A mathematical model is developed for predicting the spectra of alpha particles of distributed energy emitted at a point, transmitted through gas mixtures, and detected by a circular detector. Circular sources located coaxially with the detector are also treated. Experimental results verify the accuracy of the model. Possible applications of the model include the calibration and optimum design of all transmission-type alpha-particle gauges for specific measurements such as atmospheric densitometers. A new gauging principle based on the alpha particle stopping power is also identified and preliminary experiments indicate its feasibility. This principle consists of using a nonlinear search on the amounts of gaseous components present to obtain the best fit between experimental and calculated alpha-particle spectra. If the stopping powers at the different alpha-particle energies are different for all components, then, in principle, one can analyze for the amounts of each component. Potential application of this principle to measurements of density (free of composition interferences) and gas composition of planetary atmospheres is discussed.