Cyclotrons are not used to accelerate electrons because

1. Cyclotron operation depends on a constant frequency f = qB/(2πm). 2. As electrons gain energy, their relativistic mass m grows rapidly because the rest mass is very small. 3. The cyclotron frequency therefore drifts, breaking resonance with the AC accelerating voltage. 4. Heavier ions tolerate higher energies before relativistic effects matter; for electrons, betatrons or synchrotrons are used instead. _Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", §4.4.2_