Frankenstein's Dosimeter is so named because it's made from body parts removed from various dead radiation instruments. With a little "spark" of life from a battery, these old components come alive, making a pretty decent radiation dose measurement device.
The cylinder on the top is an ionization chamber removed from a CDV-715 Survey Meter. It is mounted on the lid of a CDV-750, manufactured by Bendix. The Bendix version has the screw in the bottom cover, which is necessary for this mod. The power connector and control are removed and appropriate holes are drilled for the ionization chamber feedthru and mounting holes.
The only component left in place is the inductor in the CDV-750; it is used to generate the high voltage for the chamber. The inverter is similar to some of my Geiger counter circuits, but only 50 volts is generated. Current consumption during normal operation is about 50 uA. The inductor is the full winding of the transformer in a CDV-750 dosimeter charger. Find the winding with the highest resistance. Other inductors, 10 mH or above, should work. The zener diodes were chosen to achieve the desired output voltage, but the voltage rating of the diode or diodes will need to be on the high side, due to the very low current. The 1N4715 is a 36 volt zener but only drops about 27 volts in this circuit. Choose a zener family designed for very low current operation. Although ordinary plastic transistors are specified, the actual unit uses metal can types. Just about any transistors will work, as long as the NPN has a breakdown voltage above 50 volts. The parts are mounted on a phenolic terminal strip that was soldered directly onto the lid. Those tiny propane torches make such soldering jobs quick and easy.
Feel free to use any other voltage generator, or even a string of small batteries. The current requirement is virtually zero. I especially like the one I used in the Modified CDV-715 . That circuit uses a 9-volt battery. The battery in this unit is a 6 volt camera type found on a clearance table at the grocery store. Stores dump batteries like these far sooner than necessary and this old battery will power this unit for the next decade.
The circuit below converts the chamber current into pulses for counting. The chamber capacitance plus a little for the JFET is about 17 pF and the CDV-715 manual states that 0.5R/Hr produces about 7 pA. Those values may be used to calculate that 0.5 mR will change the voltage on the gate by 1.5 volts. So, arranging the circuit to reset the chamber after a change of exactly 1.5 volts will yield pulses representing an accumulated dose of 0.5 mR:
500mR/Hr gives 7 pA, so 0.5mR/Hr gives 7 pA/1000 = 7 fA. The voltage on 17 pF would be changing at the rate 7 fA / 17 pF = 412 uV/sec or 1.5 volts per hour. So, 0.5 mR changes the voltage on the capacitance by 1.5 volts.
The 82k resistor sets the sawtooth height and it may be varied to achieve exactly 1.5 volts p-p on the source. You could decide to adjust the resistor for a 3 volt change to get 1 mR pulses, but the pulses are short and might be hard for some commercial counters to register. I decided to double the speed of the pulses and divide them with a flip-flop to get very wide pulses for the slow event counter module I'm using.
When testing the circuit, it's handy to have access to the underside of the chamber, where the pin comes through. That face seems to have thinner materials impeding radiation and laying a radioactive mantle or piece of uranium ore on that face will give a higher reading. Make sure to clean the JFET and guard tube with something like acetone to remove all contaminants. I sprayed a good brand of PCB cleaner up into the tube as a final rinse. If you happen to have used a metal can JFET like the 2N4117A, you can shine a light at the base of the transistor to create a little leakage. Obviously, the transistor should be in the dark when operating. That's a little-known gotcha when using those. Now I consider it a feature!
The 3.3 uF can be any value above 1 uF and it connects between circuit ground and the case. Remember, the case is at +50 volts! The 2.2 megohm and three diodes can be directly soldered to the guard tube. My "three" diodes are in a single package (old GE MPD300) and it is obscured by the metal JFET. Metal transistors were used instead of plastic ones, but just about any general-purpose types will work.
Here's how it works:
Radiation ionizes the air inside the chamber and the 50 volts attracts the resulting free electrons and negative ions to the can, and drives the positive ions to the internal plate. The positive ions steal electrons from the plate, slowly increasing the voltage. The source voltage follows the gate, and, when sufficient current flows in the source resistor, the two-transistor circuit in the drain is triggered. That "flasher" circuit suddenly and momentarily pulls the voltage low on the collector of the NPN, and therefore on the drain of the JFET through the diode. The JFET's gate becomes forward-biased and the chamber is quickly discharged. When the flasher circuit reverts to normal, the drain voltage jumps back up and the gate becomes reverse-biased again. This little trick makes it possible to discharge the chamber without any additional components that might pose leakage issues; notice that the only circuit element connected to the chamber wire is the JFET's gate. Use this little trick on homemade chambers, too.
Below is a scope screen of the voltage on the source of the JFET. The first portion of the scope display shows a single reset and slow drift due to leakage and background radiation, and the second portion shows the sawtooth waveform when I place a piece of uranium ore near the chamber. I eventually adjusted the source resistor to get achieve 1.5 volts p-p (2.5 volts p-p in photo).
The counter is a commercial module (CUB3R000) with its own internal battery. (I got a couple of these in a surplus deal, but new ones are probably too expensive.) I removed the battery and cut the case down:
There's a CD4013 flip-flop wired "dead bug" style to divide the pulses by two (1/2 of the IC). This modification is only necessary if your counter can't count the short reset pulses directly. The pulses are big; they drop from 6 volts to nearly 0 volts, so they can be counted in any number of ways, especially with 4000 series CMOS operating from the same battery. The details of the counter are left to the experimenter since my particular configuration isn't likely to be convenient to anyone else. Just find a way to count and display short, negative-going pulses. A little micro would have no trouble.
(If battery life isn't an issue, see the
OLED display at
http://www.picaxe.com. They include a PicAXE
with plenty of remaining memory space to make a counter like this. And, the
display is beautiful.)
There's also a sawtooth waveform on the source of the JFET. That's a great place to monitor when testing the circuit. The "PIC enabled" hobbyist could leave out the flasher circuit and connect the drain of the FET directly to Vcc, and use a PIC to measure the voltage on the source. An 82 k source resistor is fine and is not critical. Once the voltage reaches the desired switching point, change the PIC's pin from an input to an output and pull the source low. That will discharge the gate to near zero volts. Then, quickly change that pin back to an input and watch the voltage climb. It will jump up, then ramp at a rate determined by the radiation level. Alternatively, use a different PIC pin to drive the gate of a mosfet that shorts the source of the JFET to ground. The PIC could also measure the rate-of-change of the sawtooth for a current radiation rate measurement (giving both rate and accumulated dose).