Portable radiation detector based on an ionization chamber
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Pending tasks: Connect the back plane to guard voltage instead of can voltage, and see if noise is still present. Polarize with lower resistance (instead of multimeter's internal). Test PNP configuration (should have larger gain). Design and mill self-contained PCB that uses 12V battery.
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15/Feb/2017: It works! At first the result with the new transistors had lots of noise as well. But adding a copper plane behind the main electrode seems to have done the trick. So maybe the transistors were not the problem! The pictures show a new prototype being tested with and without a radiation source. Its response ranges from 70mV to 160mV and it is way more stable than before. Schematic is Charles Wenzel's Experimenters Ionization Chamber with an FMMT634TA NPN darlington transistor.
- 8/Feb/2017: The Darlington transistors used until now (MMBT6427, MMBTA14, MMBTA28, MMBTA63, MMBTA64) may not be suitable for detecting the low currents involved in this detection problem (nA or even pA). Yet again I have resorted to Charles Wenzel's awesome website. A proper surface-mount alternative to the MPSAW45A power darlington transistor may be the FMMT634 and FMMT734. So I've ordered a bunch of these transistors in order to test the designs again.
- Week of 7/Feb/2017: The new approach is to use an actual chamber that uses a copper electrode insulated from ground by an outer guard ring. The chamber itself is made of a standard size, commercially available PCB shielding can (in the pictures you can see my attempt at creating one with tin sheet from a metal can). The first prototype actually seemed to have some reaction during the tests. However the second prototype did not. G-code to mill the electrode.
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On a second thought, the "finger electrodes" may not be a good idea. Since there is a large contact area between each finger, there surely is a lot of leakage current that will mask-out any ion-induced current (one of the biggest concerns explained by Charles Wenzel's ion chamber website)
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19/Jan/2017 Finally got a .9uCi AM241 source to test, it will be the benchmark for the detector. Unfortunately, the first attempts with the amplifier circuit yield very noisy measurements, that mainly depend on wind and humidity. Surprisingly (to me), it is also very dependent on static electricity. I've also tested again the spark particle detector, but there seems to be absolutely no relation between the sparks and the proximity of the radiation source. Maybe the finger electrodes are too "pointy" because of the manufacture method (milling)? pic1 pic2
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Week of 9/Jan/2017: Simulation of circuit amplifiers "click type", "LED scale type"
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Self-ask the question: How to make a simple low-cost radiation detector? Begin to learn about ionization chambers thanks to Charles Wenzel's website.
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Week of 1/Jan/2017: First learn of spark particle detectors. Design, build and test a PCB electrode (but without any radiation source to be tested). video
