5 operating at a modulation frequency of 50 kHz, driven by quartz oscillator 6, and polarizer 7. After the polarizing modulator, radiation passes through analytical cell 8, interference filter 9 with the peak transmittance at λ = 254 nm, and is detected by photodetector 10. Signals from the photodetector output arrive at electric signal processing unit 11, where the signals modulated at frequency S1 and dc signals S0 are separated and an analytical signal, related to the differential cross section ΔQ by the Eq.