Patent ID: 6407394
Filing Date: 2002-06-18
Classification: G01T

Abstract:
A procedure to measure the radioactivity of radioactive materials enclosed in a container (4) by means of &ggr; ray detectors (6) positioned laterally, outside the container, characterized in that comprises the following phases:i) weighing a known volume container (4) in order to determine the average density of its contents; ii) positioning the container (4) upon a support (1); iii) positioning on respective supports (5) two &ggr; ray detectors (6) set symmetrically compared to a main transversal axis (IIâ€”II) of the container, facing the smaller walls (W) of the container (4); iv) moving (A2) the container (4), so that its main longitudinal axis coincides with the longitudinal axis (Iâ€”I) connecting the two detectors (6); v) moving (A1) the two detectors (6) on their respective supports (5) in order to stop them in a sampling position, along said longitudinal axis (Iâ€”I), symmetrical with the container (4), where the intensity of the radiation from the container is sampled in order to define the counting geometry; vi) moving (A1) the two detectors (6) upon their respective supports (5) in order to stop them in a counting position, along said longitudinal axis (Iâ€”I), symmetrical with the container (4), that proves to by suited to providing measurements of the radiations that are an optimum in terms of accuracy of the measurement and counting time, the distance(D1)of such positions generally being greater than the largest dimensions (W) of the walls of the container facing the detectors (6); vii) measuring the &ggr; radiation and calculation of the geometric mean, expressed as counts per second, of the counting ratios (C1,C2) of the two detectors (6), such a geometric mean being linked to the intensity of the corresponding &ggr; radiation by the following formula (C1Â·C2)Â½=IÂ·&bgr;Â·F1Â·F2Â·(&agr;/D2)Â·exp(âˆ’&mgr;*Â·L/2)â€ƒâ€ƒ(1) when the activity is mainly concentrated in a limited area of the container, and alternatively by the formula (C1Â·C2)Â½=IÂ·&bgr;Â·F1Â·F2Â·(&agr;/D2)Â·(1âˆ’exp(âˆ’&mgr;*Â·L))/(&mgr;*Â·L)â€ƒâ€ƒ(2) when the activity is distributed in a sensibly uniform manner within the container; viii) calculating the intensity I of the &ggr; radiation by means of one of the above formulae, where&mgr;*=(2/D)+&mgr;(H2O)Â·&rgr;, &mgr;(H2O) is the mean attenuation coefficient in the water, expressed in cm2/g, &rgr; is the average density, expressed in g/cm3, of the material gathered in the container (4), I is the intensity of the source, expressed in dps or Bq, &bgr; is the branching of the &ggr; line considered, F1 is the absorption factor of the container base, F2 is the absorption factor in air, &agr; is a calculated parameter expressed in cmâˆ’2 that depends on the energy of the photons and on the characteristics of the detectors, D is the distance between a detector and the wall of the container facing the detector, expressed in cm, including the distance covered on average by the photons inside the container, L is the height of the container, expressed in cm.