Abstract <p>Gamma activation analysis of Se was performed during its irradiation with bremsstrahlung gamma radiation at the Microtron-ST cyclic electron accelerator (<i>E</i> = 21 MeV). Irradiation was carried out at room temperature for 1 h and gamma quanta intensity of ~5 × 10¹³ γ/s. Se samples in the form of granules (balls) weighing 5–10 mg and microparticles (powder) were studied. Microparticle samples were suspended in distilled water in a plastic capsule. After irradiation of samples with gamma quanta, the induced radioactivity was measured on an SBS-75 gamma spectrometer with a semiconductor detector. Energy spectra of selenium radionuclides were obtained. Analysis of spectra of radionuclides showed the absence of impurity chemical elements in the selenium samples.</p>