Low activity nuclear density gauge

Abstract

A nuclear density gauge and test method is provided for measuring density material in a relatively thin zone beneath a surface of the material. The gauge comprises a gauge housing and a substantially planar base on said gauge housing adapted to be positioned on a surface of the material sample. A gamma radiation source having a characteristic primary energy and an activity of no more than 100 microcurie is mounted within the housing and cooperates with the base for emitting gamma radiation through the base and into an underlying material sample. An energy selective gamma radiation detector is mounted within the gauge housing and in laterally spaced apart relation from the gamma radiation source. The gamma radiation detector is operable for quantifying the energy level of the detected gamma radiation. Shielding is provided within the gauge housing between the source and the detector for preventing gamma radiation from passing directly from said source to the detector. An analyzer is connected to the detector for detecting gamma radiation counts in a predetermined energy spectrum having a lower limit of 0.1 MeV or greater and an upper limit which is less than the characteristic primary energy of said source. The density of the sample is calculated based upon the gamma radiation counts obtained by the analyzer within the predetermined energy spectrum.

Description

FIELD OF THE INVENTION[0001]This invention relates to the measurement of density, and more particularly to a test instrument and method for measuring the density of a sample using gamma radiation. The invention is especially suited for measuring the density in a relatively thin zone below the surface of a sample.BACKGROUND OF THE INVENTION[0002]In the asphalt pavement construction industry, portable nuclear gauges are frequently used for measuring the density of the asphalt pavement. Often, the asphalt paving material is applied in relatively thin layers, e.g. on the order of about one to two inches in thickness, over a prepared roadbed foundation or an existing paved roadway. Consequently, there is a need to measure density of the pavement sample in a relatively thin zone, e.g., one to three inches in depth, below the pavement surface. To this end, nuclear density gauges have been developed for directly measuring the density of a thin layer of paving material. For example, nuclear ...

AI Technical Summary

Benefits of technology

[0004]It is therefore an object of the present invention to provide a nuclear gauge suited for measuring the density in a relatively thin zone below the surface of a sample, and which uses a low activity radiation source.

[0005]It is a more specific object of the present invention to provide a gauge which can operate using a gamma radiation source having an activity in the microcurie range, and more specifically with an activity of no more than 100 microcurie, and more desirably an activity of no more than 50 microcurie. Gauges employing these low activity nuclear sources are subject to fewer and less stringent restrictions and regulations, if any.

Problems solved by technology

Prior attempts to produce nuclear gauges using low activity (microcurie) radiation sources have had limited success, primarily because of their limited levels of accuracy.

The main difficulty in developing a gauge based on a low activity gamma radiation source is that the signal to noise ratio of the gamma radiation detection is low because of the relatively low gamma radiation flux from a low activity source.

Background radiation from certain naturally occurring radioactive elements (e.g. K-40, U and Th) present in the material to be tested generate noise which cannot be ignored without sacrificing the accuracy of measurement.

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