Procedure

            I conducted the actual experiment at the University of Saskatchewan, in the Department of Physics laboratory.

            A Cesium-137 (Cs-137) source with a half-life of 30 years was used. Cs-137 has an energy of 661.66 KeV (0.66166 MeV), meaning that the vast majority of the photons that Cs-137 emits as it decays have an energy of 661.66 KeV.
            To properly simulate Compton scattering, a metal rod called a scatterer was placed in the way of the source to deflect incoming gamma rays in all directions. The more energy a photon lost after striking the scatterer, the greater the angle at which it was scattered. The scattered photon would then interact inside the crystal via the Photoelectric effect, or Compton scattering.
            A common Thallium-doped Sodium Iodine (NaI(Tl)) crystal was used inside the detector. As scintillators, NaI(Tl) crystals have a high efficiency, satisfactory resolution and they produce intense bursts of light compared to other scintillators.
           
            The experimental set up is shown below:

Diagram 4. A block diagram of the experimental set up.

Figure 2. A view of the detector, source, and the related electrical and computer components. Both the detector and source are encased in lead to prevent undesired radiation from entering or escaping. The detector may be pivoted 180 degrees about the scatterer.

            1. I selected the following ten angles at which the energy of the scattered photons was then measured:
           

            These angles were selected so that most of the  values would be evenly spaced. Angles 10 and 90 were also selected to give a complete representation of the change in the inverse of scattered energy with respect to . Zero was not selected as an angle because the radiation would be too intense for the detector to handle – it would have trouble differentiating between the various gamma rays.

            2. Before the actual measurements could be made, the equipment was calibrated by using radioactive sources with known energies.

            3. The energy of the scattered photons was measured for each one of the ten angles. A program called Maestro automatically plotted ‘channel number’ vs. ‘counts’, and was used to view the resulting spectrum.
            In order to calculate the energy, the channel number of the spectrum’s peak was first estimated by eye. Then, two channel numbers were chosen with an approximately equal number of counts, one to the left of the peak and one to the right, and the channel numbers were averaged. The average was compared to the estimated channel number, and based on these two values the most plausible channel number of the peak was selected. This channel number corresponded to a value of energy -.

            4. Microsoft Excel was used to plot  against the corresponding  values. A trendline was added to the graph, and the inverse of the trendline’s slope yielded the rest mass of the electron.

            5. Lastly, statistical analysis was used to find the weighted mean (based on all 10 values of ) and the corresponding error estimate to give a more appropriate value for the mass of the electron.