[en] The effective hyperfine magnetic fields acting on short-lived excited nuclear states (27 < τ < 127 ps) have been measured for ∼ 7.5 MeV Ir and Pt ions immediately after implantation into iron hosts at room temperature The observed field strengths decrease with the lifetime of the probe state and are consistent with the hyperfine field being absent for about 6 ps after implantation As the hyperfine field is quenched while the local temperature exceeds the Curie temperature, these results give a direct measure of the thermal-spike lifetime The results generally confirm molecular dynamics computer simulations and suggest that electron-phonon coupling does not play a major role in cooling the heat spike