I really enjoyed you piece on ion chambers. I built an AC output chamber from a soft drink can a few years back and got an excellent signal from alpha particles. It made a great radon detector and gave several counts per minute. I cut up a second can to make a piece of sheet aluminum and used it for the charged electrode by lining the inside of the ion chamber with a thin layer of poly foam for insulation and then placing the sheet aluminum on the inside of the foam. This allowed me to run the outside of the chamber at ground (safety feature) but also allowed any leakage current to flow to ground. That allowed me to use air and PCB material (no copper) as the other insulators, since the center electrode was near ground potential. I used a piece of small gauge nichrome wire as the center electrode and suspended it from a piece of monofilament line from three holes punched in the open end on the chamber (electronics at the other end). Therefore, I could open the chamber and insert test samples in it easily. The monofilament didn't cause much obstruction. Of course, you have to discharge the 400V polarizing supply first! The wire would vibrate and the chamber made a very sensitive capacitance mic. vibration detector. I could test the chamber by gently tapping the table and watching the damped sine wave! Not a practical portable tool. I used a 100,000 meg resistor from my junk box, which, along with a 2pF feedback resistor, gave me a time constant of about 200 ms. Since the time to sweep the ions out is about 10-50ms (depends on chamber voltage and geometry), that worked out great. I used a 2N4416 jfet as the amp, since an AC chamber is much less sensitive to leakage (the second amp is AC coupled to block the DC offset). Your comment about the difficulty in obtaining high value resistors stuck with me. I was recently working on another project requiring 100+ meg resistors and considering the problem again. I was thinking of using a current source in a feedback loop to simulate a resistor at low frequencies and ran a series of experiments, both thought and physical. A pin photodiode controlled by an LED works but tends to be noisy and very temperature sensitive. Also, even good photodiodes have a substantial capacitance and a leakage current floor of 0.5 nA or so. Very good photodiodes are expensive and we are back to the original problem. I tried a high frequency (low collector capacitance) bipolar transistor, but the noise was much too high. Then I started considering a miniature ionization chamber built using an alpha source from a smoke detector to supply the ion pairs. You could control the "resistance" by shielding the source with alum. foil with a pin hole in it to let through only a small percentage of the alpha particles. But that struck me as cheating, although I am sure it would work. If you don't apply voltage, the ion pairs would all recombine and there would be no current. The number of ion pairs that are separated and reach the electrodes would be dependent on the applied voltage. Therefore, a "resistor". But was there another, cheaper and easier way? Finally, it occurred to me that an old style vacuum phototube might work. Light strikes a low work function material and knocks off electrons, which are swept to the other electrode by the applied voltage and become current. It would only work as a unipolar device and would tend to be a constant current device at higher voltages, but I liked the idea. Unfortunately, those devices are collectors items these days and would be very expensive. It finally occurred to me that since I am interested in the low nanoamp to picoamp region, it wouldn't take much in the way of electrode surface to do the job. So I found an NE2 neon lamp in the junk box and connected it in in series with my DVM set to the 200mV scale and two 9V batteries. ( I don't know if you have used the DVM trick, but if you push 1nA through a 10meg resistor you get a 10 mV drop, which shows up on the display. So if the meter reads down to 0.1mV, that is 10pA. Instant auto-zero picoamp meter with a 10meg shunt.) And lo, current flowed when the bench fluorescent light was shining on the bulb! At first everything was very drifty and unstable. Then I washed the bulb in isopropyl and distilled water and heat dried it. No more noise and I had a controllable current source from about 500pA down to 10pA (measurement limit). For a constant illumination, the current varied with the square of the voltage applied across the tube wires. Handy if you want high dynamic range. I tried a flashlight with a krypton bulb and it worked as the light source. I tried a yellow led and it worked. I tried a blue led in one of those keychain lights and it worked well. ALL RIGHT! I haven't designed his gadget into anything yet, but it has to work given what I saw on the bench. The neatest part is that you can vary the "resistance" by changing the light striking the tube. Or use a photodiode feedback loop to keep it very constant. Probably a constant current drive on the led would work for most cases. Keeping the tube above room temperature keeps surface moisture from being a problem, but that comes free with the heat generated by the light source. The voltage isolation is a good as the glass that the tube is made of. Just don't get any gunk between the wires where they come out of the tube. And it is CHEAP! Maybe a buck if you buy he parts new in singles. BTW, infrared from a IR remote handheld did NOT work. I think that is because the photon energy isn't high enough(950nm) to exceed the work function of whatever material the tube electrodes are made from, but I haven't proved that. Anyway, there it is. I plan to try it out as the current return element in an ion chamber as soon as I can get around to it. It is just too cute to resist playing with. Oh, I hooked up the NE2 in place of a photodiode (10V bias) in a very sensitive amplifier to look for high frequency noise. When I illuminated it I saw DC current but NO increase in the (band limited) AC noise. Cool. Also, think of the possibilities of using a light pulse to remove charge and thereby converting the ion chamber into an "ion to frequency" converter. Should work. Just apply a fixed width, constant current, pulse the led whenever the amplifier voltage exceeds some set point and suck out a hunk of charge. Calibration should be interesting. Maybe a step voltage across a small value capacitor? If you use the thing before I do, let me know how it works out.
