Synergy effect of total ionizing dose (TID) on alpha-soft error rate (a-SER) in FDSOI 28nm SRAM has been experimentally characterized using a dedicated setup combining alpha-particle irradiation (241 Am solid source) in vacuum chamber and 10 keV X-ray irradiation. Measurements have been performed on a 3 Mbit single-port SRAM cut powered at 1V. Irradiations up to 125 krad(Si) have been achieved and their impact on the alpha-SER has been characterized from the cumulated number of bitflips as a function of the exposition time to the alpha-source. Modelling and simulation have been used to link transistor threshold voltage variations to SRAM cell stability in terms of static noise margin (SNM), critical charge (Qcrit) and finally estimated SER, in good agreement with experimental results.