An Elementary Experiment on the Energy Straggling of a Particles in Air

The a particles from a covered ²¹⁰Po source (full width at half-maximum without an absorber ~0.232 MeV and mean energy «4.71 MeV) were allowed to penetrate a fixed thickness of air and their energy spectrum was measured as a function of the pressure. The energy spectra were also calculated from the zero pressure spectrum on the assumption that statistical fluctuations in the energy loss process could be neglected. It was found that even though the full width at half-maximum increased considerably (by a factor of 1.6 in one case) the measured and calculated spectra were in good agreement. The results obtained here are in agreement with theory which predicts that once the dispersion in energy becomes large, changes in the energy distribution function due to statistical fluctuations become small compared with the changes which occur because particles with different energies lose energy at different average rates. The surprising thing is that statistical fluctuations become unimportant for an a spectrum while the full width at half-maximum is still relatively small.