Determination of instrument background of low dose rate monitor

Within the joint monitoring project carried out by the CMEA countries, the instrument background of low dose rate monitors was studied in a salt mine in the GDR. The experiment is described in detail for the dose and dose ratemeter TESLA NB 3201 with plastic scintillator featuring special modification. The obtained value was 1.82 pGy.s-1 of which the photomultiplier dark current (noise) accounted for about one half and the rest was the radioactivity of the probes including shielding and the glass of the photomultiplier. It is stated that some, albeit immeasurable external effect could come from scatterring caused by the near walls of the salt chamber and the presence of a greater number of persons and instruments. (Z.M.). 1 fig., 11 refs