The study of Doppler broadened gamma lineshape

Thermal neutron capture reactions produce excited nuclei which subsequently decay. The lifetime of different levels in γ decays are ranged from 10-18 sec up to days. In order to measure these lifetimes a wide variety of nuclear methods are used. Recently a new method was proposed, suitable for lifetime measurements of femtosecond order. It is based on high resolution crystal γ-ray spectrometers, designed and developed at ILL-Grenoble. Using it, the lineshape of a γ-ray emitted by a recoiling nucleus, moving due to a previous transition, can be measured with a resolution three order of magnitude greater than the resolution of the best semiconductor detectors. Unlike the Doppler shift attenuation method, where the Doppler shift of γ ray can amount up to several KeV (compatible with usual semiconductor detectors), in this case the initial velocity of the recoiling atom is much lower and the gamma line is not shifted but Doppler broadened. Moreover, the gamma line shape contains information both about the lifetime of the decaying nucleus level and about the slowing down process of the recoiling nucleus. The present paper presents our preliminary results concerning the information which can be obtained, using this method, about the lattice properties of the host crystal, where the slowing down process takes place. We restrict to recoiling energies of few eV, when the recoiling atom is oscillating with a high amplitude around the equilibrium position. A model of a damped oscillator is compared with the molecular dynamics calculations available. Several numerical simulations are also reported. (authors)