X-ray fluorescence analyzer

Details X-ray fluorescence analyzer X-ray fluorescence analyzer

2002-02-04

2004-07-20

Glick, Edward J. (Department: 2882)

X-ray or gamma ray systems or devices

Specific application

Fluorescence

C378S045000, C378S048000, C378S049000

Reexamination Certificate

active

06765986

ABSTRACT:

TECHNICAL FIELD AND BACKGROUND ART
Quantitative analysis of metal alloys in the field is an essential component to such commercial applications as the sorting of recyclable scrap metal, on-site sample analysis in mining facilities, non-destructive testing in specialty metal manufacturing, and positive material inspection of alloys. X-Ray Fluorescence (“XRF”) is the standard technique used to measure the composition of the major and minor elementary components with atomic number greater than about 20.
FIG. 1
is a prior art system that is used for measuring the composition of metal alloys and precious metals. As shown in
FIG. 1
, kilovolt photons
14
from a radioactive source
24
, impinge on a target
16
whose elements are to be analyzed. In front of the radioactive source is a window
17
, which is typically made out of stainless steel. The fluoresced x-rays
18
are detected in an energy-dispersing detector
20
connected to electronics
28
. The detector
20
is shielded from the radiation of source
24
and from any ambient radiation by a shield
22
. The incident photons
14
interact with the target
16
to produce the principal types of fluorescent radiation
18
including Compton scattering, Rayleigh scattering and photoelectric emission. Compton scattering produces a scattered x-ray with a lower energy than the incident x-ray; Rayleigh scattering produces an unchanged photon energy; and photoelectric emission, which occurs when an x-ray is absorbed by an element and x-rays characteristic of the element are emitted when the atom deexcites. The energy distribution of the fluorescent radiation is the sum of the characteristic x-rays from the target elements, the scattered radiation, and background radiations unconnected with the presence of the target. The energies of the gamma rays and x-rays emitted in the decays of
241
Am,
55
Fe and
109
Cd are given in Table I.
TABLE 1
Energy of XRF Radioisotopes
Isotope
Half-Life
Energy, keV
Identification
55
Fe
2.73
years
5.9
keV
K
&agr;
6.5
keV
K
&agr;
109
Cd
462
days
22.2
keV
K
&agr;
25
keV
K
&bgr;
88
keV
Gamma
241
Am
433
years
13.9
keV
L
&agr;
17.8
keV
L
&bgr;
20.8
keV
L
&ggr;
26.4
keV
Gamma
59.5
keV
Gamma
In order to analyze alloys and precious metals, XRF instruments must have high efficiency for exciting and detecting x-rays whose energies range from a few keV to approximately 35 keV. To attain such sensitivity for alloy analysis, the XRF instruments now deployed in the field, including those made by Niton Corporation, use several x-ray sources, each with an energy spectrum most sensitive to specific regions of the periodic table.
In the prior art XRF analyzers, the multiple radioactive sources are used in sequence and are changed by a changing module
32
so that each x-ray source is sequentially exposed to the material being analyzed. The three standard x-ray sources are
241
Am,
109
Cd and
55
Fe, though sometimes
253
Gd or
239
Pu are substituted for
241
Am The 59.5 keV gamma rays of
241
Am makes that source sensitive to elements in the tin region (Z=50), and efficiently covers the range of elements from rhodium (Z=45) to the rare-earth thulium (Z=69). A
109
Cd source is a strong emitter of 22.2 keV x-rays that are efficient for exciting the K x-ray spectra of elements from chromium (Z=25) to ruthenium (Z=44) as well as the L x-ray spectra of heavier elements from tungsten (Z=74) through uranium (Z=92); the 88 keV gamma ray is too weak for quick-time measurements. The 5.9 keV x-ray of
55
Fe is effective for exciting the elements titanium (Z=22), and vanadium (Z=23). The relative sensitivities of the three sources for measuring elements are given in Table 2.
TABLE 2
Relative Effectiveness of
55
Fe,
109
Cd, and
241
Am Sources
for XRF of an Iron Matrix
Ti
Cr
Fe
Zn
W(L)
Pb(L)
Zr
Mo
Ag
Ba
55
Fe
0.07
0.025
109
Cd
.014
0.034
0.08
0.05
0.06
0.16
0.68
1.2
241
Am, 59 keV
0.001
0.002
0.005
0.003
0.004
0.01
0.05
0.08
0.21
1.0
All commercially available alloy analyzers use
109
Cd sources as the primary source with
55
Fe used to increase the sensitivity to the lightest elements and
241
Am to analyze the elements in the tin region.
Multi-source instruments have several drawbacks. One drawback is cost. The individual radioactive sources are expensive and adding additional radioactive sources increases the cost proportionally. Second, when testing is performed on a material, the radioactive sources are used sequentially to minimize interference. Using the sources sequentially is very time consuming. Third, in order to use the source in a sequential manner, each source requires a source-changing mechanism, increasing the cost, size and complexity of the analyzer. Fourth, the multi-source system has issues of normalization and mechanical reproducibility.
Although a single source instrument would provide distinct advantages and overcome the inherent problems described above, certain prohibitions have caused the reliance on multi-source instruments. First, there is no known single radioactive source that provides a usable energy spectrum when used with the prior art XRF analysis methods. For example, an
241
Am source has a spectrum with strong monoenergetic photons emitted in the range from 13.9 keV to 26.4 keV and previous analytic methods were unable to quantify this region due to the interfering Rayleigh and Compton scattering intensities that depended on the material being analysed.
SUMMARY OF THE INVENTION
In a first embodiment of the invention there is provided a device for photon fluorescence. The device includes a single radioactive source, such as
241
Am. Both the emitted x-rays and gamma rays are used to determine the composition of a test material, such as a metal alloy or a precious metal that contains trace elements. An energy detector is used for receiving the fluoresced x-rays and gamma rays from the test material. The energy detector passes a signal to electronics for processing. The electronics process the signal and determine the composition of the test material based in part on the fluoresced x-rays and gamma rays. The electronics compensate for interfering Rayleigh and Compton scattering peaks by first choosing a Rayleigh scattered peak in a region of the spectrum that does not interfere with any fluoresced x-ray from the metal sample. This is the reference peak for the spectrum. For metal samples, the intensity of Rayleigh scattering through 180° is sufficiently independent of Z that the intensity of the reference peak determines the intensity of all the other Rayleigh scattered lines. Specifically, the Rayleigh scattered spectrum from a typical metal such as iron is stored in the device's computer. The intensity of the reference line in the sample spectrum is compared to the intensity of the reference line in the stored spectrum and the ratio is applied to the stored spectrum, which is then subtracted out of the sample spectrum. In this way, one accounts for the interfering Rayleigh peaks in the measured sample spectrum. The intensity of Compton scattering in the 12 keV to 20 keV range is low enough, from metals heavier than titanium, that they can be taken into account as well from the reference spectrum. The invention is illustrated with the use of
241
Am since this source is traditionally used as a source of only 59.5 keV gamma rays. The technique can be usefully employed with other sources, for example
239
Pu. If that source is used with a beryllium exit window so that the L lines are used and not absorbed, then the 12.6 keV L
&agr;
line is the appropriate normalizing Rayleigh peak.
The device further includes a shield for the radioactive source. The shield isolates the detector from direct exposure to the x-rays and gamma rays of the radioactive source so that the detector mainly receives the fluoresced radiation from the test material. The shield surrounds the radioactive source except in the direction of the test material. A source backing may be selected such as Rhodium so that the radioactive material interacts with the so

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