Determining the Energy of Alpha Particles


Stage, camera holder, source holder, polonium-210, Minipix-EDU


  1. Launch the Pixet Basic software and modify settings to the following:
    1. Min Level: 0
    2. Max Level: 100
    3. Measurement Mode: Tracking
    4. Frames: 100
    5. Exposure: 1 s
    6. Sum: uncheck
    7. Color Map: Hot
  2. Mount the MiniPix EDU camera and polonium on the stage.
  3. Keep the camera and the source as close as possible and click on the play button. 
  4. Repeat the same experiment with the distance between the camera and source to be 2 cm and 3 cm.


  1. We observe the alpha particles in the image section.
  2. For the closest distance, in the energy tab of the Spectra window, we can see the graph between counts and energy of alpha particles. We observe a peak at 4210 keV. (Fig. 1)

    Figure 1. The energy peak of alpha particles. The higher the exposure time, the more counts we observe.

  3. We may use the scroll button to focus on the peak and get the exact energy or increase the size of the spectra window by clicking on the rectangle at the top right corner of the spectra window.
  4. We also observe that with increasing distance between the camera and the source, the peak shifts towards lower energy, from 4210 keV to 3550 keV to 2550 keV, as shown in figure 2.

    Figure 2. The three different energy peaks observed for three different distances.

  5. There is another small alpha peak at 8800 keV when the camera and source are closest, as shown in figure 3.

    Figure 3. A smaller peak observed at almost twice the energy of alpha particles which is the result of two alpha particles striking the same pixel at the same moment.


  1. The average energy of alpha particles is 5500 keV but the observed energy (4210 keV) is less than the energy of alpha particles. This is because of the loss of energy in the air by ionization and part of the energy is also lost in the source itself (the radiation which comes from the inner depth of the source)
  2. The small peak observed at 8800 keV (fig. 3) is most probably because of two or more particles striking the camera at the same pixel, at the same time. If the exposure time or frame time is increased, we will observe more overlapping particles and thus will increase the 8800 keV peak.
  3. Some of the particles that strike the edges of the camera (Fig []) are not able to register their full energy and thus are detected as low energy particles. More information can be found in the experiment showing how collimator effects the detected energy.

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