:

Tue Aug 30 17:42:26 2011 + 460 ms: < PULSE,1,40789,1699440061556,1,(0,7)
Tue Aug 30 17:42:26 2011 + 462 ms: < PULSE,2,92031,1699440061557,5,(0,(1,(0,(0,(1,4),2),2),3),4)

(#3) 6:00 pm.  From North by 5 ns:

Tue Aug 30 17:59:43 2011 + 529 ms: < PULSE,2,103061,1906866999930,5,(0,(0,(0,(1,(0,5),2),2),3),3)
Tue Aug 30 17:59:43 2011 + 528 ms: < PULSE,1,45704,1906866999931,1,(0,4)

(#4) 6:17 pm.  Neither North nor South:

Tue Aug 30 18:17:12 2011 + 547 ms: < PULSE,1,50695,2116678060181,2,(0,(2,3),4)
Tue Aug 30 18:17:12 2011 + 548 ms: < PULSE,2,114408,2116678060181,3,(0,(0,(1,3),2),2)

(#5) 7:11 pm.  From North by 20 ns:

Tue Aug 30 19:11:52 2011 + 126 ms: < PULSE,1,66549,2772581913802,1,(0,4)
Tue Aug 30 19:11:52 2011 + 127 ms: < PULSE,2,149798,2772581913798,4,(0,(0,(1,(0,4),2),3),1)

(#6) 2:15 am.  From South by 5 ns:

Wed Aug 31 02:14:45 2011 + 155 ms: < PULSE,1,188649,7847208784469,1,(0,4)
Wed Aug 31 02:14:45 2011 + 157 ms: < PULSE,2,425844,7847208784468,4,(0,(1,(0,(1,3),2),2),2)

(#7) 2:36 am.  From South by 20 ns:

Wed Aug 31 02:35:55 2011 + 911 ms: < PULSE,1,194843,8101342573919,2,(0,(1,5),4)
Wed Aug 31 02:35:55 2011 + 911 ms: < PULSE,2,439464,8101342573923,1,(0,4)

(#8) 3:09 am.  From North by 15 ns:

Wed Aug 31 03:09:15 2011 + 429 ms: < PULSE,1,204543,8501263144244,1,(0,6)
Wed Aug 31 03:09:15 2011 + 431 ms: < PULSE,2,461019,8501263144241,5,(0,(1,(0,(0,(1,7),3),4),3),21)

So, this time at least we got a good mix of shower orientations (3 from the South side of sky, 3 from the North side of sky, and 2 from near the Azimuth/East/West plane, within the resolution limit.

At 7:11 pm we got a shower from near the North corner of the sky when the supernova was above the horizon - however at 20/25 = 80% of max time difference or (very roughly) 11 degrees away from the north-south PMT axis, it was probably *too* close to the North to have actually been from the supernova.

The Northwestern sky from Tallahassee (sans atmosphere) at 7:12 pm EDT on 8/30/11,
when we detected a cosmic-ray shower from close to North.

I actually had several runs in that data file, so I separated out the largest one, and am now re-running my Scilab analysis script anal-pulses.sce to plot the average pulse rates over that run.  Even just this part was still a 17.4-hour run (3:21 pm to 8:44 am), which is why we had so many more coincidences in it, compared to our short run from last Saturday.

Here is the average pulse rate data over the run:

Pulse rate data for PMT #1 (bottom), PMT #2 (middle), and both together (top).  The horizontal axis is minutes into the run, and the vertical axis is pulses binned in each minute.  The rate for PMT #1 is lower due to a different scintillator material used (we think).  This discrepancy will be (approximately) compensated for by increasing the PMT #1 base voltage in our next run.

There is no noticeable change in pulse rate after sunrise (which happens about 400 minutes into the run) of sunset (which happens 100 minutes before the end of the run).  It could be that almost all of the pulses that are energetic enough to be picked up here are originating from primary astroparticles having energies much greater than most particles in the solar wind, so that the solar contribution to this data is low.  Or, perhaps this data is dominated by terrestrial radiation (since after all, we are pretty close to sea level).

Ray mentioned that powerful (e.g. X-class) solar flares could produce high-energy particles.  We are near the sunspot minimum right now, but are on the upswing, so such flares may become increasingly common in the next few years:

14-year history of sunspot activity.  From http://www.nwra.com/spawx/ssne-cycle2324.html.

Ray got one of the scintillator paddles ready to hook up:

Scintillator paddle & PMT in gun case.

We hooked it up.  The pulses are lower & broader, which makes sense due to the thinner & wider scintillator.  We had to adjust the threshold down from 350 mV to about 300 mV to get a reasonable (not too high, not too low) event rate.  Tomorrow (or whenever we next work on this) we'll hook up the second paddle and look for coincidences.  Hoping we'll see more of them, due to the much larger area.