The present invention relates to the calibration and calibration confirmation of nuclear gauges and, more particularly, relates to a method of calibrating such gauges without standardized calibration blocks.
Nuclear radiation gauges are used to determine density and/or moisture content of soils, asphalt, and similar materials. Examples of such gauges are described in U.S. Pat. Nos. 2,781,453 and 3,544,793. In many instances, these gauges have become the industry standard because of their non-destructive testing capability and endurance. The American Society for Testing and Materials (ASTM) has established testing standards for using nuclear gauges to measure density and moisture content. The testing standards are designated D 2922-96 (density) and D 3017-88 (moisture) and are incorporated herein in their entirety.
Nuclear density gauges currently in use, for example, the Troxler Model 3400 and 4400 series gauges manufactured by the Assignee of the present invention, employ a nuclear radiation source, typically a mono-energetic source, that discharges gamma radiation into the test specimen and a radiation detector, typically a Geiger Mueller tube, that measures the scattered radiation. The gamma radiation interacts with matter in the test specimen, either by losing energy and changing direction (Compton interactions) or by terminating (photoelectric interactions). Consequently, the gamma radiation detected by the radiation detector has a continuous energy spectrum.
These gauges are designed to operate both in a xe2x80x9cbackscatterxe2x80x9d mode and in a direct transmission mode. The radiation source is vertically moveable from a backscatter position where it resides within the gauge housing to a series of direct transmission positions where it is inserted into small holes or bores in the test specimen. The gamma radiation received by the radiation detector is related to the density of the test medium by an expression of the following form.
CR=A exp(xe2x88x92BD)xe2x88x92Cxe2x80x83xe2x80x83Equation 1
where:
CR=count ratio (the accumulated photon count normalized to a reference standard photon count for purposes of eliminating long term effects of source decay and electronic drift),
D=density of test specimen, and
A, B, and C are constants.
The gauges are factory calibrated to arrive at values for constants A, B, and C for each gauge at each source depth position. The factory calibration procedure is a time-consuming iterative process, which may require several hours, or even days, to complete. In order to determine values for the three calibration parameters of the above equation, count measurements must be taken using at least three materials of different densities at each radiation source position. Typically, the three materials are solid blocks of aluminum, magnesium and a laminate of magnesium and aluminum. In some instances, as many as five calibration blocks of material have been employed in order to take into account the distinct mass attenuation coefficients of different soils. Thus, the standard factory calibration methods, often referred to as the three-block or five-block calibration methods, require a large number of individual counts in order to complete the calibration. For example, a gauge having a twelve-inch radiation source rod with seven different radiation source depth positions requires a minimum of twenty-one separate counts using the three-block calibration method. Each count is taken for a predetermined period of time, with longer periods of time producing greater precision. For example, for some gauge models, a typical count period for calibration is about four minutes for a direct transmission mode and about eight to twenty minutes for backscatter mode. Once all the counts are accumulated, values for the calibration parameters A, B, and C are calculated for each radiation source position.
The above-described calibration method is both time consuming and labor intensive because it requires numerous counts and movement of the gauge to positions overlying a plurality of blocks. The requirement that the gauge be moved from block-to-block also makes it difficult to fully automate the calibration process. Additionally, each standardized calibration block occupies a relatively large volume of space and weighs over 300 pounds, making them unwieldy and poorly suited for portability.
Further, in normal use, nuclear gauges undergo stress that can change the source-detector geometry of the gauge. Changes in geometry, as well as other factors, affect the gauge response such that, after a period of time, there is a need for recalibration of the gauge to arrive at new values for the constants A, B, and C. The standard practice in the industry has been to return the gauge to the factory, or to a regional calibration center, where the factory calibration process described above is repeated. Thus, the gauge user must go without the use of the gauge for a period of time while the gauge is recalibrated.
Efforts have been made to shorten the calibration process by using fewer standardized calibration blocks. For instance, in U.S. Pat. No. 4,587,623, incorporated by reference herein in its entirety, a calibration process using only two blocks is disclosed. This disclosed method relies on the assumption that the constant B, for a given radiation source position, does not change during the life of the gauge. However, the two-block method does sacrifice some accuracy since constant B may change slightly during the life of the gauge. Further, the two-block method still requires numerous counts and movement of the gauge between two heavy standard calibration blocks.
A calibration method using only one standard calibration block is disclosed in U.S. Pat. No. 4,791,656, which is incorporated by reference herein in its entirety. The one-block method involves a collection of counts from a single calibration block and the use of statistically derived relationships between the count rate actually obtained from the calibration block to the count rates historically obtained from at least two different calibration blocks of other known densities. By using such historically derived relationships, the expected calibration count rates for the other blocks can be estimated and used to calculate the equation parameters described above. Although the one-block method reduces the number of experimental counts that must be taken, it still requires the use of at least one heavy standardized calibration block. When using the one-block method, it is also generally advisable to confirm the calibration by testing the gauge on at least one other calibration block of substantially different density. A three block calibration process would still be required if the gauge failed to pass quality control/quality assurance tests. Further, by relying on the historically derived data, the one-block method assumes that, at a given source rod depth, there is a set of strong linear relationships that relate the magnesium, magnesium/aluminum and aluminum counts to one another. Some gauges, however, do not adhere to these relationships in a consistent manner. Such gauges would require a full three-block calibration process to ensure an adequate calibration. Finally, the one-block calibration procedure is only appropriate for calibrating gauges of identical construction as the gauges used to generate the historical data. So a one-block calibration process would be impossible for a new model or style of gauge because of the absence of historical data.
