[en] Energy spectra of a 1 MeV, 70 kA, 4μs, 140 kJ relativistic electron beam (REB) on the U1-SPIN device are measured before and after passing through gas/plasma prefilled drift chamber using multifoil analyzer. The drift chamber is a 1-long, 10 cm diameter stainless steel tube with 4T longitudinal magnetic field. The initial beam injected to the tube from the accelerator diode is monoenergetic. For high gas densities (∼10¹⁶cm^-3) the energy spectrum on the exit of the tube remains the same as it is the diode. If the gas/plasma density is less then 10¹⁵cm^-3 the energy loss up to 20% is observed. At the initiak gas density about 10¹³cm^-3 appreciable fraction (∼10%) of the beam electrons with the energy more than initial one appears. Specific features of the diagnostic technique are discussed in details. 21 refs.; 15 figs