by Doug Coulter » Tue Feb 01, 2011 9:19 am
Well, it's certainly interesting, and in fact I've even seen it done in the old decimal counters in similar fashion, in this case, one neon bulb per number. Yes, you could get a feel for rates just glancing at it. Trying to do this with leds is going to give one a lot of noise problems -- the current spikes tend to add up, and that was a real bear with the early led-driving bargraph chips.
The lack of a way to have accurate gate timing is a flaw indeed, however. For counting say, an activation you need a succession of correctly timed counts so you can curve fit and "pre-traploate" back to what the count would have been when you powered off the neutron source -- you know you've missed some decay time turning off the gear and moving the active sample to the counter. A timestamping thing tells you how much time you lost doing that, if for no other reason than that it was counting away when the power was on too from radiation the device was making -- you can see the count drop to zero, then resume at the activated sample rate when you do the move from the oven to the counter. But since you've been taking readings right along, you also can know precisely how long that took.
It's a heck of a lot of parts and complexity -- order or two of magnitude more than a PIC, one bypass cap, and rs-232 converter (and its caps), which gives gate accuracy, time stamping, and possibly a bunch of other features -- the designer can put in whatever is wanted, including a pwm output to drive an analog meter (I love those things..paint the needles with day-glo paint and put some UV on them to read them in the dark too) -- and it could even be a log function, for wider dynamic range shown on the meter than usual. You could do log->analog meter all analog as well, a couple people make the chips for that function, and that's a lot (heck of a lot) less total parts. You just have to threshold and pulse width normalize your detector input (eg comparator and one shot) then low pass filter the result to get volts proportional to count rate. This would work for all but fast phototubes, unless you had a range switch to shorten/lengthen the one-shot pulse width depending on count rate. This has been done, of course, and you can see a copy of one in Carl W's nifty demo of 3He tubes, in that video he made. His unit had a special meter as well, that had a lot more degrees of pointer deflection available than normal -- a lot of "cool" factor there.
For my uses, I need the timestamps as I need to know how long from power down to first count to really get any kind of accuracy on how activated a short life (silver) sample was right at power down, as well as *accurate* gate times I can't get with my hand on a pushbutton.
I am also a huge fan of simply hooking detector signals to an audio amp, perhaps with a bit of pulse stretching first so there's enough audio power to hear. The ear seems awfully good at this, and you can have more than one going at a time. This helps to notice correlation (such as when the thing under measurement is bursting, or you're counting EMI instead of real counts) that the eye can't do so easily. A multichannel scope is also very good for relative measurements, and in fact I'm using mine for that too -- a couple detectors, an EMI antenna, and a faraday probe in the tank all at once and time-aligned on the screen is pretty nice. PC software can capture those scope screens for later examination as well (free with the scope, or free on linux anyway, which supports most DSO's directly). Further, most of my DSO's will count pulses in a known time base for you if you like (frequency counter built in).
But the audio amp is both super simple and has one huge advantage -- you don't have to be looking at it so your eyes are free for other things. And it's as real-time as it gets. Another thing it can tell you (if you don't over stretch pulses) is when a burst of them all come in at once -- more bass in the sound! I use a little microphone transformer in my lashup to get rid of ground loop noise and stretch the pulses "just enough" due to the transformer self resonance. Works quite nicely. The kids were impressed to see (hear?) that in our recent demo for them.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.