[en] With plates of 15 different alloys of fine steel the absorption of the gamma rays of ¹³⁷Cs was measured as a function of the thickness. In 5 series of measurements the distances source - detector and source - probe were changed. The analysis of the records obtained showed that with a constant distance source - detector of about 250 mm a change of the position of the probe of more than +-20 mm has no influence to the measuring values. A distribution of the probes in 4 groups with average values of the absorption coefficients leads to deviations of the calculated values to the nominal thickness values between +0.18 mm and -0.16 mm, whereat by this distribution systematic errors occure. For exact measurements therefore the alloy specific absorption coefficients must be taken. An accuracy of better than +-0.1 mm must be obtainable. (Author)