There remains a need for a new method of calibrating and confirming the calibration of nuclear gauges that is less time-consuming, less labor-intensive, and suitable for both initial factory calibrations and recalibrations by the gauge user at locations remote from the factory.
The present invention is directed to a method and apparatus for calibrating nuclear gauges that provides the accuracy of multiple block calibration methods without requiring multiple standardized calibration blocks of known density. Using the apparatus of the present invention, a nuclear gauge may be calibrated at each radiation source depth position without moving the gauge between multiple calibration blocks. Instead, the apparatus of the present invention utilizes a variable radiation filter capable of simulating the radiation attenuation of numerous materials of varying densities. In a preferred embodiment, the variable radiation filter simulates the density of multiple materials of known density, such as solid calibration blocks, and also attenuates the incident gamma radiation in such a way that the resulting energy spectra incident upon the gamma ray detectors substantially match the energy spectra of the materials of known density.
The apparatus of the present invention is useful for calibrating a nuclear gauge having a radiation source and a radiation detector. The apparatus comprises a support for receiving a nuclear gauge of the type having a radiation source and a radiation detector. Preferably, the support has a lateral surface adapted for receiving and supporting the underside of the nuclear gauge and an opening for receiving a vertically moveable source rod of a nuclear gauge therethrough. The support additionally includes a guide operatively positioned to guide a nuclear gauge into a position wherein said source rod of said gauge is overlying said cavity.
The apparatus further comprises a variable radiation filter operatively associated with the support and being located with respect to a nuclear gauge received on this support so as to attenuate radiation emitted from the radiation source. The radiation filter provides variable extents of radiation attenuation and is capable of simulating the radiation scattering of a plurality of materials having different densities. Preferably, the variable radiation filter comprises a mass of substantially uniform density mounted for movement relative to the support to present a variable extent of radiation attenuation to the radiation detector of the nuclear gauge. In one embodiment, the moveable mass comprises a block of predetermined shape, such as cylindrical, mounted for rotation relative to the support. The block has a cavity therein positioned beneath the opening of the support and adapted for receiving the source rod of the nuclear gauge. The cavity is eccentrically located within the block and the block is positioned for rotation about the central axis of the cavity, the axis of rotation being substantially perpendicular to the planar lateral support surface. In one embodiment, a motorized drive cooperating with the block rotates the block about the central axis of the cavity.
The variable radiation filter preferably includes a fixed mass operatively positioned adjacent to the radiation detector to further attenuate radiation emitted from the radiation source of the nuclear gauge. The fixed mass preferably comprises at least one plate constructed of material selected from the group consisting of aluminum, magnesium, lead, polyethylene, cadmium, tungsten, graphite and combinations thereof.
Preferably, the variable radiation filter produces energy spectra that substantially match the energy spectra of a plurality of materials having different densities. In one embodiment the variable radiation filter has a setting that simulates the density of magnesium, wherein the setting produces a ratio of total counts detected/counts with energy over 300 keV of at least about 4.0 at a radiation source depth of 2 inches measured by a 1-inch by 1-inch NaI scintillation detector. Most preferably, the ratio is at least about 4.5. Using the same magnesium setting at a radiation source depth of 8 inches, the setting produces a ratio of total counts detected/counts with energy over 300 keV of at least about 3.5, measured by a 1-inch by 1-inch NaI scintillation detector. Most preferably, the ratio is at least about 4.0.
In one embodiment, the variable radiation filter has a setting that simulates a known density and produces a ratio of total counts detected/counts with energy over 300 keV that is at least 70% of the same ratio produced by a solid block of material of the known density, as measured by a 1-inch by 1-inch NaI scintillation detector. Preferably, the ratio is about 80% of the ratio produced by a solid block of material of the known density. For example, the solid block of material could be a magnesium or aluminum calibration block.
The present invention also provides a method of calibrating a nuclear density gauge. The method includes providing a nuclear gauge having a radiation source and a radiation detector. The gauge is positioned on a calibration stand having a variable radiation filter such that radiation emanating from the source is attenuated by the filter. An accumulated count of scattered radiation is obtained with the nuclear gauge in position on the stand. The method may further include the step of adjusting the variable radiation filter to provide a different extent of radiation attenuation and obtaining a second accumulated count of scattered radiation with a nuclear gauge in the same position on the stand. The variable radiation may be adjusted to provide further different extents of radiation attenuation and further accumulated counts of scattered radiation may be obtained. For example, three or five different extents of radiation attenuation may be used to obtain a corresponding number of accumulated counts of scattered radiation.
Where the nuclear gauge is of the type having a radiation source located in a vertically moveable source rod, the source rod may be repositioned to a different source position while the gauge remains in position on the stand. In this manner, the gauge may be calibrated by obtaining accumulated counts of scattered radiation at each source depth position. Using the method of the present invention, the nuclear gauge may be calibrated at each source depth position by adjusting the variable radiation filter to provide one or more different extents of radiation attenuation and obtaining corresponding accumulated counts of scattered radiation without moving the gauge from the lateral support surface.