Demonstration of Thoriated Electrode Activity


Stage, camera holder, electrode holder, thoriated electrodes (~2% of thorium-232), Minipix-EDU


  1. Launch the Pixet Basic software and modify settings to the following:
    1. Min Level: 0
    2. Max Level: 50
    3. Measurement Mode: Tracking
    4. Frames: 100
    5. Exposure: 1 s
    6. Sum: check
    7. Color Map: Hot
  2. Mount the MiniPix EDU camera and the thoriated electrode on the stage, as shown in figure 1.

    Figure 1. Setup of thoriated electrodes and the camera mounted on the stage

  3. Keep the camera and the source as close as possible and click on the play button.


  1. We observe three different types of tracks, as shown in figure 2.

    Figure 2. The radioactivity from thoriated electrodes showing all the three types of radiation- alpha, beta, and gamma

  2. The rarest ones are the round blobs (Fig. 4), caused by the impact of alpha particles.
  3. The worm-like structures (Fig. 5) are the result of beta radiation. 
  4. The third type of radiation is gamma radiation, which usually leaves a one-pixel large track (Fig. 6). Sometimes beta radiation might also leave such traces but then the energy of the pixel can be used to differentiate between them.
  5. Draw a rectangle around the individual alpha blob or gamma pixel to determine the energy of the individual particles (Fig. 3). The “Total” field in Image Info will show the energy in the rectangle.

    Figure 3. Radioactivity of thorium from thoriated electrodes. Drawing a rectangle around the individual particle to determine its energy


  1. The difference between the tracks of the three radiation is due to the way they interact with the camera.
  2. Alpha particles (Fig. 4) being the heaviest, are more ionizing and quickly lose their energy, thus having a short mean linear range.

    Figure 4. An individual alpha blob along with its information shown in Image Info

  3. For eg. in figure 2, the average energy of the alpha particles is 3000 keV which corresponds to the mean range of 9.33 µm in silicon. The generation of free electrons and holes at the surface of the silicon sensor (300 µm) initiate a motion of free charge to the electrodes. During such motion, the holes penetrate multiple adjacent pixels due to diffusion which results in the formation of round blobs. From the color layout of blobs, we can see that the energy absorbed at the center is higher than the edges.
  4. However, in beta radiation (Fig. 5), the electron gradually loses its energy due to collisions with electrons in silicon atoms and ionizes them. This causes a signal in neighboring pixels and the result is a curved trajectory of high energy electrons.

    Figure 5. An individual beta particle along with its information shown in Image Info

  5. Gamma radiation (Fig. 6) are photons with energy of units to tens of keV. The interaction of this photon and silicon atom releases an electron. The energy of such an electron is very low and is detected as a measurable signal by one, two, or three pixels. 

    Figure 6. An individual gamma photon along with its information shown in Image Info

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