Tube Wall Thickness Gauge With Selection Of Backscattered Gamma-Radiations

A gauge is described in which γ-rays scattered back from a thickness of metal are distinguished from primary γ-rays by virtue of their reduced energy. The selection is achieved by using a scintillation counter with a sodium iodide phosphor as detector, followed by a single channel kicksorter as energy selector. The need for screening is thus eliminated and the source and detector can be mounted close together. This results in a light-weight measuring head, capable of very local measurement of thickness. The electronic circuit is arranged to permit single compensation for source decay so that constant calibration is preserved when using a 60Co source. The instrument is suitable for measuring the tube wall or plate thicknesses up to 7/8 in. of steel and is particularly useful in detecting corrosion pitting in tubes.