Semiconductor Radioactivity Detector: Part I

Currently I'm trying to make a working version of a radioactivity detector that uses semiconductor as a sensor. It's a different approach than Geiger-Muller detectors or ionization chambers, more complicated, but also much more interesting.

While Geiger-Muller counters can only provide information about the amount of particles in a period of time, semiconductor detectors can also measure their energy, so it's possible to say much more about the nature of observed ionizing radiation. Some of the disadvantages of these detectors are that they are more expensive, complex and sensitivity may degrade over time.

The current version doesn't work, but I think it's so interesting concept that I've written this entry anyway.

semiconductor radioactivity detector

The idea is that when ionizing particle (alpha, beta or gamma) is blocked by the p-n junction, a small amount of the energy is released. It has a form of a current spike and can be observed by the next stages of the device.

The p-n junction is just a diode polarized reverse-biased. To make the working area of the p-n junction bigger, a photodiode is used. I know that there also exists specialized versions that are more sensitive, however, I couldn't find any in any online electronic shops.

In my design, the sensor is D1, it's polarized by R1, and C1, R2, L1 (those last three elements are making a low band filter to block noise from power supply, they should be as close to D1 as possible).

The first stage of an amplifier is based on a N-JFET to minimize current sink from the measured circuit, in addition, this type of transistors are extremely fast (that's why they are used widely in RF designs). To reduce parasitic currents between PCB traces, this part is mounted "in the air". EMI that could affect this stage are reduced by a small mass connected shield made from copper and aluminium tape.

Next two steps are high pass amplifiers. Since the signal is very small and those are not rail-to-rail opamps, a symmetrical power supply or virtual mass should be used. I've forgotten about that so lately I just used additional AA battery connected between negative power pin of the opamp and ground.

There are three outputs: raw, high/low (R10, R11, IC1C) and integrated over a period of time (IC1D, R12, R13, R14, C9, C10).

semiconductor radioactivity detector circuit

Below image shows the sensor, I've removed the protective glass from the photodiode to expose it better on the ionizing radiation.

The PCB looks like a nightmare because I've scratched some pads during multiple soldering and desoldering of elements, also some traces were cut and connected again, etc. It's a big blow of a mess now and I think that the story of this PCB is ended, soon I will design a new one based on the experience I've gained.

As I've said in the beginning of the article, the current version doesn't work - I can't observe anything except noise. This may be due to multiple problems. One of them is a proper shielding, tracks length, etc. It's a challenge to shield the device from EMI, but still make it sensitive to ionizing radiation.

Another problem is that I can test it only with alpha or beta particles, but they have big problems penetrating objects (are easily blocked), so it may be that they aren't even going to the pn junction, but are blocked by the case. This is something that is unclear to me at this moment.

I will continue working on this project and write a new article when I will make some progress.

15 comments:

  1. Just a thought, why not generate a simulated transient at the sensor to verify your amplification circuit is working properly? This would give you a clear focus for troubleshooting, if the amplification circuit works correctly you can focus on the sensor, for example.

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    1. Hi,

      that's a good idea, I will do that, thanks.

      Bob

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  2. Hi, There is a semiconductor geiger counter project and a lot of background information on:
    opengeiger.de
    I built their stuttgarter geigerle. It works quite well. Because of the small detector area it is not very sensitive. The main site is in German, with a lot of the information in English at:
    opengeiger.de/index_en.html

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    1. Hi,

      phenomenal materials, thanks! This will help me a lot.

      Bob

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  3. I've worked with radioactive materials and I can tell you that most beta emitters can be stopped by single peace of paper. Beta radiation has very low ionizing power. Alpha radiation has high ionizing ability but does not travel very far. Best option would be to obtain gamma emitter. Maybe some referent Cs 137 or uranium glass which can be bought legally due very low radiation.

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  4. Maybe instead diode use a single cell of PV panel, there is PN junction but on large surface.For example mono crystalline PV cell ?

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    1. With bigger surface, internal capacitance increases. Because the signal has form of very short spikes, with sufficient capacitance, it will be internally filtered out, so it won't be possible to measure.

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  5. There is an Elektor kit that use a BPW34 photodiode, I bougth it and it works well, I believe, only if shielded and in the dark. I used a Pringles can and some aluminium foil.

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    1. Thanks for info, yes I have read it (Measure Gamma Rays with a Photodiode, Radiation detector using a BPW34), but I doubted that it can work since bipolar transistor would sink a lot of current from measured circuit.

      I will try to build this!

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  6. What program are you using to make your boards? What would you recommend?

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    1. Eagle and it can do the job, but is missing key shortcuts, so it's a bit tedious. There is also open source alternative - KiCAD, and business grade Altium (it has some free version, but works only in cloud, so I din't check it).

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  7. I have built similar circuit that worked brillantly.
    It allowed to record energy spectrum of X-rays emitted from 241Am (mainly 77keV and some other lines) and other X-ray sources. I used BPW 34 photodiode at 30V bias, but any bias >5V will do.

    You just need transimpendance amplifier made with ultralow-current-noise input opamp (with input nose current around 1fA/sqrt(Hz) or less) and tens of megaohm feedback resistor (but 1Gohm is better). I have used ultra fast OPAMP - ADA4817, but you may use a slow one - LF357 or LM662 (with unbelievable low current noise).

    Using discrete FET at input may give you good results but it's tricky and you must be really good at noise calculations, because with discrete elements things are much more complicated and usually datasheets does not say anything about parameters you need to know.

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    1. Thank you! I've take a look on the transcendance amplifier, and indeed it seems to be a configuration that is needed here, I will use this in the next revision.

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