A laboratory safety officer is testing a sample of dust collected near an industrial processing plant to determine if it is contaminated solely by an alpha-emitting radionuclide or if beta/gamma emitters are also present.
The officer measures the total count rate at varying distances from the sample in air using a Geiger-Müller tube and plots the results in Figure 1 below.
Using the graph in Figure 1, explain how the results support the claim that the sample contains only an alpha-emitting source.