X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
1999-07-22
2002-03-05
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S070000, C378S071000, C378S076000
Reexamination Certificate
active
06353656
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to an apparatus for use in x-ray residual stress analysis. More specifically, this invention relates to an x-ray residual stress analysis apparatus having a radioisotope for generating x-ray emissions.
2. Description of the Related Art
X-ray residual stress analysis, a subset of x-ray diffraction, is a well-known technique for measuring stress in crystalline materials. A detailed discussion of the technique and the various apparatuses that employ this technique for the non-destructive examination of materials can be found in a text by I. C. Noyan and J. B. Cohen, entitled
Residual Stress: Measurements by Diffraction and Interpretation
, published by Springer-Verlag in 1987.
All of the x-ray residual stress analysis devices discussed by Noyan and Cohen utilize an x-ray tube composed of an anode and a cathode where electrons emitted by the cathode are accelerated at high velocities into the anode. The interaction of the electrons with the anode produces a continuum of bremsstrahlung photons with energies distributed over a wide energy range and a photon with a specific energy that is characteristic of the anode (target) material. Thus, the energy spectrum of an x-ray tube has a characteristic line superimposed on a bremsstrahlung background. For the energy spectrum to be useful, the bremsstrahlung background must be removed or reduced either electronically or by using mechanical filters.
Typically, the characteristic x-ray energies employed in residual stress vary between 5.4 and 17 keV. Elemental radioisotopes emit photons originating in the nucleus or in the atomic shell surrounding the nucleus having energies within this energy range. However, x-ray energies in excess of 10 keV present special detection problems for silicon-based solid state detectors. As photon energy increases, the stopping power of the silicon in a solid state photodiode array declines. More photons simply pass through the active volume of the silicon undetected. The use of a phosphor material as a scintillator optically coupled to a photodetector permits efficient measurement at higher photon energies. Cesium Iodide(Thallium) (CsI(Tl)) and Gadolinium Oxysulfide (GdO
2
S) are two such phosphor materials shown to have high sensitivities over a range of wavelengths extending from approximately 10 keV to approximately 100 keV.
The efficiency of a photodiode array (PDA) to directly detect x rays compared to a phosphor screen coated PDA can be examined using a few simple calculation. In a silicon-based PDA, the number of electron hole pairs per x ray directly produced by a 10 keV x ray is:
10000
eV
3.65
eV
ion
pair
=
2740
electron
hole
pairs
x
ray
(
1
)
The results in Equation 1 assume that the total energy of the x ray is absorbed in the active region of the silicon.
According to a paper published in 1991 by Valentine et al., entitled “Charge Calibration of Systems with CsI(Tl), a Photodiode and a Charge Sensitive Preamplifier,” in Nuclear Instrumentation Methods, 1991, a CsI(Tl) coated PDA will yield an average of 47,900 electron hole (e. h.) pairs per MeV of photon energy for CsI in the temperature range of −15° to 40° C. The decay constant for CsI is 6 &mgr;sec. This is important only for high x-ray interaction rates within the CsI. For 10 keV x rays interacting with the CsI, this produces:
47
,
900
e
.
h
.
pairs
MeV
·
0.010
eV
=
479
e
.
h
.
pairs
interaction
(
2
)
In both Equations 1 and 2, the total amount of charge deposited in the photodiode will be determined by the flux of the x-ray beam. For example, if it is assumed that the event rate within a photodiode is one x ray per second, then the continuous current produced in the photodiode would be:
2740
e
.
h
.
pairs
x
ray
·
1
x
ray
sec
.
·
1.6
×
10
-
19
C
=
0.00043
pA
(
3
)
For the CsI photodetector combination and an assumed interaction rate of one x ray per second, the decay time (6 &mgr;sec) of the phosphor is much shorter than the event rate within the photodetector; hence, there are no overlapping pulses. Therefore, each photon generated within the scintillator and interacting with the photodiode will be detected. This yields a peak current per x-ray event of:
479
e
.
h
.
pairs
event
·
1
6
ms
·
1
×
10
6
ms
s
·
1.6
×
10
-
19
C
=
0.0127
pA
event
(
4
)
For a 5.4 keV x ray, the current produced in a bare and a CsI coated photodetector are estimated to be 0.235 and 6.5 pA/event, respectively.
While systems that utilize phosphor screens attached to a fiber-optic bundle optically coupled to an array of photodetectors are commercially available, it is desirable to employ such a phosphor screen/detector combination with an x-ray residual stress analysis device incorporating an isotropic source.
Other x-ray diffraction apparatuses have been developed for use in x-ray diffraction studies using conventional x-ray tubes. Typical of the art are those devices disclosed in the following U.S. Patents:
U.S. Pat. No.
Inventor(s)
Issue Date
5,125,016
Korhonen, M. et al.
Jun. 23, 1992
4,095,103
Cohen, J. et al.
Jun. 13, 1978
U.S. Pat. Nos. 5,125,016 and 4,095,103 both employ a conventional x-ray tube as the emission source for the x rays used in diffraction studies. Both the '016 and the '103 patents disclose the use of x-ray tubes in conjunction with one or two position sensitive detectors for use in single or multiple exposure x-ray diffraction studies. However, conventional x-ray tubes are too large to be practical when developing a miniaturized battery powered x-ray residual stress apparatus.
Sources of monoenergetic photons have been used for many years in fluorescence analysis of materials. Typical source designs may be found in an article by K. H. Ansell and E. G. Hall, entitled “Recent Developments of Low Energy Photon Sources,” published in the text
Applications of Low Energy X and Gamma Rays
, edited by Ziegler, and published by Gordon and Breach in 1970 and a publication by F. E. LeVert and E. Helminski, entitled “Literature Review and Commercial Source Evaluation of Americium-241,” ORO-4333-1, 1973. In these cases, the radioisotope may be a direct emitter of x rays (e.g., the electron capture process in Fe-55 leading to the 5.8 keV Mn x ray) or the x rays may be generated indirectly by using a monoenergetic source of photons to excite characteristic x rays in various pure element targets. To be of analytic use, the resulting direct or indirect x-ray radiation must be highly monoenergetic with negligible background contributions.
A paper presented by William S. Toothacker and Luther E. Preuss entitled “Radioisotopes as Zero Power Sources of X-rays for X-ray Diffraction Analysis,” published in Nucleonics in Aerospace by the Instrument Society of America, 1968, discussed the use of an isotropic source in x-ray diffraction. For purposes of this application, it is important to distinguish between x-ray diffraction and x-ray residual stress analysis. X-ray diffraction is the study of the structure of crystals and complex molecules through the diffractive properties of these bodies. Toothacker taught the use of a radioisotope as an x-ray source for the study of the structure and composition of matter. Given the definition of x-ray diffraction accepted by those persons skilled in the art, Toothacker does not make obvious the use of a radioisotope as a sealed x-ray source for residual stress analysis.
Residual stress analysis is different from x-ray diffraction in that x-ray diffraction is used to identify the compositi
Bailey V. Carol
Krafsur David S.
LeVert Francis E.
Pardue E. Beth
Ho Allen C.
Kim Robert H.
Pitts & Brittian P.C.
Technology for Energy Corporation
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