Adapted from an email to NYSDOT (firstname.lastname@example.org):
The minimum green and yellow times on the signals from Burnett Blvd to Rt 55 are too short for bicycle traffic making a left turn across six traffic lanes.
The pictures show key points from our ride on 2015-07-10, returning from the Balloon Festival in Poughkeepsie. We took the DCRT around Poughkeepsie, went through Arlington to Rt 376 at Collegeview, then took Rt 376 Red Oaks Mill.
The image sequence numbers identify frames extracted from video files. The front camera (a Sony HDR-AS30V) runs at 60 fps and the rear camera (a Cycliq Fly6) at 30 fps, so you can directly calculate the time between frames. The Fly6 timestamp is one hour ahead, for reasons I don’t quite understand.
The red signals are turning off and the greens haven’t lit up yet:
One second later, the car and our bikes are starting to roll:
The yellow signals begin turning on seven seconds after the green:
The car has reached the pedestrian ladder across Rt 55, but we’re still crossing the westbound lanes of traffic. Note that I’m lined up with the lane closest to our starting point on Burnett: this is a big intersection. We may not be the fastest riders on the road, but we’re not the slowest, either.
We’ve reached the far side of the intersection just under 16 seconds from the green:
However, the opposing signals turned green while we’re still crossing the eastbound lanes of Rt 55, 15 seconds after the Burnett Blvd signals went green:
About 2.7 seconds later, cars have been accelerating across the intersection toward us as we reach the pedestrian ladder:
Setting the minimum Burnett green to 12 seconds, the minimum yellow to 10 seconds, and the minimum delay from Burnett green to Rt 55 green to 30 seconds would help cyclists (just barely) reach the far side of the intersection before opposing traffic starts rolling.
As a bonus, adjusting the sensor amplifiers on Burnett to respond to bicycles and marking the coil locations on the pavement in both lanes would help us through the intersection during low-traffic-volume times, as our bikes seem unable to trip the signals.
It’s a good thing I have a pretty deep parts stock, as one of the caps didn’t fit into its holes at all.
The Russian CI-3BG glass tube, according to the datasheet and discussion on MightyOhm, is sensitive to gamma and beta radiation, so it should serve as a simple cross-check on my ionization chamber results. It’s not clear the C8600 is applying the correct voltage to the CI-3BG tube, but it probably doesn’t make much difference; the supply is so feeble that there’s no way to actually measure the results.
A closer look at the CI-3BG suggests the active volume lies inside that spiral-wrapped section between the white insulators:
In round numbers, that section is 6 mm long and 3 mm OD. Figuring the ID at 2.5 mm, that’s a volume of 30 mm3 = 0.030 cm3. That’s maybe 1/7300 of the ionization chamber volume, so, (handwaving) assuming roughly equal sensitivity, the chamber should report three orders of magnitude more pulses than this little thing.
It’s mildly sensitive to a radium-dial watch and perks up when a watch hand lines up along the spiral-wrapped volume. Given that the radium decay sequence spits out betas and no gammas, the (scaled) count may be a bit higher than the ionization chamber produces, but there are so many other imponderables that it might not matter in the least.
We take the Wappingers section of Maloney Road from Rt 376 to the Dutchess Rail Trail; it’s our main connection to the DCRT for southbound rides.
Here’s a look at 1500 feet of Maloney, starting just uphill from the entrance to the strip mall (click on any image to embiggen and browse the gallery):
Many of those longitudinal cracks go down through multiple patch layers and through the original pavement. Rolling your wheel through them would produce an instant upset.
Most drivers give us as much room as they can, but we’ve had some very near misses. Some drivers object loudly to our presence in the middle of the lane, probably because we’re pedaling slowly up the hill, although there’s really no safe path along the right side of the roadway.
The downhill side seems slightly better, but not by very much.
Feeding the output voltage into the ‘scope, with AC coupling to strip off the DC bias, produces this:
Those cute little spikes seem to be gamma ray ionization events: they are always positive-going, there are no similar negative-going pulses, they occur irregularly at a few per second with occasional clusters, and generally seem about like random radioactive events. The picture shows a particularly busy interval; mostly, nothing happens and the baseline voltage wobbles around in a low frequency rumble.
For what it’s worth, the shielding around the circuit completely eliminates not only 60 Hz interference, but everything else, too: astonishingly good results from a fairly simple layout.
Taking a closer look at one pulse:
(Vigorous handwaving begins)
The tallest spikes are typically 20 mV above the baseline, corresponding to peak output current of 20 mV / 12 kΩ = 1.5 µA and a chamber current of 1.5 µA / 100×106 = 15 fA.
They’re generally 5 ms wide, which is orders of magnitude longer than the actual ion generation time, but the area under that spike should be more-or-less proportional to the area under the actual impulse.
If you grant that and agree those pulses look mostly triangular, their integral is:
1/2 x 15 fA x 5 ms = 40 fA·ms = 40 aC
That’s “a” for “atto” =10-18 = a billionth of a billionth = hardly anything at all.
