September 22nd, 2010

Steampunk Geiger Counter, Part 1

I haven't really done much in electronics in the last year or two, and I miss it. So when stumbling around in my scanned schematics folder tree, I came across a couple of Geiger counter circuits that I discovered while scanning Carl & Jerry out of Popular Electronics a few years ago. What struck me back then was how little there actually is to a Geiger counter circuit, and, with a Geiger tube in hand, I could have a working counter in a couple of hours or less. (A really ugly clip-lead lashup might take me half an hour.) And although Uncle Louie gave me a Raytheon counter tube when I was 11, I can no longer find it. So up I went to eBay, and discovered to my delight that somebody was selling an Amperex 75NB3 counter tube. This is significant (nay, an omen!) because I've been looking for one for a while. It's the tube called out in a circuit PE published in July 1955, with the cool Ed Valigursky prospector cover. (Scroll down to it.)

GeigerCircuit-PE-July1955-P30-500Wide.jpg

The circuit is simple; nay, minimal: Basically, a 375V DC source applied to the center element of a Geiger tube through a current-limiting resistor. Any time an energetic particle passes through the tube, it ionizes some of the gas inside (generally neon with some trace gases to sharpen the pulse by quenching the trail quickly) and for an instant the tube conducts. You can pick off a pulse through a blocking capacitor and hear it with sensitive headphones as a sharp click.

Getting 375V worth of battery is nontrivial these days, but also unnecessary. Note what happens above: The batteries do nothing but charge a couple of capacitors. A circuit I found in the July 1957 issue finesses high-voltage batteries completely by setting up an output transformer as a step-up, and applying interrupted DC to the output (low impedence) winding. The interrupted DC induces high-voltage pulses in the input (high-impedence) windings, and if you capture them in a capacitor, you can power the Geiger tube from a single D cell.

GeigerCircuitPE07-1957-500Wide.jpg

The one glitch is interesting all by itself: You have to pass the pulse through a spark gap. In this circuit it's an automotive spark plug (remember those?) but it can be anything you can crank down to a thirty-second of an inch or so. In a similar circuit published in The Boys' Second Book of Radio and Electronics, Alfred Morgan uses two small nails held in binding posts, and lacking a spark plug in the junkbox, that's probably what I'll do. The spark gap acts as a crude rectifier, making sure that only the positive excursion of the induced pulse goes to the hot side of the .05 cap. The interrupted DC is generated by repeatedly pressing and releasing a momentary-contact pushbutton switch. The transistor here is a headphone amp, but again, high-impedence headphones will make pulses audible direct from the tube.

So. Is this really a steampunk technology? In other words, could someone with some skill and knowledge have built one of these in 1900? (Again, the circuit does not require a transistor, nor even a vacuum tube to amplify the pulses.) I don't see why not. You'd need somebody who understood ionizing radiation, but that's no stretch for a 1900-era mad scientist in brass goggles. Neon gas in a graphite-coated glass tube? Transformers? Headphones? Kid stuff.

While I'm waiting for the counter tube, I'm going to lash up the cap charging circuit and see how it works. The output transformer called out in the circuit used to be present in every single All American Five clunkerjunker tube radio I found on the curb on Garbage Day, but you may have to ask around for used units, or spend (much) more on a new transformer from Antique Electronic Supply.

And to test it? In my box of gas-regulator tubes I have a couple of old units that were "salted" with something like a trace of Lead-210 to make them conduct instantly when power is applied. Such tubes aren't very radioactive anymore because the salting materials have short half-lives, and it's been 55 or 60 years since most were made. And hey, if they really are dead, there's always a few cosmic rays floating around.

I was told recently that bananas are mildly radioactive. (It's the Potassium-40.) This seems like a stretch to me, but...we'll find out.