Simple radiation detector
I decided to use a small ionization chamber with a current amplifier built on a composite transistor as a sensor.
But when I connected the base of the composite transistor directly to the sensor wire, there was practically no collector current. I expected to see some leakage current due to the floating base and the gain factor of tens of thousands. I don’t know if all composite npn transistors are as good as these MPSW45A, but the leakage current was surprisingly small, and the gain looked very high, maybe 30,000, with a base current of several dozen picoampers. (I checked the gain using a test resistor with a resistance of 100 MΩ, connected to a power supply with adjustable output voltage.) Suddenly I saw the possibility of how to use these ordinary components to make a really sensitive sensor. I added another transistor as shown below
Who needs offset resistors ?! I used a can with a diameter of about 10 cm with a hole in the bottom for the antenna wire and aluminum foil covering the open part. I quickly realized that a resistor connected to a 2N4403 base (10 kΩ) is a good idea to prevent damage from a short circuit. The performance of this circuit was excellent, it easily detected the Coleman thorium glowing lamp! So why not add another composite transistor? It seemed funny, but here's what I built:
I used a 9 V supply voltage, but recommended use a somewhat higher voltage to obtain sufficient potential in the ionization chamber. Resistors have been added to protect against an accidental short circuit, which can quickly disable the transistor or ammeter. During normal operation, they have little effect on the functioning of the circuit.
This circuit really works well and after 5-10 minutes, necessary for stabilization, it could detect the perforation grid at a distance about ten centimeters.But the scheme turned out to be sensitive to temperature changes and the ammeter reading increased with a slight increase in temperature in the room. Therefore, I decided to add temperature compensation by constructing an identical circuit, but without a sensor wire connected to the base of the transistor, and switching on a measuring device between the output points of both circuits:
It looks a bit confusing, but in fact quite easy to implement. The circuit was assembled in the same tin can as used in one of the above-described field-effect transistor (JFET) projects, and all parts of the circuit were mounted on an 8-pin circuit board. The attentive reader will notice that I actually applied 2.4 kΩ and 5.6 kΩ resistors, but these differences in the ratings do not play a big role. I also used a blocking capacitor connected in parallel to the battery, rated, for example, 10 μF. The sensor wire is directly connected to the base of the transistor and passes through a hole drilled in the bottom of a tin can. The circuit is quite sensitive to electric fields,so it's a good idea to have a shell like this.
Allow the circuit to" warm up "a few minutes after the supply voltage is applied, after which the ammeter should decrease to very small values. If the meter reads negative, switch the sensor wire to the base of the other transistor and change the polarity of the meter connection. If a noticeable voltage drops across the 2.2 kΩ resistors, maybe up to one volt, try cleaning everything with solvent and completely dry. When the readings of the ammeter become low and stable, bring the radioactive source, for example, the glowing grid, to the window covered with foil, and the readings should increase rapidly. A digital voltmeter with a scale up to 1 V or an ammeter with a scale of 100 μA can be used as a measuring instrument. The measuring device shown below already has a scale calibrated in radioactivity units, and the readings of about 2.2 are due to the effect of the gauze mesh.
This is a simple sensor, given its sensitivity ! The active experimenter can try other transistors, most likely, composite ones, for example, MPSA18, or even a voltage-controlled operational amplifier, for example, a CA3080 with open-loop feedback.
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