Patent Number: 061782181
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1, a schematic view illustrating an embodiment of the present invention is shown. A neutron source 12 capable of providing neutrons shown. Preferably the neutrons provided from the neutron source 12 have an energy of up to 14 Mev. The neutron source can be a neutron generator or accelerator, such as a MF Physics A-320 probe detector by MF Physics Corporation, which is a variation on the basic A-320 design and is useful in applications where an extremely rugged, highly portable "probe" configured system is required. Alternatively, the neutron source of the present invention could be an isotopic source, such as .sup.252 Cf. Neutrons from the neutron source are directed toward the metal test specimen 14. The metals capable of being tested using the present invention include: steel, aluminum, copper, and alloys thereof. For example, neutron activation of the copper component of many such alloys, including Alcoa 6061/T6 aircraft aluminum which contains 0.25% copper, will produce a positron emitter disposed within the metal. The .sup.63 Cu (n, gamma) reaction produces .sup.64 Cu, a positron emitter having a 12 hour half life. Also, the .sup.63 Cu (n, 2n) reaction produces .sup.62 Cu, a positron emitter having a 9.7 minute half life. Neutron activation of copper in the alloy produces sufficient positrons for fatigue measurements of parts made from aluminum alloys containing at least one tenth of a percent of copper. Other elements found in some aluminum alloys include zinc, which may be also neutron activated to serve as a positron source. Positrons from these neutron activated alloy constituents have been found to be suitable for determining the strength loss from fatigue of components built from aluminum alloys and steel. Further, positrons from these neutron activated alloy constituents permit fatigue measurements to be made at far greater depths within aluminum alloy parts than are possible with external positron sources. Therefore a significant advantage of the present invention is the ability to perform bulk analysis of a metal specimen (e.g., at a depth of up to 3.5 inches in steel) using positron annihilation, rather than being limited to surface analysis (e.g. at a depth of approximately one tenth of an inch) as is achieved by conventional positron annihilation techniques. Exposure of the aluminum alloy to a neutron flux of 1,000,000 neutrons per square centimeter per second for ten minutes has been observed to provide ample activation for measurement of fatigue and related defects in the aluminum alloy. This exposure will not cause measurable neutron embrittlement because measureable embrittlement does not occur until the alloy is subjected to a cumulative flux of 10.sup.15 neutrons per square centimeter. Therefore, use of the present invention on aircraft components can be performed in-situ and will not cause damage to the aircraft. Another neutron activated positron source formed within a metal test specimen is .sup.58 Co, which is formed by in situ neutron capture from .sup.59 Co within the metal. It has been observed that there are sufficient .sup.58 Co produced positrons present during refueling shutdowns at nuclear power plants. The .sup.58 Co is produced during normal operation of a nuclear power plant and is deposited on the primary coolant system surfaces and fixed in the approximately 0.1 micron corrosion layer. The .sup.58 Co is also embedded throughout the reactor pressure vessel wall adjacent to the reactor. Three characteristics of positrons and the radiation that they emit upon annihilation with electrons make the positron annihilation method of the present invention useful for detecting the presence and size of microscopic flaws in metals. First, the positive electrical charge cause positrons to be repelled by protons. This characteristic accounts for the positron's attraction to dislocations, vacant lattice sites, vacancy clusters, cavities and other open volumes (voids) in the metal, where the density of atomic nuclei is lower. Thus, a small increase in the number or size of the microscopic defects in a sample results in a large increase in the proportion of annihilation events occurring in the defects. Second, annihilation radiation is sensitive to the momentum distribution of the electrons with which positron annihilate. Defects contain a higher ratio of free electrons to core electrons than perfect metal. This phenomenon can be explained by the tendency of free (conduction electron) to spill over into the defect more than core electrons. Core electrons have a much higher linear momentum than do free electrons. Thus, gamma rays from annihilation events involving free electrons are more likely to approximate the energy (511 keV) and direction (180 degrees) typical of gamma rays produced by events involving positrons and electrons at rest. These characteristics make it possible to detect the presence of defects from the energy spectrum of the gamma ray emissions and from the spectrum of angles of deviation from 180 degrees. Third, because the density of electrons is lower in defects than in perfect metal, the mean lifetime of thermalized positrons trapped in defects is longer than those diffusing in perfect metal. Thus, measurement of positron lifetimes cans also be used to indicate the presence of defects in the metal. As shown in FIG. 1, the gamma rays 20 resulting from the positron annihilation are emitted from the metal specimen 16 and collimated through a variable slit collimator 22 and detected by a high purity germanium detector 24. Preferably the