Doppler broadening, which is one of the problems in in-beam gamma-ray spectroscopy in inverse kinematics, should be considered more seriously in the case where the gamma rays from short-lived isomer states with a lifetime of a few nanoseconds are detected. Decay points are distributed along the path of the particle, while such information is mandatory for complete correction of the energy spectrum for Doppler broadening effects. In this short talk, firstly an example (real data) of the energy spectrum obtained by detecting such retarded gamma rays (ground transition of the first excited 5/2+, 0.74 MeV state in 15C with a halflife of 2.61 ns) is shown. Secondly it will be shown how we can recover the energy resolution thorught identifying the decay points by using high timing resolution crystal like LaBr3, whose timing resolution is of the order of 0.2 ns. The second part is based on a Monte Carlo simulation study using GEANT code.