[en] Highlights: → CdS NCs are formed by e-beam irradiation of CdCl₂ + MPA aqueous solution. → CdS molecules are released from Cd²⁺ - MPA complex by energetic electrons. → Only QDs are formed under a MPA-rich condition. → NRs and QDs are formed under a cadmium-rich condition. - Abstract: We investigated a simple route for synthesizing CdS nanocrystals (NCs) via irradiation of a CdCl₂ + mercaptopropionic acid (MPA) aqueous solution with a 10-MeV electron beam without using any additional sulfur precursors or stabilizers. When the concentration of the MPA was fixed to 2.5 mM, the morphology and optical properties of the NCs were changed by varying the CdCl₂ concentration. Based on transmission electron microscopy (TEM) imaging, a 0.4 mM concentration of CdCl₂ (the-MPA rich case) created only spherical quantum dots (QDs) with an average diameter of 6.6 nm. When the concentration of CdCl₂ was increased to 5 mM (the cadmium-rich case), nanorods with an average diameter of 5.3 nm and an average aspect ratio of 8.2 were formed along with QDs with diameter of 4.9 nm. The crystalline structures of the NCs were verified to be hexagonal.