Hi, I thin because of the Bragg peak behavior and the overall very short reach of Alpha particles it will be quit difficult to get the exact energy value. However, you van try to use a certain distance in air to slow the alpha particles down. Here you can find the 'Bragg curve of 5.49 MeV alpha particles in air' https://en.wikipedia.org/wiki/File:Bragg_Curve_for_Alphas_in_Air.png

and the corresponding formulas in this article:

https://en.wikipedia.org/wiki/Stopping_power_%28particle_radiation%29

I hope thats helpful.

Best regards

Tobias