Side feeding patterns and nuclear lifetime determinations by the Doppler shift attenuation method in (α,nγ) reactions

Abstract

$\ensuremath{\gamma}$ rays were measured at several angles in both singles and coincidence mode in the $^{119}\mathrm{Sn}$($\ensuremath{\alpha},n\ensuremath{\gamma}$)$^{122}\mathrm{Te}$ reaction at 15 MeV on a thick target. Lifetimes of excited states in $^{122}\mathrm{Te}$ were determined from a Monte Carlo Doppler shift attenuation method (DSAM) analysis of the Doppler broadened lines shapes of $\ensuremath{\gamma}$ rays de-exciting the levels. A comparison of several deduced lifetimes with recent results obtained with the ($n,{n}^{\ensuremath{'}}$) reaction allowed us to validate the choice of a parameter used to calculate the contribution of the side feeding times. The ingredients of the DSAM line-shape analysis (stopping power, description of instrumental line shapes, and side feeding evaluation) are presented in some detail. It is concluded that with proper treatment of side feeding, a DSAM line-shape analysis of peaks in singles or coincidence spectra obtained following the ($\ensuremath{\alpha},n\ensuremath{\gamma}$) reaction is able to provide rather accurate values for the lifetimes of levels with low and medium spins.