Semiconductor Radioactivity Detector: Part III

In this post I will present a new hardware version of my sensor, older versions are described in part I and part II. In comparison to the previous one, sensitivity is roughly x10 more sensitive.

In previous version, tin foil window for photodiodes was very close to the BNC sockets and because enclosure was small, it was hard to place a sample close enough. Not it's better, however, if I would choosing again, I would use metal enclosure similar to those used in PC oscilloscopes and put BNCs on front panel, power socket on rear panel and tin foil window on top. This would allow me to easier access for debugging- now I have to desolder sockets to get to photodiodes or to bottom side of PCB (fortunately this side is empty).

Bellow you can see the diagram (click on it to enlarge), what has changed in comparison to previous version:

  • One additional photodiode (previous version has only two of them) to increase sensitivity, also the window in enclosure is much bigger
  • x10 bigger resistance of the feedback loop resistor in first stage amplifier, I tried bigger, but then osculations started
  • Bias for photodiodes using 12V batteries, I could increase it, but didn't have enough space in this enclosure
  • Buffer op-amp at the analog output
  • Digital output.

Additional changes not shown on diagram:

  • U1 is OPA657U
  • U2 is OPA656U
  • R4 is 1G
  • As close as possible to input power socket are placed in paraller 1n/16V and 100n/16V, without them the device started to oscillate randomly.
  • A Schottky diode is connected in series after mentioned above extra capacitors to reduce risk of damaging the device when power supply is connected incorrectly. I don't know if it will help enough but I have already broke one PCB of this device this way, so now it's there.

Digital output is 12V in high state, 0V in low state, this is not very useful for 3V3 logic microcontroller that I'm using for data acquisition, so I made a simple converter using additional PCB.

Here it is visible soldered. I like in those SMA Female sockets that they can be soldered to the edge of the PCB (as visible below) and this is still quite mechanically stable, but doesn't require to drill holes as in regular mounting way.

All materials (including software part presented in previous post) for this hardware revision can be downloaded from project's repository.

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