  This paper presents a new explanation of the width of gamma-ray spectra based
on Rahm's electronegativity scale. This quantitatively proves, for the first
time, that positrophilic electrons in the positron-electron annihilation
process are exactly the valence electrons. This suggests the replacement of
Full Width at Half Maximum (FWHM) of the gamma-ray spectra with the newly
defined physical quantity Average Doppler Shift (ADS). Both FWHM and ADS of the
gamma-ray spectra in light elements agree well with the corresponding Rahm's
electronegativity values, respectively. Further, ADS provides strong evidence
in favor of Rahm's electronegativity scale. It is expected that this will be
useful in understanding the mechanism of the positron-electron annihilation
process.
