Calorimetric measurement of the first excited state lifetime in 7Li

We report a calorimetric measurement of the lifetime of the first excited state in ⁷Li (478 keV). It is based on highresolution neutron spectroscopy of the ¹⁰B(n, α)⁷Li reaction in a 3 mg TiB2 crystal. The energy deposition of the 7Li recoil nucleus has been measured with an energy resolution of 5.5 keV FWHM using a superconducting transition-edge neutron spectrometer. The lifetime of the 478 keV excited state has been determined from the experimental Doppler broadening of the 2.31 MeV energy line to 10.5 keV. The broadening is modeled with the Doppler Shift Attenuation method to directly determine the energy distribution of the recoil nuclei. The life time has then been extracted by deriving the slow-down parameter for the ⁷Li ion in TiB2 within the Lindhardt-Scharff-Schiott theory. The lifetime value found by this method was of 110 ± 23 fs, which is within 7% from the evaluated data found in the literature. We discuss the theory behind the lifetime calculations derived from the experiment.