Calculations of gamma-ray spectral profiles of linear alkanes in the positron annihilation process

The positron-electron annihilation gamma-ray spectra of linear alkanes CnH2n + 2 (n = 1-12) have been studied systematically. A profile quality (PQ) parameter, χ, is introduced to assess the agreement between the obtained theoretical profiles and the experimental measurements in the entire region of energy shift of the spectra. Together with the Doppler shift (Δɛ) of the gamma-ray spectra, the two parameters, χ and Δɛ, are able to provide a more comprehensive assessment of the calculated gamma-ray spectra with respect to available experiment. Applying the recently developed docking model, the present study determines the positrophilic electrons for individual alkanes from which the gamma-ray spectral profiles are calculated. The results achieve an excellent agreement with experiment, not only with respect to the Doppler shift, but also with respect to the gamma-ray profiles in the photon energy region up to 5 keV. The study further calculates the gamma-ray spectra of other linear alkanes in the series without available experimental measurements, such as heptane (C7H16), octane (C8H18), decane (C10H22) and undecane (C11H24). The results obtained show a dominance of the positrophilic electrons in the lowest occupied valence orbital (LOVO) in the positron-electron annihilation process, in agreement with previous studies.