Indeed, seeing as how one coulomb contains 6.2×1018 electron charges, that pulse represents 250 ion pairs, at least assuming a zero-current baseline.
Gamma rays arrive with various energies, produce ionization trails of various lengths, and don’t necessarily traverse the entire chamber, so the pulses have various heights & widths; you can see smaller pulses sticking up out of the grass in the first scope shot. Assuming all those average out to five “big” pulses every second, the chamber collector electrode passes 200 aC/s into the transistor base → 200 aA → 0.20 fA. At 1 fA per 100 µR/h, that’s 20 µR/h of gamma background.
Note bene: Because 1 C = 6.241×1018 ion pairs, 2.08×109 ion pairs is 333×10-12 C and, if you do that in one second, you get 333 pA of current from your ideal 1 cm3 ionization chamber. Those two approaches should be equally close.
(Vigorous handwaving ends)
Again, I don’t trust any of the values to within an order of magnitude and surely made a major blunder in running some of the numbers, but the results seem encouraging.
The coaxial cable’s capacitance could explain why the pulses look like triangles: the capacitance integrates a rectangular current pulse into a voltage ramp. The cable measures 200 pF and the scope input adds 13 pF, but let’s call it 200 pF across the 12 kΩ emitter resistor. Raising the voltage across that capacitance by 20 mV in 2 ms requires a current of:
200x10-12 x (20 mV / 2 ms) = 2 nA
Dividing that by 100×106 gives a chamber current pulse of 20×10-18 = 20 aA: three orders of magnitude less than the original guesstimate. That suggests the (handwaved) 15 fA chamber current, amplified by the absurd gain of two stacked Darlingtons, easily drives the cable capacitance. Something else causes the ramp.
The chamber itself has 10 pF capacitance, but it’s not clear to me how (or if) that enters into the proceedings. The entire collection of ions appears in mid-air, as if by magic, whereupon the +24 V chamber bias voltage draws them (well, the positive ones, anyway) to the transistor base without appreciable voltage change.
Perhaps the triangle represents the actual arrival of the ions: a few at first from the near side of the trail, a big bunch from the main trail, stragglers from the far side, then tapering off back to the baseline.
That’s definitely more than anyone should infer from a glitch produced by a pair of transistors…
The Victoreen 710-104 ionization chamber specs say it produces 1 pA in a 100 mR/h gamma environment, which suggests the actual current will be much, much lower in the Basement Laboratory. In fact, I’m hoping to spot individual gamma rays, rather than the overall radiation background current, which will involve counting groups of electrons as they march by.
The simplest possible electrometer amplifier, an MPSA14 NPN Darlington, produced 25 nA of current that, assuming a gain of 10 k, corresponds to an unrealistically high 2.5 pA of chamber current and is, realistically, entirely leakage current.
Adding an MPSA65 PNP Darlington boosts the overall gain to maybe (10 k)2 = 100 x 106:
Granted, there’s not much to like about that circuit (“Any sufficiently sensitive instrument is indistinguishable from a thermometer”) and stuffing that much gain into a pair of inverters is basically crazy talk, but it looks like this:
The blue trimpot in the foreground drives the base of a duplicate pair of transistors in a misguided attempt to make a differential amp that would balance out some thermal effects. Turned out to be not worth the effort, due to the adjustment’s fiddly nature, but also not worth unsoldering the parts.
The black lump covers two RG-174 coaxial cables that run off to the oscilloscope; they already had BNC connectors on the end and were small enough for the job.
Some DC measurements:
The output idles at 6.5 VDC → 550 μA of Q101 collector current → 6 pA of Q102 base current. Yeah, right.
Grounding the base of Q102 → 5.9 V output → 500 µA → 50 nA leakage into Q102’s collector. Maybe.
Shorting Q101’s base to its emitter produces 350 mV at the output → 30 µA of output current. Huh.
After restoring the status quo ante, the output idled at 10.2 V. See previous comment about thermometers, modulo soldering transistor leads.
So, given the predictably absurd temperature sensitivity of this whole lashup, it’s reasonable to say the entire DC output current comes from leakage, which also agrees with the fact that I’m not dying of gamma exposure. In point of fact, an ancient CDV-715 Radiological Survey Meter with a similar ionization chamber (which, at this late date, passes its “circuit test” function and seems to be perfectly happy) reports exactly zero background on its most sensitive 500 mR/h scale.
A semitrailer load of scrap metal pulled into an I-90 rest stop just after we arrived:
Apparently, they dump the scrap into the trailer from a great height and, sometimes, a bar can gash the aluminum side wall. That slice obviously predates the current load, but you can see how it happened: dump a load atop a bar leaning against the side and you get a giant metal shear.
The trailer also had several puncture wounds:
I didn’t notice the circular feature at the bottom center until I looked at the picture, but it certainly reminded me of a bullet hole in glass plate. Close inspection of the original image suggests it’s a welded stress relief border around a drilled hole, perhaps with a boss on the inside of the trailer: