Patent GB-1414655-A (GB)
Title: Non-destructive densitometric measurement
IPC: G01N23/20
Filed: 1972-11-13
Abstract: 1414655 Measurement of electron density within a body ATOMIC ENERGY OF CANADA Ltd 13 Nov 1972 [24 Nov 1971] 52277/72 Heading G1A The electron density of a small volume of material within a body (which may be irregularly shaped and non-uniform) is measured non- destructively by passing two finely collimated beams of photons through the volume at an angle to one another and making specified measurements including ones involving the flux densities of transmitted and Compton scattered photons, and calculating the electron density. In a first method two unequal high-energy sources of X or α-radiation are arranged so that the angle between the beams results in the mean energy of the Compton-scattered photons from the more energetic source Sl received at the second detector D2 being equal to the mean energy of the photons from the less energetic source S2. The measurements taken are then the photon flux densities of the directly transmitted radiations measured individually, the photon flux densities of the radiation from source S1 scattered to the second detector D2 with source S2 cut off for the initial position of the body and with the body rotated through 180 degrees, the photon flux densities of the two sources, and the efficiencies of the detectors. These measurements together with a constant determined from the Klein- Nishina equation and the geometries of collimation of the beams give an absolute value of the electron density of the volume V. Alternatively by measuring only the specified transmitted and scattered photon flux densities together with an apparatus constant determined from a substance of known electron density, the relative electron density can be calculated. The sources may be Co-60 (1.25 MeV) and Cs-137 (667 ke V) with a scattering angle of 50 degrees. Instead of rotating the body the detectors and sources may be rotated through 180 degrees round it. In a second method two equal sources of lowenergy radiation are used the angle between the beams being such that the Compton scattered photons have substantially the same energy as the emitted photons. The measurements taken are then the photon flux densities of the transmitted and scattered radiation received by the detectors from each source used individually. These measurements together with a calculated constant give an absolute value of the electron density but a relative value can be obtained by using a constant determined from a standard sample. The sources may be Gd-153(0.1 MeV) with a scattering angle of 45 degrees. Instead of two sources a single source can be used moved between the two source positions. In either method a single detector can be used arranged for movement between the two detector positions. The methods may be used to determine the mineral content of bone inside the body, the densities in continuous flows of minerals (additional sources and detectors may be used), and to assay materials inside containers. Measurements can be made on finished castings of metal, plastics or ceramic, fluid materials in pipes and mixes of materials in containers, and also on statuary, archeological objects, and jewellery.