From left to right the peaks are due to Po, Pu, Po and Am. The fact that isotopes such as Pu and Am have more than one alpha line indicates that the (daughter) nucleus can be in different discrete energy levels.
Calibration: 
MCA does not work on energy, it works on voltage. To relate the energy to voltage one must calibrate the detection system. Here different alpha emitting sources of known energy were placed under the detector and the full energy peak is recorded.
Measurement of thickness of thin foils: 
Energies of alpha particles from radioactive sources are measured before and after passing through the thin films. By measuring difference and using SRIM we can measure the thickness of thin foils.