detector 24 is shielded from the neutron source 12 by a neutron shield 26. The collimator design required for these measurements is a variable slit collimator that allows the area of the metal being measured to be controlled so that the detector can be focused on specific areas such as a weld. The detector shielding configuration is shown schematically in FIG. 1. Interchangeable tungsten collimators with varying slit widths (nominally 1 inch long by either 1/8 inch and 5/8 inch wide) and a solid plug, are used with the shield/detector assembly for data acquisition. The detector/shield assembly is fixed in place at each measurement location with a specially designed strapping device that allows the detector to be attached to piping at any location. The collimator used was selected to achieve count rates that produced analyzer dead times less than 20%. The tungsten shield and the solid collimator plug provided at least two tenth-value layers for 1.3 MeV .sup.60 Co gamma rays. Background photopeak contributions from the solid collimator plug measurements are subtracted from those obtained with the open collimator. The measurement system components are specifically chosen to minimize rate effects on the detector and maximize resolution. In addition, a pulser system is used on the analyzer to provide assurance that the measurements are being performed without rate-dependent effects on peak shape. The detector used was an ORTEC Gamma X detector with a Canberra Inspector multichannel analyzer system being used to perform measurements on samples where the positron source was place near the surface of the metal. The detector has a tungsten backshield to prevent a gamma-ray leakage into the detector. The detector was a 59% detector with a 1.95 keV Full Width and Half Max (FWHM) for .sup.60 Co at 1332 keV. Numerous detectors were evaluated to obtain one with the required stability in variable radiation fields and the necessary resolution for performing these measurements. An example of an analysis system used in the present invention is a Canberra Inspector that had been specially modified so that pulse injection with subsequent removal to confirm that the spectrum was obtained in a stable environment and that gain shifts did not occur during data acquisition. The system had the following features: (a) pulser calibration can remain accurate for months, (b) automatic monitoring of the channel positions, shape of the pulser peaks for gain and zero shifts, extraneous noise, and (c) automatic correction for dead time and random summing. This system was temperature stable over the range 0.degree. to 100.degree. C. with a drift of less than 0.5 keV. Variation in the stability as a function of count rate is less than 3% over the range up to 135,000 counts per second. Referring now to FIG. 2, the method of the present invention is illustrated in schematic form. The data are in the form of gamma ray counts versus gamma ray energy. A parameter S, called a line-shape parameter, is used to measure the gamma spectrum width. The line-shape parameter is equal to the ratio of the number of counts in Region A to the total number of counts under the curve. The value of S increase as the number of defects within the specimen increases. The section of the gamma ray spectrum within 10 keV on each side of the 511 keV positron annihilation peak is extracted from the spectrum for analysis. This is referred to in this application as the "width" of the 511 keV peak. This section of the spectrum is integrated to determine the total number of counts in the spectrum and then is normalized to a predetermined integral quantity (nominally 10M counts). The centroid of the peak is then mathematically adjusted to a previously determined energy within 0.1 keV of the 511 keV peak. The channel contents of the channels above the adjusted centroid channel are then extracted from the spectrum section and the FWHM is calculated for the portion of the peak above the 511 keV energy. This is referred to in this application as the "high momentum structure" of the 511 keV peak. This provides an initial assessment of the peak shape and Doppler broadening of the peak when compared with standard peak shapes as defined by standard FWHM for the detector being used. The section of the spectrum above 511 keV channel is then compared on a channel by channel basis with reference spectra with know fatigue or embrittle levels. Then two spectral sections are identified that most closely bound the measured spectrum, interpolation is performed on the channel contents to determine the exact fatigue of embrittlement level by determining the average difference between the two fatigue levels and calculating the average fatigue based on interpolation of the values. A statistical uncertainty is then calculated by summing the differences in the channel contents between the measure spectrum section and the reference spectrum that is closest to the measured spectrum. The average uncertainty in the difference between the two spectral sections is calculated. This is necessary because the actual shape of the peak may vary based on temperature and other effects that may affect the shape of the peak. These uncertainties are reflected in the uncertainty associated with the fatigue measurement being performed. The fatigue or embrittlement level with an uncertainty associated that reflects how closely the measured spectrum reflects that section of the reference spectrum can then be reported and/or displayed by computer. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical application and enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.