Camera nuclear-radiation sensor: part I

In previous posts I've described a radioactivity detector based on a photodiodes. Image sensors in cameras use photoelements too, so I think that they could be also used to detect radioactivity. At this moment i didn't success in this , nevertheless I will describe here my attempts.

I'm using RaspbberyPI, to get data from the sensor, camera is some low-budget/quality clone designed for RaspberryPI. I have removed optics, and covered whole camera in black tape to block any light coming to the sensor. It's visible on the picture below.

To handle the camera on the software side, I'm using Python script (with PiCamera library). In infinitive loop it takes a photo and calculate sum of pixels value and then sums values of each RGB channel. This value with timestamp is put to CSV file that is later parsed to diagrams using R script.

Without any sample, internal noise of the sensor (and maybe background radiation) should give after some time (e.g. after a couple of hours) a Gaussian curve on the histogram. After putting measured sample to the sensor and waiting similar period, a new Gaussian curve would appear, so that the histogram would have to visible peaks. That was my assumption, hit would prove that the sensor works, however as visible below it doesn't - there is only one peek.

I will try to better isolate the sensor also from electromagnetic noise or maybe buy a new camera (less noisy). Other than that I don't have ideas to make it works.

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Weird arrays in C

Recently I've saw quite strange way to create an array in C. I will describe it bellow - looks quite interesting!

Let's assume we have an array given bellow:

int myArray[4];

We could rework it, so that each element is declared separately:

    int myArray0;
    int myArray1;
    int myArray2;
    int myArray3;

Now we can obtain pointer to first element and last element, and iterate through elements in between it as with regular array. We exploit that compiler will probably put those variables in the same order in the same place in memory.

A full example is given below:

#include <stdio.h>

void processElement(int e)
{
    printf("processing element in array, it has value %d\n", e);
}

int main()
{
    int myArray0 = 0;
    int myArray1 = 1;
    int myArray2 = 2;
    int myArray3 = 3;

    // dummy way to force compiler not to optimalize-out our array members
    printf("%d\n%d\n%d\n%d\n", &myArray0, &myArray1, &myArray2, &myArray3);

    int *start = &myArray0;
    int *stop = &myArray3;

    if(start > stop)
    {
        start = &myArray3;
        stop = &myArray0;
    }

    for(int* it= start; it <= stop; *it++)
    {
       processElement(*it);
    }
}

:)

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Semiconductor nuclear-radiation sensor: 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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Semiconductor nuclear-radiation sensor: part II

There are many ways to measure radioactivity level, semiconductor detectors sense interactions between ionizing radiation and p-n junction. Because in hobbyist area most popular are Geiger-Muller based detectors (in short: not a semiconductor but lamp based devices), I think it's a cool idea to take a look at this approach.

In this post I will present such home-made sensor and a set of software to parse collected results.

Picture below presents circuit of the sensor that I made, it consist of a photodiode that acts as a sensor, transimpedance amplifier and "regular" amplifier. I've selected op-amps that has little input noise.

Changes that I made during testing the device, that are not presented on the diagram:

• D1 is BPW34
• U1 is OPA657U
• U2 is OPA656U
• 2p capacitor is connected in parallel with R6
• 820k resistor is connected between U2 out and BNC socket.

Below is visible sensor in enclosure and without it. Because the device is sensitive for background EMI and light noise, a metal housing is needed. To increase exposure of the sensor to the radiation, where it is, in the housing, a hole is made and just a tiny adhesive metal tape is covering it.

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