Radiant energy – Calibration or standardization methods
Reexamination Certificate
1999-01-29
2002-04-09
Ham, Seungsook (Department: 2878)
Radiant energy
Calibration or standardization methods
C378S207000
Reexamination Certificate
active
06369381
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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 “backscatter” 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(−
BD
)−
C
Equation 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.
SUMMARY OF THE INVENTION
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 know
Dep Wewage Linus
Troxler Robert Ernest
Troxler, Sr. William Finch
Ham Seungsook
Lee Shun
Troxler Electronic Laboratories, Inc.
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