> Can anyone suggest to me if there is something wrong with the
>following reasoning.
There is nothing wrong with your logic. Helium would help a great deal. If
you are after high precision (e.g., better than 1 or 2%, say) in the
relative intensities with a moving detector, you must also be concerned
about the humidity in the air.
R. A. Young, Professor Emeritus
School of Physics
Georgia Institute of Technology
Atlanta, GA 30332 USA
Tel: 404-894-5208
e-mail: r.young@physics.gatech.edu