| 摘要 |
PURPOSE:To enable the measurement of a background value in the state wherein an apparatus is made to present inside an object of measurement, by covering the object of measurement with a shielding member and by determining the background value at the time when the thickness of the object of measurement is measured. CONSTITUTION:Back-scattering gamma-rays are measured by a scintillator 1 disposed in close vicinity to a plurality of test pieces 5 being different in thickness. Then a characteristic curve wherein a peak value of a scatter spectrum or the total number of counts thereof within a certain range has a proportional relationship with the plate thickness of the piece 5 is obtained. Moreover, the surface of the piece 5 is covered with a shielding member 6 when the characteristic curve is prepared, and a background A is determined from a measured value obtained in said condition. When the wall thickness of a tube 7 in the ground, which is an object of measurement, is measured, the scintillator 1 is conveyed into the tube 7, and the measurement is conducted at a position equivalent to the position at the time of measurement of the piece 5, whereby the number B of counts corresponding to the wall thickness of the tube 7 is obtained. On the occasion, the inner surface of the tube 7 is covered with the same member 6, and a background value A' is determined in this condition. Based on this value, a corrected value C is determined by an equation C=B-(A-A'), and with the value C used as the number of counts, a measured value of the wall thickness of the tube 7 is obtained from the characteristic curve. |
