This work reports a long-duration (similar to 3 years) real-time underground experiment of 65 nm SRAM technology at the underground laboratory of Modane (LSM) to quantify the impact of alpha-emitter on the Soft-Error Rate (SER). We developed an original and full analytical charge deposition based on non constant Linear Energy Transfer (LET) to accurately model the diffusion/collection approach. Monte Carlo simulation results based on this improved model have been compared to experimental data to analyze the impact of alpha-particle production inside the circuit silicon material for both single and multiple chip upsets. Finally, the respective contributions of alpha emitters and atmospheric neutrons to the circuit Soft-Error Rate (SER) are evaluated and compared, considering additional real-time measurements performed in altitude on the ASTEP platform.