An Ionisation Chamber Shield for OpenRelief

Posted on Feb 7, 2014

I was prototyping for the OpenRelief project. Based on a simple ionisation chamber, initial testing used a bank of batteries for the bias power supply and this post details the construction of an inverter to replace these. For details of the detector itself see the original post as none of th

An Ionisation Chamber Shield for OpenRelief
Click here to download the full size of the above Circuit.

e fundamental details have changed, and for more on OpenRelief see the interview with project leader, Shane Coughlan. Just to recap, the bias voltage is applied across the ionisation chamber electrodes, between which tiny currents flow when ionising radiation enters the chamber. Using four PP3 batteries in series provided a bias of 36v, which is possibly suboptimal in addition to not being terribly convenient. Once again I must give due credit to Charles Wenzel, who designed the circuit which is described here and that which the detector is based upon. The power supply schematic is below and for further details and alternative designs see Charles` excellent website. Since the detector is based around an Arduino compatible Nanode, the power supply was constructed on a prototyping shield. The top side of the shield can be seen in the picture at the top of this post, and the underside below. With the shield completed I wanted to check the current consumption, which as can be seen below is around 360uA at 5v input, giving an output of around 90v. Since the current flow across the chamber electrodes will always be incredibly small ” in the order of nano/picoamps ” and the circuit is efficient, the power drawn by the shield should be barely noticeable. Having confirmed that there were no shorts, the power supply operated as expected and did not draw too much current, it was time to assemble the detector ready for testing. The...

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