And a follow-up:
I was going to use the "neon bulb as resistor" idea in a non-ion chamber design I am working on and dug out a different bulb of the same make and model (GE 2E molded into the glass at the pinch seal where the wires come out). I hooked it up in a simple bench test first to see if there were any difference and it showed NO PHOTOELECTRIC EFFECT at all! I messed with it for about 45 minutes and could get no change in current flow from light to no light. The results with the first bulb were easily measured so I kept thinking I was making some mistake, but I never found any problem with the test setup. The test is so simple that I can't imagine any serious mistakes, just a 9V battery and a bench DVM on the 200mV scale in series with the bulb. So now I am puzzled, to say the least. Could it require running the bulb for a while to clean or "condition" the electrodes from sputtering? Don't know, but since you published my "experiment", I figured I owed it to anybody out there to follow up with this new result least they try it and wonder what the h--l is going on.
So there it is, a non-repeatable experiment. The most interesting kind! I need to hook up the dud bulb to the 110 line with a series resistor for a 100 hours or so and then try it again, I guess. Very intriguing.
Craig (one experiment proves nothing) Taylor
Here is a letter from another reader:
Hello Charles (and Craig),
I work in the semiconductor industry as an ion implant specialist and I also work with various types of x-ray systems and high voltage on the side. I own a Victoreen model 500 electrometer with 3 sizes of ionization chambers. It has been out of production for a few years and has some problems. I was looking on the web for information about my electrometer when I found your very interesting site. The Victoreen 500 has a dose integration (totalizing) feature that I need for my work. If you think about it, knowing the dose rate is fine but accurately quantifying the total dose exposure emitted by an x-ray scanner or flash source is a more practical problem for an ionization chamber instrument. I have a flash x-ray source that emits 2.5 mR with each 60 uSec long pulse. I think that only film, thermoluminescents and integrating ion chambers can measure x-ray flashes.
I found reading about Craig's use of an NE2 neon bulb as a miniature photo tube i.e. photo resistor to be interesting. There was one thing that struck me though. Craig seems to have overlooked the fact that for the photoelectric effect to work well one must consider the level of vacuum or rather presence of gas (Ne in this case) to impede his total work function. With those blue photons he may be emitting photoelectrons well enough but, if they soon collide with Ne and other gas molecules he may be reading a current that is a resulting composite of photoelectrons and ionization pair cascades between his electrodes. Perhaps the reason Craig had trouble repeating his original results is due to a non uniformity in the gas pressure and stoichiometry (or mix). Additionally, I would imagine that a Ne bulb filled to about 20 Torr may also behave as an ion pump to actually trap some gasses in one of its' electrode's surfaces. Craig's light controlled discharge is a very good idea and used in his application is perhaps a patentable one too. I would suggest that he instead think of a similar device that lacks a deliberate gas fill such as an acorn or peanut vacuum tube.
One thing that I've enjoyed for several years is designing, building and using small, light weight, high freq. regulated high voltage power supplies. As you can imagine, for a HV supply to regulate its' output voltage it must be able to sense that voltage for comparison to a control setting. As long as a 100 KV power supply is rated at perhaps 1 KW you don't need to be very concerned about your HV divider loading the output by a couple of mA. Some industrial HVPS will actually heat up an output metering resistor. However, if the PS is rated at 50 W maximum, (500 uA) you'll have very little power, if any remaining with which to drive your x-ray tube once you've metered its' output voltage. That's where thick film high voltage resistors come in. For my supplies I have used 3 Gohm, 6 W @ 1 or 2% in my voltage dividers. I have found that it is hard to find these and other larger values such as 100 G ohm and higher unless they're ordered from a manufacturer. I wonder, would you and or Craig be interested in buying a small amount of these if I were to purchase a small lot of them? My needs are slightly different but I may be able to include the types you need in an order. I'll look into finding a source of these. For my needs the resistors must also be precision and highly stable values. Usually 500M to 1G is best so I can tie several in series in order to have the assembly able to hold off lots of high voltage. I have designed a bipolar HVPS that outputs plus and minus, zero to 62.5 KV DC for a total of 125 KV to drive an x-ray tube to a max of 1 mA. The current is typically kept to about 100 to 200 uA because I've been using Gen II military night vision image intensifiers with a small disk of scintillator screen cut from a damaged fast response medical radiography cassette and optically coupled to the II's input face plate. The night vision II tubes come with all of the HV PS they need to power their photo multiplying mosaic micro channel plates. When a very small x-ray flux strikes this assembly called a LIXI scope, its' output phosphor screen lights up brightly. In fact, in the absence of an x-ray source one can "see" a "slice" of the nuclear cascades occurring as a result of incoming cosmic particles. You can watch as long streaks, series of successive dots in neat rows and very bright random flashes appear. The US Mil. II tubes need just two AA batteries to operate for many hours. I personally find this use to be thousands of times more interesting than simply looking around outdoors in starlight at night.
Thanks for sharing your interesting work with us Charles. I have
been wanting to build an ionization instrument that is sensitive, calibratable,
integrating. The trouble is that I lack the time to design it and could not use it to
measure an x-ray system for sale.
Thanks, Joe Sitter. Gilbert, AZ.